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| Starbursts & star formation in external galaxies | |||||||||||||||||||||||||
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Abstract We report ALMA observations of 12CO(3-2) and 13CO(3-2) in the gas-poor dwarf galaxy NGC 5253. These 0.3"(5.5 pc) resolution images reveal small, dense molecular gas clouds that are located in kinematically distinct, extended filaments. Some of the filaments appear to be falling into the galaxy and may be fueling its current star formation. The most intense CO(3-2) emission comes from the central ~100 pc region centered on the luminous radio-infrared HII region known as the supernebula. The CO(3-2) clumps within the starburst region are anti-correlated with Hα on~5 pc scales, but are well-correlated with radio free-free emission. Cloud D1, which enshrouds the supernebula, has a high 12CO/13CO ratio, as does another cloud within the central 100 pc starburst region, possibly because the clouds are hot. CO(3-2) emission alone does not allow determination of cloud masses as molecular gas temperature and column density are degenerate at the observed brightness, unless combined with other lines such as 13CO.
Abstract We present observations of CO(3–2) and 13CO(3–2) emission near the supernebula in the dwarf galaxy NGC 5253, which contains one of the best examples of a potential globular cluster in formation. The 0farcs3 resolution images reveal an unusual molecular cloud, "Cloud D1," that is coincident with the radio-infrared supernebula. The ~6 pc diameter cloud has a linewidth, Δ v = 21.7km s-1, that reflects only the gravitational potential of the star cluster residing within it. The corresponding virial mass is 2.5 × 15 M⊙. The cluster appears to have a top-heavy initial mass function, with M* ≳ 1–2M⊙. Cloud D1 is optically thin in CO(3–2), probably because the gas is hot. Molecular gas mass is very uncertain but constitutes <35% of the dynamical mass within the cloud boundaries. In spite of the presence of an estimated ~1500–2000 O stars within the small cloud, the CO appears relatively undisturbed. We propose that Cloud D1 consists of molecular clumps or cores, possibly star-forming, orbiting with more evolved stars in the core of the giant cluster.
Abstract We present observations of the 12CO(J=6-5) line and 686 GHz continuum emission in NGC 253 with the Submillimeter Array at an angular resolution of ~4". The 12CO(J=6-5) emission is clearly detected along the disk and follows the distribution of the lower 12CO line transitions with little variations of the line ratios. A large-velocity gradient analysis suggests a two-temperature model of the molecular gas in the disk, likely dominated by a combination of low-velocity shocks and the disk wide PDRs. Only marginal 12CO(J=6-5) emission is detected in the vicinity of the expanding shells at the eastern and western edges of the disk. While the eastern shell contains gas even warmer (Tkin >300 K) than the hot gas component (Tkin =300 K) of the disk, the western shell is surrounded by gas much cooler (Tkin =60 K) than the eastern shell but somewhat hotter than the cold gas component of the disk (for similar H2 and CO column densities), indicative of different (or differently efficient) heating mechansisms. The continuum emission at 686 GHz in the disk agrees well in shape and size with that at lower (sub-)millimeter frequencies, exhibiting a spectral index consistent with thermal dust emission. We find dust temperatures of ~10-30 K and largely optically thin emission. However, our fits suggest a second (more optically thick) dust component at higher temperatures (Td > 60 K), similar to the molecular gas. We estimate a global dust mass of ~106 Mo (solar mass) for the disk translating into a gas-to-dust mass ratio of a few hundred consistent with other nearby active galaxies.
Abstract A local dwarf galaxy, NGC 5253, has a young super star cluster that may provide an example of highly efficient star formation. Here we report the detection and imaging, with the Submillimeter Array, of the J= 3-2 rotational transition of CO at the location of the massive cluster associated with the supernebula. The gas cloud is hot, dense, quiescent, and extremely dusty. Its gas-to-dust ratio is lower than the Galactic value, which we attribute to dust enrichment by Wolf-Rayet stars within the embedded star cluster. Its star formation efficiency exceeds 50%, ten times higher than clouds in the Milky Way: this cloud is a factory of stars and soot. We suggest that high efficiency results from the force-feeding of star formation by a streamer of gas falling into the galaxy.
Abstract
We present the results from the observations of Arp 220 in
multiple CO spectral line transitions of J = 6-5, J = 3-2, J = 2-1 and
the isotope 13CO at J = 2-1 at 690, 340 and 230 GHz using the
Submillimeter Array (SMA) with the angular resolutions of sub-arcsec to
a few arcsec. Based on the measured line ratios, we modeled the excitation
conditions of the molecular clouds in the nuclear region of Arp 220,
suggesting that the CO J = 6-5 emission arises from dense
(n(H2)~105 cm-3) and warm
(Tk~60 K) cloud components. Each of the four
CO J = 6-5 clumps contains at least a few times 108
M
Abstract Using the Very Large Array (VLA), 1 we have observed the H92 alpha line in the nuclear regions of NGC 3628 and Arp 299 (IC 694 + NGC 3690) with an angular resolution of 1". The radio recombination lines could provide quite important constraints to the physical conditions for the ionized gas in the nuclear regions. For the central nuclear components in NGC 3628, IC 694, and NGC 3690, we find that the electron density of the ionized gas is in the range 5 x 102 to 5 x 104 cm-3, with corresponding geometrical sizes ranging from 25 to 0.01 pc. For the regions with a large line-to-continuum ratio, such as the anomalous kinematic components in IC 694 and NGC 3628, the electron density of the H II regions is well constrained. The derived parameters suggest the existence of ~100 high-density, compact H II regions (ne ~ 104 cm-3 and l ~ 1--2 pc) in these two components. The large number of high-density, compact H II regions deduced for these components can be regarded as evidence for enhanced massive star formation in anomalous kinematic structures that are often produced in interacting systems. Our new H92 alpha line observations reveal the presence of rotating circumnuclear ionized gas disks in both NGC 3628 and IC 694. From the analysis of the nuclear kinematics, we find that a total dynamical mass of 3 x 108 Msun exists within a radius R = 120 pc in NGC 3628 and 7 x 108 Msun within R = 200 pc in IC 694. The kinematics of the ionized gas and the models with a collection of H II regions infer that the ratio of H II to dynamical mass in the starburst nuclei varies from 1 x 104 cm-3 (for the high-density, compact H II region model) to 1 x 102 cm-3(for the low-density, large H II region model).
Abstract Using the Very Large Array (VLA) with an angular resolution of 3", we have detected the hydrogen recombination line H92alpha from the galaxies Arp 220, M83, and NGC 2146. The line emission arises from the nuclear regions with a line-to-continuum ratio of 1% or less. In order to fit both the observed H92alpha line and continuum data in the nuclear regions, we have considered two types of models. First, we utilize a model with a collection of H II regions. A large number of compact H II regions are required in this model. With electron temperatures in the range 5 x 103 - 1 x 104 K and a range of electron densities, this model can account for both the line intensity and the continuum spectrum. In most cases, the H92alpha line is dominated by internal stimulated emission due to free-free continuum arising within the H II regions. In a low-density case (ne = 50 cm-3) for Arp 220, about half the line emission comes from external stimulated emission due to the background nonthermal source. Typical rates of ionizing photons predicted from these models are ~5 x 1052 s-1 for M83, ~4 x 1053 s-1 for NGC 2146, and 5 x 1054 s-1 for Arp 220. We infer that 105 O5 stars are required in Arp 220, which is an order of magnitude greater than in NGC 2146 and 2 orders of magnitude greater than in M83. Alternatively, several uniform slab models with Te >= 5 x 103 and ne in the range of 50 - 1 x 104 cm-3 appear to fit both the H92alpha line and continuum data of Arp 220 and M83. In the low-density models, stimulated emission by the background nonthermal radiation appears to be dominant at low frequencies, and the lines at higher frequencies arise primarily from spontaneous emission. The uniform slab model requires a higher ionizing photon rate than the H II region model. No slab models with reasonable Te can fit the data observed in M83 and NGC 2146. Combining previous published data with these new observations, a sample of 13 galaxies has been observed for radio recombination lines (RRLs) with the VLA. Nine out of the 13 galaxies have been detected in the H92alpha line. While the H9alpha line luminosity appears to be correlated with the Bralpha line luminosity, we find that nearly all the RRL galaxies show a significant excess in H92alpha line compared to the expected LTE value. The excess in the H92alpha line flux suggests that non-LTE effects are important for the H92alpha line in these starburst nuclei. A strong correlation between H92alpha and the molecular lines of HCN/HCO+ is also found, indicating that the RRL emitters may be spatially associated with the dense molecular cores. The inferred high electron density also suggests an intimate relation between the RRLs and the dense molecular medium in these galaxies.
Abstract Using the Very Large Array with an angular resolution of 3", we have detected the hydrogen recombination line H92alpha (nurest = 8309.38 MHz) from the starburst nuclei in the galaxies NGC 3628 and IC 694 and also from the Seyfert II nucleus in NGC 1365. In each case the line-emitting region extends over a few hundred parsecs. The detected lines have peak intensities in the range 0.5-1.5 mJy and widths (FWHM) 200- 400 km s-1. The line was not detected in three other galaxies (NGC 262, NGC 1068, and NGC 3079) that were observed to a similar sensitivity level. We present a model in which a collection of H II regions in the nuclear region accounts for the observed H92alpha line. The required number of H II regions, their temperature, electron density, and linear size are constrained by the observed line flux density, line width, continuum spectrum, and size of the line-emitting region. If the temperature of the H II regions is above 5000 K, then electron densities in the range 5-50 x 103 cm-3 are permitted by the available constraints. Several hundred H II regions of a few parsecs in size, with a total mass of a few times 105 Msun, are required to account for the observed line flux density. The rate of production of Lyman continuum photons required to maintain the ionization is a few times 1054 s-1. Much of the line emission comes from internal stimulated emission due to the continuum generated within the H II regions which account for 5%-30% of the observed total continuum at 5 GHz. Predictions are made for the expected recombination line and continuum flux density as a function of frequency from the nuclear region of the galaxies.
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![[sun]](http://scitation.aip.org/stockgif3/odot-script.gif)
molecular gas.
The dense and warm molecular clouds are likely the
consequence of inelastic collisions between the molecular
clouds in the counterrotating nuclear disks.
Properties determined from these high-density regions suggest
that they are in the early stage of star formation.
The study of Arp 220 could provide us detailed information on
the astrophysical processes in the galaxies at high red-shifts.
