Website of Sandro Tacchella

Research and Publications

In my research, I aim at understanding the physical processes that lead to the formation and evolution of galaxies. I perform observations with the Hubble Space Telescope and ground-based facilities to determine the structural evolution of galaxies and how star formation is regulated. Simultaneously, I lead the development and analysis of numerical and analytical models to interpret my observations and to draw inspiration for new observational projects. The most significant result from my previous research is the discovery that the most massive galaxies 3 billion years after the Big Bang sustain their high star-formation rates at large radii in rotating disk components, while hosting fully-grown and already mature bulges in their cores. Additionally, using cosmological hydrodynamical simulations, I found that the simulated galaxies oscillate about the star-formation equilibrium state, where the spatial distribution of star formation and the buildup of bulges are linked to these galactic-scale oscillations. In the future, with the upcoming James Webb Space Telescope (JWST), I am excited to explore how star formation is regulated in the first galaxies and how the first bulges and disks are assembled at high redshifts.

Refereed first-author publications:

  1. "Stochastic modelling of star-formation histories II: star-formation variability from molecular clouds and gas inflow"
    Tacchella, Forbes & Caplar 2020 MNRAS 497, 698
  2. "Morphology and Star Formation in IllustrisTNG: the Buildup of Spheroids and Discs"
    Tacchella et al 2019 MNRAS 487, 5416
  3. "A Redshift-independent Efficiency Model: Star Formation and Stellar Masses in Dark Matter Halos at z ≳ 4"
    Tacchella et al 2018 ApJ 868, 92
  4. "Dust Attenuation, Bulge Formation, and Inside-out Quenching of Star Formation in Star-forming Main Sequence Galaxies at z~2"
    Tacchella et al 2018 ApJ 859, 56
  5. "On the Evolution of the Central Density of Quiescent Galaxies"
    Tacchella et al 2017 ApJL 844, 1
  6. "The Confinement of Star-Forming Galaxies into a Main Sequence Through Episodes of Gas Compaction, Depletion and Replenishment"
    Tacchella et al 2016 MNRAS 457, 2790
  7. "Evolution of Density Profiles in High-z Galaxies: Compaction and Quenching Inside-Out"
    Tacchella et al 2016 MNRAS 458, 242
  8. "Evidence for Mature Bulges and an Inside-Out Quenching Phase 3 Billion Years After the Big Bang"
    Tacchella et al 2015 Science 348, 314
  9. "SINS/zC-SINF Survey of z~2 Galaxy Kinematics: Rest-frame Morphology, Structure, and Colors from Near-infrared Hubble Space Telescope Imaging"
    Tacchella et al 2015 ApJ 802, 101
  10. "A Physical Model for the 0 < z < 8 Redshift Evolution of the Galaxy Ultraviolet Luminosity and Stellar Mass Functions"
    Tacchella et al 2013 ApJL 768, L37

The Formation and Evolution of Bulges and Disks in High-Redshift Galaxies

Complementing many surveys of large numbers of high-z galaxies, which provide statistical measures based on integrated information on individual galaxies, our SINS/zC-SINF program has studied a smaller sample of typical star-forming galaxies at z~2 on resolved scales of ~1 kpc. Namely, we combine HST imaging data and near-IR Adaptive-Optics integral-field spectroscopy from VLT/SINFONI in order to constrain, at kpc resolution, ionized-gas kinematics, Hα and UV (dust-corrected) star-formation rates, dust attenuation maps, stellar mass densities and star-formation histories This enables us to investigate the physical processes that occur within galaxies at such early epochs.

Combining space-based HST imaging data with ground-based VLT/SINFONI IFU observations.

We measure the stellar mass, dust attenuation and SFR surface density distributions in these SFGs with ~1 kpc resolution. The individual dust attenuation radial profiles in the rest-frame V-band scatter around an average profile that gently decreases from 1.8 mag in the center down to ~0.6 mag at ~3-4 half-mass radii. We use these measurements to correct UV- and Hα-based SFRs, which agree well with each other. At masses >10^11 solar masses, the dust-corrected sSFR profiles are on average radially constant at a mass-doubling timescale of ~300 Myr, pointing at a synchronous growth of bulge and disk components in such galaxies. This is consistent with galaxies evolving along a well-defined relation between the central stellar mass density, an indicator for the strength of the bulge component, and the total stellar mass.

At masses ≥10^11 solar masses, we discover that the sSFR profiles are typically centrally-suppressed by a factor of ~10 relative to the galaxy outskirts. This indicates that at least a fraction of massive z~2 Main Sequence galaxies have started quenching inside-out that will move them to the quiescent sequence. By using spatially resolved stellar mass continuity, we find that, in the most massive galaxies, star formation is quenched from the inside out, on timescales less than 1 billion years in the inner regions, up to a few billion years in the outer disks. These galaxies sustain high star-formation activity at large radii, while hosting fully grown and already quiescent bulges in their cores: with central stellar mass densities of > 10^10 Msun/kpc^2 they are comparable to z~0 massive early-type galaxies. These results are published in the journal Science and received extensive national and international press coverage (European Southern Observatory, Hubble Space Telescope, ETH Zurich, The Conversation).

Furthermore, galaxies above and below the ridge of the Main Sequence relation have respectively centrally-enhanced and centrally-suppressed sSFRs relative to their outskirts, supporting a picture developed from the cosmological simulations where bulges are built due to gas compaction, leading to a high central SFR as galaxies move towards the upper envelope of the Main Sequence. These trends and the shapes of the stellar mass and SFR profiles from the observations are in agreement with the ones of the simulated galaxies, indicating a consistent scenario of a gas-rich compaction leading to inside-out quenching and the subsequent saturation of a dense stellar bulge.

Growth of Galaxies in Cosmological Simulations: Confinement of Starforming Galaxies into a Main Sequence

Observations of galaxies spanning the last 13 billion years of cosmic time show that the majority of star-forming follow a tight relation between their SFR and total stellar mass, the socalled Main Sequence. A key implication of this tight relation is that star-forming galaxies sustain their SFRs for extended periods in quasi-steady state of gas inflow, gas outflow and gas consumption, rather than short-lived periods of merger-induced starburst peaks. A natural way to understand the decline of the normalization of the Main Sequence with time is provided by the aforementioned model, where the decline of gas accretion rate onto the galaxies is closely related to the evolution of the cosmological accretion rate into dark matter halos.

In order to understand the physics that shapes the efficiency of converting gas into stars and to constrain the mechanisms that confine the individual star-forming to a Main Sequence, we need to expand upon this analytical model: a self-consistent treatment of star formation and feedback processes is required. We achieve this by using state-of-the-art cosmological simulations. We find that the mechanisms of gas compaction, depletion and replenishment confine the star-forming galaxies to the narrow Main Sequence. The star-forming galaxies oscillate about the Main Sequence ridge. The propagation upwards is due to gas compaction, triggered, e.g., by mergers, counter-rotating streams, and/or violent disk instabilities. The downturn at the upper envelope is due to central gas depletion by peak star formation and outflows while inflow from the shrunken gas disk is suppressed. An upturn at the lower envelope can occur once the extended disk has been replenished by fresh gas and a new compaction can be triggered, namely as long as the replenishment time is shorter than the depletion time. Full quenching occurs in massive halos and/or at low redshifts, where the replenishment time is long compared to the depletion time.

We link these MS oscillations in the cosmological simulations to the morphological evolution of the high-redshift galaxies by investigating the evolution of surface density profiles. Following a gas-rich compaction event, the gas develops a cusp inside the effective radius. The associated peak in SFR, in the absence of further gas inflow to the center, marks the onset of gas depletion from the center, leading to quenching of the central SFR. Before the central quenching process, the stellar density profile grows self-similarly, maintaining its log-log shape because the specific SFR is similar at all radii. During the quenching process, the stellar density in the central bulge region saturates to a constant value, while the total stellar mass still increases.

A Better Understanding of the Evolution of the Cosmic Star-Formation Rate Density by Connecting Galaxies to Their Dark Matter Halos

One of the most fundamental observables of the universe is the cosmic star-formation rate density. We present an analytical model to understand the redshift evolution of the UV luminosity function of galaxies and thereby constrain the physical processes that drive the redshift evolution of the cosmic star-formation rate density. Our approach is based on the assumption that the luminosity and stellar mass of a galaxy is related to its dark matter halo assembly and gas accretion rate. The model correctly predicts the evolution of the cosmic star-formation rate density as well as the cosmic stellar mass density from z=0 to z=10. While the details of star-formation efficiency and feedback are hidden within our calibrated luminosity versus halo mass relation, our study highlights that the primary driver of galaxy evolution across cosmic time is the buildup of dark matter halos, without the need to invoke a redshift-dependent efficiency in converting gas into stars. In particular, the increase in the cosmic star-formation rate density at high redshifts can be explained by the increase in the number density of halos that host star-forming galaxies, while the decline from z~2 to today is due to the reduced accretion rate onto dark matter halos, resulting in the decline of the star-formation rate of typical star-forming galaxies and star-formation quenching of individual galaxies. This model has been used as a benchmark in many observational works.

Quiescent Galaxies - the Descendants of Star-Forming Galaxies

The massive z~2 star-forming galaxies have central stellar mass densities that are saturated to those of similar mass quiescent spheroids in the local universe. These galaxies will become members the quiescent galaxy population soon. In order to quantify the role of the addition of newly quenched galaxies in driving the apparent evolution of population-averaged quantities of quiescent galaxies (progenitor bias effect), such as the size and the central stellar mass density, we measure the relation between stellar age and central density at z~0. As with star-forming galaxies, the quiesent galaxies shape a narrow locus in the central density - stellar mass plane, which we refer to as the S1 ridgeline. At fixed stellar mass, old quiescent galaxies on the S1 ridgeline have higher central density than young quiescent galaxies. This shows explicitly that galaxies landing on the S1 ridgeline today arrive with lower central density, which tends to drive the population-averaged zeropoint of the ridgeline down with time. We conclude that the zeropoint evolution of the ridgeline is mainly driven by progenitor bias effects.