arXiv subjects

Puragra Guhathakurta

Publications and source records attributed to Puragra Guhathakurta.

At least 55 records · Page 3Linked to original sources

arXiv2023

The Panchromatic Hubble Andromeda Treasury: Triangulum Extended Region (PHATTER). V. The Structure of M33 in Resolved Stellar Populations

We present a detailed analysis of the the structure of the Local Group flocculent spiral galaxy M33, as measured using the Panchromatic Hubble Andromeda Treasury Triangulum Extended Region (PHATTER) survey. Leveraging the multiwavelength coverage of PHATTER, we find that the oldest populations are dominated by a smooth exponential disk with two distinct spiral arms and a classical central bar $-$ completely distinct from what is seen in broadband optical imaging, and the first-ever confirmation of a bar in M33. We estimate a bar extent of $\sim$1 kpc. The two spiral arms are asymmetric in orientation and strength, and likely represent the innermost impact of the recent tidal interaction responsible for M33's warp at larger scales. The flocculent multi-armed morphology for which M33 is known is only visible in the young upper main sequence population, which closely tracks the morphology of the ISM. We investigate the stability of M33's disk, finding $Q{\sim}1$ over the majority of the disk. We fit multiple components to the old stellar density distribution and find that, when considering recent stellar kinematics, M33's bulk structure favors the inclusion of an accreted halo component, modeled as a broken power-law. The best-fit halo model has an outer power-law index of $-$3 and accurately describes observational evidence of M33's stellar halo from both resolved stellar spectroscopy in the disk and its stellar populations at large radius. Integrating this profile yields a total halo stellar mass of ${\sim}5{\times}10^8\ M_{\odot}$, giving a total stellar halo mass fraction of 16%, most of which resides in the innermost 2.5 kpc.

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arXiv2023

The Panchromatic Hubble Andromeda Treasury XXI. The Legacy Resolved Stellar Photometry Catalog

We present the final legacy version of stellar photometry for the Panchromatic Hubble Andromeda Treasury (PHAT) survey. We have reprocessed all of the Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3) and Advanced Camera for Surveys (ACS) near ultraviolet (F275W, F336W), optical (F475W, F814W), and near infrared (F110W, F160W) imaging from the PHAT survey using an improved method that optimized the survey depth and chip gap coverage by including all overlapping exposures in all bands in the photometry. An additional improvement was gained through the use of charge transfer efficiency (CTE) corrected input images, which provide more complete star finding as well as more reliable photometry for the NUV bands, which had no CTE correction in the previous version of the PHAT photometry. While this method requires significantly more computing resources and time than earlier versions where the photometry was performed on individual pointings, it results in smaller systematic instrumental completeness variations as demonstrated by cleaner maps in stellar density, and it results in optimal constraints on stellar fluxes in all bands from the survey data. Our resulting catalog has 138 million stars, 18% more than the previous catalog, with lower density regions gaining as much as 40% more stars. The new catalog produces nearly seamless population maps which show relatively well-mixed distributions for populations associated with ages older than 1-2 Gyr, and highly structured distributions for the younger populations.

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arXiv2023

RomAndromeda: The Roman Survey of the Andromeda Halo

As our nearest large neighbor, the Andromeda Galaxy provides a unique laboratory for investigating galaxy formation and the distribution and substructure properties of dark matter in a Milky Way-like galaxy. Here, we propose an initial 2-epoch ($Δt\approx 5$yr), 2-band Roman survey of the entire halo of Andromeda, covering 500 square degrees, which will detect nearly every red giant star in the halo (10$σ$ detection in F146, F062 of 26.5, 26.1AB mag respectively) and yield proper motions to $\sim$25 microarcsec/year (i.e., $\sim$90 km/s) for all stars brighter than F146 $\approx 23.6$ AB mag (i.e., reaching the red clump stars in the Andromeda halo). This survey will yield (through averaging) high-fidelity proper motions for all satellites and compact substructures in the Andromeda halo and will enable statistical searches for clusters in chemo-dynamical space. Adding a third epoch during the extended mission will improve these proper motions by $\sim t^{-1.5}$, to $\approx 11$ km/s, but this requires obtaining the first epoch in Year 1 of Roman operations. In combination with ongoing and imminent spectroscopic campaigns with ground-based telescopes, this Roman survey has the potential to yield full 3-d space motions of $>$100,000 stars in the Andromeda halo, including (by combining individual measurements) robust space motions of its entire globular cluster and most of its dwarf galaxy satellite populations. It will also identify high-velocity stars in Andromeda, providing unique information on the processes that create this population. These data offer a unique opportunity to study the immigration history, halo formation, and underlying dark matter scaffolding of a galaxy other than our own.

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arXiv2023

A non-interacting Galactic black hole candidate in a binary system with a main-sequence star

We describe the discovery of a solar neighborhood (d=468 pc) binary system with a main-sequence sunlike star and a massive non-interacting black hole candidate. The spectral energy distribution (SED) of the visible star is described by a single stellar model. We derive stellar parameters from a high signal-to-noise Magellan/MIKE spectrum, classifying the star as a main-sequence star with $T_{\rm eff} = 5972 \rm K$, $\log{g} = 4.54$, and $M = 0.91$ \msun. The spectrum shows no indication of a second luminous component. To determine the spectroscopic orbit of the binary, we measured radial velocities of this system with the Automated Planet Finder, Magellan, and Keck over four months. We show that the velocity data are consistent with the \textit{Gaia} astrometric orbit and provide independent evidence for a massive dark companion. From a combined fit of our spectroscopic data and the astrometry, we derive a companion mass of $11.39^{+1.51}_{-1.31}$\msun. We conclude that this binary system harbors a massive black hole on an eccentric $(e =0.46 \pm 0.02)$, $185.4 \pm 0.1$ d orbit. These conclusions are independent of \cite{ElBadry2022Disc}, who recently reported the discovery of the same system. A joint fit to all available data (including \cite{ElBadry2022Disc}'s) yields a comparable period solution, but a lower companion mass of $9.32^{+0.22}_{-0.21} M_{\odot}$. Radial velocity fits to all available data produce a unimodal solution for the period that is not possible with either data set alone. The combination of both data sets yields the most accurate orbit currently available.

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arXiv2023

The Next Generation Virgo Cluster Survey (NGVS). XXXV. First Kinematical Clues of Overly-Massive Dark Matter Halos in Several Ultra-Diffuse Galaxies in the Virgo Cluster

We present Keck/DEIMOS spectroscopy of the first complete sample of ultra-diffuse galaxies (UDGs) in the Virgo cluster. We select all UDGs in Virgo that contain at least 10 globular cluster (GC) candidates and are more than $2.5σ$ outliers in scaling relations of size, surface brightness, and luminosity (a total of 10 UDGs). We use the radial velocity of their GC satellites to measure the velocity dispersion of each UDG. We find a mixed bag of galaxies: from one UDG that shows no signs of dark matter, to UDGs that follow the luminosity-dispersion relation of early-type galaxies, to the most extreme examples of heavily dark matter dominated galaxies that break well-known scaling relations such as the luminosity-dispersion or the U-shaped total mass-to-light ratio relations. This is indicative of a number of mechanisms at play forming these peculiar galaxies. Some of them may be the most extended version of dwarf galaxies, while others are so extreme that they seem to populate dark matter halos consistent with that of the Milky-Way or even larger. Even though Milky-Way stars and other GC interlopers contaminating our sample of GCs cannot be fully ruled-out, our assessment of this potential problem and simulations indicate that the probability is low and, if present, unlikely to be enough to explain the extreme dispersions measured. Further confirmation from stellar kinematics studies in these UDGs would be desirable. The lack of such extreme objects in any of the state-of-the-art simulations, opens an exciting avenue of new physics shaping these galaxies.

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arXiv2023

HALO7D III: Chemical Abundances of Milky Way Halo Stars from Medium Resolution Spectra

The Halo Assembly in Lambda Cold Dark Matter: Observations in 7 Dimensions (HALO7D) survey measures the kinematics and chemical properties of stars in the Milky Way (MW) stellar halo to learn about the formation of our Galaxy. HALO7D consists of Keck II/DEIMOS spectroscopy and Hubble Space Telescope-measured proper motions of MW halo main sequence turn-off (MSTO) stars in the four CANDELS fields. HALO7D consists of deep pencil beams, making it complementary to other contemporary wide-field surveys. We present the [Fe/H] and [$α$/Fe] abundances for 113 HALO7D stars in the Galactocentric radial range of $\sim 10-40$ kpc. Using the full 7D chemodynamical data (3D positions, 3D velocities, and abundances) of HALO7D, we measure the velocity anisotropy, $β$, of the halo velocity ellipsoid for each field and for different metallicity-binned subsamples. We find that two of the four fields have stars on very radial orbits, while the remaining two have stars on more isotropic orbits. Separating the stars into high, mid, and low [Fe/H] bins at $-2.2$ dex and $-1.1$ dex for each field separately, we find differences in the anisotropies between the fields and between the bins; some fields appear dominated by radial orbits in all bins while other fields show variation between the [Fe/H] bins. These chemodynamical differences are evidence that the HALO7D fields have different fractional contributions from the progenitors that built up the MW stellar halo. Our results highlight the additional information that is available on smaller spatial scales when compared to results from a spherical average of the stellar halo.

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arXiv2023

The Neon Gap: Probing Ionization with Dwarf Galaxies at z~1

We present measurements of [NeIII]λ3869 emission in z~1 low-mass galaxies taken from the Keck/DEIMOS spectroscopic surveys HALO7D and DEEPWinds. We identify 167 individual galaxies with significant [NeIII] emission lines, including 112 "dwarf" galaxies with log(M_{\star}/M_{\odot}) < 9.5, with 0.3 < z < 1.4. We also measure [NeIII] emission from composite spectra derived from all [OII]λλ3727,3729 line emitters in this range. This provides a unique sample of [NeIII]-emitters in the gap between well-studied emitters at z = 0 and 2 < z < 3. To study evolution in ionization conditions in the ISM over this time, we analyze the log([NeIII]λ3869/[OII]λλ3727,3729) ratio (Ne3O2) as a function of the stellar mass and of the log([OIII]λλ4959,5007/[OII]λλ3727,3729) ratio (O32). We find that the typical star-forming dwarf galaxy at this redshift, as measured from the composite spectra, shares the Ne3O2-M_{\star} relation with local galaxies, but have higher O32 at given Ne3O2. This finding implies that the ionization and metallicity characteristics of the z~1 dwarf population do not evolve substantially from z~1 to z=0, suggesting that the known evolution in those parameter from z~2 has largely taken place by z~1. Individual [NeIII]-detected galaxies have emission characteristics situated between local and z~2 galaxies, with elevated Ne3O2 and O32 emission potentially explained by variations in stellar and nebular metallicity. We also compare our dwarf sample to similarly low-mass z > 7 galaxies identified in JWST Early Release Observations, finding four HALO7D dwarfs with similar size, metallicity, and star formation properties.

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arXiv2022

Resolved SPLASH Chemodynamics in Andromeda's PHAT Stellar Halo and Disk: On the Nature of the Inner Halo Along the Major Axis

Stellar kinematics and metallicity are key to exploring formation scenarios for galactic disks and halos. In this work, we characterized the relationship between kinematics and photometric metallicity along the line-of-sight to M31's disk. We combined optical HST/ACS photometry from the Panchromatic Hubble Andromeda Treasury (PHAT) survey with Keck/DEIMOS spectra from the Spectroscopic and Photometric Landscape of Andromeda's Stellar Halo (SPLASH) survey. The resulting sample of 3512 individual red giant branch stars spans 4-19 projected kpc, making it a useful probe of both the disk and inner halo. We separated these stars into disk and halo populations by modeling the line-of-sight velocity distributions as a function of position across the disk region, where $\sim$73% stars have a high likelihood of belonging to the disk and $\sim$14% to the halo. Although stellar halos are typically thought to be metal-poor, the kinematically identified halo contains a significant population of stars ($\sim$29%) with disk-like metallicity ([Fe/H]$_{\rm phot}$ $\sim$ $-0.10$). This metal-rich halo population lags the gaseous disk to a similar extent as the rest of the halo, indicating that it does not correspond to a canonical thick disk. Its properties are inconsistent with those of tidal debris originating from the Giant Stellar Stream merger event. Moreover, the halo is chemically distinct from the phase-mixed component previously identified along the minor axis (i.e., away from the disk), implying contributions from different formation channels. These metal-rich halo stars provide direct chemodynamical evidence in favor of the previously suggested "kicked-up" disk population in M31's inner stellar halo.

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arXiv2022

Elemental abundances in M31: Individual and Coadded Spectroscopic [Fe/H] and [α/Fe] throughout the M31 Halo with SPLASH

We present spectroscopic chemical abundances of red giant branch (RGB) stars in Andromeda (M31), using medium resolution ($R\sim6000$) spectra obtained via the Spectroscopic and Photometric Landscape of Andromeda's Stellar Halo (SPLASH) survey. In addition to individual chemical abundances, we coadd low signal-to-noise ratio (S/N) spectra of stars to obtain a high enough to measure average [Fe/H] and [$α$/Fe] abundances. We obtain individual and coadded measurements for [Fe/H] and [$α$/Fe] for M31 halo stars, covering a range of 9--180 kpc in projected radius from the center of M31. With these measurements, we greatly increase the number of outer halo ($R_{\mathrm{proj}} > 50$ kpc) M31 stars with spectroscopic [Fe/H] and [$α$/Fe], adding abundance measurements for 45 individual stars and 33 coadds from a pool of an additional 174 stars. We measure the spectroscopic metallicity ([Fe/H]) gradient, finding a negative radial gradient of $-0.0050\pm0.0003$ for all stars in the halo, consistent with gradient measurements obtained using photometric metallicities. Using the first measurements of [$α$/Fe] for M31 halo stars covering a large range of projected radii, we find a positive gradient ($+0.0026\pm0.0004$) in [$α$/Fe] as a function of projected radius. We also explore the distribution in [Fe/H]--[$α$/Fe] space as a function of projected radius for both individual and coadded measurements in the smooth halo, and compare these measurements to those stars potentially associated with substructure. These spectroscopic abundance distributions highlight the substantial evidence that M31 has had an appreciably different formation and merger history compared to our own Galaxy.

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arXiv2022

The survey of planetary nebulae in Andromeda (M31). IV. Radial oxygen and argon abundance gradients of the thin and thicker disc

We obtain a magnitude-limited sample of Andromeda (M 31) disc PNe with chemical abundance estimated through the direct detection of the [OIII] 4363 $\mathring{\mathrm A}$ line. This leads to $205$ and $200$ PNe with oxygen and argon abundances respectively. We find that high- and low-extinction M 31 disc PNe have statistically distinct argon and oxygen abundance distributions. In the radial range $2-30$ kpc, the older low-extinction disc PNe are metal-poorer on average with a slightly positive radial oxygen abundance gradient ($0.006 \pm 0.003$ dex/kpc) and slightly negative for argon ($-0.005 \pm 0.003$ dex/kpc), while the younger high-extinction disc PNe are metal-richer on average with steeper radial abundance gradients for both oxygen ($-0.013 \pm 0.006$ dex/kpc) and argon ($-0.018 \pm 0.006$ dex/kpc), similar to the gradients computed for the M 31 HII regions. The M 31 disc abundance gradients are consistent with values computed from major merger simulations, with the majority of the low-extinction PNe being the older pre-merger disc stars in the thicker disc, and the majority of the high-extinction PNe being younger stars in the thin disc, formed during and after the merger event. The chemical abundance of the M 31 thicker disc has been radially homogenized because of the major merger. Accounting for disc scale-lengths, the positive radial oxygen abundance gradient of the M 31 thicker disc is in sharp contrast to the negative one of the MW thick disc. However, the thin discs of the MW and M 31 have remarkably similar negative oxygen abundance gradients.

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arXiv2022

The Hubble Space Telescope Survey of M31 Satellite Galaxies I. RR Lyrae-based Distances and Refined 3D Geometric Structure

We measure homogeneous distances to M31 and 38 associated stellar systems ($-$16.8$\le M_V \le$ $-$6.0), using time-series observations of RR Lyrae stars taken as part of the Hubble Space Telescope Treasury Survey of M31 Satellites. From $>700$ orbits of new/archival ACS imaging, we identify $>4700$ RR Lyrae stars and determine their periods and mean magnitudes to a typical precision of 0.01 days and 0.04 mag. Based on Period-Wesenheit-Metallicity relationships consistent with the Gaia eDR3 distance scale, we uniformly measure heliocentric and M31-centric distances to a typical precision of $\sim20$ kpc (3%) and $\sim10$ kpc (8%), respectively. We revise the 3D structure of the M31 galactic ecosystem and: (i) confirm a highly anisotropic spatial distribution such that $\sim80$% of M31's satellites reside on the near side of M31; this feature is not easily explained by observational effects; (ii) affirm the thin (rms $7-23$ kpc) planar "arc" of satellites that comprises roughly half (15) of the galaxies within 300 kpc from M31; (iii) reassess physical proximity of notable associations such as the NGC 147/185 pair and M33/AND XXII; and (iv) illustrate challenges in tip-of-the-red-giant branch distances for galaxies with $M_V > -9.5$, which can be biased by up to 35%. We emphasize the importance of RR Lyrae for accurate distances to faint galaxies that should be discovered by upcoming facilities (e.g., Rubin Observatory). We provide updated luminosities and sizes for our sample. Our distances will serve as the basis for future investigation of the star formation and orbital histories of the entire known M31 satellite system.

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arXiv2022

The Panchromatic Hubble Andromeda Treasury: Triangulum Extended Region (PHATTER). IV. Star Cluster Catalog

We construct a catalog of star clusters from Hubble Space Telescope images of the inner disk of the Triangulum Galaxy (M33) using image classifications collected by the Local Group Cluster Search, a citizen science project hosted on the Zooniverse platform. We identify 1214 star clusters within the Hubble Space Telescope imaging footprint of the Panchromatic Hubble Andromeda Treasury: Triangulum Extended Region (PHATTER) survey. Comparing this catalog to existing compilations in the literature, 68% of the clusters are newly identified. The final catalog includes multi-band aperture photometry and fits for cluster properties via integrated light SED fitting. The cluster catalog's 50% completeness limit is ~1500 solar masses at an age of 100 Myr, as derived from comprehensive synthetic cluster tests.

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arXiv2022

The survey of planetary nebulae in Andromeda (M 31) V. Chemical enrichment of the thin and thicker discs of Andromeda. Oxygen to argon abundance ratios for planetary nebulae and H II regions

We use oxygen and argon abundances for planetary nebulae (PNe) with low internal extinction (progenitor ages of (>4.5 Gyr) and high extinction (progenitor ages <2.5 Gyr), as well as those of the H II regions, to constrain the chemical enrichment and star formation efficiency in the thin and thicker discs of M31. The argon element is produced in larger fraction by Type Ia supernovae (SNe) than oxygen. We find that the mean log(O/Ar) values of PNe as a function of their argon abundances, 12 + log(Ar/H), trace the inter-stellar matter (ISM) conditions at the time of birth of the M 31 disc PN progenitors. Thus the chemical enrichment and star formation efficiency information encoded in the [alpha/Fe] vs. [Fe/H] distribution of stars is also imprinted in the oxygen-to-argon abundance ratio log(O/Ar) vs. argon abundance for the nebular emissions of the different stellar evolution phases. We propose to use the log(O/Ar) vs. (12 + log(Ar/H)) distribution of PNe with different ages to constrain the star-formation histories of the parent stellar populations in the thin and thicker M31 discs. For the inner M31 disc (R_{GC} < 14 kpc), the chemical evolution model that reproduces the mean log(O/Ar) values as function of argon abundance for the high- and low-extinction PNe requires a second infall of metal poorer gas during a gas-rich (wet) satellite merger. In M31, the thin disc is younger and less radially extended, formed stars at a higher star formation efficiency, and had a faster chemical enrichment timescale than the more extended, thicker disc. Both the thin and thicker disc in M31 reach similar high argon abundances ( 12 + log(Ar/H) ) ~ 6.7. The chemical and structural properties of the thin/thicker discs in M31 are thus remarkably different from those determined for the Milky Way thin and thick discs.

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arXiv2022

The Dwarf Galaxy Population at $z\sim 0.7$: A Catalog of Emission Lines and Redshifts from Deep Keck Observations

We present a catalog of spectroscopically measured redshifts over $0 < z < 2$ and emission line fluxes for 1440 galaxies. The majority ($\sim$65\%) of the galaxies come from the HALO7D survey, with the remainder from the DEEPwinds program. This catalog includes redshifts for 646 dwarf galaxies with $\log(M_{\star}/M_{\odot}) < 9.5$. 810 catalog galaxies did not have previously published spectroscopic redshifts, including 454 dwarf galaxies. HALO7D used the DEIMOS spectrograph on the Keck II telescope to take very deep (up to 32 hours exposure, with a median of $\sim$7 hours) optical spectroscopy in the COSMOS, EGS, GOODS-North, and GOODS-South CANDELS fields, and in some areas outside CANDELS. We compare our redshift results to existing spectroscopic and photometric redshifts in these fields, finding only a 1\% rate of discrepancy with other spectroscopic redshifts. We measure a small increase in median photometric redshift error (from 1.0\% to 1.3\%) and catastrophic outlier rate (from 3.5\% to 8\%) with decreasing stellar mass. We obtained successful redshift fits for 75\% of massive galaxies, and demonstrate a similar 70-75\% successful redshift measurement rate in $8.5 < \log(M_{\star}/M_{\odot}) < 9.5$ galaxies, suggesting similar survey sensitivity in this low-mass range. We describe the redshift, mass, and color-magnitude distributions of the catalog galaxies, finding HALO7D galaxies representative of CANDELS galaxies up to \textit{i}-band magnitudes of 25. The catalogs presented will enable studies of star formation (SF), the mass-metallicity relation, SF-morphology relations, and other properties of the $z\sim0.7$ dwarf galaxy population.

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arXiv2022

Young, blue, and isolated stellar systems in the Virgo Cluster. II. A new class of stellar system

We discuss five blue stellar systems in the direction of the Virgo cluster, analogous to the enigmatic object SECCO 1 (AGC 226067). These objects were identified based on their optical and UV morphology and followed up with HI observations with the VLA (and GBT), MUSE/VLT optical spectroscopy, and HST imaging. These new data indicate that one system is a distant group of galaxies. The remaining four are extremely low mass ($M_\ast \sim 10^5 \; \mathrm{M_\odot}$), are dominated by young, blue stars, have highly irregular and clumpy morphologies, are only a few kpc across, yet host an abundance of metal-rich, $12 + \log (\mathrm{O/H}) > 8.2$, HII regions. These high metallicities indicate that these stellar systems formed from gas stripped from much more massive galaxies. Despite the young age of their stellar populations, only one system is detected in HI, while the remaining three have minimal (if any) gas reservoirs. Furthermore, two systems are surprisingly isolated and have no plausible parent galaxy within $\sim$30' ($\sim$140 kpc). Although tidal stripping cannot be conclusively excluded as the formation mechanism of these objects, ram pressure stripping more naturally explains their properties, in particular their isolation, owing to the higher velocities, relative to the parent system, that can be achieved. Therefore, we posit that most of these systems formed from ram pressure stripped gas removed from new infalling cluster members, and survived in the intracluster medium long enough to become separated from their parent galaxies by hundreds of kiloparsecs, and that they thus represent a new type of stellar system.

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arXiv2022

The Panchromatic Hubble Andromeda Treasury: Triangulum Extended Region (PHATTER) II. The Spatially Resolved Recent Star Formation History of M33

We measure the spatially resolved recent star formation history (SFH) of M33 using optical images taken with the Hubble Space Telescope as part of the Panchromatic Hubble Andromeda Treasury: Triangulum Extended Region (PHATTER) survey. The area covered by the observations used in this analysis covers a de-projected area of $\sim$38 kpc$^{2}$ and extends to $\sim$3.5 and $\sim$2 kpc from the center of M33 along the major and semi-major axes, respectively. We divide the PHATTER optical survey into 2005 regions that measure 24 arcsec, $\sim$100 pc, on a side and fit color magnitude diagrams for each region individually to measure the spatially resolved SFH of M33 within the PHATTER footprint. There are significant fluctuations in the SFH on small spatial scales and also galaxy-wide scales that we measure back to about 630 Myr ago. We observe a more flocculent spiral structure in stellar populations younger than about 80 Myr, while the structure of the older stellar populations is dominated by two spiral arms. We also observe a bar in the center of M33, which dominates at ages older than about 80 Myr. Finally, we find that the mean star formation rate (SFR) over the last 100 Myr within the PHATTER footprint is 0.32$\pm$0.02 M$_{\odot}$ yr$^{-1}$. We measure a current SFR (over the last 10 Myr) of 0.20$\pm$0.03 M$_{\odot}$ yr$^{-1}$. This SFR is slightly higher than previous measurements from broadband estimates, when scaled to account for the fraction of the D25 area covered by the PHATTER survey footprint.

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arXiv2022

The Next Generation Virgo Cluster Survey. XXXIII. Stellar Population Gradients in the Virgo Cluster Core Globular Cluster System

We present a study of the stellar populations of globular clusters (GCs) in the Virgo Cluster core with a homogeneous spectroscopic catalog of 692 GCs within a major axis distance $R_{\rm maj} = $ 840 kpc from M87. We investigate radial and azimuthal variations in the mean age, total metallicity, [Fe/H], and $α$-element abundance, of blue (metal-poor) and red (metal-rich) GCs using their co-added spectra. We find that the blue GCs have a steep radial gradient in [Z/H] within $R_{\rm maj} =$ 165 kpc, with roughly equal contributions from [Fe/H] and [$α$/Fe], and flat gradients beyond. By contrast, the red GCs show a much shallower gradient in [Z/H], which is entirely driven by [Fe/H]. We use GC-tagged Illustris simulations to demonstrate an accretion scenario where more massive satellites (with more metal- and $α$-rich GCs) sink further into the central galaxy than less massive ones, and where the gradient flattening occurs because of the low GC occupation fraction of low-mass dwarfs disrupted at larger distances. The dense environment around M87 may also cause the steep [$α$/Fe] gradient of the blue GCs, mirroring what is seen in the dwarf galaxy population. The progenitors of red GCs have a narrower mass range than those of blue GCs, which makes their gradients shallower. We also explore spatial inhomogeneity in GC abundances, finding that the red GCs to the northwest of M87 are slightly more metal-rich. Future observations of GC stellar population gradients will be useful diagnostics of halo merger histories.

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arXiv2022

The Baltimore Oriole's Nest: Cool Winds from the Inner and Outer Parts of a Star-Forming Galaxy at z=1.3

Strong galactic winds are ubiquitous at $z\gtrsim 1$. However, it is not well known where inside galaxies these winds are launched from. We study the cool winds ($\sim 10^4$\,K) in two spatial regions of a massive galaxy at $z=1.3$, which we nickname the "Baltimore Oriole's Nest." The galaxy has a stellar mass of $10^{10.3\pm 0.3} M_\odot$, is located on the star-forming main sequence, and has a morphology indicative of a recent merger. Gas kinematics indicate a dynamically complex system with velocity gradients ranging from 0 to 60 $\mathrm{km}\cdot\mathrm{s}^{-1}$. The two regions studied are: a dust-reddened center (Central region), and a blue arc at 7 kpc from the center (Arc region). We measure the \ion{Fe}{2} and \ion{Mg}{2} absorption line profiles from deep Keck/DEIMOS spectra. Blueshifted wings up to 450 km$\cdot$s$^{-1}$ are found for both regions. The \ion{Fe}{2} column densities of winds are $10^{14.7\pm 0.2}\,\mathrm{cm}^{-2}$ and $10^{14.6\pm 0.2}\,\mathrm{cm}^{-2}$ toward the Central and Arc regions, respectively. Our measurements suggest that the winds are most likely launched from both regions. The winds may be driven by the spatially extended star formation, the surface density of which is around 0.2 $M_\odot\,\mathrm{yr}^{-1}\cdot \mathrm{kpc}^{-2}$ in both regions. The mass outflow rates are estimated to be $4\,M_\odot\,\mathrm{yr}^{-1}$ and $3\,M_\odot\,\mathrm{yr}^{-1}$ for the Central and Arc regions, with uncertainties of one order-of-magnitude or more. Findings of this work and a few previous studies suggest that the cool galactic winds at $z\gtrsim 1$ might be commonly launched from the entire spatial extents of their host galaxies due to extended galaxy star formation.

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