arXiv subjects

Rupali Chandar

Publications and source records attributed to Rupali Chandar.

At least 55 records · Page 3Linked to original sources

arXiv2021

The Star Formation History of a Post-Starburst Galaxy Determined From Its Cluster Population

The recent star formation histories (SFHs) of post-starburst galaxies have been determined almost exclusively from detailed modeling of their composite star light. This has provided important but limited information on the number, strength, and duration of bursts of star formation. In this work, we present a direct and independent measure of the recent SFH of S12 (plate-mjd-fiber for SDSS 623-52051-207; designated EAS12 in Smercina et al. 2018) from its star cluster population. We detect clusters from high resolution, $UBR$ optical observations from HST, and compare their luminosities and colors with stellar population models to estimate the ages and masses of the clusters. No clusters younger than $\sim$70 Myr are found, indicating star formation shut off at this time. Clusters formed $\sim$120 Myr ago reach masses up to $\sim \mbox{few}\times10^7~M_{\odot}$, several times higher than similar age counterparts formed in actively merging galaxies like the Antennae and NGC 3256. We develop a new calibration based on known properties for 8 nearby galaxies to estimate the star formation rate (SFR) of a galaxy from the mass of the most massive cluster, $M_{\rm max}$. The cluster population indicates that S12 experienced an extremely intense but short-lived burst $\sim$120 Myr ago, with an estimated peak of $500^{+500}_{-250}~M_{\odot}~\mbox{yr}^{-1}$ and duration of $50\pm25$ Myr, one of the highest SFRs estimated for any galaxy in the nearby universe. Prior to the recent, intense burst, S12 was forming stars at a moderate rate of $\sim 3{-}5~M_{\odot}~\mbox{yr}^{-1}$, typical of spiral galaxies. However, the system also experienced an earlier burst approximately $1{-}3$ Gyr ago. While fairly uncertain, we estimate that the SFR during this earlier burst was $\sim20{-}30~M_{\odot}~\mbox{yr}^{-1}$, similar to the current SFR in the Antennae and NGC 3256.

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arXiv2021

Predicting the spectrum of UGC 2885, Rubin's Galaxy with machine learning

Wu & Peek (2020) predict SDSS-quality spectra based on Pan-STARRS broad-band \textit{grizy} images using machine learning (ML). In this letter, we test their prediction for a unique object, UGC 2885 ("Rubin's galaxy"), the largest and most massive, isolated disk galaxy in the local Universe ($D<100$ Mpc). After obtaining the ML predicted spectrum, we compare it to all existing spectroscopic information that is comparable to an SDSS spectrum of the central region: two archival spectra, one extracted from the VIRUS-P observations of this galaxy, and a new, targeted MMT/Binospec observation. Agreement is qualitatively good, though the ML prediction prefers line ratios slightly more towards those of an active galactic nucleus (AGN), compared to archival and VIRUS-P observed values. The MMT/Binospec nuclear spectrum unequivocally shows strong emission lines except H$β$, the ratios of which are consistent with AGN activity. The ML approach to galaxy spectra may be a viable way to identify AGN supplementing NIR colors. How such a massive disk galaxy ($M^* = 10^{11}$ M$_\odot$), which uncharacteristically shows no sign of interaction or mergers, manages to fuel its central AGN remains to be investigated.

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arXiv2021

PHANGS-ALMA: Arcsecond CO(2-1) Imaging of Nearby Star-Forming Galaxies

We present PHANGS-ALMA, the first survey to map CO J=2-1 line emission at ~1" ~ 100pc spatial resolution from a representative sample of 90 nearby (d<~20 Mpc) galaxies that lie on or near the z=0 "main sequence" of star-forming galaxies. CO line emission traces the bulk distribution of molecular gas, which is the cold, star-forming phase of the interstellar medium. At the resolution achieved by PHANGS-ALMA, each beam reaches the size of a typical individual giant molecular cloud (GMC), so that these data can be used to measure the demographics, life-cycle, and physical state of molecular clouds across the population of galaxies where the majority of stars form at z=0. This paper describes the scientific motivation and background for the survey, sample selection, global properties of the targets, ALMA observations, and characteristics of the delivered ALMA data and derived data products. As the ALMA sample serves as the parent sample for parallel surveys with VLT/MUSE, HST, AstroSat, VLA, and other facilities, we include a detailed discussion of the sample selection. We detail the estimation of galaxy mass, size, star formation rate, CO luminosity, and other properties, compare estimates using different systems and provide best-estimate integrated measurements for each target. We also report the design and execution of the ALMA observations, which combine a Cycle~5 Large Program, a series of smaller programs, and archival observations. Finally, we present the first 1" resolution atlas of CO emission from nearby galaxies and describe the properties and contents of the first PHANGS-ALMA public data release.

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arXiv2021

PHANGS-ALMA Data Processing and Pipeline

We describe the processing of the PHANGS-ALMA survey and present the PHANGS-ALMA pipeline, a public software package that processes calibrated interferometric and total power data into science-ready data products. PHANGS-ALMA is a large, high-resolution survey of CO J=2-1 emission from nearby galaxies. The observations combine ALMA's main 12-m array, the 7-m array, and total power observations and use mosaics of dozens to hundreds of individual pointings. We describe the processing of the u-v data, imaging and deconvolution, linear mosaicking, combining interferometer and total power data, noise estimation, masking, data product creation, and quality assurance. Our pipeline has a general design and can also be applied to VLA and ALMA observations of other spectral lines and continuum emission. We highlight our recipe for deconvolution of complex spectral line observations, which combines multiscale clean, single scale clean, and automatic mask generation in a way that appears robust and effective. We also emphasize our two-track approach to masking and data product creation. We construct one set of "broadly masked" data products, which have high completeness but significant contamination by noise, and another set of "strictly masked" data products, which have high confidence but exclude faint, low signal-to-noise emission. Our quality assurance tests, supported by simulations, demonstrate that 12-m+7-m deconvolved data recover a total flux that is significantly closer to the total power flux than the 7-m deconvolved data alone. In the appendices, we measure the stability of the ALMA total power calibration in PHANGS--ALMA and test the performance of popular short-spacing correction algorithms.

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arXiv2021

Calibrating X-ray binary luminosity functions via optical reconnaissance I. The case of M83

Building on recent work by Chandar et al. (2020), we construct X-ray luminosity functions (XLFs) for different classes of X-ray binary (XRB) donors in the nearby star-forming galaxy M83 through a novel methodology: rather than classifying low- vs. high-mass XRBs based on the scaling of the number of X-ray sources with stellar mass and star formation rate, respectively, we utilize multi-band Hubble Space Telescope imaging data to classify each Chandra-detected compact X-ray source as a low-mass (i.e. donor mass <~ 3 solar masses), high-mass (donor mass >~ 8 solar masses) or intermediate-mass XRB based on either the location of its candidate counterpart on optical color-magnitude diagrams or the age of its host star cluster. In addition to the the standard (single and/or truncated) power-law functional shape, we approximate the resulting XLFs with a Schechter function. We identify a marginally significant (at the 1-to-2 sigma level) exponential downturn for the high-mass XRB XLF, at logLx ~ 38.48^{+0.52}_{-0.33} (in log CGS units). In contrast, the low- and intermediate-mass XRB XLFs, as well as the total XLF of M83, are formally consistent with sampling statistics from a single power-law. Our method suggests a non-negligible contribution from low- and possibly intermediate-mass XRBs to the total XRB XLF of M83, i.e. between 20 and 50%, in broad agreement with X-ray based XLFs. More generally, we caution against considerable contamination from X-ray emitting supernova remnants to the published, X-ray based XLFs of M83, and possibly all actively star-forming galaxies.

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arXiv2021

Feedback in Forming Star Clusters: The Mass-Radius Relation and Mass Function of Molecular Clumps in the Large Magellanic Cloud

We derive the mass-radius relation and mass function of molecular clumps in the Large Magellanic Cloud (LMC) and interpret them in terms of the simple feedback model proposed by Fall, Krumholz, and Matzner (FKM). Our work utilizes the dendrogram-based catalog of clumps compiled by Wong et al. from $^{12}$CO and $^{13}$CO maps of six giant molecular clouds in the LMC observed with the Atacama Large Millimeter Array (ALMA). The Magellanic Clouds are the only external galaxies for which this type of analysis is possible at the necessary spatial resolution ($\sim1$ pc). We find that the mass-radius relation and mass function of LMC clumps have power-law forms, $R \propto M^α$ and $dN/dM \propto M^β$, with indices $α= 0.36 \pm 0.03$ and $β= -1.8 \pm 0.1 $ over the mass ranges $10^2 M_\odot \lesssim M \lesssim 10^5 M_\odot$ and $10^2 M_\odot \lesssim M \lesssim 10^4 M_\odot$, respectively. With these values of $α$ and $β$ for the clumps (i.e., protoclusters), the predicted index for the mass function of young LMC clusters from the FKM model is $β\approx 1.7$, in good agreement with the observed index. The situation portrayed here for clumps and clusters in the LMC replicates that in the Milky Way.

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arXiv2021

PHANGS-HST: Star Cluster Spectral Energy Distribution Fitting with CIGALE

The sensitivity and angular resolution of photometric surveys executed by the Hubble Space Telescope (HST) enable studies of individual star clusters in galaxies out to a few tens of megaparsecs. The fitting of spectral energy distributions (SEDs) of star clusters is essential for measuring their physical properties and studying their evolution. We report on the use of the publicly available Code Investigating GALaxy Emission (CIGALE) SED fitting package to derive ages, stellar masses, and reddenings for star clusters identified in the Physics at High Angular resolution in Nearby GalaxieS-HST (PHANGS-HST) survey. Using samples of star clusters in the galaxy NGC 3351, we present results of benchmark analyses performed to validate the code and a comparison to SED fitting results from the Legacy ExtraGalactic Ultraviolet Survey (LEGUS). We consider procedures for the PHANGS-HST SED fitting pipeline, e.g., the choice of single stellar population models, the treatment of nebular emission and dust, and the use of fluxes versus magnitudes for the SED fitting. We report on the properties of clusters in NGC 3351 and find, on average, the clusters residing in the inner star-forming ring of NGC 3351 are young ($< 10$ Myr) and massive ($10^{5} M_{\odot}$) while clusters in the stellar bulge are significantly older. Cluster mass function fits yield $β$ values around -2, consistent with prior results with a tendency to be shallower at the youngest ages. Finally, we explore a Bayesian analysis with additional physically-motivated priors for the distribution of ages and masses and analyze the resulting cluster distributions.

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arXiv2020

A Comparison of Star-Forming Clumps and Tidal Tails in Local Mergers and High Redshift Galaxies

The Clusters, Clumps, Dust, and Gas in Extreme Star-Forming Galaxies (CCDG) survey with the Hubble Space Telescope includes multi-wavelength imaging of 13 galaxies less than 100 Mpc away spanning a range of morphologies and sizes, from Blue Compact Dwarfs (BCDs) to luminous infrared galaxies (LIRGs), all with star formation rates in excess of hundreds of solar masses per year. Images of 7 merging galaxies in the CCDG survey were artificially redshifted to compare with galaxies at z=0.5, 1, and 2. Most redshifted tails have surface brightnesses that would be visible at z=0.5 or 1 but not at z=2 due to cosmological dimming. Giant star clumps are apparent in these galaxies; the 51 measured have similar sizes, masses and colors as clumps in observed high-z systems in UDF, GEMS, GOODS, and CANDELS surveys. These results suggest that some clumpy high-z galaxies without observable tidal features could be the result of mergers. The local clumps also have the same star formation rate per unit area and stellar surface density as clumps observed at intermediate and high redshift, so they provide insight into the substructure of distant clumps. A total of 1596 star clusters brighter than MV = -9 were identified within the boundaries of the local clumps. The cluster magnitude distribution function is a power law with approximately the same slope (approximately -1 for a number-log luminosity plot) for all the galaxies both inside and outside the clumps and independent of clump surface brightness.

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arXiv2020

Distances to PHANGS Galaxies: New Tip of the Red Giant Branch Measurements and Adopted Distances

PHANGS-HST is an ultraviolet-optical imaging survey of 38 spiral galaxies within ~20 Mpc. Combined with the PHANGS-ALMA, PHANGS-MUSE surveys and other multiwavelength data, the dataset will provide an unprecedented look into the connections between young stars, HII regions, and cold molecular gas in these nearby star-forming galaxies. Accurate distances are needed to transform measured observables into physical parameters (e.g., brightness to luminosity, angular to physical sizes of molecular clouds, star clusters and associations). PHANGS-HST has obtained parallel ACS imaging of the galaxy halos in the F606W and F814W bands. Where possible, we use these parallel fields to derive tip of the red giant branch (TRGB) distances to these galaxies. In this paper, we present TRGB distances for 11 galaxies from ~4 to ~15 Mpc, based on the first year of PHANGS-HST observations. Five of these represent the first published TRGB distance measurements (IC 5332, NGC 2835, NGC 4298, NGC 4321, and NGC 4328), and eight of which are the best available distances to these targets. We also provide a compilation of distances for the 118 galaxies in the full PHANGS sample, which have been adopted for the first PHANGS-ALMA public data release.

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arXiv2020

Deep Transfer Learning for Star Cluster Classification: I. Application to the PHANGS-HST Survey

We present the results of a proof-of-concept experiment which demonstrates that deep learning can successfully be used for production-scale classification of compact star clusters detected in HST UV-optical imaging of nearby spiral galaxies (D < 20 Mpc) in the PHANGS-HST survey. Given the relatively small nature of existing, human-labelled star cluster samples, we transfer the knowledge of state-of-the-art neural network models for real-object recognition to classify star clusters candidates into four morphological classes. We perform a series of experiments to determine the dependence of classification performance on: neural network architecture (ResNet18 and VGG19-BN); training data sets curated by either a single expert or three astronomers; and the size of the images used for training. We find that the overall classification accuracies are not significantly affected by these choices. The networks are used to classify star cluster candidates in the PHANGS-HST galaxy NGC 1559, which was not included in the training samples. The resulting prediction accuracies are 70%, 40%, 40-50%, 50-70% for class 1, 2, 3 star clusters, and class 4 non-clusters respectively. This performance is competitive with consistency achieved in previously published human and automated quantitative classification of star cluster candidate samples (70-80%, 40-50%, 40-50%, and 60-70%). The methods introduced herein lay the foundations to automate classification for star clusters at scale, and exhibit the need to prepare a standardized dataset of human-labelled star cluster classifications, agreed upon by a full range of experts in the field, to further improve the performance of the networks introduced in this study.

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arXiv2020

LEGUS and Halpha-LEGUS Observations of Star Clusters in NGC 4449: Improved Ages and the Fraction of Light in Clusters as a Function of Age

We present a new catalog and results for the cluster system of the starburst galaxy NGC 4449 based on multi-band imaging observations taken as part of the LEGUS and Halpha-LEGUS surveys. We improve the spectral energy fitting method used to estimate cluster ages and find that the results, particularly for older clusters, are in better agreement with those from spectroscopy. The inclusion of Halpha measurements, the role of stochasticity for low mass clusters, the assumptions about reddening, and the choices of SSP model and metallicity all have important impacts on the age-dating of clusters. A comparison with ages derived from stellar color-magnitude diagrams for partially resolved clusters shows reasonable agreement, but large scatter in some cases. The fraction of light found in clusters relative to the total light (i.e., T_L) in the U, B, and V filters in 25 different ~kpc-size regions throughout NGC 4449 correlates with both the specific Region Luminosity, R_L, and the dominant age of the underlying stellar population in each region. The observed cluster age distribution is found to decline over time as dN/dt ~ t^g, with g=-0.85+/-0.15, independent of cluster mass, and is consistent with strong, early cluster disruption. The mass functions of the clusters can be described by a power law with dN/dM ~ M^b and b=-1.86+/-0.2, independent of cluster age. The mass and age distributions are quite resilient to differences in age-dating methods. There is tentative evidence for a factor of 2-3 enhancement in both the star and cluster formation rate ~100 - 300 Myr ago, indicating that cluster formation tracks star formation generally. The enhancement is probably associated with an earlier interaction event.

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arXiv2020

Mass Functions of Giant Molecular Clouds and Young Star Clusters in Six Nearby Galaxies

We compare the mass functions of young star clusters (ages $\leq 10$ Myr) and giant molecular clouds (GMCs) in six galaxies that cover a large range in mass, metallicity, and star formation rate (LMC, M83, M51, NGC 3627, the Antennae, and NGC 3256). We perform maximum-likelihood fits of the Schechter function, $ψ(M) = dN/dM \propto M^β \exp(-M/M_*)$, to both populations. We find that most of the GMC and cluster mass functions in our sample are consistent with a pure power-law distribution ($M_* \rightarrow \infty$). M51 is the only galaxy that shows some evidence for an upper cutoff ($M_*$) in both populations. Therefore, physical upper mass cutoffs in populations of both GMCs and clusters may be the exception rather than the rule. When we perform power-law fits, we find a range of indices $β_{\rm PL}=-2.3\pm0.3$ for our GMC sample and $β_{\rm PL}=-2.0\pm0.3$ for the cluster sample. This result, that $β_{\rm Clusters} \approx β_{\rm GMC} \approx -2$, is consistent with theoretical predictions for cluster formation and suggests that the star-formation efficiency is largely independent of mass in the GMCs.

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arXiv2020

Candidate LBV stars in galaxy NGC 7793 found via HST photometry + MUSE spectroscopy

Only about 19 Galactic and 25 extra-galactic bona-fide Luminous Blue Variables (LBVs) are known to date. This incomplete census prevents our understanding of this crucial phase of massive star evolution which leads to the formation of heavy binary black holes via the classical channel. With large samples of LBVs one could better determine the duration and maximum stellar luminosity which characterize this phase. We search for candidate LBVs (cLBVs) in a new galaxy, NGC 7793. For this purpose, we combine high spatial resolution images from two Hubble Space Telescope (HST) programs with optical spectroscopy from the Multi Unit Spectroscopic Explorer (MUSE). By combining PSF-fitting photometry measured on F547M, F657N, and F814W images, with restrictions on point-like appearance (at HST resolution) and Hα luminosity, we find 100 potential cLBVs, 36 of which fall in the MUSE fields. Five of the latter 36 sources are promising cLBVs which have M(V) less than or equal to -7 and a combination of: Hα with a P-Cygni profile; no [O I] 6300 emission; weak or no [O III] 5007 emission; large [N II]/Hα relative to H II regions; and [S II] 6716 / [S II] 6731 similar to 1. It is not clear if these five cLBVs are isolated from O-type stars, which would favor the binary formation scenario of LBVs. Our study, which approximately covers one fourth of the optical disc of NGC 7793, demonstrates how by combining the above HST surveys with multi-object spectroscopy from 8-m class telescopes, one can efficiently find large samples of cLBVs in nearby galaxies.

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arXiv2020

The Strikingly Metal-Rich Halo of the Sombrero Galaxy

The nature of the Sombrero galaxy (M 104 = NGC 4594) has remained elusive despite many observational studies at a variety of wavelengths. Here we present Hubble Space Telescope imaging of two fields at $\sim$16 and 33 kpc along the minor axis to examine stellar metallicity gradients in the extended spheroid. We use this imaging, extending more than 2 mag below the tip of the red giant branch (TRGB), in combination with artificial star tests to forward model observed color-magnitude diagrams (CMDs), measuring metallicity distribution functions (MDFs) at different radii along the minor axis. An important and unexpected result is that the halo of the Sombrero is strikingly metal-rich: even the outer field, located at $\sim$17 effective radii of the bulge, has a median metallicity [Z/H]$\sim$-0.15 and the fraction of stars with [Z/H]<-1.0 is negligible. This is unprecedented among massive galaxy halos studied to date, even among giant ellipticals. We find significant radial metallicity gradients, characterized by an increase in the fraction of metal-poor stars with radius and a gradient in median metallicity of $\sim$-0.01 dex/kpc. The density profile is well fit by power laws with slopes that exhibit a dependence on metallicity, with flatter slopes for more metal-poor stars. We discuss our results in the context of recent stellar MDF studies of other nearby galaxies and potential formation scenarios for the Sombrero galaxy.

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arXiv2019

Another Servicing Mission to Extend Hubble Space Telescope's Science past the Next Decade

The Hubble Space Telescope has produced astonishing science over the past thirty years. Hubble's productivity can continue to soar for years to come provided some worn out components get upgraded. While powerful new ground-based and space telescopes are expected to come online over the next decade, none of them will have the UV capabilities that make Hubble a unique observatory. Without Hubble, progress in UV and blue optical astrophysics will be halted. Observations at these wavelengths are key for a range of unresolved astrophysics questions, ranging from the characterization of solar system planets to understanding interaction of galaxies with the intergalactic medium and the formation history of the universe. Hubble will remain our only source of high-angular resolution UV imaging and high-sensitivity UV spectroscopy for the next two decades, offering the ability for continued unique science and maximizing the science return from complementary observatories. Therefore, we recommend that NASA, ESA, and the private sector study the scientific merit, technical feasibility, and risk of a new servicing mission to Hubble to boost its orbit, fix aging components, and expand its instrumentation. Doing so would: 1) keep Hubble on its path to reach its unmet full potential, 2) extend the mission's lifetime past the next decade, which will maximize the synergy of Hubble with other upcoming facilities, and 3) enable and enhance the continuation of scientific discoveries in UV and optical astrophysics.

astro-ph.IM↗

arXiv2019

Towards a high accuracy measurement of the local black hole occupation fraction in low mass galaxies

This document illustrates the feasibility of a few per cent level measurement of the local black hole occupation fraction in low mass galaxies through wide-field, high angular resolution X-ray imaging observations of local volume galaxies. The occupation fraction, particularly at the low end of the galaxy luminosity function, is a key benchmark for any model which aims to reproduce the formation and growth of super-massive black holes and their host galaxies. Our proposed measurement will complement orthogonal efforts that are planned in X-rays at high red-shifts, as well as in the local Universe with ground-based facilities.

astro-ph.HE↗

arXiv2018

Constraints on Upper Cutoffs in the Mass Functions of Young Star Clusters

We test claims that the power-law mass functions of young star clusters (ages $\lesssim\mbox{few}\times10^8$~yr) have physical upper cutoffs at $M_*\sim10^5~M_{\odot}$. Specifically, we perform maximum-likelihood fits of the Schechter function, $ψ(M)=dN/dM\propto M^β~\mbox{exp}(-M/M_*)$, to the observed cluster masses in eight well-studied galaxies (LMC, SMC, NGC 4214, NGC 4449, M83, M51, Antennae, and NGC 3256). In most cases, we find that a wide range of cutoff mass is permitted ($10^5~M_\odot \lesssim M_* < \infty$). We find a weak detection at $M_* \sim 10^5~M_\odot$ in one case (M51) and strong evidence against this value in two cases. However, when we include realistic errors in cluster masses in our analysis, the constraints on $M_*$ become weaker and there are no significant detections (even for M51). Our data are generally consistent with much larger cutoffs, at $M_*\sim\mbox{few}\times10^6~M_{\odot}$. This is the predicted cutoff from dynamical models in which old globular clusters and young clusters observed today formed by similar physical processes with similar initial mass functions.

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arXiv2018

A comparison of young star properties with local galactic environment for LEGUS/LITTLE THINGS dwarf irregular galaxies

We have explored the role environmental factors play in determining characteristics of young stellar objects in nearby dwarf irregular and Blue Compact Dwarf galaxies. Star clusters are characterized by concentrations, masses, and formation rates, OB associations by mass and mass surface density, O stars by their numbers and near-ultraviolet absolute magnitudes, and HII regions by Halpha surface brightnesses. These characteristics are compared to surrounding galactic pressure, stellar mass density, HI surface density, and star formation rate surface density. We find no trend of cluster characteristics with environmental properties, implying that larger scale effects are more important in determining cluster characteristics or that rapid dynamical evolution erases memory of the initial conditions. On the other hand, the most massive OB associations are found at higher pressure and HI surface density, and there is a trend of higher HII region Halpha surface brightness with higher pressure, suggesting that a higher concentration of massive stars and gas are found preferentially in regions of higher pressure. At low pressures we find massive stars but not bound clusters and OB associations. We do not find evidence for an increase of cluster formation efficiency as a function of star formation rate density. However, there is an increase in the ratio of the number of clusters to number of O stars with pressure, perhaps reflecting an increase in clustering properties with star formation rate.

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