arXiv subjects

Rupali Chandar

Publications and source records attributed to Rupali Chandar.

At least 19 recordsLinked to original sources

arXiv2026

Nowhere Left to Hide: Uncovering All of the Massive Young Embedded Star Clusters in the Antennae with JWST

The Antennae galaxies merger produces the brightest infrared emission of any galaxy within ~20 Mpc, mostly from intense star formation taking place in supergiant molecular cloud complexes in the overlap region. Here, we present new, high-resolution NIRCam and MIRI images of the Antennae galaxies taken with the F150W, F187N, F335M, F360M, F410M, and F770W filters on JWST to search for the predicted but as-yet-undiscovered population of deeply embedded, optically obscured star clusters. We identify a population of 45 sources, 40 previously unknown, with high Bralpha/Halpha and Paalpha/Halpha flux ratios which are likely very young clusters still embedded or just emerging from their natal cocoons, and estimate their age, extinction (A_V), and mass. We find that all are extremely young (< 2.5 Myr), have A_V between 2 and 10 mag, and masses between ~ 10^4 and several x 10^6~Msun. We believe we have now uncovered all clusters with M > 3 x 10^4 Msun and A_V > 2 mag in the Antennae. While our sample represents a small fraction(~15%) of clusters younger than 3~Myr by number, it dominates the ionizing photon luminosity across the galaxy pair (~60%). We find elevated H_2/PAH ratios of the ISM surrounding the most massive pair of embedded clusters, supporting the idea that merger-induced shock-heated gas play an important role in the formation of extremely massive clusters.

astro-ph.GA↗

arXiv2026

Discovery of Ancient Globular Cluster Candidates in the Relic, a Quiescent Galaxy at $z=2.5$

Globular clusters (GCs) are some of the oldest bound structures in the Universe, holding clues to the earliest epochs of star formation and galaxy assembly. However, accurate age measurements of ancient clusters are challenging due to the age-metallicity degeneracy. Here, we report the discovery of 36 compact stellar systems within the 'Relic', a massive, quiescent galaxy at $z=2.53$. The Relic resides in an overdensity behind the Abell~2744 cluster, with a prominent tidal tail extending towards two low-mass companions. Using deep data from the UNCOVER/MegaScience JWST Surveys, we find that clusters formed in age intervals ranging from 8~Myr up to $\sim2$~Gyr, suggesting a rich formation history starting at $z\sim10$. While the cluster-based star formation history is broadly consistent with the high past star formation rates derived from the diffuse host galaxy light, one potential discrepancy is a tentative $\sim2-3\times$ higher rate in the cluster population for the past Gyr. Taken together with the spatial distribution and low inferred metallicities of these young-to-intermediate age clusters, we may be seeing direct evidence for the accretion of star clusters in addition to their early in situ formation. The cluster masses are high, $\sim10^6-10^7~M_{\odot}$, which may explain why we are able to detect them around this likely post-merger galaxy. Overall, the Relic clusters are consistent with being precursors of the most-massive present-day GCs. This unique laboratory enables the first connection between long-lived, high-redshift clusters and local stellar populations, offering insights into the early stages of GC evolution and the broader processes of galaxy assembly.

astro-ph.GA↗

arXiv2026

Localized Deviations from the CO-PAH Relation in PHANGS-JWST Galaxies: Faint PAH Emission or Elevated CO Emissivity?

Polycyclic aromatic hydrocarbon (PAH) emission is widely used to trace the distribution of molecular gas in the interstellar medium, exhibiting a tight correlation with CO(2-1) emission across nearby galaxies. Using PHANGS-JWST and PHANGS-ALMA data, we identify localized regions where this correlation fails, with CO flux exceeding that predicted from 7.7$μ$m PAH emission by more than an order of magnitude. These outlier regions are found in 20 out of 70 galaxies and are located in galaxy centers and bars, without signs of massive star formation. We explore two scenarios to explain the elevated CO-to-PAH ratios, which can either be due to suppressed PAH emission or enhanced CO emissivity. We examine PAH emission in other bands (3.3$μ$m and 11.3$μ$m) and the dust continuum dominated bands (10$μ$m and 21$μ$m), finding consistently high CO-to-PAH (or CO-to-dust continuum) emission ratios, suggesting that 7.7$μ$m PAH emission is not particularly suppressed. In some outlier regions, PAH sizes and spectral energy distribution of the radiation differ slightly from nearby control regions with normal CO-to-PAH ratios, though without a consistent trend. We find that the outlier regions show higher CO velocity dispersions ($Δv_{\mathrm{CO}}$). This increase in $Δv_{\mathrm{CO}}$ lowers CO optical depth and raises its emissivity for a given gas mass. Our results favor a scenario where shear along the bar lanes and shocks at the bar ends elevate CO emissivity, leading to the breakdown of the CO-PAH correlation. Future JWST spectroscopy and deep ALMA observations of CO isotopologues will provide critical tests of this scenario.

astro-ph.GA↗

arXiv2025

PAH Marks the Spot: Digging for Buried Clusters in Nearby Star-forming Galaxies

The joint capabilities of the Hubble Space Telescope (HST) and JWST allow for an unparalleled look at the early lives of star clusters at near- and mid-infrared wavelengths. We present here a multiband analysis of embedded young stellar clusters in 11 nearby, star-forming galaxies, using the PHANGS-JWST and PHANGS-HST datasets. We use the Zooniverse citizen science platform to conduct an initial by-eye search for embedded clusters in near-UV/optical/near-infrared images that trace stellar continuum emission, the Paschen$α$ and H$α$ recombination lines, and the 3.3 $μ$m polycyclic aromatic hydrocarbon feature and its underlying continuum. With this approach, we identify 292 embedded cluster candidates for which we characterize their ages, masses, and levels of line-of-sight extinction by comparing the photometric data to predictions from stellar population models. The embedded cluster candidates have a median age of 4.5 Myr and an average line-of-sight extinction $\left< A_V \right> = 6.0$ mag. We determine lower limits on source stellar masses, resulting in a median stellar mass of $10^3$ $M_{\odot}$. We use this sample of embedded cluster candidates to train multiple convolutional neural network models to carry out deep transfer learning-based searches for embedded clusters. With the aim of optimizing models for future catalog production, we compare results for four variations of training data using two neural networks. Confusion matrices for all eight model configurations, as well as inter-model identification trends, are presented. With refinement of the training sample, we determine that optimized models could serve as a pathway for future embedded cluster identification beyond our 11 galaxy sample.

astro-ph.GA↗

arXiv2025

Empirical SED Templates for Star Clusters Observed with HST and JWST: No Strong PAH or IR Dust Emission after Five Myr

JWST observations, when combined with HST data, promise to improve age estimates of star clusters in nearby spiral galaxies. However, feedback from young cluster stars pushes out the natal gas and dust, making cluster formation and evolution a challenge to model. Here, we use JWST + HST observations of the nearby spiral galaxy NGC 628 to produce spectral energy distribution (SED) templates of compact star clusters spanning 275 nm through 21 μm. These preliminary SEDs capture the cluster stars and associated gas and dust within radii of 0.12" to 0.67" (corresponding to 6 to 33 pc at the distance of NGC 628). One important finding is that the SEDs of 1, 2, 3, and 4 Myr clusters can be differentiated in the infrared. Another is that in 80-90% of the cases we study, the PAH and H_alpha emission track one another, with the dust responsible for the 3.3 μm PAH emission largely removed by 4 Myr, consistent with pre-supernova stellar feedback acting quickly on the surrounding gas and dust. Nearly-embedded cluster candidates have infrared SEDs which are quite similar to optically visible 1 to 3 Myr clusters. In nearly all cases we find there is a young star cluster within a few tenths of an arcsec (10 - 30 pc) of the nearly embedded cluster, suggesting the formation of the cluster was triggered by its presence. The resulting age estimates from the empirical templates are compatible both with dynamical estimates based on CO superbubble expansion velocities, and the TODDLERS models which track spherical evolution of homogeneous gas clouds around young stellar clusters.

astro-ph.GA↗

arXiv2025

Linking stellar populations to HII regions across nearby galaxies. II. Infrared Reprocessed and UV Direct Radiation Pressure in HII Regions

Radiation pressure is a key mechanism by which stellar feedback disrupts molecular clouds and drives HII region expansion. This includes direct radiation pressure exerted by UV photons on dust grains, pressure associated with photoionization, and infrared (IR) radiation pressure on grains due to dust-reprocessed IR photons. We present a new method that combines high resolution mid-IR luminosities from JWST-MIRI, optical attenuation and nebular line measurements from VLT-MUSE, and HST H$α$-based region sizes to estimate the strength of radiation pressure in $\approx 18,000$ HII regions across 19 nearby star-forming galaxies. This is the most extensive and direct estimate of these terms beyond the Local Group to date. In the disks of galaxies, we find that the total reprocessed IR pressure is on average 5% of the direct UV radiation pressure. This fraction rises to 10% in galaxy centers. We expect reprocessed IR radiation pressure to dominate over UV radiation pressure in regions where $L_{\rm F2100W}/L_{\rm Hα}^{\rm corr} \gtrsim 75$. Radiation pressure due to H ionizations is lower than pressure on dust in our sample, but appears likely to dominate the radiation pressure budget in dwarf galaxies similar to the Small Magellanic Cloud. The contribution from all radiation pressure terms appears to be subdominant compared to thermal pressure from ionized gas, reinforcing the view that radiation pressure is most important in compact, heavily embedded, and young regions.

astro-ph.GA↗

arXiv2025

The PHANGS-HST-Halpha Survey: Warm Ionized Gas Physics at High Angular resolution in Nearby GalaxieS with the Hubble Space Telescope

The PHANGS project is assembling a comprehensive, multi-wavelength dataset of nearby (~5-20 Mpc), massive star-forming galaxies to enable multi-phase, multi-scale investigations into the processes that drive star formation and galaxy evolution. To date, large survey programs have provided molecular gas (CO) cubes with ALMA, optical IFU spectroscopy with VLT/MUSE, high-resolution NUV--optical imaging in five broad-band filters with HST, and infrared imaging in NIRCAM+MIRI filters with JWST. Here, we present PHANGS-HST-Halpha, which has obtained high-resolution (~2-10 pc), narrow-band imaging in the F658N or F657N filters with the HST/WFC3 camera of the warm ionized gas in the first 19 nearby galaxies observed in common by all four of the PHANGS large programs. We summarize our data reduction process, with a detailed discussion of the production of flux-calibrated, Milky Way extinction corrected, continuum-subtracted Halpha maps. PHANGS-MUSE IFU spectroscopy data are used to background subtract the HST-Halpha maps, and to determine the [NII] correction factors for each galaxy. We describe our public data products and highlight a few key science cases enabled by the PHANGS-HST-Halpha observations.

astro-ph.GA↗

arXiv2025

PAH Feature Ratios Around Stellar Clusters and Associations in 19 Nearby Galaxies

We present a comparison of observed polycyclic aromatic hydrocarbon (PAH) feature ratios in 19 nearby galaxies with a grid of theoretical expectations for near- and mid-infrared dust emission. The PAH feature ratios are drawn from Cycle 1 JWST observations and are measured for 7224 stellar clusters and 29176 stellar associations for which we have robust ages and mass estimates from HST five-band photometry. Though there are galaxy-to-galaxy variations, the observed PAH feature ratios largely agree with the theoretical models, particularly those that are skewed toward more ionized and larger PAH size distributions. For each galaxy we also extract PAH feature ratios for 200 pc-wide circular regions in the diffuse interstellar medium, which serve as a non-cluster/association control sample. Compared to what we find for stellar clusters and associations, the 3.3um/7.7um and 3.3um/11.3um ratios from the diffuse interstellar medium are $\sim 0.10-0.15$ dex smaller. When the observed PAH feature ratios are compared to the radiation field hardness as probed by the [OIII]/H$β$ ratio, we find anti-correlations for nearly all galaxies in the sample. These results together suggest that the PAH feature ratios are driven by the shape intensity of the radiation field, and that the smallest PAHs -- observed via JWST F335M imaging -- are increasingly 'processed' or destroyed in regions with the most intense and hard radiation fields.

astro-ph.GA↗

arXiv2024

Tracing the earliest stages of star and cluster formation in 19 nearby galaxies with PHANGS-JWST and HST: compact 3.3 $μ$m PAH emitters and their relation to the optical census of star clusters

The earliest stages of star and cluster formation are hidden within dense cocoons of gas and dust, limiting their detection at optical wavelengths. With the unprecedented infrared capabilities of JWST, we can now observe dust-enshrouded star formation with $\sim$10 pc resolution out to $\sim$20 Mpc. Early findings from PHANGS-JWST suggest that 3.3 $μ$m polycyclic aromatic hydrocarbon (PAH) emission can identify star clusters in their dust-embedded phases. Here, we extend this analysis to 19 galaxies from the PHANGS-JWST Cycle 1 Treasury Survey, providing the first characterization of compact sources exhibiting 3.3$μ$m PAH emission across a diverse sample of nearby star-forming galaxies. We establish selection criteria, a median color threshold of F300M-F335M=0.67 at F335M=20, and identify of 1816 sources. These sources are predominantly located in dust lanes, spiral arms, rings, and galaxy centers, with $\sim$87% showing concentration indices similar to optically detected star clusters. Comparison with the PHANGS-HST catalogs suggests that PAH emission fades within $\sim$3 Myr. The H$α$ equivalent width of PAH emitters is 1-2.8 times higher than that of young PHANGS-HST clusters, providing evidence that PAH emitters are on average younger. Analysis of the bright portions of luminosity functions (which should not suffer from incompleteness) shows that young dusty clusters may increase the number of optically visible $\leq$ 3 Myr-old clusters in PHANGS-HST by a factor between $\sim$1.8x-8.5x.

astro-ph.GA↗

arXiv2024

Clusters, Clumps, Dust, & Gas (CCDG) in NGC1614: Bench-marking Cluster Demographics in Extreme Systems

Observations of young star clusters in a variety of galaxies have been used to constrain basic properties related to star-formation, such as the fraction of stars found in clusters (Gam) and the shape of the cluster mass function. However, the results can depend heavily on the reliability of the cluster age-dating process and other assumptions. One of the biggest challenges for successful age-dating lies in breaking the age-reddening degeneracy, where older, dust-free clusters and young, reddened clusters can have similar broad-band colors. While this degeneracy affects cluster populations in all galaxies, it is particularly challenging in dusty, extreme star-forming environments systems. We study the cluster demographics in the luminous infrared galaxy NGC1614 using Hubble imaging taken in 8 optical-NIR passbands. For age-dating, we adopt a spectral energy distribution fitting process that limits the maximum allowed reddening by region, and includes Ha photometry directly. We find that without these assumptions, essentially all clusters in the dust-free UV-bright arm which should have ages 50-250Myr are incorrectly assigned ages younger than 10Myr. We find this method greatly reduces the number of clusters in the youngest (tau<10Myrs) age bin and shows a fairly uniform distribution of massive clusters, the most massive being few10^7M. A maximum likelihood fit shows that the cluster mass function is well fitted by a power-law with an index -1.8, with no statistically significant high-mass cutoff. We calculate the fraction of stars born in clusters to be Gam1-10=22.4+_5.7%. The fraction of stars in clusters decreases quickly over time, with Gam10-100= 4.5+_1.1% and Gam100-400=1.7+_0.4%, suggesting that clusters dissolve rapidly over the first ~0.5Gyr. The decreasing fraction of stars in clusters is consistent with the declining shape observed for the cluster age distribution.

astro-ph.GA↗

arXiv2024

H-alpha emission and HII regions at the locations of recent supernovae in nearby galaxies

We present a statistical analysis of the local, approximately 50-100 pc scale, H-alpha emission at the locations of recent (less than 125 years) supernovae (SNe) in nearby star-forming galaxies. Our sample consists of 32 SNe in 10 galaxies that are targets of the PHANGS-MUSE survey. We find that 41% (13/32) of these SNe occur coincident with a previously identified HII region. For comparison, HII regions cover 32% of the area within 1 kpc of any recent SN. Contrasting this local covering fraction with the fraction of SNe coincident with HII regions, we find a statistical excess of 7.6% +/- 8.7% of all SNe to be associated with HII regions. This increases to an excess of 19.2% +/- 10.4% when considering only core-collapse SNe. These estimates appear to be in good agreement with qualitative results from new, higher resolution HST H-alpha imaging, which also suggest many CCSNe detonate near but not in HII regions. Our results appear consistent with the expectation that only a modest fraction of stars explode during the first 5 Myr of the life of a stellar population, when H-alpha emission is expected to be bright. Of the HII region associated SNe, 8% (11/13) also have associated detected CO(2-1) emission, indicating the presence of molecular gas. The HII region associated SNe have typical Av extinctions approximately equal to 1 mag, consistent with a significant amount of pre-clearing of gas from the region before the SNe explode.

astro-ph.GA↗

arXiv2024

PHANGS-HST: Globular Cluster Systems in 17 Nearby Spiral Galaxies

We present new catalogs of likely globular clusters (GCs) in 17 nearby spiral galaxies studied as part of the PHANGS-HST Treasury Survey. The galaxies were imaged in five broad-band filters from the near-ultraviolet through the $I$ band. PHANGS-HST has produced catalogs of stellar clusters of all ages by selecting extended sources (from multiple concentration index measurements) followed by morphological classification (centrally concentrated and symmetric or asymmetric, multiple peaks, contaminant) by visually examining the V-band image and separately by a machine-learning algorithm which classified larger samples to reach fainter limits. From both cluster catalogs, we select an initial list of candidate GCs to have $B-V \geq 0.5$ and $V-I \geq 0.73$~mag, then remove likely contaminants (including reddened young clusters, background galaxies misclassified by the neural network, and chance superpositions/blends of stars) after a careful visual inspection. We find that $\approx86$ % of the color-selected candidates classified as spherically symmetric, and $\approx68$ of those classified as centrally concentrated but asymmetric are likely to be GCs. The luminosity functions of the GC candidates in 2 of our 17 galaxies, NGC 628 and NGC 3627, are atypical, and continue to rise at least 1~mag fainter than the expected turnover near $M_V \sim -7.4$. These faint candidate GCs have more extended spatial distributions than their bright counterparts, and may reside in the disk rather than the bulge/halo, similar to faint GCs previously discovered in M101. These faint clusters may be somewhat younger since the age-metallicity degeneracy makes it difficult to determine precise cluster ages from integrated colors once they reach $\approx1$~Gyr.

astro-ph.GA↗

arXiv2024

PHANGS-HST catalogs for $\sim$100,000 star clusters and compact associations in 38 galaxies: I. Observed properties

We present the largest catalog to-date of star clusters and compact associations in nearby galaxies. We have performed a V-band-selected census of clusters across the 38 spiral galaxies of the PHANGS-HST Treasury Survey, and measured integrated, aperture-corrected NUV-U-B-V-I photometry. This work has resulted in uniform catalogs that contain $\sim$20,000 clusters and compact associations which have passed human inspection and morphological classification, and a larger sample of $\sim$100,000 classified by neural network models. Here, we report on the observed properties of these samples, and demonstrate that tremendous insight can be gained from just the observed properties of clusters, even in the absence of their transformation into physical quantities. In particular, we show the utility of the UBVI color-color diagram, and the three principal features revealed by the PHANGS-HST cluster sample: the young cluster locus, the middle-age plume, and the old globular cluster clump. We present an atlas of maps of the 2D spatial distribution of clusters and compact associations in the context of the molecular clouds from PHANGS-ALMA. We explore new ways of understanding this large dataset in a multi-scale context by bringing together once-separate techniques for the characterization of clusters (color-color diagrams and spatial distributions) and their parent galaxies (galaxy morphology and location relative to the galaxy main sequence). A companion paper presents the physical properties: ages, masses, and dust reddenings derived using improved spectral energy distribution (SED) fitting techniques.

astro-ph.GA↗

arXiv2023

Universal Upper End of the Stellar Initial Mass Function in the Young and Compact LEGUS clusters

We investigate the variation in the upper end of stellar initial mass function (uIMF) in 375 young and compact star clusters in five nearby galaxies within $\sim 5$ Mpc. All the young stellar clusters (YSCs) in the sample have ages $\lesssim 4$ Myr and masses above 500 $M_{\odot}$, according to standard stellar models. The YSC catalogs were produced from Hubble Space Telescope images obtained as part of the Legacy ExtraGalactic UV Survey (LEGUS) Hubble treasury program. They are used here to test whether the uIMF is universal or changes as a function of the cluster's stellar mass. We perform this test by measuring the H$α$ luminosity of the star clusters as a proxy for their ionizing photon rate, and charting its trend as a function of cluster mass. Large cluster numbers allow us to mitigate the stochastic sampling of the uIMF. The advantage of our approach relative to previous similar attempts is the use of cluster catalogs that have been selected independently of the presence of H$α$ emission, thus removing a potential sample bias. We find that the uIMF, as traced by the H$α$ emission, shows no dependence on cluster mass, suggesting that the maximum stellar mass that can be produced in star clusters is universal, in agreement with previous findings.

astro-ph.GA↗

arXiv2023

The X-ray Binary-Star Cluster Connection in Late-Type Galaxies

We conduct one of the largest systematic investigations of bright X-ray binaries (XRBs) in both young star clusters and ancient globular clusters (GCs) using a sample of six nearby, star-forming galaxies. Combining complete CXO X-ray source catalogs with optical PHANGS-HST cluster catalogs, we identify a population of 33 XRBs within or near their parent clusters. We find that GCs that host XRBs in spiral galaxies appear to be brighter, more compact, denser, and more massive than the general GC population. However, these XRB hosts do not appear preferentially redder or more metal-rich, pointing to a possible absence of the metallicity-boosted formation of low-mass X-ray binaries (LMXBs) that is observed in the GCs of older galaxies. We also find that a smaller fraction of LMXBs is found in spiral GC systems when compared with those in early-type galaxies: between 8 and 50%, or an average of 20% across galaxies in our sample. Although there is a non-negligible probability of a chance superposition between an XRB and an unrelated young cluster, we find that among clusters younger than 10 Myr, which most likely host high-mass XRBs, the fraction of clusters associated with an XRB increases at higher cluster masses and densities. The X-ray luminosity of XRBs appears to increase with the mass of the cluster host for clusters younger than ~400 Myr, while the inverse relation is found for XRBs in GCs.

astro-ph.HE↗

arXiv2023

A Tale of 3 Dwarfs: No Extreme Cluster Formation in Extreme Star-Forming Galaxies

Nearly all current simulations predict that outcomes of the star formation process, such as the fraction of stars that form in bound clusters (Gamma), depend on the intensity of star formation activity (SigmaSFR) in the host galaxy. The exact shape and strength of the predicted correlations, however, vary from simulation to simulation. Observational results also remain unclear at this time, because most works have mixed estimates made from very young clusters for galaxies with higher SigmaSFR with those from older clusters for galaxies with lower SigmaSFR. The three blue compact dwarf (BCD) galaxies ESO185-IG13, ESO338-IG04, and Haro11 have played a central role on the observational side because they have some of the highest known SigmaSFR and published values of Gamma. We present new estimates of Gamma for these BCDs in three age intervals (1-10 Myr, 10-100 Myr, 100-400 Myr), based on age-dating which includes Halpha photometry to better discriminate between clusters younger and older than ~10 Myr. We find significantly lower values for Gamma (1-10 Myr) than published previously. The likely reason for the discrepancy is that previous estimates appear to be based on age-reddening results that underestimated ages and overestimated reddening for many clusters, artificially boosting Gamma (1-10 Myr). We also find that fewer stars remain in clusters over time, with ~15-39% in 1-10 Myr, ~5-7% in 10-100 Myr, and ~1-2% in 100-400 Myr clusters. We find no evidence that Gamma increases with SigmaSFR. These results imply that cluster formation efficiency does not vary with star formation intensity in the host galaxy. If confirmed, our results will help guide future assumptions in galaxy-scale simulations of cluster formation and evolution.

astro-ph.GA↗

arXiv2023

Improving Star Cluster Age Estimates in PHANGS-HST Galaxies and the Impact on Cluster Demographics in NGC 628

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.

astro-ph.GA↗

arXiv2023

PHANGS-JWST First Results: The Dust Filament Network of NGC 628 and its Relation to Star Formation Activity

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$μ$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.

astro-ph.GA↗