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

Dense Molecular Gas in the Nearby Low Metallicity Dwarf Starburst Galaxy IC 10

Dense molecular gas and star formation are correlated in galaxies. The effect of low metallicity on this relationship is crucial for interpreting observations of high redshift galaxies, which have lower metallicities than galaxies today. However, it remains relatively unexplored because dense molecular gas tracers like HCN and HCO+ are faint in low metallicity systems. We present Green Bank Telescope observations of HCN(1-0) and HCO+(1-0) on giant molecular cloud (34pc) scales in the nearby low metallicity ($12+\log({\rm O/H})=8.2$) starburst IC 10 and compare them to those in other galaxies. We detect HCN and HCO+ in one and three of five pointings, respectively. The $I_{\rm HCN}/I_{\rm HCO+}$ values are within the range seen in other galaxies, but are most similar to those seen in other low metallicity sources and in starbursts. The detections follow the fiducial $L_{\rm IR}$-$L_{\rm HCN}$ and $L_{\rm IR}$-$L_{\rm HCO+}$ relationships. These trends suggest that HCN and HCO+ can be used to trace dense molecular gas at metallicities of 1/4 $Z_\odot$, to first order. The dense gas fraction is similar to that in spiral galaxies, but lower than that in U/LIRGs. The dense molecular gas star formation efficiency, however, is on the upper end of those in normal galaxies and consistent with those in U/LIRGs. These results suggest that the CO and HCN/HCO+ emission occupy the same relative volumes as at higher metallicity, but that the entire emitting structure is reduced in size. Dense gas mass estimates for high redshift galaxies may need to be corrected for this effect.Comment: Accepted to Ap

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)

Optical depth estimates and effective critical densities of dense gas tracers in the inner parts of nearby galaxy discs

High critical density molecular lines like HCN (1-0) or HCO+ (1-0) represent our best tool to study currently star-forming, dense molecular gas at extragalactic distances. The optical depth of these lines is a key ingredient to estimate the effective density required to excite emission. However, constraints on this quantity are even scarcer in the literature than measurements of the high-density tracers themselves. Here, we combine new observations of HCN, HCO+ and HNC(1-0) and their optically thin isotopologuesH13CN,H13CO+ andHN13C (1-0) to measure isotopologue line ratios.We use IRAM30-m observations from the large programme EMPIRE and new Atacama Large Millimetre/submillimetre Array observations, which together target six nearby star-forming galaxies. Using spectral stacking techniques, we calculate or place strong upper limits on the HCN/H13CN, HCO+/H13CO+ and HNC/HN13C line ratios in the inner parts of these galaxies. Under simple assumptions, we use these to estimate the optical depths of HCN (1-0) and HCO+ (1-0) to be t ~ 2-11 in the active, inner regions of our targets. The critical densities are consequently lowered to values between 5 and 20 × 105 cm-3, 1 and 3 × 105 cm-3 and 9 × 104 cm-3 for HCN, HCO+ and HNC, respectively.We study the impact of having different beam-filling factors, η, on these estimates and find that the effective critical densities decrease by a factor of η12 /η13 τ12. A comparison to existing work in NGC 5194 and NGC 253 shows the HCN/H13CN and HCO+/H13CO+ ratios in agreement with our measurements within the uncertainties. The same is true for studies in other environments such as the Galactic Centre or nuclear regions of active galactic nucleus dominated nearby galaxie

Dense Gas, Dynamical Equilibrium Pressure, and Star Formation in Nearby Star-Forming Galaxies

We use new ALMA observations to investigate the connection between dense gas fraction, star formation rate, and local environment across the inner region of four local galaxies showing a wide range of molecular gas depletion times. We map HCN (1-0), HCO$^+$ (1-0), CS (2-1), $^{13}$CO (1-0), and C$^{18}$O (1-0) across the inner few kpc of each target. We combine these data with short spacing information from the IRAM large program EMPIRE, archival CO maps, tracers of stellar structure and recent star formation, and recent HCN surveys by Bigiel et al. and Usero et al. We test the degree to which changes in the dense gas fraction drive changes in the SFR. $I_{HCN}/I_{CO}$ (tracing the dense gas fraction) correlates strongly with $I_{CO}$ (tracing molecular gas surface density), stellar surface density, and dynamical equilibrium pressure, $P_{DE}$. Therefore, $I_{HCN}/I_{CO}$ becomes very low and HCN becomes very faint at large galactocentric radii, where ratios as low as $I_{HCN}/I_{CO} \sim 0.01$ become common. The apparent ability of dense gas to form stars, $\Sigma_{SFR}/\Sigma_{dense}$ (where $\Sigma_{dense}$ is traced by the HCN intensity and the star formation rate is traced by a combination of H$\alpha$ and 24$\mu$m emission), also depends on environment. $\Sigma_{SFR}/\Sigma_{dense}$ decreases in regions of high gas surface density, high stellar surface density, and high $P_{DE}$. Statistically, these correlations between environment and both $\Sigma_{SFR}/\Sigma_{dense}$ and $I_{HCN}/I_{CO}$ are stronger than that between apparent dense gas fraction ($I_{HCN}/I_{CO}$) and the apparent molecular gas star formation efficiency $\Sigma_{SFR}/\Sigma_{mol}$. We show that these results are not specific to HCN.Comment: 31 pages, 13 figures, accepted for publication in The Astrophysical Journal, email for access to data table before publicatio

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=

¹³CO/C¹⁸O Gradients across the Disks of Nearby Spiral Galaxies

We use the IRAM Large Program EMPIRE and new high-resolution ALMA data to measure 13CO(1-0)/C18O(1-0) intensity ratios across nine nearby spiral galaxies. These isotopologues of 12CO are typically optically thin across most of the area in galaxy disks, and this ratio allows us to gauge their relative abundance due to chemistry or stellar nucleosynthesis effects. Resolved 13CO/C18O gradients across normal galaxies have been rare due to the faintness of these lines. We find a mean 13CO/C18O ratio of 6.0 ± 0.9 for the central regions of our galaxies. This agrees well with results in the Milky Way, but differs from results for starburst galaxies (3.4 ± 0.9) and ultraluminous infrared galaxies (1.1 ± 0.4). In our sample, the 13CO/C18O ratio consistently increases with increasing galactocentric radius and decreases with increasing star formation rate surface density. These trends could be explained if the isotopic abundances are altered by fractionation; the sense of the trends also agrees with those expected for carbon and oxygen isotopic abundance variations due to selective enrichment by massive stars.F.B., M.J., and D.C. acknowledge support from DFG grant BI 1546/1-1. A.H. acknowledges support from the Centre National d’Etudes Spatiales (CNES). A.U. acknowledges support from Spanish MINECO grants FIS2012-32096 and ESP2015-68964. The work of A.K.L. is partially supported by the National Science Foundation under grants No. 1615105 and 1615109. M.R.K. acknowledges support from ARC DP160100695. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. This paper makes use of the following ALMA data: ADS/JAO. ALMA #2011.0.00004.SV. ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada), NSC and ASIAA (Taiwan), and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ