Millimeter-Wave Line Ratios and Sub-beam Volume Density Distributions

We explore the use of mm-wave emission line ratios to trace molecular gas density when observations integrate over a wide range of volume densities within a single telescope beam. For observations targeting external galaxies, this case is unavoidable. Using a framework similar to that of Krumholz and Thompson (2007), we model emission for a set of common extragalactic lines from lognormal and power law density distributions. We consider the median density of gas producing emission and the ability to predict density variations from observed line ratios. We emphasize line ratio variations, because these do not require knowing the absolute abundance of our tracers. Patterns of line ratio variations have the prospect to illuminate the high-end shape of the density distribution, and to capture changes in the dense gas fraction and median volume density. Our results with and without a high density power law tail differ appreciably; we highlight better knowledge of the PDF shape as an important area. We also show the implications of sub-beam density distributions for isotopologue studies targeting dense gas tracers. Differential excitation often implies a significant correction to the naive case. We provide tabulated versions of many of our results, which can be used to interpret changes in mm-wave line ratios in terms of changes in the underlying density distributions.Comment: 24 pages, 16 figure, Accepted for publication in the Astrophysical Journal, two online tables temporarily available at http://www.astronomy.ohio-state.edu/~leroy.42/densegas_table2.txt and http://www.astronomy.ohio-state.edu/~leroy.42/densegas_table3.tx

The EMPIRE Survey: Systematic Variations in the Dense Gas Fraction and Star Formation Efficiency from Full-Disk Mapping of M51

We present the first results from the EMPIRE survey, an IRAM large program that is mapping tracers of high density molecular gas across the disks of nine nearby star-forming galaxies. Here, we present new maps of the 3-mm transitions of HCN, HCO+, and HNC across the whole disk of our pilot target, M51. As expected, dense gas correlates with tracers of recent star formation, filling the "luminosity gap" between Galactic cores and whole galaxies. In detail, we show that both the fraction of gas that is dense, f_dense traced by HCN/CO, and the rate at which dense gas forms stars, SFE_dense traced by IR/HCN, depend on environment in the galaxy. The sense of the dependence is that high surface density, high molecular gas fraction regions of the galaxy show high dense gas fractions and low dense gas star formation efficiencies. This agrees with recent results for individual pointings by Usero et al. 2015 but using unbiased whole-galaxy maps. It also agrees qualitatively with the behavior observed contrasting our own Solar Neighborhood with the central regions of the Milky Way. The sense of the trends can be explained if the dense gas fraction tracks interstellar pressure but star formation occurs only in regions of high density contrast.Comment: 7 pages, 5 figures, ApJL accepte

Resolved low-J 12^{12}CO excitation at 190 parsec resolution across NGC 2903 and NGC 3627

The low-$J$ rotational transitions of $^{12}$CO are commonly used to trace the distribution of molecular gas in galaxies. Their ratios are sensitive to excitation and physical conditions in the molecular gas. Spatially resolved studies of CO ratios are still sparse and affected by flux calibration uncertainties, especially since most do not have high angular resolution or do not have short-spacing information and hence miss any diffuse emission. We compare the low-$J$ CO ratios across the disk of two massive, star-forming spiral galaxies NGC2903 and NGC3627 to investigate whether and how local environments drive excitation variations at GMC scales. We use Atacama Large Millimeter Array (ALMA) observations of the three lowest-$J$ CO transitions at a common angular resolution of 4$''$ (190pc). We measure median line ratios of $R_{21}=0.67^{+0.13}_{-0.11}$, $R_{32}=0.33^{+0.09}_{-0.08}$, and $R_{31}=0.24^{+0.10}_{-0.09}$ across the full disk of NGC3627. We see clear CO line ratio variation across the galaxy consistent with changes in temperature and density of the molecular gas. In particular, toward the center, $R_{21}$, $R_{32}$, and $R_{31}$ increase by 35\%, 50\%, and 66\%, respectively compared to their average disk values. The overall line ratio trends suggest that CO(3-2) is more sensitive to changes in the excitation conditions than the two lower-$J$ transitions. Furthermore, we find a similar radial $R_{32}$ trend in NGC2903, albite a larger disk-wide average of $\langle R_{32}\rangle=0.47^{+0.14}_{-0.08}$. We conclude that the CO low-$J$ line ratios vary across environments in such a way that they can trace changes in the molecular gas conditions, with the main driver being changes in temperature.Comment: accepted for publication in MNRAS, 17 pages, 16 figure

Full-disc 13CO(1-0) mapping across nearby galaxies of the EMPIRE survey and the CO-to-H2 conversion factor

Carbon monoxide (CO) provides crucial information about the molecular gas properties of galaxies. While 12CO has been targeted extensively, isotopologues such as 13CO have the advantage of being less optically thick and observations have recently become accessible across full galaxy discs. We present a comprehensive new data set of 13CO(1-0) observations with the IRAM30-m telescope of the full discs of nine nearby spiral galaxies from the EMPIRE survey at a spatial resolution of ~1.5 kpc. 13CO(1-0) is mapped out to 0.7 - 1 r25 and detected at high signal-to-noise ratio throughout our maps. We analyse the 12CO(1-0)-to-13CO(1-0) ratio (R) as a function of galactocentric radius and other parameters such as the 12CO(2-1)-to- 12CO(1-0) intensity ratio, the 70-to-160 μm flux density ratio, the star formation rate surface density, the star formation efficiency, and the CO-to-H2 conversion factor.We find that R varies by a factor of 2 at most within and amongst galaxies, with a median value of 11 and larger variations in the galaxy centres than in the discs.We argue that optical depth effects, most likely due to changes in the mixture of diffuse/dense gas, are favoured explanations for the observed R variations, while abundance changes may also be at play. We calculate a spatially resolved 13CO(1-0)-to-H2 conversion factor and find an average value of 1.0×1021 cm-2 (K km s-1)-1 over our sample with a standard deviation of a factor of 2. We find that 13CO(1-0) does not appear to be a good predictor of the bulk molecular gas mass in normal galaxy discs due to the presence of a large diffuse phase, but it may be a better tracer of the mass than 12CO(1-0) in the galaxy centres where the fraction of dense gas is larger.DC is supported by the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement no. 702622. DC also acknowledges support from the DAAD/PROCOPE projects 57210883/35265PE. MJJD and FB acknowledge support from the German Research Foundation (DFG) grant BI 1546/1-1. FB acknowledges funding from the European Union’s Horizon 2020 research and innovation programme (grant agreement no. 726384 – EMPIRE). The work of MG and AKL is partially supported by the National Science Foundation under grants nos 1615105, 1615109, and 1653300. ER is supported by a Discovery Grant from the Natural Sciences and Engineering Research Council of Canada (NSERC) of Canada. ES acknowledges funding from the European Research Council under the European Unions Horizon 2020 research and innovation programme (grant agreement no. 694343)

Super Star Clusters in the Central Starburst of NGC 4945

The nearby (3.8Mpc) galaxy NGC 4945 hosts a nuclear starburst and Seyfert type 2 active galactic nucleus (AGN). We use the Atacama Large Millimeter/submillimeter Array (ALMA) to image the 93 GHz (3.2 mm) free-free continuum and hydrogen recombination line emission (H40 alpha and H42 alpha) at 2.2 pc (0 12) resolution. Our observations reveal 27 bright, compact sources with FWHM sizes of 1.4-4.0 pc, which we identify as candidate super star clusters. Recombination line emission, tracing the ionizing photon rate of the candidate clusters, is detected in 15 sources, six of which have a significant synchrotron component to the 93 GHz continuum. Adopting an age of similar to 5Myr, the stellar masses implied by the ionizing photon luminosities are log(10) (M*/M-circle dot) approximate to 4.7-6.1. We fit a slope to the cluster mass distribution and find beta = -1.8 +/-.0.4. The gas masses associated with these clusters, derived from the dust continuum at 350 GHz, are typically an order of magnitude lower than the stellar mass. These candidate clusters appear to have already converted a large fraction of their dense natal material into stars and, given their small freefall times of similar to 0.05 Myr, are surviving an early volatile phase. We identify a pointlike source in 93 GHz continuum emission that is presumed to be the AGN. We do not detect recombination line emission from the AGN and place an upper limit on the ionizing photons that leak into the starburst region of Q(0).<.10(52) s(-1)

Dynamical Equilibrium in the Molecular ISM in 28 Nearby Star-forming Galaxies

We compare the observed turbulent pressure in molecular gas, P_(turb), to the required pressure for the interstellar gas to stay in equilibrium in the gravitational potential of a galaxy, P_(DE). To do this, we combine arcsecond resolution CO data from PHANGS-ALMA with multiwavelength data that trace the atomic gas, stellar structure, and star formation rate (SFR) for 28 nearby star-forming galaxies. We find that P_(turb) correlates with—but almost always exceeds—the estimated P_(DE) on kiloparsec scales. This indicates that the molecular gas is overpressurized relative to the large-scale environment. We show that this overpressurization can be explained by the clumpy nature of molecular gas; a revised estimate of P_(DE) on cloud scales, which accounts for molecular gas self-gravity, external gravity, and ambient pressure, agrees well with the observed P_(turb) in galaxy disks. We also find that molecular gas with cloud-scale P_(turb) ≈ P_(DE) ≳ 10⁵ kB K cm⁻³ in our sample is more likely to be self-gravitating, whereas gas at lower pressure it appears more influenced by ambient pressure and/or external gravity. Furthermore, we show that the ratio between P_(turb) and the observed SFR surface density, Σ_(SFR), is compatible with stellar feedback-driven momentum injection in most cases, while a subset of the regions may show evidence of turbulence driven by additional sources. The correlation between Σ_(SFR) and kpc-scale P_(DE) in galaxy disks is consistent with the expectation from self-regulated star formation models. Finally, we confirm the empirical correlation between molecular-to-atomic gas ratio and kpc-scale P_(DE) reported in previous works

Molecular Gas Properties on Cloud Scales across the Local Star-forming Galaxy Population

Using the PHANGS–ALMA CO(2–1) survey, we characterize molecular gas properties on ~100 pc scales across 102,778 independent sightlines in 70 nearby galaxies. This yields the best synthetic view of molecular gas properties on cloud scales across the local star-forming galaxy population obtained to date. Consistent with previous studies, we observe a wide range of molecular gas surface densities (3.4 dex), velocity dispersions (1.7 dex), and turbulent pressures (6.5 dex) across the galaxies in our sample. Under simplifying assumptions about subresolution gas structure, the inferred virial parameters suggest that the kinetic energy of the molecular gas typically exceeds its self-gravitational binding energy at ~100 pc scales by a modest factor (1.3 on average). We find that the cloud-scale surface density, velocity dispersion, and turbulent pressure (1) increase toward the inner parts of galaxies, (2) are exceptionally high in the centers of barred galaxies (where the gas also appears less gravitationally bound), and (3) are moderately higher in spiral arms than in inter-arm regions. The galaxy-wide averages of these gas properties also correlate with the integrated stellar mass, star formation rate, and offset from the star-forming main sequence of the host galaxies. These correlations persist even when we exclude regions with extraordinary gas properties in galaxy centers, which contribute significantly to the inter-galaxy variations. Our results provide key empirical constraints on the physical link between molecular cloud populations and their galactic environment

Star Formation Laws and Efficiencies across 80 Nearby Galaxies

We measure empirical relationships between the local star formation rate (SFR) and properties of the star-forming molecular gas on 1.5 kpc scales across 80 nearby galaxies. These relationships, commonly referred to as "star formation laws," aim at predicting the local SFR surface density from various combinations of molecular gas surface density, galactic orbital time, molecular cloud free-fall time, and the interstellar medium dynamical equilibrium pressure. Leveraging a multiwavelength database built for the PHANGS survey, we measure these quantities consistently across all galaxies and quantify systematic uncertainties stemming from choices of SFR calibrations and the CO-to-H$_2$ conversion factors. The star formation laws we examine show 0.3-0.4 dex of intrinsic scatter, among which the molecular Kennicutt-Schmidt relation shows a $\sim$10% larger scatter than the other three. The slope of this relation ranges $\beta\approx0.9{-}1.2$, implying that the molecular gas depletion time remains roughly constant across the environments probed in our sample. The other relations have shallower slopes ($\beta\approx0.6{-}1.0$), suggesting that the star formation efficiency (SFE) per orbital time, the SFE per free-fall time, and the pressure-to-SFR surface density ratio (i.e., the feedback yield) may vary systematically with local molecular gas and SFR surface densities. Last but not least, the shapes of the star formation laws depend sensitively on methodological choices. Different choices of SFR calibrations can introduce systematic uncertainties of at least 10-15% in the star formation law slopes and 0.15-0.25 dex in their normalization, while the CO-to-H$_2$ conversion factors can additionally produce uncertainties of 20-25% for the slope and 0.10-0.20 dex for the normalization.Comment: 10 pages main text + 2 appendices. ApJL in press. Data products available at https://www.canfar.net/storage/list/phangs/RELEASES/Sun_etal_2023 . Slides summarizing key results can be found at https://www.dropbox.com/s/5gsegexeo9n0t05/Sun_et_PHANGS_2023.pptx?dl=

Multiwavelength Characterization of an ACT-Selected, Lensed Dusty Star-Forming Galaxy at zeta 2.64

We present C I(21) and multi-transition C-12 O observations of a dusty star-forming galaxy, ACT J2029+0120,which we spectroscopically confirm to lie at zeta = 2.64. We detect CO(3-2), CO(5-4), CO(7-6), CO(8-7), and C I(2-1) at high significance, tentatively detect HCO+(4-3), and place strong upper limits on the integrated strength of dense gas tracers (HCN(4-3) and CS(7-6)). Multi-transition CO observations and dense gas tracers can provide valuable constraints on the molecular gas content and excitation conditions in high-redshift galaxies. We therefore use this unique data set to construct a CO spectral line energy distribution (SLED) of the source, which is most consistent with that of a ULIRG Seyfert or QSO host object in the taxonomy of the Herschel Comprehensive ULIRG Emission Survey. We employ RADEX models to fit the peak of the CO SLED, inferring a temperature of T approximately 117 K and n(sub H2) approximately 10(exp5) cm(exp -3), most consistent with a ULIRGQSO object and the presence of high-density tracers. We also find that the velocity width of the C I line is potentially larger than seen in all CO transitions forth is object, and that the L'(sub Ci(2-1))/L'(sub CO(3-2))ratio is also larger than seen in other lensed and unlensed submillimeter galaxies and QSO hosts; if confirmed, this anomaly could be an effect of differential lensing of a shocked molecular outflow

EMPIRE: The IRAM 30-m Dense Gas Survey of Nearby Galaxies

International audienceWe present EMPIRE, an IRAM 30-m large program that mapped $\lambda = 3{-}4$ mm dense gas tracers at $\sim 1{-}2\,$kpc resolution across the whole star-forming disk of nine nearby, massive, spiral galaxies. We describe the EMPIRE observing and reduction strategies and show new whole-galaxy maps of HCN(1-0), HCO$^+$(1-0), HNC(1-0) and CO(1-0). We explore how the HCN-to-CO and IR-to-HCN ratios, observational proxies for the dense gas fraction and dense gas star formation efficiency, depend on host galaxy and local environment. We find that the fraction of dense gas correlates with stellar surface density, gas surface density, molecular-to-atomic gas ratio, and dynamical equilibrium pressure. In EMPIRE, the star formation rate per unit dense gas anti-correlates with these same environmental parameters. Thus, although dense gas appears abundant the central regions of many spiral galaxies, this gas appears relatively inefficient at forming stars. These results qualitatively agree with previous work on nearby galaxies and the Milky Way's Central Molecular Zone. To first order, EMPIRE demonstrates that the conditions in a galaxy disk set the gas density distribution and that the dense gas traced by HCN shows an environment-dependent relation to star formation. However, our results also show significant ($\pm 0.2$ dex) galaxy-to-galaxy variations. We suggest that gas structure below the scale of our observations and dynamical effects likely also play an important role