We demonstrate a new method for measuring the escape fraction of ionizing photons using Hubble Space Telescope imaging of resolved stars in NGC 4214, a local analog of high-redshift starburst ...galaxies that are thought to be responsible for cosmic reionization. Specifically, we forward model the UV through near-IR spectral energy distributions of ∼83,000 resolved stars to infer their individual ionizing flux outputs. We constrain the local escape fraction by comparing the number of ionizing photons produced by stars to the number that are either absorbed by dust or consumed by ionizing the surrounding neutral hydrogen in individual star-forming regions. We find substantial spatial variation in the escape fraction (0%-40%). Integrating over the entire galaxy yields a global escape fraction of 25 − 15 + 16 %. This value is much higher than previous escape fractions of zero reported for this galaxy. We discuss the sources of this apparent tension and demonstrate that the viewing angle and 3D interstellar medium geometric effects are the cause. If we assume that NGC 4214 has no internal dust, like many high-redshift galaxies, we find an escape fraction of 59% (an upper limit for NGC 4214). This is the first nonzero escape fraction measurement for UV-faint (M FUV = −15.9) galaxies at any redshift and supports the idea that starburst UV-faint dwarf galaxies can provide a sufficient number of ionizing photons to the intergalactic medium.
Abstract H ii region electron temperatures are a critical ingredient in metallicity determinations, and recent observations have revealed systematic variations in the temperatures measured using ...different ions. We present electron temperatures ( T e ) measured using the optical auroral lines (N ii λ 5756, O ii λ λ 7320, 7330, S ii λ λ 4069, 4076, O iii λ 4363, and S iii λ 6312) for a sample of H ii regions in seven nearby galaxies. We use observations from the Physics at High Angular resolution in Nearby Galaxies survey (PHANGS) obtained with integral field spectrographs on Keck (Keck Cosmic Web Imager) and the Very Large Telescope (Multi-Unit Spectroscopic Explorer). We compare the different T e measurements with H ii region and ISM environmental properties such as electron density, ionization parameter, molecular gas velocity dispersion, and stellar association/cluster mass and age obtained from PHANGS. We find that the temperatures from O ii and S ii are likely overestimated due to the presence of electron density inhomogeneities in H ii regions. We measure high O iii temperatures in a subset of regions with high molecular gas velocity dispersion and low ionization parameter, which may be explained by the presence of low-velocity shocks. In agreement with previous studies, the T e – T e between N ii and S iii temperatures have the lowest observed scatter and follow predictions from photoionization modeling, which suggests that these tracers reflect H ii region temperatures across the various ionization zones better than O ii , S ii , and O iii .
ABSTRACT
Observations indicate dust populations vary between galaxies and within them, suggesting a complex life cycle and evolutionary history. Here we investigate the evolution of galactic dust ...populations across cosmic time using a suite of cosmological zoom-in simulations from the Feedback in Realistic Environments project, spanning $M_{\rm vir}=10^{9-12}{M}_{\odot };\, M_{*}=10^{6-11}\, {M}_{\odot }$. Our simulations incorporate a dust evolution model that accounts for the dominant sources of dust production, growth, and destruction and follows the evolution of specific dust species. All galactic dust populations in our suite exhibit similar evolutionary histories, with gas–dust accretion being the dominant producer of dust mass for all but the most metal-poor galaxies. Similar to previous works, we find the onset of efficient gas–dust accretion occurs above a ‘critical’ metallicity threshold (Zcrit). Due to this threshold, our simulations reproduce observed trends between galactic D/Z and metallicity and element depletion trends in the interstellar medium. However, we find Zcrit varies between dust species due to differences in key element abundances, dust physical properties, and life cycle processes resulting in $Z_{\rm crit}\sim 0.05{\rm Z}_{\odot },\, 0.2{\rm Z}_{\odot },\, 0.5{\rm Z}_{\odot }$ for metallic iron, silicates, and carbonaceous dust, respectively. These variations could explain the lack of small carbonaceous grains observed in the Magellanic Clouds. We also find a delay between the onset of gas–dust accretion and when a dust population reaches equilibrium, which we call the equilibrium time-scale (τequil). The relation between τequil and the metal enrichment time-scale of a galaxy, determined by its recent evolutionary history, can contribute to the scatter in the observed relation between galactic D/Z and metallicity.
Abstract
The PHANGS collaboration has been building a reference data set for the multiscale, multiphase study of star formation and the interstellar medium (ISM) in nearby galaxies. With the ...successful launch and commissioning of JWST, we can now obtain high-resolution infrared imaging to probe the youngest stellar populations and dust emission on the scales of star clusters and molecular clouds (∼5–50 pc). In Cycle 1, PHANGS is conducting an eight-band imaging survey from 2 to 21
μ
m of 19 nearby spiral galaxies. Optical integral field spectroscopy, CO(2–1) mapping, and UV-optical imaging for all 19 galaxies have been obtained through large programs with ALMA, VLT-MUSE, and Hubble. PHANGS–JWST enables a full inventory of star formation, accurate measurement of the mass and age of star clusters, identification of the youngest embedded stellar populations, and characterization of the physical state of small dust grains. When combined with Hubble catalogs of ∼10,000 star clusters, MUSE spectroscopic mapping of ∼20,000 H
ii
regions, and ∼12,000 ALMA-identified molecular clouds, it becomes possible to measure the timescales and efficiencies of the earliest phases of star formation and feedback, build an empirical model of the dependence of small dust grain properties on local ISM conditions, and test our understanding of how dust-reprocessed starlight traces star formation activity, all across a diversity of galactic environments. Here we describe the PHANGS–JWST Treasury survey, present the remarkable imaging obtained in the first few months of science operations, and provide context for the initial results presented in the first series of PHANGS–JWST publications.
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.
We present the results of modeling dust spectral energy distributions (SEDs) across the Small Magellanic Cloud (SMC) with the aim of mapping the distribution of polycyclic aromatic hydrocarbons ...(PAHs) in a low-metallicity environment. Using Spitzer Survey of the SMC photometry from 3.6 to 160 {mu}m over the main star-forming regions of the Wing and Bar of the SMC along with spectral mapping observations from 5 to 38 {mu}m from the Spitzer Spectroscopic Survey of the SMC in selected regions, we model the dust SED and emission spectrum to determine the fraction of dust in PAHs across the SMC. We use the regions of overlapping photometry and spectroscopy to test the reliability of the PAH fraction as determined from SED fits alone. The PAH fraction in the SMC is low compared to the Milky Way and variable-with relatively high fractions (q {sub PAH} {approx}1%-2%) in molecular clouds and low fractions in the diffuse interstellar medium (ISM; average (q {sub PAH}) = 0.6%). We use the map of PAH fraction across the SMC to test a number of ideas regarding the production, destruction, and processing of PAHs in the ISM. We find weak or no correlation between the PAH fraction and the distribution of carbon asymptotic giant branch stars, the location of supergiant H I shells and young supernova remnants, and the turbulent Mach number. We find that the PAH fraction is correlated with CO intensity, peaks in the dust surface density and the molecular gas surface density as determined from 160 {mu}m emission. The PAH fraction is high in regions of active star formation, as predicted by its correlation with molecular gas, but is suppressed in H II regions. Because the PAH fraction in the diffuse ISM is generally very low-in accordance with previous work on modeling the integrated SED of the SMC-and the PAH fraction is relatively high in molecular regions, we suggest that PAHs are destroyed in the diffuse ISM of the SMC and/or PAHs are forming in molecular clouds. We discuss the implications of these observations for our understanding of the PAH life cycle, particularly in low-metallicity and/or primordial galaxies.
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.
ABSTRACT
Previous work has argued that atomic gas mass estimates of galaxies from 21-cm H i emission are systematically low due to a cold opaque atomic gas component. If true, this opaque component ...necessitates a $\sim 35{{\ \rm per\ cent}}$ correction factor relative to the mass from assuming optically thin H i emission. These mass corrections are based on fitting H i spectra with a single opaque component model that produces a distinct ‘top-hat’ shaped line profile. Here, we investigate this issue using deep, high spectral resolution H i VLA observations of M31 and M33 to test if these top-hat profiles are instead superpositions of multiple H i components along the line of sight. We fit both models and find that ${\gt}80{{\ \rm per\ cent}}$ of the spectra strongly prefer a multicomponent Gaussian model while ${\lt}2{{\ \rm per\ cent}}$ prefer the single opacity-corrected component model. This strong preference for multiple components argues against previous findings of lines of sight dominated by only cold H i. Our findings are enabled by the improved spectral resolution (0.42 ${\rm km\, s^{-1}}$), whereas coarser spectral resolution blends multiple components together. We also show that the inferred opaque atomic ISM mass strongly depends on the goodness-of-fit definition and is highly uncertain when the inferred spin temperature has a large uncertainty. Finally, we find that the relation of the H i surface density with the dust surface density and extinction has significantly more scatter when the inferred H i opacity correction is applied. These variations are difficult to explain without additionally requiring large variations in the dust properties. Based on these findings, we suggest that the opaque H i mass is best constrained by H i absorption studies.
ABSTRACT
Young stellar objects (YSOs) are the gold standard for tracing star formation in galaxies but have been unobservable beyond the Milky Way and Magellanic Clouds. But that all changed when the ...JWST was launched, which we use to identify YSOs in the Local Group galaxy M33, marking the first time that individual YSOs have been identified at these large distances. We present Mid-Infrared Instrument (MIRI) imaging mosaics at 5.6 and 21 $\mu$m that cover a significant portion of one of M33’s spiral arms that has existing panchromatic imaging from the Hubble Space Telescope and deep Atacama Large Millimeter/submillimeter Array CO measurements. Using these MIRI and Hubble Space Telescope images, we identify point sources using the new dolphot MIRI module. We identify 793 candidate YSOs from cuts based on colour, proximity to giant molecular clouds (GMCs), and visual inspection. Similar to Milky Way GMCs, we find that higher mass GMCs contain more YSOs and YSO emission, which further show YSOs identify star formation better than most tracers that cannot capture this relationship at cloud scales. We find evidence of enhanced star formation efficiency in the southern spiral arm by comparing the YSOs to the molecular gas mass.
Abstract
Polycyclic aromatic hydrocarbons (PAHs) play a critical role in the reprocessing of stellar radiation and balancing the heating and cooling processes in the interstellar medium but appear to ...be destroyed in H
ii
regions. However, the mechanisms driving their destruction are still not completely understood. Using PHANGS–JWST and PHANGS–MUSE observations, we investigate how the PAH fraction changes in about 1500 H
ii
regions across four nearby star-forming galaxies (NGC 628, NGC 1365, NGC 7496, and IC 5332). We find a strong anticorrelation between the PAH fraction and the ionization parameter (the ratio between the ionizing photon flux and the hydrogen density) of H
ii
regions. This relation becomes steeper for more luminous H
ii
regions. The metallicity of H
ii
regions has only a minor impact on these results in our galaxy sample. We find that the PAH fraction decreases with the H
α
equivalent width—a proxy for the age of the H
ii
regions—although this trend is much weaker than the one identified using the ionization parameter. Our results are consistent with a scenario where hydrogen-ionizing UV radiation is the dominant source of PAH destruction in star-forming regions.
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