Subjects: Astronomy >> Astrophysical processes submitted time 2023-02-19
Abstract: We combine archival HST and new JWST imaging data, covering the ultraviolet to mid-infrared regime, to morphologically analyze the nuclear star cluster (NSC) of NGC 628, a grand-design spiral galaxy. The cluster is located in a 200 pc x 400 pc cavity, lacking both dust and gas. We find roughly constant values for the effective radius (r_eff ~ 5 pc) and ellipticity ({\epsilon} ~ 0.05), while the S\'ersic index (n) and position angle (PA) drop from n ~ 3 to ~ 2 and PA ~ 130{\deg} to 90{\deg}, respectively. In the mid-infrared, r_eff ~ 12pc, {\epsilon} ~ 0.4, and n ~ 1-1.5, with the same PA ~ 90{\deg}. The NSC has a stellar mass of log10 (M_nsc / M_Sun) = 7.06 +- 0.31, as derived through B-V, confirmed when using multi-wavelength data, and in agreement with the literature value. Fitting the spectral energy distribution, excluding the mid-infrared data, yields a main stellar population's age of (8 +- 3) Gyr with a metallicity of Z = 0.012 +- 0.006. There is no indication of any significant star formation over the last few Gyr. Whether gas and dust were dynamically kept out or evacuated from the central cavity remains unclear. The best-fit suggests an excess of flux in the mid-infrared bands, with further indications that the center of the mid-infrared structure is displaced with respect to the optical center of the NSC. We discuss five potential scenarios, none of them fully explaining both the observed photometry and structure.
Peer Review Status:Awaiting Review
Subjects: Astronomy >> Astrophysical processes submitted time 2023-02-19
Abstract: We present a comparison of theoretical predictions of dust continuum and polycyclic aromatic hydrocarbon (PAH) emission with new JWST observations in three nearby galaxies: NGC 628, NGC 1365, and NGC 7496. Our analysis focuses on a total of 1063 compact stellar clusters and 2654 stellar associations previously characterized by HST in the three galaxies. We find that the distributions and trends in the observed PAH-focused infrared colors generally agree with theoretical expectations, and that the bulk of the observations is more aligned with models of larger, ionized PAHs. These JWST data usher in a new era of probing interstellar dust and studying how the intense radiation fields near stellar clusters and associations play a role in shaping the physical properties of PAHs.
Peer Review Status:Awaiting Review
Subjects: Astronomy >> Astrophysical processes submitted time 2023-02-19
Abstract: Mapping star-formation rates (SFR) within galaxies is key to unveiling their assembly and evolution. Calibrations exist for computing SFR from a combination of ultraviolet and infrared bands for galaxies as integrated systems, but their applicability to sub-galactic (kpc) scales remains largely untested. Here we use integral field spectroscopy of 19 nearby ($D <$ 20 Mpc) galaxies obtained by PHANGS-MUSE to derive accurate Balmer decrements (H$\alpha$/H$\beta$) and attenuation-corrected H$\alpha$ maps. We combine this information with mid-infrared maps from WISE at 22 $\rm \mu m$, and ultraviolet maps from GALEX in the far-UV band, to derive SFR surface densities in nearby galaxies on resolved (kpc) scales. Using the H$\alpha$ attenuation-corrected SFR as a reference, we find that hybrid recipes from the literature overestimate the SFR in regions of low SFR surface density, low specific star-formation rate (sSFR), low attenuation and old stellar ages. We attribute these trends to heating of the dust by old stellar populations (IR cirrus). We calibrate this effect by proposing functional forms for the coefficients in front of the IR term which depend on band ratios sensitive to the sSFR. These calibrations prove reliable as a function of physical scale. In particular, they agree within 10% with the attenuation corrections computed from the Balmer decrement on 100 pc scales. Despite small quantitative differences, our calibrations are also applicable to integrated galaxy scales probed by the MaNGA survey, albeit with a larger scatter (up to 0.22 dex). Observations with JWST open up the possibility to calibrate these relations in nearby galaxies with cloud-scale ($\sim$100 pc) resolution mid-IR imaging.
Peer Review Status:Awaiting Review
Subjects: Astronomy >> Astrophysical processes submitted time 2023-02-19
Abstract: A long-standing problem when deriving the physical properties of stellar populations is the degeneracy between age, reddening, and metallicity. When a single metallicity is used for all star clusters in a galaxy, this degeneracy can result in $`$catastrophic$'$ errors for old globular clusters. Typically, approximately 10 - 20 % of all clusters detected in spiral galaxies can have ages that are incorrect by a factor of ten or more. In this paper we present a pilot study for four galaxies (NGC 628, NGC 1433, NGC 1365, and NGC 3351) from the PHANGS-HST survey. We describe methods to correct the age-dating for old globular clusters, by first identifying candidates using their colors, and then reassigning ages and reddening based on a lower metallicity solution. We find that young $`$interlopers$'$ can be identified from their Halpha flux. CO (2-1) intensity or the presence of dust can also be used, but our tests show that they do not work as well. Improvements in the success fraction are possible at the $\sim$ 15 % level (reducing the fraction of catastrophic age-estimates from between 13 - 21 % to 3 - 8 %). A large fraction of the incorrectly age-dated globular clusters are systematically given ages around 100 Myr, polluting the younger populations as well. Incorrectly age-dated globular clusters significantly impact the observed cluster age distribution in NGC 628, which affects the physical interpretation of cluster disruption in this galaxy. For NGC 1365, we also demonstrate how to fix a second major age-dating problem, where very dusty young clusters with E(B-V) $>$ 1.5 mag are assigned old, globular-cluster like ages. Finally, we note the discovery of a dense population of $\sim$ 300 Myr clusters around the central region of NGC 1365. and discuss how this results naturally from the dynamics in a barred galaxy.
Peer Review Status:Awaiting Review
Subjects: Astronomy >> Astrophysical processes submitted time 2023-02-19
Abstract: We compare mid-infrared (mid-IR), extinction-corrected H$\alpha$, and CO (2-1) emission at 70--160 pc resolution in the first four PHANGS-JWST targets. We report correlation strengths, intensity ratios, and power law fits relating emission in JWST's F770W, F1000W, F1130W, and F2100W bands to CO and H$\alpha$. At these scales, CO and H$\alpha$ each correlate strongly with mid-IR emission, and these correlations are each stronger than the one relating CO to H$\alpha$ emission. This reflects that mid-IR emission simultaneously acts as a dust column density tracer, leading to the good match with the molecular gas-tracing CO, and as a heating tracer, leading to the good match with the H$\alpha$. By combining mid-IR, CO, and H$\alpha$ at scales where the overall correlation between cold gas and star formation begins to break down, we are able to separate these two effects. We model the mid-IR above $I_\nu = 0.5$~MJy sr$^{-1}$ at F770W, a cut designed to select regions where the molecular gas dominates the interstellar medium (ISM) mass. This bright emission can be described to first order by a model that combines a CO-tracing component and an H$\alpha$-tracing component. The best-fitting models imply that $\sim 50\%$ of the mid-IR flux arises from molecular gas heated by the diffuse interstellar radiation field, with the remaining $\sim 50\%$ associated with bright, dusty star forming regions. We discuss differences between the F770W, F1000W, F1130W bands and the continuum dominated F2100W band and suggest next steps for using the mid-IR as an ISM tracer.
Peer Review Status:Awaiting Review
Subjects: Astronomy >> Astrophysical processes submitted time 2023-02-19
Abstract: We develop a method to identify and determine the physical properties of stellar associations using Hubble Space Telescope (HST) NUV-U-B-V-I imaging of nearby galaxies from the PHANGS-HST survey. We apply a watershed algorithm to density maps constructed from point source catalogues Gaussian smoothed to multiple physical scales from 8 to 64 pc. We develop our method on two galaxies that span the distance range in the PHANGS-HST sample: NGC 3351 (10 Mpc), NGC 1566 (18 Mpc). We test our algorithm with different parameters such as the choice of detection band for the point source catalogue (NUV or V), source density image filtering methods, and absolute magnitude limits. We characterise the properties of the resulting multi-scale associations, including sizes, number of tracer stars, number of associations, photometry, as well as ages, masses, and reddening from Spectral Energy Distribution fitting. Our method successfully identifies structures that occupy loci in the UBVI colour-colour diagram consistent with previously published catalogues of clusters and associations. The median ages of the associations increases from log(age/yr) = 6.6 to log(age/yr) = 6.9 as the spatial scale increases from 8 pc to 64 pc for both galaxies. We find that the youngest stellar associations, with ages < 3 Myr, indeed closely trace H ii regions in H$\alpha$ imaging, and that older associations are increasingly anti-correlated with the H$\alpha$ emission. Owing to our new method, the PHANGS-HST multi-scale associations provide a far more complete census of recent star formation activity than found with previous cluster and compact association catalogues. The method presented here will be applied to the full sample of 38 PHANGS-HST galaxies.
Peer Review Status:Awaiting Review
Subjects: Astronomy >> Astrophysical processes submitted time 2023-02-19
Abstract: We present new HCN and HCO$^+$ ($J$=3-2) images of the nearby star-forming galaxies (SFGs) NGC 3351, NGC 3627, and NGC 4321. The observations, obtained with the Morita ALMA Compact Array, have a spatial resolution of $\sim$290-440 pc and resolve the inner $R_\textrm{gal} \lesssim$ 0.6-1 kpc of the targets, as well as the southern bar end of NGC 3627. We complement this data set with publicly available images of lower excitation lines of HCN, HCO$^+$, and CO and analyse the behaviour of a representative set of line ratios: HCN(3-2)/HCN(1-0), HCN(3-2)/HCO$^+$(3-2), HCN(1-0)/CO(2-1), and HCN(3-2)/CO(2-1). Most of these ratios peak at the galaxy centres and decrease outwards. We compare the HCN and HCO$^+$ observations with a grid of one-phase, non-local thermodynamic equilibrium (non-LTE) radiative transfer models and find them compatible with models that predict subthermally excited and optically thick lines. We study the systematic variations of the line ratios across the targets as a function of the stellar surface density ($\Sigma_\textrm{star}$), the intensity-weighted CO(2-1) ($\langle I_\text{CO}\rangle$), and the star formation rate surface density ($\Sigma_\text{SFR}$). We find no apparent correlation with $\Sigma_\text{SFR}$, but positive correlations with the other two parameters, which are stronger in the case of $\langle I_\text{CO}\rangle$. The HCN/CO-$\langle I_\text{CO}\rangle$ relations show $\lesssim$0.3 dex galaxy-to-galaxy offsets, with HCN(3-2)/CO(2-1)-$\langle I_\text{CO}\rangle$ being $\sim$2 times steeper than HCN(1-0)/CO(2-1). In contrast, the HCN(3-2)/HCN(1-0)-$\langle I_\text{CO}\rangle$ relation exhibits a tighter alignment between galaxies. We conclude that the overall behaviour of the line ratios cannot be ascribed to variations in a single excitation parameter (e.g. density or temperature).
Peer Review Status:Awaiting Review
Subjects: Astronomy >> Astrophysical processes submitted time 2023-02-19
Abstract: We present maps tracing the fraction of dust in the form of polycyclic aromatic hydrocarbons (PAHs) in IC 5332, NGC 628, NGC 1365, and NGC 7496 from JWST/MIRI observations. We trace the PAH fraction by combining the F770W ($7.7~\mu$m) and F1130W ($11.3~\mu$m) filters to track ionized and neutral PAH emission, respectively, and comparing the PAH emission to F2100W which traces small, hot dust grains. We find average $R{\rm_{PAH} = (F770W+F1130W)/F2100W}$ values of 3.3, 4.7, 5.1, and 3.6 in IC 5332, NGC 628, NGC 1365, and NGC 7496, respectively. We find that H II regions traced by MUSE H$\alpha$ show a systematically low PAH fraction. The PAH fraction remains relatively constant across other galactic environments, with slight variations. We use CO + H I + H$\alpha$ to trace the interstellar gas phase and find that the PAH fraction decreases above a value of I$_{H\alpha}/\Sigma_{H~I+H_2}$ $\sim~10^{37.5}$ erg s$^{-1}$ kpc$^{-2}$ (M$_\odot$ pc$^{-2}$)$^{-1}$, in all four galaxies. Radial profiles also show a decreasing PAH fraction with increasing radius, correlated with lower metallicity, in line with previous results showing a strong metallicity dependence to the PAH fraction. Our results suggest that the process of PAH destruction in ionized gas operates similarly across the four targets.
Peer Review Status:Awaiting Review
Subjects: Astronomy >> Astrophysical processes submitted time 2023-02-19
Abstract: Ratios of polycyclic aromatic hydrocarbon (PAH) vibrational bands are a promising tool for measuring the properties of the PAH population and their effect on star formation. The photometric bands of the MIRI and NIRCam instruments on JWST provide the opportunity to measure PAH emission features across entire galaxy disks at unprecedented resolution and sensitivity. Here we present the first results of this analysis in a sample of three nearby galaxies: NGC 628, NGC 1365, and NGC 7496. Based on the variations observed in the 3.3, 7.7, and 11.3 $\mu$m features, we infer changes to the average PAH size and ionization state across the different galaxy environments. High values of F335M$_{\rm PAH}$/F1130W and low values of F1130W/F770W are measured in H II regions in all three galaxies. This suggests that these regions are populated by hotter PAHs, and/or that the PAH ionization fraction is larger. We see additional evidence of heating and/or changes in PAH size in regions with higher molecular gas content as well as increased ionization in regions with higher H$\alpha$ intensity.
Peer Review Status:Awaiting Review
Subjects: Astronomy >> Astrophysical processes submitted time 2023-02-19
Abstract: We present maps of the 3.3 micron polycyclic aromatic hydrocarbon (PAH) emission feature in NGC 628, NGC 1365, and NGC 7496 as observed with the Near-Infrared Camera (NIRCam) imager on JWST from the PHANGS-JWST Cycle 1 Treasury project. We create maps that isolate the 3.3 micron PAH feature in the F335M filter (F335M$_{\rm PAH}$) using combinations of the F300M and F360M filters for removal of starlight continuum. This continuum removal is complicated by contamination of the F360M by PAH emission and variations in the stellar spectral energy distribution slopes between 3.0 and 3.6 micron. We modify the empirical prescription from Lai et al. (2020) to remove the starlight continuum in our highly resolved galaxies, which have a range of starlight- and PAH-dominated lines-of-sight. Analyzing radially binned profiles of the F335M$_{\rm PAH}$ emission, we find that between 5-65% of the F335M intensity comes from the 3.3 micron feature within the inner 0.5 $r_{25}$ of our targets. This percentage systematically varies from galaxy to galaxy, and shows radial trends within the galaxies related to each galaxy's distribution of stellar mass, interstellar medium, and star formation. The 3.3 micron emission is well correlated with the 11.3 micron PAH feature traced with the MIRI F1130W filter, as is expected, since both features arise from C-H vibrational modes. The average F335M$_{\rm PAH}$/F1130W ratio agrees with the predictions of recent models by Draine et al. (2021) for PAHs with size and charge distributions shifted towards larger grains with normal or higher ionization.
Peer Review Status:Awaiting Review
Subjects: Astronomy >> Astrophysical processes submitted time 2023-02-19
Abstract: We compare embedded young massive star clusters (YMCs) to (sub-)millimeter line observations tracing the excitation and dissociation of molecular gas in the starburst ring of NGC 1365. This galaxy hosts one of the strongest nuclear starbursts and richest populations of YMCs within 20 Mpc. Here we combine near-/mid-IR PHANGS-JWST imaging with new ALMA multi-J CO (1-0, 2-1 and 4-3) and [CI](1-0) mapping, which we use to trace CO excitation via R42 = I_CO(4-3)/I_CO(2-1) and R21 = I_CO(2-1)/I_CO(1-0) and dissociation via RCICO = I_[CI](1-0)/I_CO(2-1) at 330 pc resolution. We find that the gas flowing into the starburst ring from northeast to southwest appears strongly affected by stellar feedback, showing decreased excitation (lower R42) and increased signatures of dissociation (higher RCICO) in the downstream regions. There, radiative transfer modeling suggests that the molecular gas density decreases and temperature and [CI/CO] abundance ratio increase. We compare R42 and RCICO with local conditions across the regions and find that both correlate with near-IR 2 um emission tracing the YMCs and with both PAH (11.3 um) and dust continuum (21 um) emission. In general, RCICO exhibits ~ 0.1 dex tighter correlations than R42, suggesting CI to be a more sensitive tracer of changing physical conditions in the NGC 1365 starburst than CO (4-3). Our results are consistent with a scenario where gas flows into the two arm regions along the bar, becomes condensed/shocked, forms YMCs, and then these YMCs heat and dissociate the gas.
Peer Review Status:Awaiting Review
Subjects: Astronomy >> Astrophysical processes submitted time 2023-02-19
Abstract: Mergers are thought to be a fundamental channel for galaxy growth, perturbing the gas dynamics and the magnetic fields (B-fields) in the interstellar medium (ISM). However, the mechanisms that amplify and dissipate B-fields during a merger remain unclear. We characterize the morphology of the ordered B-fields in the multi-phase ISM of the closest merger of two spiral galaxies, the Antennae galaxies. We compare the inferred B-fields using $154~\mu$m thermal dust and $11$ cm radio synchrotron emission polarimetric observations. We find that the $154~\mu$m B-fields are more ordered across the Antennae galaxies than the $11$ cm B-fields. The turbulent-to-ordered $154~\mu$m B-field increases at the galaxy cores and star-forming regions. The relic spiral arm has an ordered spiral $154~\mu$m B-field, while the $11$ cm B-field is radial. The $154~\mu$m B-field may be dominated by turbulent dynamos with high $^{12}$CO(1-0) velocity dispersion driven by star-forming regions, while the $11$ cm B-field is cospatial with high HI velocity dispersion driven by galaxy interaction. This result shows the dissociation between the warm gas mainly disturbed by the merger, and the dense gas still following the dynamics of the relic spiral arm. We find a $\sim8.9$ kpc scale ordered B-field connecting the two galaxies. The base of the tidal tail is cospatial with the HI and $^{12}$CO(1-0) emission and has compressed and/or sheared $154~\mu$m and $11$ cm B-fields driven by the merger. We suggest that amplify B-fields, with respect to the rest of the system and other spiral galaxies, may be supporting the gas flow between both galaxies and the tidal tail.
Peer Review Status:Awaiting Review
Subjects: Astronomy >> Astrophysical processes submitted time 2023-02-19
Abstract: 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.
Peer Review Status:Awaiting Review
Subjects: Astronomy >> Astrophysical processes submitted time 2023-02-19
Abstract: We use new HCN(1-0) data from the ALMOND (ACA Large-sample Mapping Of Nearby galaxies in Dense gas) survey to trace the kpc-scale molecular gas density structure and CO(2-1) data from PHANGS-ALMA to trace the bulk molecular gas across 25 nearby, star-forming galaxies. At 2.1 kpc scale, we measure the density-sensitive HCN/CO line ratio and the SFR/HCN ratio to trace the star formation efficiency in the denser molecular medium. At 150 pc scale, we measure structural and dynamical properties of the molecular gas via CO(2-1) line emission, which is linked to the lower resolution data using an intensity-weighted averaging method. We find positive correlations (negative) of HCN/CO (SFR/HCN) with the surface density, the velocity dispersion and the internal turbulent pressure of the molecular gas. These observed correlations agree with expected trends from turbulent models of star formation, which consider a single free-fall time gravitational collapse. Our results show that the kpc-scale HCN/CO line ratio is a powerful tool to trace the 150 pc scale average density distribution of the molecular clouds. Lastly, we find systematic variations of the SFR/HCN ratio with cloud-scale molecular gas properties, which are incompatible with a universal star formation efficiency. Overall, these findings show that mean molecular gas density, molecular cloud properties and star formation are closely linked in a coherent way, and observations of density-sensitive molecular gas tracers are a useful tool to analyse these variations, linking molecular gas physics to stellar output across galaxy discs.
Peer Review Status:Awaiting Review
Subjects: Astronomy >> Astrophysical processes submitted time 2023-02-19
Abstract: The processes of star formation and feedback, regulating the cycle of matter between gas and stars on the scales of giant molecular clouds (GMCs; $\sim$100pc), play a major role in governing galaxy evolution. Measuring the time-scales of GMC evolution is important to identify and characterise the specific physical mechanisms that drive this transition. By applying a robust statistical method to high-resolution CO and narrow-band H$\alpha$ imaging from the PHANGS survey, we systematically measure the evolutionary timeline from molecular clouds to exposed young stellar regions on GMC scales, across the discs of an unprecedented sample of 54 star-forming main-sequence galaxies (excluding their unresolved centres). We find that clouds live for about $1{-}3$ GMC turbulence crossing times ($5{-}30$Myr) and are efficiently dispersed by stellar feedback within $1{-}5$Myr once the star-forming region becomes partially exposed, resulting in integrated star formation efficiencies of $1{-}8$%. These ranges reflect physical galaxy-to-galaxy variation. In order to evaluate whether galactic environment influences GMC evolution, we correlate our measurements with average properties of the GMCs and their local galactic environment. We find several strong correlations that can be physically understood, revealing a quantitative link between galactic-scale environmental properties and the small-scale GMC evolution. Notably, the measured CO-visible cloud lifetimes become shorter with decreasing galaxy mass, mostly due to the increasing presence of CO-dark molecular gas in such environment. Our results represent a first step towards a comprehensive picture of cloud assembly and dispersal, which requires further extension and refinement with tracers of the atomic gas, dust, and deeply-embedded stars.
Peer Review Status:Awaiting Review
Subjects: Astronomy >> Astrophysical processes submitted time 2023-02-19
Abstract: We present new neutral atomic carbon [CI](3P1-3P0) mapping observations within the inner ~7 kpc and ~4 kpc of the disks of NGC3627 and NGC4321 at a spatial resolution of 190 pc and 270 pc, respectively, using the ALMA Atacama Compact Array (ACA). We combine these with the CO(2-1) data from PHANGS-ALMA, and literature [CI] and CO data for two other starburst and/or active galactic nucleus (AGN) galaxies (NGC1808, NGC7469), to study: a) the spatial distributions of CI and CO emission; b) the observed line ratio RCICO = I_[CI](1-0)/I_CO(2-1) as a function of various galactic properties; and c) the abundance ratio of [CI/CO]. We find excellent spatial correspondence between CI and CO emission and nearly uniform RCICO ~0.1 across the majority of the star-forming disks of NGC3627 and NGC4321. However, RCICO strongly varies from ~0.05 at the centre of NGC4321 to >0.2-0.5 in NGC1808's starburst centre and NGC7469's centre with an X-ray AGN. Meanwhile, RCICO does not obviously vary with $U$, similar to the prediction of PDR models. We also find a mildly decreasing RCICO with an increasing metallicity over 0.7-0.85 solar metallicity, consistent with the literature. Assuming various typical ISM conditions representing GMCs, active star-forming regions and strong starbursting environments, we calculate the LTE radiative transfer and estimate the [CI/CO] abundance ratio to be ~0.1 across the disks of NGC3627 and NGC4321, similar to previous large-scale findings in Galactic studies. However, this abundance ratio likely has a substantial increase to ~1 and >1-5 in NGC1808's starburst and NGC7469's strong AGN environments, respectively, in line with the expectations for cosmic-ray dominated region (CRDR) and X-ray dominated region (XDR) chemistry. Finally, we do not find a robust evidence for a generally CO-dark, CI-bright gas in the disk areas we probed. (abbreviated)
Peer Review Status:Awaiting Review
Subjects: Astronomy >> Astrophysical processes submitted time 2023-02-19
Abstract: We present a rich, multiwavelength, multiscale database built around the PHANGS-ALMA CO$\,$(2-1) survey and ancillary data. We use this database to present the distributions of molecular cloud populations and sub-galactic environments in 80 PHANGS galaxies, to characterize the relationship between population-averaged cloud properties and host galaxy properties, and to assess key timescales relevant to molecular cloud evolution and star formation. We show that PHANGS probes a wide range of kpc-scale gas, stellar, and star formation rate (SFR) surface densities, as well as orbital velocities and shear. The population-averaged cloud properties in each aperture correlate strongly with both local environmental properties and host galaxy global properties. Leveraging a variable selection analysis, we find that the kpc-scale surface densities of molecular gas and SFR tend to possess the most predictive power for the population-averaged cloud properties. Once their variations are controlled for, galaxy global properties contain little additional information, which implies that the apparent galaxy-to-galaxy variations in cloud populations are likely mediated by kpc-scale environmental conditions. We further estimate a suite of important timescales from our multiwavelength measurements. The cloud-scale free-fall time and turbulence crossing time are ${\sim}5{-}20$ Myr, comparable to previous cloud lifetime estimates. The timescales for orbital motion, shearing, and cloud-cloud collisions are longer, ${\sim}100$ Myr. The molecular gas depletion time is $1{-}3$ Gyr and shows weak to no correlations with the other timescales in our data. We publish our measurements online and expect them to have broad utility to future studies of molecular clouds and star formation.
Peer Review Status:Awaiting Review
Subjects: Astronomy >> Astrophysical processes submitted time 2023-02-19
Abstract: Carbon monoxide (CO) emission is the most widely used tracer of the bulk molecular gas in the interstellar medium (ISM) in extragalactic studies. The CO-to-H$_2$ conversion factor, $\alpha_{\rm CO}$, links the observed CO emission to the total molecular gas mass. However, no single prescription perfectly describes the variation of $\alpha_{\rm CO}$ across all environments across galaxies as a function of metallicity, molecular gas opacity, line excitation, and other factors. Using resolved spectral line observations of CO and its isotopologues, we can constrain the molecular gas conditions and link them to a variation in the conversion factor. We present new IRAM 30-m 1mm and 3mm line observations of $^{12}$CO, $^{13}$CO, and C$^{18}$O} across the nearby galaxy M101. Based on the CO isotopologue line ratios, we find that selective nucleosynthesis and opacity changes are the main drivers of the variation in the line emission across the galaxy. Furthermore, we estimated $\alpha_{\rm CO(1-0)}$ using different approaches, including (i) the dust mass surface density derived from far-IR emission as an independent tracer of the total gas surface density and (ii) LTE-based measurements using the optically thin $^{13}$CO(1-0) intensity. We find an average value of $\alpha_{\rm CO}=4.4{\pm}0.9\rm\,M_\odot\,pc^{-2}(K\,km\,s^{-1})^{-1}$ across the galaxy, with a decrease by a factor of 10 toward the 2 kpc central region. In contrast, we find LTE-based values are lower by a factor of 2-3 across the disk relative to the dust-based result. Accounting for $\alpha_{\rm CO}$ variations, we found significantly reduced molecular gas depletion time by a factor 10 in the galaxy's center. In conclusion, our result suggests implications for commonly derived scaling relations, such as an underestimation of the slope of the Kennicutt Schmidt law, if $\alpha_{\rm CO}$ variations are not accounted for.
Peer Review Status:Awaiting Review
Subjects: Astronomy >> Astrophysical processes submitted time 2023-02-19
Abstract: JWST observations of polycyclic aromatic hydrocarbon (PAH) emission provide some of the deepest and highest resolution views of the cold interstellar medium (ISM) in nearby galaxies. If PAHs are well mixed with the atomic and molecular gas and illuminated by the average diffuse interstellar radiation field, PAH emission may provide an approximately linear, high resolution, high sensitivity tracer of diffuse gas surface density. We present a pilot study that explores using PAH emission in this way based on MIRI observations of IC 5332, NGC 628, NGC 1365, and NGC 7496 from the PHANGS-JWST Treasury. Using scaling relationships calibrated in Leroy et al. (2022), scaled F1130W provides 10--40 pc resolution and 3$\sigma$ sensitivity of $\Sigma_{\rm gas} \sim 2$ M$_\odot$ pc$^{-2}$. We characterize the surface densities of structures seen at $< 7$ M$_\odot$ pc$^{-2}$ in our targets, where we expect the gas to be HI-dominated. We highlight the existence of filaments, inter-arm emission, and holes in the diffuse ISM at these low surface densities. Below $\sim 10$ M$_\odot$ pc$^{-2}$ for NGC 628, NGC 1365, and NGC 7496 the gas distribution shows a ``Swiss cheese''-like topology due to holes and bubbles pervading the relatively smooth distribution of diffuse ISM. Comparing to recent galaxy simulations, we observe similar topology for the low surface density gas, though with notable variations between simulations with different setups and resolution. Such a comparison of high resolution, low surface density gas with simulations is not possible with existing atomic and molecular gas maps, highlighting the unique power of JWST maps of PAH emission.
Peer Review Status:Awaiting Review
Subjects: Astronomy >> Astrophysical processes submitted time 2023-02-19
Abstract: PHANGS-JWST mid-infrared (MIR) imaging of nearby spiral galaxies has revealed ubiquitous filaments of dust emission in intricate detail. We present a pilot study to systematically map the dust filament network (DFN) at multiple scales between 25-400 pc in NGC 628. MIRI images at 7.7, 10, 11.3 and 21$\mu$m of NGC 628 are used to generate maps of the filaments in emission, while PHANGS-HST B-band imaging yields maps of dust attenuation features. We quantify the correspondence between filaments traced by MIR thermal continuum / polycyclic aromatic hydrocarbon (PAH) emission and filaments detected via extinction / scattering of visible light; the fraction of MIR flux contained in the DFN; and the fraction of HII regions, young star clusters and associations within the DFN. We examine the dependence of these quantities with the physical scale at which the DFN is extracted. With our highest resolution DFN maps (25 pc filament width), we find that filaments in emission and attenuation are co-spatial in 40% of sight lines, often exhibiting detailed morphological agreement; that ~30% of the MIR flux is associated with the DFN; and that 75-80% of HII regions and 60% of star clusters younger than 5 Myr are contained within the DFN. However, the DFN at this scale is anti-correlated with looser associations of stars younger than 5 Myr identified using PHANGS-HST near-UV imaging. We discuss the impact of these findings for studies of star formation and the ISM, and the broad range of new investigations enabled with multi-scale maps of the DFN.
Peer Review Status:Awaiting Review