ABSTRACT The Magellanic Clouds provide the only laboratory to study the effects of metallicity and galaxy mass on molecular gas and star formation at high ( 20 pc) resolution. We use the dust ...emission from HERITAGE Herschel data to map the molecular gas in the Magellanic Clouds, avoiding the known biases of CO emission as a tracer of H 2 . Using our dust-based molecular gas estimates, we find molecular gas depletion times ( dep mol ) of 0.4 Gyr in the Large Magellanic Cloud and 0.6 in the Small Magellanic Cloud at 1 kpc scales. These depletion times fall within the range found for normal disk galaxies, but are shorter than the average value, which could be due to recent bursts in star formation. We find no evidence for a strong intrinsic dependence of the molecular gas depletion time on metallicity. We study the relationship between the gas and the star formation rate across a range of size scales from 20 pc to 1 kpc, including how the scatter in dep mol changes with the size scale, and discuss the physical mechanisms driving the relationships. We compare the metallicity-dependent star formation models of Ostriker et al. and Krumholz to our observations and find that they both predict the trend in the data, suggesting that the inclusion of a diffuse neutral medium is important at lower metallicity.
Abstract
The Small Magellanic Cloud (SMC) provides the only laboratory to study the structure of molecular gas at high resolution and low metallicity. We present results from the
Herschel
...Spectroscopic Survey of the SMC (HS
3
), which mapped the key far-IR cooling lines C
ii
, O
i
, N
ii
, and O
iii
in five star-forming regions, and new ALMA 7 m array maps of
and
with coverage overlapping four of the five HS
3
regions. We detect C
ii
and O
i
throughout all of the regions mapped. The data allow us to compare the structure of the molecular clouds and surrounding photodissociation regions using
,
, C
ii
, and O
i
emission at
(
pc) scales. We estimate
using far-IR thermal continuum emission from dust and find that the CO/C
ii
ratios reach the Milky Way value at high
in the centers of the clouds and fall to
the Milky Way value in the outskirts, indicating the presence of translucent molecular gas not traced by bright
emission. We estimate the amount of molecular gas traced by bright C
ii
emission at low
and bright
emission at high
. We find that most of the molecular gas is at low
and traced by bright C
ii
emission, but that faint
emission appears to extend to where we estimate that the
-to-H
i
transition occurs. By converting our
gas estimates to a CO-to-
conversion factor (
X
CO
), we show that
X
CO
is primarily a function of
, consistent with simulations and models of low-metallicity molecular clouds.
We compare atomic gas, molecular gas, and the recent star formation rate (SFR) inferred from H Delta *a in the Small Magellanic Cloud (SMC). By using infrared dust emission and local dust-to-gas ...ratios, we construct a map of molecular gas that is independent of CO emission. This allows us to disentangle conversion factor effects from the impact of metallicity on the formation and star formation efficiency of molecular gas. On scales of 200 pc to 1 kpc (where the distributions of H2 and star formation match well) we find a characteristic molecular gas depletion time of Delta *tmol dep ~ 1.6 Gyr, similar to that observed in the molecule-rich parts of large spiral galaxies on similar spatial scales. This depletion time shortens on much larger scales to ~0.6 Gyr because of the presence of a diffuse H Delta *a component, and lengthens on much smaller scales to ~7.5 Gyr because the H Delta *a and H2 distributions differ in detail. We estimate the systematic uncertainties in our dust-based Delta *tmol dep measurement to be a factor of ~2-3. We suggest that the impact of metallicity on the physics of star formation in molecular gas has at most this magnitude, rather than the factor of ~40 suggested by the ratio of SFR to CO emission. The relation between SFR and neutral () gas surface density is steep, with a power-law index 2.2 ? 0.1, similar to that observed in the outer disks of large spiral galaxies. At a fixed total gas surface density the SMC has a 5-10 times lower molecular gas fraction (and star formation rate) than large spiral galaxies. We explore the ability of the recent models by Krumholz et al. and Ostriker et al. to reproduce our observations. We find that to explain our data at all spatial scales requires a low fraction of cold, gravitationally bound gas in the SMC. We explore a combined model that incorporates both large-scale thermal and dynamical equilibrium and cloud-scale photodissociation region structure and find that it reproduces our data well, as well as predicting a fraction of cold atomic gas very similar to that observed in the SMC.
The spatial variations of the gas-to-dust ratio (GDR) provide constraints on the chemical evolution and life-cycle of dust in galaxies. We examine the relation between dust and gas at 10-50 pc ...resolution in the Large and Small Magellanic Clouds (LMC and SMC) based on Herschel far-infrared (FIR), H I 21 cm, CO, and H alpha observations. We investigate the range of CO-to-Hsub 2 conversion factor to best account for all the molecular gas in the beam of the observations, and find upper limits on XCO to be 6 x 10sup 20 cmsup -2 Ksup -1 kmsup -1 s in the LMC (Z = 0.5Z) at 15 pc resolution, and 4 x 1021 cmsup -2 Ksup -1 kmsup -1 s in the SMC (Z = 0.2Z) at 45 pc resolution. Our analysis demonstrates that obtaining robust ISM masses remains a non-trivial endeavor even in the local Universe using state-of-the-art maps of thermal dust emission.
The CO-to-H2 conversion factor (αCO) is central to measuring the amount and properties of molecular gas. It is known to vary with environmental conditions, and previous studies have revealed lower ...αCO in the centers of some barred galaxies on kiloparsec scales. To unveil the physical drivers of such variations, we obtained Atacama Large Millimeter/submillimeter Array bands (3), (6), and (7) observations toward the inner ∼2 kpc of NGC 3627 and NGC 4321 tracing 12CO, 13CO, and C18O lines on ∼100 pc scales. Our multiline modeling and Bayesian likelihood analysis of these data sets reveal variations of molecular gas density, temperature, optical depth, and velocity dispersion, which are among the key drivers of αCO. The central 300 pc nuclei in both galaxies show strong enhancement of temperature Tk ≳ 100 K and density nH2>103 cm−3. Assuming a CO-to-H2 abundance of 3 × 10−4, we derive 4–15 times lower αCO than the Galactic value across our maps, which agrees well with previous kiloparsec-scale measurements. Combining the results with our previous work on NGC 3351, we find a strong correlation of αCO with low-J12CO optical depths (τCO), as well as an anticorrelation with Tk. The τCO correlation explains most of the αCO variation in the three galaxy centers, whereas changes in Tk influence αCO to second order. Overall, the observed line width and 12CO/13CO 2–1 line ratio correlate with τCO variation in these centers, and thus they are useful observational indicators for αCO variation. We also test current simulation-based αCO prescriptions and find a systematic overprediction, which likely originates from the mismatch of gas conditions between our data and the simulations.
The dust properties in the Large and Small Magellanic clouds (LMC/SMC) are studied using the HERITAGE Herschel Key Project photometric data in five bands from 100 to 500 mu m. Three simple models of ...dust emission were fit to the observations: a single temperature blackbody modified by a power-law emissivity (SMBB), a single temperature blackbody modified by a broken power-law emissivity (BEMBB), and two blackbodies with different temperatures, both modified by the same power-law emissivity (TTMBB). Using these models, we investigate the origin of the submillimeter excess, defined as the submillimeter emission above that expected from SMBB models fit to observations <200 mu m. We find that the BEMBB model produces the lowest fit residuals with pixel-averaged 500 mu m submillimeter excesses of 27% and 43% for the LMC and SMC, respectively. Adopting gas masses from previous works, the gas-to-dust ratios calculated from our fitting results show that the TTMBB fits require significantly more dust than are available even if all the metals present in the interstellar medium (ISM) were condensed into dust. This indicates that the submillimeter excess is more likely to be due to emissivity variations than a second population of colder dust. We derive integrated dust masses of (7.3 + or - 1.7) x 10 super(5) and (8.3 + or - 2.1) x 10 super(4) M sub(middot in circle) for the LMC and SMC, respectively. We find significant correlations between the submillimeter excess and other dust properties; further work is needed to determine the relative contributions of fitting noise and ISM physics to the correlations.
Centaurus A, with its gas-rich elliptical host galaxy, NGC 5128, is the nearest radio galaxy at a distance of 3.8 Mpc. Its proximity allows us to study the interaction among an active galactic ...nucleus, radio jets, and molecular gas in great detail. We present ALMA observations of low-J transitions of three CO isotopologues, HCN, HCO+, HNC, CN, and CCH toward the inner projected 500 pc of NGC 5128. Our observations resolve physical sizes down to 40 pc. By observing multiple chemical probes, we determine the physical and chemical conditions of the nuclear interstellar medium of NGC 5128. This region contains molecular arms associated with the dust lanes and a circumnuclear disk (CND) interior to the molecular arms. The CND is approximately 400 pc by 200 pc and appears to be chemically distinct from the molecular arms. It is dominated by dense gas tracers while the molecular arms are dominated by 12CO and its rare isotopologues. The CND has a higher temperature, elevated CN/HCN and HCN/HNC intensity ratios, and much weaker 13CO and C18O emission than the molecular arms. This suggests an influence from the AGN on the CND molecular gas. There is also absorption against the AGN with a low velocity complex near the systemic velocity and a high velocity complex shifted by about 60 km s−1. We find similar chemical properties between the CND in emission and both the low and high velocity absorption complexes, implying that both likely originate from the CND. If the HV complex does originate in the CND, then that gas would correspond to gas falling toward the supermassive black hole.
To understand the impact of low metallicities on giant molecular cloud (GMC) structure, we compare far-infrared dust emission, CO emission, and dynamics in the star-forming complex N83 in the Wing of ...the Small Magellanic Cloud (SMC). Dust emission (measured by Spitzer as part of the Spitzer Survey of the SMC and Surveying the Agents of a Galaxy's Evolution in the SMC surveys) probes the total gas column independent of molecular line emission and traces shielding from photodissociating radiation. We calibrate a method to estimate the dust column using only the high-resolution Spitzer data and verify that dust traces the interstellar medium in the H I-dominated region around N83. This allows us to resolve the relative structures of H2, dust, and CO within a GMC complex, one of the first times such a measurement has been made in a low-metallicity galaxy. Our results support the hypothesis that CO is photodissociated while H2 self-shields in the outer parts of low-metallicity GMCs, so that dust/self-shielding is the primary factor determining the distribution of CO emission. Four pieces of evidence support this view. First, the CO-to-H2 conversion factor averaged over the whole cloud is very high 4-11 X 1021 cm-2 (K km s-1)-1, or 20-55 times the Galactic value. Second, the CO-to-H2 conversion factor varies across the complex, with its lowest (most nearly Galactic) values near the CO peaks. Third, bright CO emission is largely confined to regions of relatively high line-of-sight extinction, AV 2 mag, in agreement with photodissociation region models and Galactic observations. Fourth, a simple model in which CO emerges from a smaller sphere nested inside a larger cloud can roughly relate the H2 masses measured from CO kinematics and dust.
The CO-to-H2 conversion factor (αCO) is critical to studying molecular gas and star formation in galaxies. The value of αCO has been found to vary within and between galaxies, but the specific ...environmental conditions that cause these variations are not fully understood. Previous observations on ~kiloparsec scales revealed low values of αCO in the centers of some barred spiral galaxies, including NGC 3351. We present new Atacama Large Millimeter/submillimeter Array Band 3, 6, and 7 observations of 12CO, 13CO, and C18O lines on 100 pc scales in the inner ∼2 kpc of NGC 3351. Using multiline radiative transfer modeling and a Bayesian likelihood analysis, we infer the H2 density, kinetic temperature, CO column density per line width, and CO isotopologue abundances on a pixel-by-pixel basis. Our modeling implies the existence of a dominant gas component with a density of 2–3 × 103 cm−3 in the central ∼1 kpc and a high temperature of 30–60 K near the nucleus and near the contact points that connect to the bar-driven inflows. Assuming a CO/H2 abundance of 3 × 10−4, our analysis yields αCO ∼ 0.5–2.0 M⊙ (K km s−1 pc2)−1 with a decreasing trend with galactocentric radius in the central ∼1 kpc. The inflows show a substantially lower αCO ≲ 0.1 M⊙ (K km s−1 pc2)−1, likely due to lower optical depths caused by turbulence or shear in the inflows. Over the whole region, this gives an intensity-weighted αCO of ∼1.5 M⊙ (K km s−1 pc2)−1, which is similar to previous dust-modeling-based results at kiloparsec scales. This suggests that low αCO on kiloparsec scales in the centers of some barred galaxies may be due to the contribution of low-optical-depth CO emission in bar-driven inflows.
Context. M 33 is a gas rich spiral galaxy of the Local Group. Its vicinity allows us to study its interstellar medium (ISM) on linear scales corresponding to the sizes of individual giant molecular ...clouds. Aims. We investigate the relationship between the two major gas cooling lines and the total infrared (TIR) dust continuum. Methods. We mapped the emission of gas and dust in M 33 using the far-infrared lines of C II and O I(63 μm) and the total infrared continuum. The line maps were observed with the PACS spectrometer on board the Herschel Space Observatory. These maps have 50 pc resolution and form a ∼370 pc wide stripe along its major axis covering the sites of bright H II regions, but also more quiescent arm and inter-arm regions from the southern arm at 2 kpc galacto-centric distance to the south out to 5.7 kpc distance to the north. Full-galaxy maps of the continuum emission at 24 μm from Spitzer/MIPS, and at 70 μm, 100 μm, and 160 μm from Herschel/PACS were combined to obtain a map of the TIR. Results. TIR and C II intensities are correlated over more than two orders of magnitude. The range of TIR translates to a range of far ultraviolet (FUV) emission of G0, obs ∼ 2 to 200 in units of the average Galactic radiation field. The binned C II/TIR ratio drops with rising TIR, with large, but decreasing scatter. The contribution of the cold neutral medium to the C II emission, as estimated from VLA H I data, is on average only 10%. Fits of modified black bodies to the continuum emission were used to estimate dust mass surface densities and total gas column densities. A correction for possible foreground absorption by cold gas was applied to the O I data before comparing it with models of photon dominated regions. Most of the ratios of C II/O I and (C II+O I)/TIR are consistent with two model solutions. The median ratios are consistent with one solution at n ∼ 2 × 102 cm−3, G0 ∼ 60, and a second low-FUV solution at n ∼ 104 cm−3, G0 ∼ 1.5. Conclusions. The bulk of the gas along the lines-of-sight is represented by a low-density, high-FUV phase with low beam filling factors ∼1. A fraction of the gas may, however, be represented by the second solution.
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