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Introducing the THESAN project: radiation-magnetohydrodynamic simulations of the epoch of reionization

Rahul Kannan, Enrico Garaldi, Aaron, Smith, Ruediger Pakmor, Volker Springel, Mark Vogelsberger, Lars Hernquist
Journal Paper MNRAS 2021, tmp.3440

Abstract

We introduce the THESAN project, a suite of large volume (Lbox = 95.5 cMpc) radiation-magneto-hydrodynamic simulations that simultaneously model the large-scale statistical properties of the intergalactic medium (IGM) during reionization and the resolved characteristics of the galaxies responsible for it. The flagship simulation has dark matter and baryonic mass resolutions of 3.1 × 106 M⊙ and 5.8 × 105 M⊙, respectively. The gravitational forces are softened on scales of 2.2 ckpc with the smallest cell sizes reaching 10 pc at z = 5.5, enabling predictions down to the atomic cooling limit. The simulations use an efficient radiation hydrodynamics solver (AREPO-RT) that precisely captures the interaction between ionizing photons and gas, coupled to well-tested galaxy formation (IllustrisTNG) and dust models to accurately predict the properties of galaxies. Through a complementary set of medium resolution simulations we investigate the changes to reionization introduced by different assumptions for ionizing escape fractions, varying dark matter models, and numerical convergence. The fiducial simulation and model variations are calibrated to produce realistic reionization histories that match the observed evolution of the global neutral hydrogen fraction and electron scattering optical depth to reionization. They also match a wealth of high-redshift observationally inferred data, including the stellar-to-halo-mass relation, galaxy stellar mass function, star formation rate density, and the mass-metallicity relation, despite the galaxy formation model being mainly calibrated at z = 0. We demonstrate that different reionization models give rise to varied bubble size distributions that imprint unique signatures on the 21 cm emission, especially on the slope of the power spectrum at large spatial scales, enabling current and upcoming 21 cm experiments to accurately characterise the sources that dominate the ionizing photon budget.

H-alpha emission in local galaxies: star formation, time variability and the diffuse ionized gas

Sandro Tacchella, Aaron Smith, Rahul Kannan, Federico Marinacci, Lars Hernquist, Mark Vogelsberger, Paul Torrey, Laura Sales, Hui Li
Journal Paper MNRAS sumitted, arXiv:2112.00027

Abstract

The nebular recombination line Hα is widely used as a star-formation rate (SFR) indicator in the local and high-redshift Universe. We present a detailed Hα radiative transfer study of high-resolution isolated Milky-Way and Large Magellanic Cloud simulations that include radiative transfer, non-equilibrium thermochemistry, and dust evolution. We focus on the spatial morphology and temporal variability of the Hα emission, and its connection to the underlying gas and star formation properties. The Hα and Hβ radial and vertical surface brightness profiles are in excellent agreement with observations of nearby galaxies. We find that the fraction of Hα emission from collisional excitation amounts to fcol∼5−10%, only weakly dependent on radius and vertical height, and that scattering boosts the Hα luminosity by ∼40%. The dust correction via the Balmer decrement works well (intrinsic Hα emission recoverable within 25%), though the dust attenuation law depends on the amount of attenuation itself both on spatially resolved and integrated scales. Important for the understanding of the Hα-SFR connection is the dust and helium absorption of ionizing radiation (Lyman continuum [LyC] photons), which are about fabs≈28% and fHe≈9%, respectively. Together with an escape fraction of fesc≈6%, this reduces the available budget for hydrogen line emission by nearly half (fH≈57%). We discuss the impact of the diffuse ionized gas, showing - among other things - that the extraplanar Hα emission is powered by LyC photons escaping the disc. Future applications of this framework to cosmological (zoom-in) simulations will assist in the interpretation of spectroscopy of high-redshift galaxies with the upcoming James Webb Space Telescope.

The physics of Lyman-alpha escape from disc-like galaxies

Aaron Smith, Rahul Kannan, Sandro Tacchella, Mark Vogelsberger, Lars Hernquist, Federico Marinacci, Laura Sales, Paul Torrey, Hui Li, Yuan-Chen Yeh, Jia, Qi
Journal Paper MNRAS submitted, arXiv:2111.13721

Abstract

Hydrogen emission lines can provide extensive information about star-forming galaxies in both the local and high-redshift Universe. We present a detailed Lyman continuum (LyC), Lyman-alpha (Ly{\alpha}), and Balmer line (H{\alpha} and H\b{eta}) radiative transfer study of a high-resolution isolated Milky-Way simulation using the Arepo-RT radiation hydrodynamics code with the SMUGGLE galaxy formation model. The realistic framework includes stellar feedback, non-equilibrium thermochemistry, and dust grain evolution in the interstellar medium (ISM). We extend our Cosmic Ly{\alpha} Transfer (COLT) code with photoionization equilibrium Monte Carlo radiative transfer for self-consistent end-to-end (non-)resonant line predictions. Accurate LyC reprocessing to recombination emission requires modelling pre-absorption by dust (27.5%), helium ionization (8.7%), and anisotropic escape fractions (7.9%), as these reduce the available budget for hydrogen line emission (55.9%). We investigate the role of the multiphase dusty ISM, disc geometry, gas kinematics, and star formation activity in governing the physics of emission and escape, focusing on the time variability, gas phase structure, and spatial, spectral, and viewing angle dependence of the emergent photons. Isolated disc simulations are well-suited for comprehensive observational comparisons with local H{\alpha} surveys, but would require a proper cosmological circumgalactic medium (CGM) environment as well as less dust absorption and rotational broadening to serve as analogs for high-redshift Ly{\alpha} emitting galaxies. Future applications of our framework to next-generation cosmological simulations of galaxy formation including radiation-hydrodynamics that resolve 10 pc multiphase ISM and 1 kpc CGM structures will provide crucial insights and predictions for current and upcoming Ly{\alpha} observations.

The THESAN project: predictions for multi-tracer line intensity mapping in the Epoch of Reionization

Rahul Kannan, Aaron Smith, Enrico Garaldi, Xuejian Shen, Mark Vogelsberger, Ruediger Pakmor, Volker Springel, Lars Hernquist
Journal Paper MNRAS submitted, arXiv:2111.02411

Abstract

Line intensity mapping (LIM) is rapidly emerging as a powerful technique to study galaxy formation and cosmology in the high-redshift Universe. We present LIM estimates of select spectral lines originating from the interstellar medium (ISM) of galaxies and 21 cm emission from neutral hydrogen gas in the Universe using the large volume, high resolution THESAN reionization simulations. A combination of sub-resolution photo-ionization modelling for HII regions and Monte Carlo radiative transfer calculations is employed to estimate the dust-attenuated spectral energy distributions (SEDs) of high-redshift galaxies (z≳5.5). We show that the derived photometric properties such as the ultraviolet (UV) luminosity function and the UV continuum slopes match observationally inferred values, demonstrating the accuracy of the SED modelling. We provide fits to the luminosity--star formation rate relation (L-SFR) for the brightest emission lines and find that important differences exist between the derived scaling relations and the widely used low-z ones because the interstellar medium of reionization era galaxies is generally less metal-enriched than in their low redshift counterparts. We use these relations to construct line intensity maps of nebular emission lines and cross correlate with the 21 cm emission. Interestingly, the wavenumber at which the correlation switches sign (ktransition) depends heavily on the reionization model and to a lesser extent on the targeted emission line, which is consistent with the picture that ktransition probes the typical sizes of ionized regions. The derived scaling relations and intensity maps represent a timely state-of-the-art framework for forecasting and interpreting results from current and upcoming LIM experiments.

The THESAN project: Lyman-alpha emission and transmission during the Epoch of Reionization

Aaron Smith, Rahul Kannan, Enrico Garaldi, Mark Vogelsberger, Ruediger Pakmor, Volker Springel, Lars Hernquist
Journal Paper MNRAS submitted, arXiv:2110.02966

Abstract

The visibility of high-redshift Lyman-alpha emitting galaxies (LAEs) provides important constraints on galaxy formation processes and the Epoch of Reionization (EoR). However, predicting realistic and representative statistics for comparison with observations represents a significant challenge in the context of large-volume cosmological simulations. The THESAN project offers a unique framework for addressing such limitations by combining state-of-the-art galaxy formation (IllustrisTNG) and dust models with the Arepo-RT radiation-magneto-hydrodynamics solver. In this initial study we present Lyman-alpha centric analysis for the flagship simulation that resolves atomic cooling haloes throughout a (95.5 cMpc)^3 region of the Universe. To avoid numerical artifacts we devise a novel method for accurate frequency-dependent line radiative transfer in the presence of continuous Hubble flow, transferable to broader astrophysical applications as well. Our scalable approach highlights the utility of LAEs and red damping-wing transmission as probes of reionization, which reveal nontrivial trends across different galaxies, sightlines, and frequency bands that can be modelled in the framework of covering fractions. In fact, after accounting for environmental factors influencing large-scale ionized bubble formation such as redshift and UV magnitude, the variation across galaxies and sightlines mainly depends on random processes including peculiar velocities and self-shielded systems that strongly impact unfortunate rays more than others. Throughout the EoR local and cosmological optical depths are often greater than or less than unity such that the exp(-tau) behavior leads to anisotropic and bimodal transmissivity. Future surveys will benefit by targeting both rare bright objects and Goldilocks zone LAEs to infer the presence of these (un)predictable (dis)advantages.

The THESAN project: properties of the intergalactic medium and its connection to Reionization-era galaxies

Enrico Garaldi, Rahul Kannan, Aaron Smith, Volker Springel, Ruediger Pakmor, Mark Vogelsberger, Lars Hernquist
Journal Paper MNRAS submitted, arXiv:2110.01628

Abstract

The high-redshift intergalactic medium (IGM) and the primeval galaxy population are rapidly becoming the new frontier of extra-galactic astronomy. We investigate the IGM properties and their connection to galaxies at z≥5.5 under different assumptions for the ionizing photon escape and the nature of dark matter, employing our novel THESAN radiation-hydrodynamical simulation suite, designed to provide a comprehensive picture of the emergence of galaxies in a full reionization context. Our simulations have realistic `late' reionization histories, match available constraints on global IGM properties and reproduce the recently-observed rapid evolution of the mean free path of ionizing photons. We additionally examine high-z Lyman-α transmission. The optical depth evolution is consistent with data, and its distribution suggests an even-later reionization than simulated, although with a strong sensitivity to the source model. We show that the effects of these two unknowns can be disentangled by characterising the spectral shape and separation of Lyman-α transmission regions, opening up the possibility to observationally constrain both. For the first time in simulations, THESAN reproduces the modulation of the Lyman-α flux as a function of galaxy distance, demonstrating the power of coupling a realistic galaxy formation model with proper radiation-hydrodynamics. We find this feature to be extremely sensitive on the timing of reionization, while being relatively insensitive to the source model. Overall, THESAN produces a realistic IGM and galaxy population, providing a robust framework for future analysis of the high-z Universe.

Real and counterfeit cores: how feedback expands halos and disrupts tracers of inner gravitational potential in dwarf galaxies

Ethan Jahn, Laura Sales, Federico Marinacci, Mark Vogelsberger, Paul Torrey, Jia Qi, Aaron Smith, Hui Li, Rahul Kannan, Jan Burger, Jesus Zavala
Journal Paper MNRAS submitted, arXiv:2110.00142

Abstract

The tension between the diverging density profiles in Lambda Cold Dark Matter (ΛCDM) simulations and the constant-density inner regions of observed galaxies is a long-standing challenge known as the `core-cusp' problem. We demonstrate that the \texttt{SMUGGLE} galaxy formation model implemented in the \textsc{Arepo} moving mesh code forms constant-density cores in idealized dwarf galaxies of M⋆≈8×107 M⊙ with initially cuspy dark matter halos of M200≈1010 M⊙. Identical initial conditions run with the Springel and Hernquist (2003; SH03) feedback model preserve cuspiness. Literature on the subject has pointed to the low density threshold for star formation, ρth, in SH03-like models as an obstacle to baryon-induced core formation. Using a \texttt{SMUGGLE} run with equal ρth to SH03, we demonstrate that core formation can proceed at low density thresholds, indicating that ρth is insufficient on its own to determine whether a galaxy develops a core. We suggest that the ability to resolve a multiphase interstellar medium at sufficiently high densities is a more reliable indicator of core formation than any individual model parameter. In \texttt{SMUGGLE}, core formation is accompanied by large degrees of non-circular motion, with gas rotational velocity profiles that consistently fall below the circular velocity vcirc=GM/R−−−−−−√ out to ∼2 kpc. This may artificially mimic larger core sizes when derived from observable quantities compared to the size measured from the dark matter distribution (∼0.5 kpc), highlighting the need for careful modeling in the inner regions of dwarfs to infer the true distribution of dark matter.

Characterizing hydrostatic mass bias with MOCK-X

David Barnes, Mark Vogelsberger, Francesca Pearce, Ana-Roxana Pop, Rahul Kannan, Kaili Cao, Scott Kay, Lars Hernquist
Journal Paper MNRAS 2021, 506, 2533

Abstract

Surveys in the next decade will deliver large samples of galaxy clusters that transform our understanding of their formation. Cluster astrophysics and cosmology studies will become systematics limited with samples of this magnitude. With known properties, hydrodynamical simulations of clusters provide a vital resource for investigating potential systematics. However, this is only realized if we compare simulations to observations in the correct way. Here we introduce the MOCK-X analysis framework, a multiwavelength tool that generates synthetic images from cosmological simulations and derives halo properties via observational methods. We detail our methods for generating optical, Compton-y and X-ray images. Outlining our synthetic X-ray image analysis method, we demonstrate the capabilities of the framework by exploring hydrostatic mass bias for the IllustrisTNG, BAHAMAS, and MACSIS simulations. Using simulation derived profiles we find an approximately constant bias b ≍ 0.13 with cluster mass, independent of hydrodynamical method, or subgrid physics. However, the hydrostatic bias derived from synthetic observations is mass-dependent, increasing to b = 0.3 for the most massive clusters. This result is driven by a single temperature fit to a spectrum produced by gas with a wide temperature distribution in quasi-pressure equilibrium. The spectroscopic temperature and mass estimate are biased low by cooler gas dominating the emission, due to its quadratic density dependence. The bias and the scatter in estimated mass remain independent of the numerical method and subgrid physics. Our results are consistent with current observations and future surveys will contain sufficient samples of massive clusters to confirm the mass dependence of the hydrostatic bias.

Dust entrainment in galactic winds

Rahul Kannan, Mark Vogelsberger, Federico Marinacci, Laura Sales, Paul Torrey, Lars Hernquist
Journal Paper MNRAS 2021, 503, 336

Abstract

Winds driven by stellar feedback are an essential part of the galactic ecosystem and are the main mechanism through which low-mass galaxies regulate their star formation. These winds are generally observed to be multiphase with detections of entrained neutral and molecular gas. They are also thought to enrich the circumgalactic medium around galaxies with metals and dust. This ejected dust encodes information about the integrated star formation and outflow history of the galaxy. Therefore it is important to understand how much dust is entrained and driven out of the disc by galactic winds. Here, we demonstrate that stellar feedback is efficient in driving dust-enriched winds and eject enough material to account for the amount of extraplanar dust observed in nearby galaxies. The amount of ejected dust depends on the sites from where they are launched, with dustier galaxies launching more dust-enriched outflows. Moreover, the outflowing cold and dense gas is significantly more dust enriched than the volume filling hot and tenuous material. These results provide an important new insight into the dynamics, structure, and composition of galactic winds and their role in determining the dust content of the extragalactic gas in galaxies.

Simulating dust grain-radiation coupling on a moving mesh

Ryan McKinnon, Rahul Kannan, Mark Vogelsberger, Stephanie O'Neil, Paul Torrey, Hui Li
Journal Paper MNRAS 2021, 502, 1344

Abstract

We present a model for the interaction between dust and radiation fields in the radiation hydrodynamic code AREPO-RT, which solves the moment-based radiative transfer equations on an unstructured moving mesh. Dust is directly treated using live simulation particles, each of which represent a population of grains that are coupled to hydrodynamic motion through a drag force. We introduce methods to calculate radiation pressure on and photon absorption by dust grains. By including a direct treatment of dust, we are able to calculate dust opacities and update radiation fields self-consistently based on the local dust distribution. This hybrid scheme coupling dust particles to an unstructured mesh for radiation is validated using several test problems with known analytic solutions, including dust driven via spherically-symmetric flux from a constant luminosity source and photon absorption from radiation incident on a thin layer of dust. Our methods are compatible with the multifrequency scheme in AREPO-RT, which treats UV and optical photons as single-scattered and IR photons as multi-scattered. At IR wavelengths, we model heating of and thermal emission from dust. Dust and gas are not assumed to be in local thermodynamic equilibrium but transfer energy through collisional exchange. We estimate dust temperatures by balancing these dust-radiation and dust-gas energy exchange rates. This framework for coupling dust and radiation can be applied in future radiation hydrodynamic simulations of galaxy formation.

AREPO-MCRT: Monte Carlo Radiation Hydrodynamics on a Moving Mesh

Aaron Smith, Rahul Kannan, Benny Tsang, Mark Vogelsberger, Ruediger Pakmor
Journal Paper ApJ 2020, 905, 27

Abstract

We present a model for the interaction between dust and radiation fields in the radiation hydrodynamic code AREPO-RT, which solves the moment-based radiative transfer equations on an unstructured moving mesh. Dust is directly treated using live simulation particles, each of which represent a population of grains that are coupled to hydrodynamic motion through a drag force. We introduce methods to calculate radiation pressure on and photon absorption by dust grains. By including a direct treatment of dust, we are able to calculate dust opacities and update radiation fields self-consistently based on the local dust distribution. This hybrid scheme coupling dust particles to an unstructured mesh for radiation is validated using several test problems with known analytic solutions, including dust driven via spherically-symmetric flux from a constant luminosity source and photon absorption from radiation incident on a thin layer of dust. Our methods are compatible with the multifrequency scheme in AREPO-RT, which treats UV and optical photons as single-scattered and IR photons as multi-scattered. At IR wavelengths, we model heating of and thermal emission from dust. Dust and gas are not assumed to be in local thermodynamic equilibrium but transfer energy through collisional exchange. We estimate dust temperatures by balancing these dust-radiation and dust-gas energy exchange rates. This framework for coupling dust and radiation can be applied in future radiation hydrodynamic simulations of galaxy formation.

Radiative AGN feedback on a moving mesh: the impact of the galactic disc and dust physics on outflow properties

David Barnes, Rahul Kannan , Mark Vogelsberger, Federico Marinacci
Journal Paper MNRAS, 2020, 194, 1143

Abstract

Feedback from accreting supermassive black holes, active galactic nuclei (AGN), is now a cornerstone of galaxy formation models. In this work, we present radiation-hydrodynamic simulations of radiative AGN feedback using the novel AREPO-RT. A central black hole emits radiation at a constant luminosity and drives an outflow via radiation pressure on dust grains. Utilising an isolated NFW halo we validate our setup in the single and multi-scattering regimes, with the simulated shock front propagation in excellent agreement with the expected analytic result. For a spherically symmetric NFW halo, an examination of the simulated outflow properties generated by radiative feedback demonstrates that they are lower than typically observed at a fixed AGN luminosity, regardless of the collimation of the radiation. We then explore the impact of a central disc galaxy and the assumed dust model on the outflow properties. The contraction of the halo during the galaxy's formation and modelling the production of dust grains results in a factor 100 increase in the halo's optical depth. Radiation is then able to couple momentum more efficiently to the gas, driving a stronger shock and producing a mass-loaded $10^3 \, \mathrm{M}_\odot \, \mathrm{yr}^{-1}$ outflow with a velocity of $\sim 2000 \, \mathrm{km \, s}^{-1}$, in agreement with observations. However, the inclusion of dust destruction mechanisms, like thermal sputtering, leads to the rapid destruction of dust grains within the outflow, reducing its properties below typically observed values. We conclude that radiative AGN feedback can drive outflows, but a thorough numerical and physical treatment is required to assess its true impact.

Simulating the interstellar medium of galaxies with radiative transfer, non-equilibrium thermochemistry, and dust

Kannan, Marinacci, Vogelsberger, Sales, Torrey, Springel, Hernquist
Journal Paper MNRAS, 2020, 499, 5732

Abstract

We present a novel framework to self-consistently model the effects of radiation fields, dust physics and molecular chemistry (H 2 ) in the interstellar medium (ISM) of galaxies. The model combines a state-of-the-art radiation hydrodynamics module with a non-equilibrium thermochemistry module that accounts for H 2 coupled to a realistic dust formation and destruction model, all integrated into the new stellar feedback framework SMUGGLE. We test this model on high-resolution isolated Milky-Way (MW) simulations. We show that photoheating from young stars makes stellar feedback more efficient, but this effect is quite modest in low gas surface density galaxies like the MW. The multi-phase structure of the ISM, however, is highly dependent on the strength of the interstellar radiation field. We are also able to predict the distribution of H 2 , that allow us to match the molecular Kennicutt-Schmidt (KS) relation, without calibrating for it. We show that the dust distribution is a complex function of density, temperature and ionization state of the gas which cannot be reproduced by simple scaling relations often used in the literature. Our model is only able to match the observed dust temperature distribution if radiation from the old stellar population is considered, implying that these stars have a non-negligible contribution to dust heating in the ISM. Our state-of-the-art model is well-suited for performing next generation cosmological galaxy formation simulations, which will be able to predict a wide range of resolved ( ∼10 pc) properties of galaxies.

Efficacy of early stellar feedback in low gas surface density environments

Rahul Kannan , Federico Marinacci, Christine M. Simpson, Simon C. O. Glover, Lars Hernquist
Journal Paper MNRAS, 2020, 491, 2088

Abstract

We present a suite of high resolution radiation hydrodynamic simulations of a small patch (1 kpc2) of the inter-stellar medium (ISM) performed with AREPO-RT, with the aim to quantify the efficacy of various feedback processes like supernovae explosions (SN), photoheating and radiation pressure in low gas surface density galaxies (Σgas ≃ 10 M☉ pc-2). We show that radiative feedback decrease the star formation rate and therefore the total stellar mass formed by a factor of ̃2. This increases the gas depletion timescale and brings the simulated Kennicutt-Schmidt relation closer to the observational estimates. Radiation feedback coupled with SN is more efficient at driving outflows with the mass and energy loading increasing by a factor of ̃10. This increase is mainly driven by the additional entrainment of medium density (10-2 ≤ n < 1 cm-3), warm (300 K ≤ T < 8000 K) material. Therefore including radiative feedback tends to launch colder, denser and more mass and energy loaded outflows. This is because photoheating of the high density gas around a newly formed star over-pressurises the region, causing it to expand. This reduces the ambient density in which the SN explode by a factor of 10 - 100 which in turn increases their momentum output by a factor of ̃1.5 - 2.5. Finally, we note that in these low gas surface density environments, radiative feedback primarily impact the ISM via photoheating and radiation pressure has only a minimal role in regulating star formation.

Imprints of temperature fluctuations on the z~5 Lyman-α forest: a view from radiation-hydrodynamic simulations of reionization

Wu, McQuinn, Kannan, D'Aloisio, Bird, Marinacci, Davé, Hernquist
Journal Paper MNRAS, 2019, 490, 3177

Abstract

Reionization leads to large spatial fluctuations in the intergalactic temperature that can persist well after its completion. We study the imprints of such fluctuations on the z ̃ 5 Ly α forest flux power spectrum using a set of radiation-hydrodynamic simulations that model different reionization scenarios. We find that large-scale coherent temperature fluctuations bring {̃}20-60{{ per cent}} extra power at k ̃ 0.002 s km-1, with the largest enhancements in the models where reionization is extended or ends the latest. On smaller scales (k ≳ 0.1 s km-1), we find that temperature fluctuations suppress power by {≲}10{{ per cent}}. We find that the shape of the power spectrum is mostly sensitive to the reionization mid-point rather than temperature fluctuations from reionization's patchiness. However, for all of our models with reionization mid-points of z ≤ 8 (z ≤ 12), the shape differences are {≲}20{{ per cent}} ({≲}40{{ per cent}}) because of a surprisingly well-matched cancellation between thermal broadening and pressure smoothing that occurs for realistic thermal histories. We also consider fluctuations in the ultraviolet background, finding their impact on the power spectrum to be much smaller than temperature fluctuations at k ≳ 0.01 s km-1. Furthermore, we compare our models to power spectrum measurements, finding that none of our models with reionization mid-points of z < 8 is strongly preferred over another and that all of our models with mid-points of z ≥ 8 are excluded at 2.5σ. Future measurements may be able to distinguish between viable reionization models if they can be performed at lower k or, alternatively, if the error bars on the high-k power can be reduced by a factor of 1.5.

Local photoionization feedback effects on galaxies

Obreja, Macciò, Moster, Udrescu, Buck, Kannan, Dutton, Blank
Journal Paper MNRAS, 2019, 490, 1518

Abstract

We implement an optically thin approximation for the effects of the local radiation field from stars and hot gas on the gas heating and cooling in the N-body smoothed particle hydrodynamics code GASOLINE2. We resimulate three galaxies from the NIHAO project: one dwarf, one Milky Way-like, and one massive spiral, and study what are the local radiation field effects on various galaxy properties. We also study the effects of varying the ultraviolet background (UVB) model, by running the same galaxies with two different UVBs. Galaxy properties at z = 0 like stellar mass, stellar effective mass radius, H I mass, and radial extent of the H I disc show significant changes between the models with and without the local radiation field, and smaller differences between the two UVB models. The intrinsic effect of the local radiation field through cosmic time is to increase the equilibrium temperature at the interface between the galaxies and their circumgalactic media (CGM), moving this boundary inwards, while leaving relatively unchanged the gas inflow rate. Consequently, the temperature of the inflow increases when considering the local radiation sources. This temperature increase is a function of total galaxy mass, with a median CGM temperature difference of one order of magnitude for the massive spiral. The local radiation field suppresses the stellar mass growth by 20 per cent by z = 0 for all three galaxies, while the H I mass is roughly halved. The differences in the gas phase diagrams, significantly impact the H I column densities, shifting their peaks in the distributions towards lower NH I.

Enhancing AGN efficiency and cool-core formation with anisotropic thermal conduction

Barnes, Kannan , Vogelsberger, Pfrommer, Puchwein, Weinberger, Springel, Pakmor, Nelson, Marinacci, Pillepich, Torrey, Hernquist
Journal Paper MNRAS, 2019, 488, 3003

Abstract

Understanding how baryonic processes shape the intracluster medium (ICM) is of critical importance to the next generation of galaxy cluster surveys. However, many models of structure formation neglect potentially important physical processes, like anisotropic thermal conduction (ATC). We explore the impact of ATC on the prevalence of cool-cores (CCs) via 12 pairs of magnetohydrodynamical galaxy cluster simulations, using the IllustrisTNG model with and without ATC. Examining their properties we find that the addition of ATC has a negligible impact on the median rotation measure, plasma β, the magnetic field-radial direction angle, and the effective Spitzer value. However, the scatter in the angle and effective Spitzer value is 50 per cent larger with ATC because the magnetic field aligns with the azimuthal direction to a greater extent in relaxed clusters. ATC's impact varies from cluster to cluster and with CC criterion, but its inclusion produces a systematic shift to larger CC fractions at z = 0 for all CC criteria considered. Additionally, the inclusion of ATC flattens the CC fraction redshift evolution, helping to ease the tension with the observed evolution. With ATC, the energy required for the central black hole to self-regulate is reduced by 24 per cent and the gas fraction at 0.01 r_{500} increases by 100 per cent, producing larger CC fractions. ATC makes the ICM unstable to perturbations and the increased efficiency of AGN feedback suggests that its inclusion results in a greater level of mixing in the ICM, demonstrated by the 10 per cent reduction in central metallicity for clusters with ATC.

Simulating the effect of photoheating feedback during reionization

Xiaohan Wu, Rahul Kannan , Fedrico Marinacci, Mark Vogelsberger, Lars Hernquist
Journal Paper MNRAS, 2019, 488, 419

Abstract

We present self-consistent radiation hydrodynamic simulations of hydrogen reionization performed with AREPO-RT complemented by a state-of-the-art galaxy formation model. We examine how photoheating feedback, due to reionization, shapes the galaxies properties. Our fiducial model completes reionization by z ≈ 6 and matches observations of the Ly α forest, the cosmic microwave background electron scattering optical depth, the high-redshift ultraviolet (UV) luminosity function, and stellar mass function. Contrary to previous works, photoheating suppresses star formation rates by more than 50{{ per cent}} only in haloes less massive than ̃108.4 M☉ (̃108.8 M☉) at z = 6 (z = 5), suggesting inefficient photoheating feedback from photons within galaxies. The use of a uniform UV background that heats up the gas at z ≈ 10.7 generates an earlier onset of suppression of star formation compared to our fiducial model. This discrepancy can be mitigated by adopting a UV background model with a more realistic reionization history. In the absence of stellar feedback, photoheating alone is only able to quench haloes less massive than ̃109 M☉ at z ≳ 5, implying that photoheating feedback is sub-dominant in regulating star formation. In addition, stellar feedback, implemented as a non-local galactic wind scheme in the simulations, weakens the strength of photoheating feedback by reducing the amount of stellar sources. Most importantly, photoheating does not leave observable imprints in the UV luminosity function, stellar mass function, or the cosmic star formation rate density. The feasibility of using these observables to detect imprints of reionization therefore requires further investigation.

Dust in and around galaxies: dust in cluster environments and its impact on gas cooling

Mark Vogelsberger, Ryan McKinnon, Stephanie O'Neil, Federico Marinacci, Paul Torrey, Rahul Kannan
Journal Paper MNRAS, 2019, 487, 4870

Abstract

Simulating the dust content of galaxies and their surrounding gas is challenging due to the wide range of physical processes affecting the dust evolution. Here we present cosmological hydrodynamical simulations of a cluster of galaxies, M_200,crit=6 × 10^{14}{ M_\odot }, including a novel dust model for the moving mesh code AREPO. This model includes dust production, growth, supernova-shock-driven destruction, ion-collision-driven thermal sputtering, and high-temperature dust cooling through far-infrared reradiation of collisionally deposited electron energies. Adopting a rather low thermal sputtering rate, we find, consistent with observations, a present-day overall dust-to-gas ratio of ̃2 × 10-5, a total dust mass of {̃ } 2× 10^9{ M_\odot }, and a dust mass fraction of ̃3 × 10-6. The typical thermal sputtering time-scales within {̃ } 100 kpc are around {̃ } 10 Myr, and increase towards the outer parts of the cluster to {̃ } 10^3 Myr at a cluster-centric distance of 1 Mpc. The condensation of gas-phase metals into dust grains reduces high-temperature metal-line cooling, but also leads to additional dust infrared cooling. The additional infrared cooling changes the overall cooling rate in the outer parts of the cluster, beyond {̃ } 1 Mpc, by factors of a few. This results in noticeable changes of the entropy, temperature, and density profiles of cluster gas once dust formation is included. The emitted dust infrared emission due to dust cooling is consistent with observational constraints.

AREPO-RT: radiation hydrodynamics on a moving mesh

Rahul Kannan , Mark Vogelsberger, Federico Marinacci, Ryan Mckinnon, Ruediger Pakmor, Volker Springel
Journal Paper MNRAS, 2019, 485, 117

Abstract

We introduce AREPO-RT, a novel radiation hydrodynamic (RHD) solver for the unstructured moving-mesh code AREPO. Our method solves the moment-based radiative transfer equations using the M1 closure relation. We achieve second-order convergence by using a slope-limited linear spatial extrapolation and a first-order time prediction step to obtain the values of the primitive variables on both sides of the cell interface. A Harten-Lax-van Leer flux function, suitably modified for moving meshes, is then used to solve the Riemann problem at the interface. The implementation is fully conservative and compatible with the individual time-stepping scheme of AREPO. It incorporates atomic hydrogen (H) and helium (He) thermochemistry, which is used to couple the ultraviolet radiation field to the gas. Additionally, infrared (IR) radiation is coupled to the gas under the assumption of local thermodynamic equilibrium between the gas and the dust. We successfully apply our code to a large number of test problems, including applications such as the expansion of H II regions, radiation pressure-driven outflows, and the levitation of optically thick layer of gas by trapped IR radiation. The new implementation is suitable for studying various important astrophysical phenomena, such as the effect of radiative feedback in driving galactic scale outflows, radiation-driven dusty winds in high-redshift quasars, or simulating the reionization history of the Universe in a self-consistent manner.

A census of cool core galaxy clusters in IllustrisTNG

Barnes, Vogelsberger, Kannan , Marinacci, Weinberger, Springel, Torrey, Pillepich, Nelson, Pakmor, Naiman, Hernquist, McDonald
Journal Paper MNRAS, 2018, 481, 1809

Abstract

The thermodynamic structure of hot gas in galaxy clusters is sensitive to astrophysical processes and typically difficult to model with galaxy formation simulations. We explore the fraction of cool-core (CC) clusters in a large sample of 370 clusters from IllustrisTNG, examining six common CC definitions. IllustrisTNG produces continuous CC criteria distributions, the extremes of which are classified as CC and non-cool-core (NCC), and the criteria are increasingly correlated for more massive clusters. At z=0 the CC fraction is systematically lower than observed for the complete sample, selecting massive systems increases the CC fraction for 3 criteria and reduces it for others. This result is partly driven by systematic differences between the simulated and observed gas fraction profiles. The simulated CC fraction increases more rapidly with redshift than observed, independent of mass or redshift range, and the CC fraction is overpredicted at z>1. The conversion of CCs to NCCs begins later and acts more rapidly in the simulations. Examining the fraction of CCs and NCCs defined as relaxed we find no evidence that CCs are more relaxed, suggesting that mergers are not solely responsible for disrupting CCs. A comparison of the median thermodynamic profiles defined by different CC criteria shows that the extent to which they evolve in the cluster core is dependent on the CC criteria. We conclude that the thermodynamic structure of galaxy clusters in IllustrisTNG shares many similarities with observations, but achieving better agreement most likely requires modifications of the underlying galaxy formation model.

Simulating galactic dust grain evolution on a moving mesh

Ryan McKinnon, Mark Vogelsberger, Paul Torrey, Federico Marinacci, Rahul Kannan
Journal Paper MNRAS, 2018, 481, 1809

Abstract

Interstellar dust is an important component of the galactic ecosystem, playing a key role in multiple galaxy formation processes. We present a novel numerical framework for the dynamics and size evolution of dust grains implemented in the moving-mesh hydrodynamics code AREPO suited for cosmological galaxy formation simulations. We employ a particle-based method for dust subject to dynamical forces including drag and gravity. The drag force is implemented using a second-order semi-implicit integrator and validated using several dust-hydrodynamical test problems. Each dust particle has a grain-size distribution, describing the local abundance of grains of different sizes. The grain-size distribution is discretized with a second-order piecewise linear method and evolves in time according to various dust physical processes, including accretion, sputtering, shattering, and coagulation. We present a novel scheme for stochastically forming dust during stellar evolution and new methods for sub-cycling of dust physics time-steps. Using this model, we simulate an isolated disc galaxy to study the impact of dust physical processes that shape the interstellar grain-size distribution. We demonstrate, for example, how dust shattering shifts the grain-size distribution to smaller sizes, resulting in a significant rise of radiation extinction from optical to near-ultraviolet wavelengths. Our framework for simulating dust and gas mixtures can readily be extended to account for other dynamical processes relevant in galaxy formation, like magnetohydrodynamics, radiation pressure, and thermochemical processes.

Non-ideal magnetohydrodynamics on a moving mesh

Federico Marinacci, Mark Vogelsberger, Rahul Kannan , Philip Mocz, Rüdiger Pakmor, Volker Springel
Journal Paper MNRAS, 2018, 476, 2476

Abstract

In certain astrophysical systems the commonly employed ideal magnetohydrodynamics (MHD) approximation breaks down. Here, we introduce novel explicit and implicit numerical schemes of ohmic resistivity terms in the moving-mesh code AREPO. We include these non-ideal terms for two MHD techniques: the Powell 8-wave formalism and a constrained transport scheme, which evolves the cell-centred magnetic vector potential. We test our implementation against problems of increasing complexity, such as one- and two-dimensional diffusion problems, and the evolution of progressive and stationary Alfv\'en waves. On these test problems, our implementation recovers the analytic solutions to second-order accuracy. As first applications, we investigate the tearing instability in magnetized plasmas and the gravitational collapse of a rotating magnetized gas cloud. In both systems, resistivity plays a key role. In the former case, it allows for the development of the tearing instability through reconnection of the magnetic field lines. In the latter, the adopted (constant) value of ohmic resistivity has an impact on both the gas distribution around the emerging protostar and the mass loading of magnetically driven outflows. Our new non-ideal MHD implementation opens up the possibility to study magneto-hydrodynamical systems on a moving mesh beyond the ideal MHD approximation.

Toward the Dynamical Classification of Galaxies: Principal Component Analysis of SAURON and CALIFA circular velocity curves

Kalinova, Colombo, Rosolowsky, Kannan, Lasker, Galbany, GarcÌa-Benito, Gonzalez Delgado, Sanchez, Ruiz- Lara and the CALIFA collaboration
Journal Paper MNRAS, 2017, 469 2539

Abstract

We present a dynamical classification system for galaxies based on the shapes of their circular velocity curves (CVCs). We derive the CVCs of 40 SAURON and 42 CALIFA galaxies across Hubble sequence via a full line-of-sight integration as provided by solutions of the axisymmetric Jeans equations. We use Principal Component Analysis (PCA) applied to the circular curve shapes to find characteristic features and use a k-means classifier to separate circular curves into classes. This objective classification method identifies four different classes, which we name Slow-Rising (SR), Flat (F), Sharp-Peaked (SP) and Round-Peaked (RP) circular curves. SR-CVCs are mostly represented by late-type spiral galaxies (Scd-Sd) with no prominent spheroids in the central parts and slowly rising velocities; F-CVCs span almost all morphological types (E,S0,Sab,Sb-Sbc) with flat velocity profiles at almost all radii; SP-CVCs are represented by early-type and early-type spiral galaxies (E,S0,Sb-Sbc) with prominent spheroids and sharp peaks in the central velocities. RP-CVCs are represented by only two morphological types (E,Sa-Sab) with prominent spheroids, but RP-CVCs have much rounder peaks in the central velocities than SP-CVCs. RP-CVCs are typical for high-mass galaxies, while SR-CVCs are found for low-mass galaxies. Intermediate-mass galaxies usually have F-CVCs and SP-CVCs. Circular curve classification presents an alternative to typical morphological classification and may be more tightly linked to galaxy evolution.

On the OVI Abundance in the Circumgalactic Medium of Low-Redshift Galaxies

Joshua Suresh, Kate H. R. Rubin, Rahul Kannan, Jessica K. Werk, Lars Hernquist, Mark Vogelsberger
Journal Paper MNRAS, 2017, 465 2966

Abstract

We analyze the mass, temperature, metal enrichment, and OVI abundance of the circumgalactic medium (CGM) around $z\sim 0.2$ galaxies of mass $10^9 M_\odot M_\bigstar < 10^{11.5} M_\odot$ in the Illustris simulation. Among star-forming galaxies, the mass, temperature, and metallicity of the CGM increase with stellar mass, driving an increase in the OVI column density profile of $\sim 0.5$ dex with each $0.5$ dex increase in stellar mass. Observed OVI column density profiles exhibit a weaker mass dependence than predicted: the simulated OVI abundance profiles are consistent with those observed for star-forming galaxies of mass $M_\bigstar = 10^{10.5-11.5} M_\odot$, but underpredict the observed OVI abundances by $\gtrsim 0.8$ dex for lower-mass galaxies. We suggest that this discrepancy may be alleviated with additional heating of the abundant cool gas in low-mass halos, or with increased numerical resolution capturing turbulent/conductive mixing layers between CGM phases. Quenched galaxies of mass $M_\bigstar = 10^{10.5-11.5} M_\odot$ are found to have 0.3-0.8 dex lower OVI column density profiles than star-forming galaxies of the same mass, in qualitative agreement with the observed OVI abundance bimodality. This offset is driven by AGN feedback, which quenches galaxies by heating the CGM and ejecting significant amounts of gas from the halo. Finally, we find that the inclusion of the central galaxy's radiation field may enhance the photoionization of the CGM within $\sim 50$ kpc, further increasing the predicted OVI abundance around star-forming galaxies.

Increasing blackhole feedback induced quenching with anisotropic thermal conduction

Rahul Kannan, Mark Vogelsberger, Christoph Pfrommer, Rainer Weinberger, Volker Springel, Lars Hernquist, Ewald Puchwein, Ruediger Pakmor
Journal Paper ApJ, 2017, 837 L18

Abstract

Feedback from the central supermassive blackhole has generally been invoked to explain the low star formation rates in massive galaxies. However, exactly how this injected feedback energy couples with the intracluster medium is still unclear. Using high resolution cosmological simulations, we show that it is considerably easier to induce mixing in an anisotropically conducting plasma, fostering the efficient isotropization of the feedback energy. This leads to an earlier termination of a cool core, reduces the star formation rates by more than an order of magnitude and leads to earlier quenching, despite lower amounts of feedback energy being injected into the cluster core. The increased mixing also lowers the metallicity gradients and dispersions, bringing them closer to observational estimates. These results illustrate the importance of thermal conduction and its role in quenching and maintaining quiescence of massive galaxies.

Semi-implicit anisotropic cosmic ray transport on an unstructured moving mesh

Ruediger Pakmor, Christoph Pfrommer, Christine M. Simpson, Rahul Kannan, Volker Springel
Journal Paper MNRAS, 2016, 462, 2603

Abstract

In the interstellar medium of galaxies and the intracluster gas of galaxy clusters, the charged particles making up cosmic rays are moving almost exclusively along (but not across) magnetic field lines. The resulting anisotropic transport of cosmic rays in the form of diffusion or streaming not only affects the gas dynamics but also rearranges the magnetic fields themselves. The coupled dynamics of magnetic fields and cosmic rays can thus impact the formation and evolution of galaxies and the thermal evolution of galaxy clusters in critical ways. Numerically studying these effects requires solvers for anisotropic diffusion that are accurate, efficient, and robust, requirements that have proved difficult to be satisfied in practice. Here, we present an anisotropic diffusion solver on an unstructured moving mesh that is conservative, does not violate the entropy condition, allows for semi-implicit time integration with individual timesteps, and only requires solving a single linear system of equations per timestep. We apply our new scheme to a large number of test problems and show that it works as well or better than previous implementations. Finally, we demonstrate for a numerically demanding simulation of the formation of an isolated disc galaxy that our local time-stepping scheme reproduces the results obtained with global time-stepping at a fraction of the computational cost.

Galaxy formation with local photoionization feedback - II. Effect of X-ray emission from binaries and hot gas

Rahul Kannan, Mark Vogelsberger, Greg S. Stinson, Joseph F. Hennawi, Federico Marinacci, Volker Springel, Andrea V. Maccio
Journal Paper MNRAS, 2016, 458, 2516

Abstract

We study how X-rays from stellar binary systems and the hot intracluster medium (ICM) affect the radiative cooling rates of gas in galaxies. Our study uses a novel implementation of gas cooling in the moving-mesh hydrodynamics code AREPO. X-rays from stellar binaries do not affect cooling at all as their emission spectrum is too hard to effectively couple with galactic gas. In contrast, X-rays from the ICM couple well with gas in the temperature range 10^4-10^6 K. Idealized simulations show that the hot halo radiation field has minimal impact on the dynamics of cooling flows in clusters because of the high virial temperature ( > 10^7 K), making the interaction between the gas and incident photons very ineffective. Satellite galaxies in cluster environments, on the other hand, experience a high radiation flux due to the emission from the host halo. Low-mass satellites (< 10^12 M⊙) in particular have virial temperatures that are exactly in the regime where the effect of the radiation field is maximal. Idealized simulations of satellite galaxies including only the effect of host halo radiation (no ram pressure stripping or tidal effects) fields show a drastic reduction in the amount of cool gas formed (~40 per cent) on a short time-scale of about 0.5 Gyr. A galaxy merger simulation including all the other environmental quenching mechanisms, shows about 20 per cent reduction in the stellar mass of the satellite and about ~30 per cent reduction in star formation rate after 1 Gyr due to the host hot halo radiation field. These results indicate that the hot halo radiation fields potentially play an important role in quenching galaxies in cluster environments.

Accurately simulating anisotropic thermal conduction on a moving mesh

Rahul Kannan, Volker Springel, Ruediger Pakmor, Federico Marinacci, Mark Vogelsberger
Journal Paper MNRAS, 2016, 458, 410

Abstract

We present a novel implementation of an extremum preserving anisotropic diffusion solver for thermal conduction on the unstructured moving Voronoi mesh of the AREPO code. The method relies on splitting the one-sided facet fluxes into normal and oblique components, with the oblique fluxes being limited such that the total flux is both locally conservative and extremum preserving. The approach makes use of harmonic averaging points and a simple, robust interpolation scheme that works well for strong heterogeneous and anisotropic diffusion problems. Moreover, the required discretization stencil is small. Efficient fully implicit and semi-implicit time integration schemes are also implemented. We perform several numerical tests that evaluate the stability and accuracy of the scheme, including applications such as point explosions with heat conduction and calculations of convective instabilities in conducting plasmas. The new implementation is suitable for studying important astrophysical phenomena, such as the conductive heat transport in galaxy clusters, the evolution of supernova remnants, or the distribution of heat from black hole-driven jets into the intracluster medium.

From discs to bulges: effect of mergers on the morphology of galaxies

Rahul Kannan, Andrea V. Maccio, Fabio Fontanot, Benjamin P. Moster, Wouter Karman, Rachel S. Somerville
Journal Paper MNRAS, 2015, 452, 4347

Abstract

We study the effect of mergers on the morphology of galaxies by means of the simulated merger tree approach first proposed by Moster et al. This method combines N-body cosmological simulations and semi-analytic techniques to extract realistic initial conditions for galaxy mergers. These are then evolved using high-resolution hydrodynamical simulations, which include dark matter, stars, cold gas in the disc and hot gas in the halo. We show that the satellite mass accretion is not as effective as previously thought, as there is substantial stellar stripping before the final merger. The fraction of stellar disc mass transferred to the bulge is quite low, even in the case of a major merger, mainly due to the dispersion of part of the stellar disc mass into the halo. We confirm the findings of Hopkins et al., that a gas-rich disc is able to survive major mergers more efficiently. The enhanced star formation associated with the merger is not localized to the bulge of galaxy, but a substantial fraction takes place in the disc too. The inclusion of the hot gas reservoir in the galaxy model contributes to reducing the efficiency of bulge formation. Overall, our findings suggest that mergers are not as efficient as previously thought in transforming discs into bulges. This possibly alleviates some of the tensions between observations of bulgeless galaxies and the hierarchical scenario for structure formation.

Star formation in mergers with cosmologically motivated initial conditions

Wouter Karman, Andrea V. Maccio, Rahul Kannan, Benjamin P. Moster, Rachel S. Somerville
Journal Paper MNRAS, 2015, 452, 2984

Abstract

We use semi-analytic models and cosmological merger trees to provide the initial conditions for multimerger numerical hydrodynamic simulations, and exploit these simulations to explore the effect of galaxy interaction and merging on star formation (SF). We compute numerical realizations of 12 merger trees from z = 1.5 to 0. We include the effects of the large hot gaseous halo around all galaxies, following recent observations and predictions of galaxy formation models. We find that including the hot gaseous halo has a number of important effects. First, as expected, the star formation rate on long time-scales is increased due to cooling of the hot halo and refuelling of the cold gas reservoir. Secondly, we find that interactions do not always increase the SF in the long term. This is partially due to the orbiting galaxies transferring gravitational energy to the hot gaseous haloes and raising their temperature. Finally, we find that the relative size of the starburst, when including the hot halo, is much smaller than previous studies showed. Our simulations also show that the order and timing of interactions are important for the evolution of a galaxy. When multiple galaxies interact at the same time, the SF enhancement is less than when galaxies interact in series. All these effects show the importance of including hot gas and cosmologically motivated merger trees in galaxy evolution models.

On the dependence of galaxy morphologies on galaxy mergers

Fabio Fontanot, Andrea V. Macciò, Michaela Hirschmann, Gabriella De Lucia, Rahul Kannan, Rachel S. Somerville, Dave Wilman
Journal Paper MNRAS, 2015, 451, 2968

Abstract

The distribution of galaxy morphological types is a key test for models of galaxy formation and evolution, providing strong constraints on the relative contribution of different physical processes responsible for the growth of the spheroidal components. In this paper, we make use of a suite of semi-analytic models to study the efficiency of galaxy mergers in disrupting galaxy discs and building galaxy bulges. In particular, we compare standard prescriptions usually adopted in semi-analytic models, with new prescriptions proposed by Kannan et al., based on results from high-resolution hydrodynamical simulations, and we show that these new implementations reduce the efficiency of bulge formation through mergers. In addition, we compare our model results with a variety of observational measurements of the fraction of spheroid-dominated galaxies as a function of stellar and halo mass, showing that the present uncertainties in the data represent an important limitation to our understanding of spheroid formation. Our results indicate that the main tension between theoretical models and observations does not stem from the survival of purely disc structures (i.e. bulgeless galaxies), rather from the distribution of galaxies of different morphological types, as a function of their stellar mass.

The MaGICC volume: reproducing statistical properties of high-redshift galaxies

Rahul Kannan, Greg S. Stinson, Andrea V. Macciò, Chris Brook, Simone M. Weinmann , James Wadsley , Hugh M. P. Couchman
Journal Paper MNRAS, 2014, 437, 3529

Abstract

We present a cosmological hydrodynamical simulation of a representative volume of the Universe, as part of the Making Galaxies in a Cosmological Context (MaGICC) project. MaGICC uses a thermal implementation for supernova and early stellar feedback. This work tests the feedback model at lower resolution across a range of galaxy masses, morphologies and merger histories. The simulated sample compares well with observations of high-redshift galaxies including the stellar mass-halo mass relation, the galaxy stellar mass function (GSMF) at low masses and the number density evolution of low-mass galaxies. The poor match of M⋆-Mh and the GSMF at high masses indicates that supernova feedback is insufficient to limit star formation in these haloes. At z = 0, our model produces too many stars in massive galaxies and slightly underpredicts the stellar mass around milky way mass galaxy. Altogether our results suggest that early stellar feedback, in conjunction with supernova feedback, plays a major role in regulating the properties of low-mass galaxies at high redshift.

Galaxy formation with local photoionization feedback - I. Methods

Rahul Kannan, G. S. Stinson, A. V. Maccio, J.F.Hennawi, R. Woods, J.Wadsley, S. Shen, T. Robitaille, S. Cantalupo, T. Quinn, C. Christensen
Journal Paper MNRAS, 2014, 437, 2882

Abstract

We present a first study of the effect of local photoionizing radiation on gas cooling in smoothed particle hydrodynamics simulations of galaxy formation. We explore the combined effect of ionizing radiation from young and old stellar populations. The method computes the effect of multiple radiative sources using the same tree algorithm as used for gravity, so it is computationally efficient and well resolved. The method foregoes calculating absorption and scattering in favour of a constant escape fraction for young stars to keep the calculation efficient enough to simulate the entire evolution of a galaxy in a cosmological context to the present day. This allows us to quantify the effect of the local photoionization feedback through the whole history of a galaxy's formation. The simulation of a Milky Way-like galaxy using the local photoionization model forms ~40 per cent less stars than a simulation that only includes a standard uniform background UV field. The local photoionization model decreases star formation by increasing the cooling time of the gas in the halo and increasing the equilibrium temperature of dense gas in the disc. Coupling the local radiation field to gas cooling from the halo provides a preventive feedback mechanism which keeps the central disc light and produces slowly rising rotation curves without resorting to extreme feedback mechanisms. These preliminary results indicate that the effect of local photoionizing sources is significant and should not be ignored in models of galaxy formation.

Interaction between Dark Matter Sub-halos and a Galactic Gaseous Disk

Rahul Kannan, Andrea V. Macciò, Benjamin P. Moster, Anna Pasquali, Fabian Walter
Journal Paper ApJ, 2012, 746, 10

Abstract

We investigate the idea that the interaction of dark matter (DM) sub-halos with the gaseous disks of galaxies can be the origin for the observed holes and shells found in their neutral hydrogen (HI) distributions. We use high-resolution hydrodynamic simulations to show that pure DM sub-halos impacting a galactic disk are not able to produce holes; on the contrary, they result in high-density regions in the disk. However, sub-halos containing a small amount of gas (a few percent of the total DM mass of the sub-halo) are able to displace the gas in the disk and form holes and shells. The sizes and lifetimes of these holes depend on the sub-halo gas mass, density, and impact velocity. A DM sub-halo, of mass 10^8 M⊙ and a gas mass fraction of ~3%, is able to create a kiloparsec-scale hole with a lifetime similar to those observed in nearby galaxies. We also register an increase in the star formation rate at the rim of the hole, again in agreement with observations. Even though the properties of these simulated structures resemble those found in observations, we find that the number of predicted holes (based on mass and orbital distributions of DM halos derived from cosmological N-body simulations) falls short compared to the observations. Only a handful of holes are produced per gigayear. This leads us to conclude that DM halo impact is not the major channel through which these holes are formed.

Turbulence in rotating Rayleigh-Bénard convection in low-Prandtl-number fluids

Hirdesh K. Pharasi, Rahul Kannan, Krishna Kumar, Jayanta K. Bhattacharjee
Journal Paper Phys. Rev. E, 2011, 84, 047301

Abstract

The heat flux in rotating Rayleigh-Bénard convection in a fluid of Prandtl number Pr=0.1 enclosed between free-slip top and bottom boundaries is investigated using direct numerical simulation in a wide range of Rayleigh numbers (104≤Ra≤108) and Taylor numbers (0≤Ta≤108). The Nusselt number Nu scales with the Rayleigh number Ra as Ra^β with β=2/7 for values of Nu greater than a critical value Nuc, which occurs for Ta/Ra∼1. The exponent β is not universal for Nu < Nuc (for Ta/Ra > 1) but a function of Ta showing a minimum for some intermediate value of Ta. The critical Nusselt number Nuc and the corresponding critical Rossby number Roc scale with Ta as Ta^0.277±0.001 and Ta^−0.015±0.003, respectively.

Frame Dragging and the Kinematics of Galactic-Center Stars

Rahul Kannan, Prasenjit Saha
Journal Paper ApJ, 2009, 690, 1553

Abstract

We calculate the effects of frame dragging on the Galactic-Center stars. Assuming the stars are only slightly relativistic, we derive an approximation to the Kerr metric, which turns out to be a weak-field Schwarzschild metric plus a frame-dragging term. By numerically integrating the resulting geodesic equations, we compute the effect on Keplerian elements and the kinematics. We find that the kinematic effect at pericenter passage is proportional to (a(1 - e^2))^-2. For known Galactic-center stars it is of order 10 m/s. If observed, this would provide a measurement of the spin of the black hole.

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