arXiv subjects

Kristen B. W. McQuinn

Publications and source records attributed to Kristen B. W. McQuinn.

At least 73 records · Page 4Linked to original sources

arXiv2023

The JWST Resolved Stellar Populations Early Release Science Program III: Photometric Star-Galaxy Separations for NIRCam

We present criteria for separately classifying stars and unresolved background galaxies in photometric catalogs generated with the point spread function (PSF) fitting photometry software DOLPHOT from images taken of Draco II, WLM, and M92 with the Near Infrared Camera (NIRCam) on JWST. Photometric quality metrics from DOLPHOT in one or two filters can recover a pure sample of stars. Conversely, colors formed between short-wavelength (SW) and long-wavelength (LW) filters can be used to effectively identify pure samples of galaxies. Our results highlight that the existing DOLPHOT output parameters can be used to reliably classify stars in our NIRCam data without the need to resort to external tools or more complex heuristics.

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arXiv2023

The JWST Resolved Stellar Populations Early Release Science Program II. Survey Overview

We present the JWST Resolved Stellar Populations Early Release Science (ERS) science program. We obtained 27.5 hours of NIRCam and NIRISS imaging of three targets in the Local Group (Milky Way globular cluster M92, ultra-faint dwarf galaxy Draco II, star-forming dwarf galaxy WLM), which span factors of $\sim10^5$ in luminosity, $\sim10^4$ in distance, and $\sim10^5$ in surface brightness. We describe the survey strategy, scientific and technical goals, implementation details, present select NIRCam color-magnitude diagrams (CMDs), and validate the NIRCam exposure time calculator (ETC). Our CMDs are among the deepest in existence for each class of target. They touch the theoretical hydrogen burning limit in M92 ($<0.08$ $M_{\odot}$; SNR $\sim5$ at $m_{F090W}\sim28.2$; $M_{F090W}\sim+13.6$), include the lowest-mass stars observed outside the Milky Way in Draco II (0.09 $M_{\odot}$; SNR $=10$ at $m_{F090W}\sim29$; $M_{F090W}\sim+12.1$), and reach $\sim1.5$ magnitudes below the oldest main sequence turnoff in WLM (SNR $=10$ at $m_{F090W}\sim29.5$; $M_{F090W}\sim+4.6$). The PARSEC stellar models provide a good qualitative match to the NIRCam CMDs, though are $\sim0.05$ mag too blue compared to M92 F090W$-$F150W data. The NIRCam ETC (v2.0) matches the SNRs based on photon noise from DOLPHOT stellar photometry in uncrowded fields, but the ETC may not be accurate in more crowded fields, similar to what is known for HST. We release beta versions of DOLPHOT NIRCam and NIRISS modules to the community. Results from this ERS program will establish JWST as the premier instrument for resolved stellar populations studies for decades to come.

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arXiv2022

A Panchromatic Study of Massive Stars in the Extremely Metal-Poor Local Group Dwarf Galaxy Leo A

We characterize massive stars (M>8 M_sun) in the nearby (D~0.8 Mpc) extremely metal-poor (Z~5% Z_sun) galaxy Leo A using Hubble Space Telescope ultra-violet (UV), optical, and near-infrared (NIR) imaging along with Keck/LRIS and MMT/Binospec optical spectroscopy for 18 main sequence OB stars. We find that: (a) 12 of our 18 stars show emission lines, despite not being associated with an H II region, suggestive of stellar activity (e.g., mass loss, accretion, binary star interaction), which is consistent with previous predictions of enhanced activity at low metallicity; (b) 6 are Be stars, which are the first to be spectroscopically studied at such low metallicity -- these Be stars have unusual panchromatic SEDs; (c) for stars well-fit by the TLUSTY non-local thermodynamic equilibrium (non-LTE) models, the photometric and spectroscopic values of T_eff and log(g) agree to within ~0.01 dex and ~0.18 dex, respectively, indicating that NUV/optical/NIR imaging can be used to reliably characterize massive (M ~ 8-30 M_sun) main sequence star properties relative to optical spectroscopy; (d) the properties of the most massive stars in H II regions are consistent with constraints from previous nebular emission line studies; and (e) 13 stars with M>8 M_sun are >40 pc from a known star cluster or H II region. Our sample comprises ~50% of all known massive stars at Z < 10% Z_sun with derived stellar parameters, high-quality optical spectra, and panchromatic photometry.

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arXiv2022

WALLABY Pilot Survey: HI gas kinematics of galaxy pairs in cluster environment

We examine the HI gas kinematics of galaxy pairs in two clusters and a group using Australian Square Kilometre Array Pathfinder (ASKAP) WALLABY pilot survey observations. We compare the HI properties of galaxy pair candidates in the Hydra I and Norma clusters, and the NGC 4636 group, with those of non-paired control galaxies selected in the same fields. We perform HI profile decomposition of the sample galaxies using a tool, {\sc baygaud} which allows us to de-blend a line-of-sight velocity profile with an optimal number of Gaussian components. We construct HI super-profiles of the sample galaxies via stacking of their line profiles after aligning the central velocities. We fit a double Gaussian model to the super-profiles and classify them as kinematically narrow and broad components with respect to their velocity dispersions. Additionally, we investigate the gravitational instability of HI gas disks of the sample galaxies using Toomre Q parameters and HI morphological disturbances. We investigate the effect of the cluster environment on the HI properties of galaxy pairs by dividing the cluster environment into three subcluster regions (i.e., outskirts, infalling and central regions). We find that the denser cluster environment (i.e., infalling and central regions) is likely to impact the HI gas properties of galaxies in a way of decreasing the amplitude of the kinematically narrow HI gas ($M_{\rm{narrow}}^{\rm{HI}}$/$M_{\rm{total}}^{\rm{HI}}$), and increasing the Toomre Q values of the infalling and central galaxies. This tendency is likely to be more enhanced for galaxy pairs in the cluster environment.

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arXiv2022

The Ionizing Spectra of Extremely Metal-Poor O Stars: Constraints from the Only HII Region in Leo P

Metal-poor, star-forming dwarf galaxies produce extreme nebular emission and likely played a major role in cosmic reionization. Yet, determining their contribution to the high-redshift ionizing photon budget is hampered by the lack of observations constraining the ionizing spectra of individual massive stars more metal-poor than the Magellanic Clouds (20-50%$\,Z_\odot$). We present new Keck Cosmic Web Imager (KCWI) optical integral field unit spectroscopy of the only HII region in Leo P (3%$\,Z_\odot$), which is powered by a single O star. We calculate the required production rate of photons capable of ionizing H and He from the observed H$β$ and HeI$\,λ$4471 emission-line fluxes. Remarkably, we find that the ionizing photon production rate and spectral hardness predicted by a TLUSTY model fit to the stellar SED agrees with our observational measurements within the uncertainties. We then fit Cloudy photoionization models to the full suite of optical emission lines in the KCWI data and show that the shape of the same TLUSTY ionizing continuum simultaneously matches lines across a wide range of ionization energies. Finally, we detect OIII] and NIII] nebular emission in the Hubble Space Telescope far-ultraviolet spectrum of the Leo P HII region, and highlight that the rarely observed NIII] emission cannot be explained by our Cloudy models. These results provide the first observational evidence that widely used, yet purely theoretical, model spectra accurately predict the ionizing photon production rate from late-O stars at very low metallicity, validating their use to model metal-poor galaxies both locally and at high redshift.

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arXiv2022

WALLABY Pre-Pilot and Pilot Survey: the Tully Fisher Relation in Eridanus, Hydra, Norma and NGC4636 fields

The WALLABY pilot survey has been conducted using the Australian SKA Pathfinder (ASKAP). The integrated 21-cm HI line spectra are formed in a very different manner compared to usual single-dish spectra Tully-Fisher measurements. It is thus extremely important to ensure that slight differences (e.g. biases due to missing flux) are quantified and understood in order to maximise the use of the large amount of data becoming available soon. This article is based on four fields for which the data are scientifically interesting by themselves. The pilot data discussed here consist of 614 galaxy spectra at a rest wavelength of 21cm. Of these spectra, 472 are of high enough quality to be used to potentially derive distances using the Tully-Fisher relation. We further restrict the sample to the 251 galaxies whose inclination is sufficiently close to edge-on. For these, we derive Tully-Fisher distances using the deprojected WALLABY velocity widths combined with infrared (WISE W1) magnitudes. The resulting Tully-Fisher distances for the Eridanus, Hydra, Norma and NGC 4636 clusters are 21.5, 53.5, 69.4 and 23.0 Mpc respectively, with uncertainties of 5--10\%, which are better or equivalent to the ones obtained in studies using data obtained with giant single dish telescopes. The pilot survey data show the benefits of WALLABY over previous giant single-dish telescope surveys. WALLABY is expected to detect around half a million galaxies with a mean redshift of $z = 0.05 (200 Mpc)$. This study suggests that about 200,000 Tully-Fisher distances might result from the survey.

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arXiv2022

The Turn-Down of the Baryonic Tully-Fisher Relation and Changing Baryon Fractions at Low Galaxy Masses

The ratio of baryonic-to-dark matter in present-day galaxies constrains galaxy formation theories and can be determined empirically via the baryonic Tully-Fisher relation (BTFR), which compares a galaxy's baryonic mass (Mbary) to its maximum rotation velocity (Vmax). The BTFR is well-determined at Mbary >10^8 Msun, but poorly constrained at lower masses due to small samples and the challenges of measuring rotation velocities in this regime. For 25 galaxies with high-quality data and Mbary <~10^8 Msun, we estimate Mbary from infrared and HI observations and Vmax from the HI gas rotation. Many of the Vmax values are lower limits because the velocities are still rising at the edge of the detected HI disks (Rmax); consequently, most of our sample has lower velocities than expected from extrapolations of the BTFR at higher masses. To estimate Vmax, we map each galaxy to a dark matter halo assuming density profiles with and without cores. In contrast to non-cored profiles, we find the cored profile rotation curves are still rising at Rmax values, similar to the data. When we compare the Vmax values derived from the cored density profiles to our Mbary measurements, we find a turndown of the BTFR at low masses that is consistent with LCDM predictions and implying baryon fractions of 1-10% of the cosmic value. Although we are limited by the sample size and assumptions inherent in mapping measured rotational velocities to theoretical rotation curves, our results suggest that galaxy formation efficiency drops at masses below Mbary~10^8 Msun, corresponding to Mhalo~10^10 Msun.

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arXiv2022

Determining the Timescale over Which Stellar Feedback Drives Turbulence in the ISM: A Study of four Nearby Dwarf Irregular Galaxies

Stellar feedback is fundamental to the modeling of galaxy evolution as it drives turbulence and outflows in galaxies. Understanding the timescales involved are critical for constraining the impact of stellar feedback on the interstellar medium (ISM). We analyzed the resolved star formation histories along with the spatial distribution and kinematics of the atomic and ionized gas of four nearby star-forming dwarf galaxies (NGC 4068, NGC 4163, NGC 6789, UGC 9128) to determine the timescales over which stellar feedback drives turbulence. The four galaxies are within 5 Mpc and have a range of properties including current star formation rates of 0.0005 to 0.01 M$_{\odot}$ yr$^{-1}$, log(M$_*$/M$_{\odot}$) between 7.2 and 8.2, and log(M$_{HI}$/M$_\odot$) between 7.2 and 8.3. Their Color-Magnitude Diagram (CMD) derived star formation histories over the past 500 Myrs were compared to their atomic and ionized gas velocity dispersion and HI energy surface densities as indicators of turbulence. The Spearman's rank correlation coefficient was used to identify any correlations between their current turbulence and their past star formation activity on local scales ($\sim$400 pc). The strongest correlation found was between the HI turbulence measures and the star formation rate 100-200 Myrs ago. This suggests a coupling between the star formation activity and atomic gas on this timescale. No strong correlation between the ionized gas velocity dispersion and the star formation activity between 5-500 Myrs ago was found. The sample and analysis are the foundation of a larger program aimed at understanding the timescales over which stellar feedback drives turbulence.

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arXiv2021

Far-Ultraviolet Spectra of Main-Sequence O Stars at Extremely Low Metallicity

Metal-poor massive stars dominate the light we observe from star-forming dwarf galaxies and may have produced the bulk of energetic photons that reionized the universe at high redshift. Yet, the rarity of observations of individual O stars below the $20\%$ solar metallicity ($Z_\odot$) of the Small Magellanic Cloud (SMC) hampers our ability to model the ionizing fluxes of metal-poor stellar populations. We present new Hubble Space Telescope far-ultraviolet (FUV) spectra of three O-dwarf stars in the galaxies Leo P ($3\%\,Z_\odot$), Sextans A ($6\%\,Z_\odot$), and WLM ($14\%\,Z_\odot$). We quantify equivalent widths of photospheric metal lines and strengths of wind-sensitive features, confirming that both correlate with metallicity. We infer the stars' fundamental properties by modeling their FUV through near-infrared spectral energy distributions and identify stars in the SMC with similar properties to each of our targets. Comparing to the FUV spectra of the SMC analogs suggests that (1) the star in WLM has an SMC-like metallicity, and (2) the most metal-poor star in Leo P is driving a much weaker stellar wind than its SMC counterparts. We measure projected rotation speeds and find that the two most metal-poor stars have high $v \,\mathrm{sin}(i)\,\geq\,290\,\mathrm{km}\,\mathrm{s}^{-1}$, and estimate just a $3-6\%$ probability of finding two fast rotators if the metal-poor stars are drawn from the same $v \,\mathrm{sin}(i)$ distribution observed for O dwarfs in the SMC. These observations suggest that models should include the impact of rotation and weak winds on ionizing flux to accurately interpret observations of metal-poor galaxies in both the near and distant universe.

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arXiv2021

Galaxy Properties at the Faint End of the HI Mass Function

The Survey of HI in Extremely Low-mass Dwarfs (SHIELD) includes a volumetrically complete sample of 82 gas-rich dwarfs with M_HI~<10^7.2 Msun selected from the ALFALFA survey. We are obtaining extensive follow-up observations of the SHIELD galaxies to study their gas, stellar, and chemical content, and to better understand galaxy evolution at the faint end of the HI mass function. Here, we investigate the properties of 30 SHIELD galaxies using Hubble Space Telescope imaging of their resolved stars and Westerbork Synthesis Radio Telescope observations of their neutral hydrogen. We measure tip of the red giant branch (TRGB) distances, star formation activity, and gas properties. The TRGB distances are up to 4x greater than estimates from flow models, highlighting the importance of velocity-independent distance indicators in the nearby universe. The SHIELD galaxies are in under-dense regions, with 23% located in voids; one galaxy appears paired with a more massive dwarf. We quantify galaxy properties at low masses including stellar and HI masses, SFRs, sSFRs, SFEs, birthrate parameters, and gas fractions. The lowest mass systems lie below the mass thresholds where stellar mass assembly is predicted to be impacted by reionization. Even so, we find the star formation properties follow the same trends as higher mass gas-rich systems, albeit with a different normalization. The HI disks are small ( <0.7 kpc) making it difficult to measure the HI rotation using standard techniques; we develop a new methodology and report the velocity extent, and its associated spatial extent, with robust uncertainties.

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arXiv2021

HETDEX [OIII] Emitters I: A spectroscopically selected low-redshift population of low-mass, low-metallicity galaxies

We assemble a sample of 17 low metallicity (7.45 < log(O/H)+12 < 8.12) galaxies with z < 0.1 found spectroscopically, without photometric pre-selection, in early data from the Hobby Eberly Telescope Dark Energy Experiment (HETDEX). Star forming galaxies that occupy the lowest mass and metallicity end of the mass-metallicity relation tend to be under sampled in continuum-based surveys as their spectra are typically dominated by emission from newly forming stars. We search for galaxies with high [OIII]$λ$5007 / [OII]$λ$3727, implying highly ionized nebular emission often indicative of low metallicity systems. With the Second Generation Low Resolution Spectrograph on the Hobby Eberly Telescope we acquired follow-up spectra, with higher resolution and broader wavelength coverage, of each low-metallicity candidate in order to confirm the redshift, measure the H$α$ and [NII] line strengths and, in many cases, obtain deeper spectra of the blue lines. We find our galaxies are consistent with the mass-metallicity relation of typical low mass galaxies. However, galaxies in our sample tend to have similar specific star formation rates (sSFRs) as the incredibly rare "blueberry" galaxies found in (Yang et. al. 2017). We illustrate the power of spectroscopic surveys for finding low mass and metallicity galaxies and reveal that we find a sample of galaxies that are a hybrid between the properties of typical dwarf galaxies and the more extreme blueberry galaxies.

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arXiv2021

WALLABY Pilot Survey: the diversity of ram pressure stripping of the galactic HI gas in the Hydra Cluster

This study uses HI image data from the WALLABY pilot survey with the ASKAP telescope, covering the Hydra cluster out to 2.5$r_{200}$. We present the projected phase-space distribution of HI-detected galaxies in Hydra, and identify that nearly two thirds of the galaxies within $1.25r_{200}$ may be in the early stages of ram pressure stripping. More than half of these may be only weakly stripped, with the ratio of strippable HI (i.e., where the galactic restoring force is lower than the ram pressure in the disk) mass fraction (over total HI mass) distributed uniformly below 90%. Consequently, the HI mass is expected to decrease by only a few 0.1 dex after the currently strippable portion of HI in these systems has been stripped. A more detailed look at the subset of galaxies that are spatially resolved by WALLABY observations shows that, while it typically takes less than 200 Myr for ram pressure stripping to remove the currently strippable portion of HI, it may take more than 600 Myr to significantly change the total HI mass. Our results provide new clues to understanding the different rates of HI depletion and star formation quenching in cluster galaxies.

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arXiv2021

Star Formation Histories from SEDs and CMDs Agree: Evidence for Synchronized Star Formation in Local Volume Dwarf Galaxies over the Past 3 Gyr

Star Formation Histories (SFHs) reveal physical processes that influence how galaxies form their stellar mass. We compare the SFHs of a sample of 36 nearby (D $\leq$ 4 Mpc) dwarf galaxies from the ACS Nearby Galaxy Survey Treasury (ANGST), inferred from the Color Magnitude Diagrams (CMDs) of individually resolved stars in these galaxies, with those reconstructed by broad-band Spectral Energy Distribution (SED) fitting using the Dense Basis SED fitting code. When comparing individual SFHs, we introduce metrics for evaluating SFH reconstruction techniques. For both the SED and CMD methods, the median normalized SFH of galaxies in the sample shows a period of quiescence at lookback times of 3-6 Gyr followed by rejuvenated star formation over the past 3 Gyr that remains active until the present day. To determine if these represent special epochs of star formation in the D $\leq$ 4 Mpc portion of the Local Volume, we break this ANGST dwarf galaxy sample into subsets based on specific star formation rate and spatial location. Modulo offsets between the methods of about 1 Gyr, all subsets show significant decreases and increases in their median normalized SFHs at the same epochs, and the majority of the individual galaxy SFHs are consistent with these trends. These results motivate further study of potential synchronized star formation quiescence and rejuvenation in the Local Volume as well as development of a hybrid method of SFH reconstruction that combines CMDs and SEDs, which have complementary systematics.

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arXiv2020

Exploring Chemical Homogeneity in Dwarf Galaxies: A VLT-MUSE study of JKB 18

Deciphering the distribution of metals throughout galaxies is fundamental in our understanding of galaxy evolution. Nearby, low-metallicity, star-forming dwarf galaxies in particular can offer detailed insight into the metal-dependent processes that may have occurred within galaxies in the early Universe. Here we present VLT/MUSE observations of one such system, JKB 18, a blue diffuse dwarf galaxy with a metallicity of only 12+log(O/H)=7.6$\pm$0.2 (~0.08 Zsol). Using high spatial-resolution integral field spectroscopy of the entire system, we calculate chemical abundances for individual HII regions using the direct method and derive oxygen abundance maps using strong-line metallicity diagnostics. With large-scale dispersions in O/H, N/H and N/O of ~0.5-0.6 dex and regions harbouring chemical abundances outside this 1$σ$ distribution, we deem JKB 18 to be chemically inhomogeneous. We explore this finding in the context of other chemically inhomogeneous dwarf galaxies and conclude that neither the accretion of metal-poor gas, short mixing timescales, or self-enrichment from Wolf-Rayet stars are accountable. Using a galaxy-scale, multi-phase, hydrodynamical simulation of a low-mass dwarf galaxy, we find that chemical inhomogeneities of this level may be attributable to the removal of gas via supernovae and the specific timing of the observations with respect to star-formation activity. This study not only draws attention to the fact that dwarf galaxies can be chemically inhomogeneous, but also that the methods used in the assessment of this characteristic can be subject to bias.

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arXiv2019

Galactic Winds in Low-Mass Galaxies

Stellar-feedback driven outflows are predicted to play a fundamental role in the baryon cycle of low-mass galaxies. However, observational constraints of winds in nearby dwarf galaxies are limited as outflows are transient, intrinsically low-surface brightness features, and, thus, difficult to detect. Using deep Hapha observations, we search for winds in a sample of twelve nearby dwarfs (M_* ~ 10^7 - 10^9.3 Msun) which host on-going or recent starbursts. We detect features which we classify as winds in 6 galaxies, fountain candidates in 5 galaxies, and diffuse ISM in 1 system. Winds are found preferentially in galaxies with centrally concentrated star formation, while fountains are found in galaxies with spatially distributed star formation. We suggest that the concentration of star formation is a predictor for whether a low-mass galaxy will develop a wind. The spatial extent of all detected ionized gas is limited (<1/10 virial radius) and would still be considered the ISM by cosmological simulations. Our observations suggest that the majority of material expelled from dwarfs does not escape to the intergalactic medium but remains in the halo and may be recycled to the galaxies. Derived mass-loading factors range from 0.2-7 (with only a weak dependency on circular velocity or stellar mass), in tension with higher values in simulations needed to reproduce realistic low-mass galaxies and resolve discrepancies with LambdaCDM. The sample is part of the panchromatic STARBurst IRegular Dwarf Survey - STARBIRDS - designed to characterize the starburst phenomenon in dwarf galaxies. We also report a previously uncatalogued nearby galaxy (J1118+7913).

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arXiv2019

The Chemical Evolution of Carbon, Nitrogen, and Oxygen in Metal-Poor Dwarf Galaxies

Ultraviolet nebular emission lines are important for understanding the time evolution and nucleosynthetic origins of their associated elements, but the underlying trends of their relative abundances are unclear. We present UV spectroscopy of 20 nearby low-metallicity, high-ionization dwarf galaxies obtained using the Hubble Space Telescope. Building upon previous studies, we analyze the C/O relationship for a combined sample of 40 galaxies with significant detections of the UV O+2/C+2 collisionally-excited lines and direct-method oxygen abundance measurements. Using new analytic carbon ionization correction factor relationships, we confirm the flat trend in C/O versus O/H observed for local metal-poor galaxies. We find an average log(C/O) = -0.71 with an intrinsic dispersion of σ = 0.17 dex. The C/N ratio also appears to be constant at log(C/N) = 0.75, plus significant scatter (σ = 0.20 dex), with the result that carbon and nitrogen show similar evolutionary trends. This large and real scatter in C/O over a large range in O/H implies that measuring the UV C and O emission lines alone does not provide a reliable indicator of the O/H abundance. By modeling the chemical evolution of C, N, and O of individual targets, we find that the C/O ratio is very sensitive to both the detailed star formation history and to supernova feedback. Longer burst durations and lower star formation efficiencies correspond to low C/O ratios, while the escape of oxygen atoms in supernovae winds produces decreased effective oxygen yields and larger C/O ratios. Further, a declining C/O relationship is seen with increasing baryonic mass due to increasing effective oxygen yields.

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arXiv2019

The Enigmatic (Almost) Dark Galaxy Coma P: Distance Measurement and Stellar Populations from HST Imaging

We present Hubble Space Telescope (HST) observations of the low surface brightness (SB) galaxy Coma P. This system was first discovered in the Arecibo Legacy Fast ALFA HI survey and was cataloged as an (almost) dark galaxy because it did not exhibit any obvious optical counterpart in the available survey data (e.g., Sloan Digital Sky Survey). Subsequent WIYN pODI imaging revealed an ultra-low SB stellar component located at the center of the HI detection. We use the HST images to produce a deep color-magnitude diagram (CMD) of the resolved stellar population present in Coma P. We clearly detect a red stellar sequence that we interpret to be a red giant branch, and use it to infer a tip of the red giant branch (TRGB) distance of 5.50$^{+0.28}_{-0.53}$ Mpc. The new distance is substantially lower than earlier estimates and shows that Coma P is an extreme dwarf galaxy. Our derived stellar mass is only 4.3 $\times$ 10$^5$ $M_\odot$, meaning that Coma P has an extreme HI-to-stellar mass ratio of 81. We present a detailed analysis of the galaxy environment within which Coma P resides. We hypothesize that Coma P formed within a local void and has spent most of its lifetime in a low-density environment. Over time, the gravitational attraction of the galaxies located in the void wall has moved it to the edge, where it had a recent "fly-by" interaction with M64. We investigate the possibility that Coma P is at a farther distance and conclude that the available data are best fit by a distance of 5.5 Mpc.

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arXiv2018

Delayed Stellar Mass Assembly in the Low Surface Brightness Dwarf Galaxy KDG215

We present HI spectral line and optical broadband images of the nearby low surface brightness dwarf galaxy KDG215. The HI images, acquired with the Karl G. Jansky Very Large Array (VLA), reveal a dispersion dominated ISM with only weak signatures of coherent rotation. The HI gas reaches a peak mass surface density of 6 M$_{\odot}$ pc$^{-2}$ at the location of the peak surface brightness in the optical and the UV. Although KDG215 is gas-rich, the H$α$ non-detection implies a very low current massive star formation rate. In order to investigate the recent evolution of this system, we have derived the recent and lifetime star formation histories from archival Hubble Space Telescope images. The recent star formation history shows a peak star formation rate $\sim$1 Gyr ago, followed by a decreasing star formation rate to the present day quiescent state. The cumulative star formation history indicates that a significant fraction of the stellar mass assembly in KDG215 has occurred within the last 1.25 Gyr. KDG215 is one of only a few known galaxies which demonstrates such a delayed star formation history. While the ancient stellar population (predominantly red giants) is prominent, the look-back time by which 50% of the mass of all stars ever formed had been created is among the youngest of any known galaxy.

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