arXiv subjects

Jeremy Bailin

Publications and source records attributed to Jeremy Bailin.

At least 55 records · Page 3Linked to original sources

arXiv2018

Search for a metallicity spread in the multiple population Large Magellanic Cloud cluster NGC 1978

We report on the spread of [Fe/H] values in the massive Large Magellanic Cloud cluster NGC1978, recently confirmed to harbor multiple populations of nearly the same age. We used accurate Stromgren photometry of carefully selected cluster red giant branch stars along with a high-dispersion spectroscopy-based calibration of the metallicity-sensitive index m1. Once accounting for the photometry quality, assessed from extensive artificial stars tests to trace the photometric uncertainties as a function of the position to the cluster's center as well as the stellar brightness, and those from the metallicity calibration, we found that NGC1978 exhibits a small metallicity spread of 0.035 dex (+- 0.019-0.023), depending on whether stars with individual sigma[Fe/H] < 0.15 dex or those located in the cluster's outer areas are considered. Such a spread in [Fe/H] is consistent with a cluster formation model with self-enrichment, if mass loss higher than 90 per cent due to stellar evolutionary and galactic tidal effects is assumed. Nevertheless, scenarios where the apparent [Fe/H] variation reflects CN abundance anomalies or less extreme mass loss models with environmentally-dependent self enrichment should not be ruled out.

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arXiv2018

A Model for Clumpy Self-Enrichment in Globular Clusters

Detailed observations of globular clusters (GCs) have revealed evidence of self-enrichment: some of the heavy elements that we see in stars today were produced by cluster stars themselves. Moreover, GCs have internal subpopulations with different elemental abundances, including, in some cases, in elements such as iron that are produced by supernovae. This paper presents a theoretical model for GC formation motivated by observations of Milky Way star forming regions and simulations of star formation, where giant molecular clouds fragment into multiple clumps which undergo star formation at slightly different times. Core collapse supernovae from earlier-forming clumps can enrich later-forming clumps to the degree that the ejecta can be retained within the gravitational potential well, resulting in subpopulations with different total metallicities once the clumps merge to form the final cluster. The model matches the mass-metallicity relation seen in GC populations around massive elliptical galaxies, and predicts metallicity spreads within clusters in excellent agreement with those seen in Milky Way GCs, even for those whose internal abundance spreads are so large that their entire identity as a GC is in question. The internal metallicity spread serves as an excellent measurement of how much self-enrichment has occurred in a cluster, a result that is very robust to variation in the model parameters.

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arXiv2017

d1005+68: A New Faint Dwarf Galaxy in the M81 Group

We present the discovery of d1005+68, a new faint dwarf galaxy in the M81 Group, using observations taken with the Subaru Hyper Suprime-Cam. d1005+68's color-magnitude diagram is consistent with a distance of $3.98_{-0.43}^{+0.39}$ Mpc, establishing group membership. We derive an absolute $V$-band magnitude, from stellar isochrone fitting, of $M_{V} = -7.94_{-0.50}^{+0.38}$, with a half-light radius of $r_{h} = 188_{-41}^{+39}$ pc. These place d1005+68 within the radius-luminosity locus of Local Group and M81 satellites and among the faintest confirmed satellites outside the Local Group. Assuming an age of 12 Gyr, d1005+68's red giant branch is best fit by an isochrone of [Fe/H] $= -1.90 \pm 0.24$. It has a projected separation from nearby M81 satellite BK5N of only 5 kpc. As this is well within BK5N's virial radius, we speculate that d1005+68 may be a satellite of BK5N. If confirmed, this would make d1005+68 one of the first detected satellites-of-a-satellite.

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arXiv2017

Galaxies grow their bulges and black holes in diverse ways

Galaxies with Milky Way-like stellar masses have a wide range of bulge and black hole masses; in turn, these correlate with other properties such as star formation history. While many processes may drive bulge formation, major and minor mergers are expected to play a crucial role. Stellar halos offer a novel and robust measurement of galactic merger history; cosmologically-motivated models predict that mergers with larger satellites produce more massive, higher metallicity stellar halos, reproducing the recently-observed stellar halo metallicity-mass relation. We quantify the relationship between stellar halo mass and bulge or black hole prominence using a sample of eighteen Milky Way-mass galaxies with newly-available measurements of (or limits on) stellar halo properties. There is an order of magnitude range in bulge mass, and two orders of magnitude in black hole mass, at a given stellar halo mass (or, equivalently, merger history). Galaxies with low mass bulges show a wide range of quiet merger histories, implying formation mechanisms that do not require intense merging activity. Galaxies with massive 'classical' bulges and central black holes also show a wide range of merger histories. While three of these galaxies have massive stellar halos consistent with a merger origin, two do not - merging appears to have had little impact in making these two massive 'classical' bulges. Such galaxies may be ideal laboratories to study massive bulge formation through pathways such as early gas-rich accretion, violent disk instabilities or misaligned infall of gas throughout cosmic time.

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arXiv2016

Characterizing dw1335-29, a recently discovered dwarf satellite of M83

The number, distribution, and properties of dwarf satellites are crucial probes of the physics of galaxy formation at low masses and the response of satellite galaxies to the tidal and gas dynamical effects of their more massive parent.To make progress, it is necessary to augment and solidify the census of dwarf satellites of galaxies outside the Local Group. Müller et al. (2015) presented 16 dwarf galaxy candidates near M83, but lacking reliable distances, it is unclear which candidates are M83 satellites. Using red giant branch stars from the HST/GHOSTS survey in conjunction with ground-based images from VLT/VIMOS, we confirm that one of the candidates, dw1335-29-- with a projected distance of 26 kpc from M83 and a distance modulus of $(m - M)_0 = 28.5^{+0.3}_{-0.1}$ -- is a satellite of M83. We estimate an absolute magnitude $M_V = -10.1 \pm{0.4}$, an ellipticity of $0.40^{+0.14}_{-0.22}$, a half light radius of $656^{+121}_{-170 }$ pc, and [Fe/H] = $-1.3^{+0.3}_{-0.4}$. Owing to dw1335-29's somewhat irregular shape and possible young stars, we classify this galaxy as a dwarf irregular or transition dwarf. This is curious, as with a projected distance of 26 kpc from M83, dw1335-29 is expected to lack recent star formation. Further study of M83's dwarf population will reveal if star formation in its satellites is commonplace (suggesting a lack of a hot gas envelope for M83 that would quench star formation) or rare (suggesting that dw1335-29 has a larger M83-centric distance, and is fortuitously projected to small radii).

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arXiv2016

Diverse Stellar Haloes in Nearby Milky Way-Mass Disc Galaxies

We have examined the resolved stellar populations at large galactocentric distances along the minor axis (from 10 kpc up to between 40 and 75 kpc), with limited major axis coverage, of six nearby highly-inclined Milky Way-mass disc galaxies using HST data from the GHOSTS survey. We select red giant branch stars to derive stellar halo density profiles. The projected minor axis density profiles can be approximated by power laws with projected slopes of between $-2$ and $-3.7$ and a diversity of stellar halo masses of $1-6\times 10^{9}M_{\odot}$, or $2-14\%$ of the total galaxy stellar masses. The typical intrinsic scatter around a smooth power law fit is $0.05-0.1$ dex owing to substructure. By comparing the minor and major axis profiles, we infer projected axis ratios $c/a$ at $\sim 25$ kpc between $0.4-0.75$. The GHOSTS stellar haloes are diverse, lying between the extremes charted out by the (rather atypical) haloes of the Milky Way and M31. We find a strong correlation between the stellar halo metallicities and the stellar halo masses. We compare our results with cosmological models, finding good agreement between our observations and accretion-only models where the stellar haloes are formed by the disruption of dwarf satellites. In particular, the strong observed correlation between stellar halo metallicity and mass is naturally reproduced. Low-resolution hydrodynamical models have unrealistically high stellar halo masses. Current high-resolution hydrodynamical models appear to predict stellar halo masses somewhat higher than observed but with reasonable metallicities, metallicity gradients and density profiles.

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arXiv2016

The impact of baryons on the direct detection of dark matter

The spatial and velocity distributions of dark matter particles in the Milky Way Halo affect the signals expected to be observed in searches for dark matter. Results from direct detection experiments are often analyzed assuming a simple isothermal distribution of dark matter, the Standard Halo Model (SHM). Yet there has been skepticism regarding the validity of this simple model due to the complicated gravitational collapse and merger history of actual galaxies. In this paper we compare the SHM to the results of cosmological hydrodynamical simulations of galaxy formation to investigate whether or not the SHM is a good representation of the true WIMP distribution in the analysis of direct detection data. We examine two Milky Way-like galaxies from the MaGICC cosmological simulations (a) with dark matter only and (b) with baryonic physics included. The inclusion of baryons drives the shape of the DM halo to become more spherical and makes the velocity distribution of dark matter particles less anisotropic especially at large heliocentric velocities, thereby making the SHM a better fit. We also note that we do not find a significant disk-like rotating dark matter component in either of the two galaxy halos with baryons that we examine, suggesting that dark disks are not a generic prediction of cosmological hydrodynamical simulations. We conclude that in the Solar neighborhood, the SHM is in fact a good approximation to the true dark matter distribution in these cosmological simulations (with baryons) which are reasonable representations of the Milky Way, and hence can also be used for the purpose of dark matter direct detection calculations.

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arXiv2016

The GHOSTS survey. II. The diversity of Halo Color and Metallicity Profiles of Massive Disk Galaxies

We study the stellar halo color properties of six nearby massive highly inclined disk galaxies using Hubble Space Telescope Advanced Camera for Surveys and Wide Field Camera 3 observations in both F606W and F814W filters from the GHOSTS survey. The observed fields, placed both along the minor and major axis of each galaxy, probe the stellar outskirts out to projected distances of ~ 50-70 kpc from their galactic centre along the minor axis. The 50% completeness levels of the color magnitude diagrams are typically at two mag below the tip of the red giant branch. We find that all galaxies have extended stellar halos out to ~ 50 kpc and two out to ~ 70 kpc. We determined the halo color distribution and color profile for each galaxy using the median colors of stars in the RGB. Within each galaxy we find variations in the median colors as a function of radius which likely indicates population variations, reflecting that their outskirts were built from several small accreted objects. We find that half of the galaxies (NGC 0891, NGC 4565, and NGC 7814) present a clear negative color gradient, reflecting a declining metallicity in their halos; the other have no significant color or population gradient. In addition, notwithstanding the modest sample size of galaxies, there is no strong correlation between their halo color/metallicity or gradient with galaxy's properties such as rotational velocity or stellar mass. The diversity in halo color profiles observed in the GHOSTS galaxies qualitatively supports the predicted galaxy-to-galaxy scatter in halo stellar properties; a consequence of the stochasticity inherent in the assembling history of galaxies.

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arXiv2015

Origin of the Metallicity Distribution in the Thick Disc

Aims. Using a suite of cosmological chemodynamical disc galaxy simulations, we assess how (a) radial metallicity gradients evolve with scaleheight; (b) the vertical metallicity gradients change through the thick disc; and (c) the vertical gradient of the stellar rotation velocity varies through the disc. We compare with the Milky Way to search for analogous trends. Methods. We analyse five simulated spiral galaxies with masses comparable to the Milky Way. The simulations span a range of star formation and energy feedback strengths and prescriptions, particle- and grid-based hydrodynamical implementations, as well as initial conditions/assembly history. Results. Consistently, we find that the steeper, negative, radial metallicity gradients seen in the mid-plane flatten with increasing height away from the plane. In simulations with stronger (and/or more spatially-extended) feedback, the negative radial gradients invert, becoming positive for heights in excess of 1 kpc. Such behaviour is consistent with that inferred from recent observations. Our measurements of the vertical metallicity gradients show no clear correlation with galactocentric radius, and are in good agreement with those observed in the Milky Way's thick disc (locally). Conclusions. Simulations employing stronger/more extended feedback prescriptions possess radial and vertical metallicity and kinematic gradients more in line with recent observations. The inverted, positive, radial metallicity gradients seen in the simulated thick stellar discs originate from a population of younger, more metal-rich, stars formed in-situ, superimposed upon a background population of older migrators from the inner disc; the contrast provided by the former increases radially, due to the inside-out growth of the disc. A similar behaviour may be responsible for the same flattening seen in the radial gradients with scaleheight in the Milky Way.

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arXiv2015

Stellar orbital properties as diagnostics of the origin of the stellar halo

We examine metallicities, ages and orbital properties of halo stars in a Milky-Way like disk galaxy formed in the cosmological hydrodynamical MaGICC simulations. Halo stars were either accreted from satellites or they formed in situ in the disk or bulge of the galaxy and were then kicked up into the halo ("in situ/ kicked-up" stars). Regardless of where they formed both types show surprisingly similar orbital properties: the majority of stars of both types are on short-axis tubes with the same sense of rotation as the disk -- implying that a large fraction of satellites are accreted onto the halo with the same sense of angular momentum as the disk.

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arXiv2015

Extragalactic archeology with the GHOSTS Survey I. - Age-resolved disk structure of nearby low-mass galaxies

We study the individual evolution histories of three nearby low-mass edge-on galaxies (IC 5052, NGC4244, and NGC5023). Using resolved stellar populations, we constructed star count density maps for populations of different ages and analyzed the change of structural parameters with stellar age within each galaxy. We do not detect a separate thick disk in any of the three galaxies, even though our observations cover a wider range in equivalent surface brightness than any integrated light study. While scale heights increase with age, each population can be well described by a single disk. Two of the galaxies contain a very weak additional component, which we identify as the faint halo. The mass of these faint halos is lower than 1% of the mass of the disk. The three galaxies show low vertical heating rates, which are much lower than the heating rate of the Milky Way. This indicates that heating agents, such as giant molecular clouds and spiral structure, are weak in low-mass galaxies. All populations in the three galaxies exhibit no or only little flaring. While this finding is consistent with previous integrated light studies, it poses strong constraints on galaxy simulations, where strong flaring is often found as a result of interactions or radial migration.

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arXiv2014

Globular Cluster Systems in Brightest Cluster Galaxies: A Near-Universal Luminosity Function?

We present the first results from our HST Brightest Cluster Galaxy (BCG) survey of seven central supergiant cluster galaxies and their globular cluster (GC) systems. We measure a total of 48000 GCs in all seven galaxies, representing the largest single GC database. We find that a log-normal shape accurately matches the observed luminosity function (LF) of the GCs down to the GCLF turnover point, which is near our photometric limit. In addition, the LF has a virtually identical shape in all seven galaxies. Our data underscore the similarity in the formation mechanism of massive star clusters in diverse galactic environments. At the highest luminosities (log L > 10^7 L_Sun) we find small numbers of "superluminous" objects in five of the galaxies; their luminosity and color ranges are at least partly consistent with those of UCDs (Ultra-Compact Dwarfs). Lastly, we find preliminary evidence that in the outer halo (R > 20 kpc), the LF turnover point shows a weak dependence on projected distance, scaling as L_0 ~ R^-0.2, while the LF dispersion remains nearly constant.

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arXiv2014

Discovery of a Gas-Rich Companion to the Extremely Metal-Poor Galaxy DDO 68

We present HI spectral-line imaging of the extremely metal-poor galaxy DDO 68. This system has a nebular oxygen abundance of only 3% Z$_{\odot}$, making it one of the most metal-deficient galaxies known in the local volume. Surprisingly, DDO 68 is a relatively massive and luminous galaxy for its metal content, making it a significant outlier in the mass-metallicity and luminosity-metallicity relationships. The origin of such a low oxygen abundance in DDO 68 presents a challenge for models of the chemical evolution of galaxies. One possible solution to this problem is the infall of pristine neutral gas, potentially initiated during a gravitational interaction. Using archival HI spectral-line imaging obtained with the Karl G. Jansky Very Large Array, we have discovered a previously unknown companion of DDO 68. This low-mass (M$_{\rm HI}$ $=$ 2.8$\times$10$^{7}$ M$_{\odot}$), recently star-forming (SFR$_{\rm FUV}$ $=$ 1.4$\times$10$^{-3}$ M$_{\odot}$ yr$^{-1}$, SFR$_{\rm Hα}$ $<$ 7$\times$10$^{-5}$ M$_{\odot}$ yr$^{-1}$) companion has the same systemic velocity as DDO 68 (V$_{\rm sys}$ $=$ 506 km s$^{-1}$; D $=$ 12.74$\pm$0.27 Mpc) and is located at a projected distance of 42 kpc. New HI maps obtained with the 100m Robert C. Byrd Green Bank Telescope provide evidence that DDO 68 and this companion are gravitationally interacting at the present time. Low surface brightness HI gas forms a bridge between these objects.

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arXiv2014

Systematic Problems With Using Dark Matter Simulations to Model Stellar Halos

The limits of available computing power have forced models for the structure of stellar halos to adopt one or both of the following simplifying assumptions: (1) stellar mass can be "painted" onto dark matter particles in progenitor satellites; (2) pure dark matter simulations that do not form a luminous galaxy can be used. We estimate the magnitude of the systematic errors introduced by these assumptions using a controlled set of stellar halo models where we independently vary whether we look at star particles or painted dark matter particles, and whether we use a simulation in which a baryonic disk galaxy forms or a matching pure dark matter simulation that does not form a baryonic disk. We find that the "painting" simplification reduces the halo concentration and internal structure, predominantly because painted dark matter particles have different kinematics than star particles even when both are buried deep in the potential well of the satellite. The simplification of using pure dark matter simulations reduces the concentration further, but increases the internal structure, and results in a more prolate stellar halo. These differences can be a factor of 1.5-7 in concentration (as measured by the half-mass radius) and 2-7 in internal density structure. Given this level of systematic uncertainty, one should be wary of overinterpreting differences between observations and the current generation of stellar halo models based on dark matter only simulations when such differences are less than an order of magnitude.

astro-ph.CO↗

arXiv2014

On the relation between metallicity and RGB color in HST/ACS data

The determination of stellar metallicity and its gradient in external galaxies is a difficult task, but crucial for the understanding of galaxy formation and evolution. The color of the Red Giant Branch (RGB) can be used to determine metallicities of stellar populations that have only shallow photometry. We will quantify the relation between metallicity and color in the widely used HST ACS filters F606W and F814W. We use a sample of globular clusters from the ACS Globular Cluster Survey and measure their RGB color at given absolute magnitudes to derive the color-metallicity relation. We especially investigate the scatter and the uncertainties in this relation and show its limitations. There is a clear relation between metallicity and RGB color. A comparison with isochrones shows reasonably good agreement with BaSTI models, a small offset to Dartmouth models, and a larger offset to Padua models. Even for the best globular cluster data available, the metallicity of a simple stellar population can be determined from the RGB alone only with an accuracy of 0.3dex for [M/H]< -1, and 0.15dex for [M/H]> -1. For mixed populations, as they are observed in external galaxies, the uncertainties will be even larger due to uncertainties in extinction, age, etc. Therefore caution is necessary when interpreting photometric metallicities.

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arXiv2013

Constraining the age of the NGC 4565 HI Disk Warp: Determining the Origin of Gas Warps

We have mapped the distribution of young and old stars in the gaseous HI warp of NGC 4565. We find a clear correlation of young stars (<600 Myr) with the warp, but no coincident old stars (>1 Gyr), which places an upper limit on the age of the structure. The formation rate of the young stars, which increased ~300 Myr ago relative to the surrounding regions, is (6.3 +2.5/-1.5) x 10^-5 M_sol/yr/kpc^2. This implies a ~60+/-20 Gyr depletion time of the HI warp, similar to the timescales calculated for the outer HI disks of nearby spiral galaxies. While some stars associated with the warp fall into the asymptotic giant branch (AGB) region of the color magnitude diagram, where stars could be as old as 1 Gyr, further investigation suggests that they may be interlopers rather than real AGB stars. We discuss the implications of these age constraints for the formation of HI warps, and the gas fueling of disk galaxies.

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arXiv2013

GHOSTS I: A New Faint very Isolated Dwarf Galaxy at D = 12 +/- 2 Mpc

We report the discovery of a new faint dwarf galaxy, GHOSTS I, using HST/ACS data from one of our GHOSTS (Galaxy Halos, Outer disks, Substructure, Thick disk, and Star clusters) fields. Its detected individual stars populate an approximately one magnitude range of its luminosity function (LF). Using synthetic color-magnitude diagrams (CMDs) to compare with the galaxy's CMD, we find that the colors and magnitudes of GHOSTS I's individual stars are most consistent with being young helium-burning and asymptotic giant branch stars at a distance of 12 +/- 2 Mpc. Morphologically, GHOSTS I appears to be actively forming stars, so we tentatively classify it as a dwarf irregular (dIrr) galaxy, although future HST observations deep enough to resolve a larger magnitude range in its LF are required to make a more secure classification. GHOSTS I's absolute magnitude is $M_V = -9.85^{+ 0.40}_{- 0.33}$, making it one of the least luminous dIrr galaxies known, and its metallicity is lower than [Fe/H] =-1.5 dex. The half-light radius of GHOSTS I is 226 +/- 38 pc and its ellipticity is 0.47 +/- 0.07, similar to Milky Way and M31 dwarf satellites at comparable luminosity. There are no luminous massive galaxies or galaxy clusters within ~ 4 Mpc from GHOSTS I that could be considered as its host, making it a very isolated dwarf galaxy in the Local Universe.

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arXiv2013

Halo orbits in cosmological disk galaxies: tracers of formation history

We analyze the orbits of stars and dark matter particles in the halo of a disk galaxy formed in a cosmological hydrodynamical simulation. The halo is oblate within the inner ~20 kpc and triaxial beyond this radius. About 43% of orbits are short axis tubes - the rest belong to orbit families that characterize triaxial potentials (boxes, long-axis tubes and chaotic orbits), but their shapes are close to axisymmetric. We find no evidence that the self-consistent distribution function of the nearly oblate inner halo is comprised primarily of axisymmetric short-axis tube orbits. Orbits of all families, and both types of particles are highly eccentric with mean eccentricity >0.6. We find that randomly selected samples of halo stars show no substructure in "integrals of motion" space. However individual accretion events can be clearly identified in plots of metallicity versus formation time. Dynamically young tidal debris is found primarily on a single type of orbit. However, stars associated with older satellites become chaotically mixed during the formation process (possibly due to scattering by the central bulge and disk, and baryonic processes), and appear on all four types of orbits. We find that the tidal debris in cosmological hydrodynamical simulations experiences significantly more chaotic evolution than in collisionless simulations, making it much harder to identify individual progenitors using phase space coordinates alone. However by combining information on stellar ages and chemical abundances with the orbital properties of halo stars in the underlying self-consistent potential, the identification of progenitors is likely to be possible.

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