arXiv subjects

Eric F. Bell

Publications and source records attributed to Eric F. Bell.

At least 199 records · Page 11Linked to original sources

arXiv2014

Serendipitous Discovery of a Thin Stellar Stream near the Galactic Bulge in the Pan-STARRS1 3Pi Survey

We report the discovery of a thin stellar stream found in Pan-STARRS1 photometry near the Galactic bulge in the constellation of Ophiuchus. It appears as a coherent structure in the colour-selected stellar density maps produced to search for tidal debris around nearby globular clusters. The stream is exceptionally short and narrow; it is about 2.5° long and 6' wide in projection. The colour-magnitude diagram of this object, which harbours a blue horizontal-branch, is consistent with an old and relatively metal-poor population ([Fe/H]~-1.3) located 9.5 +/- 0.9 kpc away at (l,b) ~ (5°,+32°), and 5.0 +/- 1.0 kpc from the Galactic centre. These properties argue for a globular cluster as progenitor. The finding of such a prominent, nearby stream suggests that many streams could await discovery in the more densely populated regions of our Galaxy.

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arXiv2014

A new population of recently quenched elliptical galaxies in the SDSS

We use the Sloan Digital Sky Survey to investigate the properties of massive elliptical galaxies in the local Universe (z\leq0.08) that have unusually blue optical colors. Through careful inspection, we distinguish elliptical from non-elliptical morphologies among a large sample of similarly blue galaxies with high central light concentrations (c_r\geq2.6). These blue ellipticals comprise 3.7 per cent of all c_r\geq2.6 galaxies with stellar masses between 10^10 and 10^11 h^{-2} {\rm M}_{\sun}. Using published fiber spectra diagnostics, we identify a unique subset of 172 non-star-forming ellipticals with distinctly blue urz colors and young (< 3 Gyr) light-weighted stellar ages. These recently quenched ellipticals (RQEs) have a number density of 2.7-4.7\times 10^{-5}\,h^3\,{\rm Mpc}^{-3} and sufficient numbers above 2.5\times10^{10} h^{-2} {\rm M}_{\sun} to account for more than half of the expected quiescent growth at late cosmic time assuming this phase lasts 0.5 Gyr. RQEs have properties that are consistent with a recent merger origin (i.e., they are strong `first-generation' elliptical candidates), yet few involved a starburst strong enough to produce an E+A signature. The preferred environment of RQEs (90 per cent reside at the centers of < 3\times 10^{12}\,h^{-1}{\rm M}_{\sun} groups) agrees well with the `small group scale' predicted for maximally efficient spiral merging onto their halo center and rules out satellite-specific quenching processes. The high incidence of Seyfert and LINER activity in RQEs and their plausible descendents may heat the atmospheres of small host halos sufficiently to maintain quenching.

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arXiv2014

Galactic Globular and Open Cluster Fiducial Sequences in the Pan-STARRS1 Photometric System

We present the fiducial sequences of a sample of Galactic star clusters in the five bands of the Pan-STARRS1 (PS1) photometric system (g_P1, r_P1, i_P1, z_P1, y_P1). These empirical sequences -- which include the red giant and sub-giant branches, the main sequence, and the horizontal branch -- were defined from deep colour-magnitude diagrams reaching below the oldest main-sequence turn-offs of 13 globular and 3 old open clusters covering a wide range of metallicities (-2.4 < [Fe/H] < +0.4). We find excellent agreement for the nine clusters in common with previous studies in similar photometric systems when transformed to the PS1 system. Because the photometric and spectroscopic properties of these stellar populations are accurately known, the fiducials provide a solid basis for the interpretation of observations in the PS1 system, as well as valuable constraints to improve the empirical colour--$T_{eff}$ relations.

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arXiv2014

The Panchromatic Hubble Andromeda Treasury. VI. The reliability of far-ultraviolet flux as a star formation tracer on sub-kpc scales

We have used optical observations of resolved stars from the Panchromatic Hubble Andromeda Treasury (PHAT) to measure the recent (< 500 Myr) star formation histories (SFHs) of 33 FUV-bright regions in M31. The region areas ranged from ~$10^4$ to $10^6$ pc$^2$, which allowed us to test the reliability of FUV flux as a tracer of recent star formation on sub-kpc scales. The star formation rates (SFRs) derived from the extinction-corrected observed FUV fluxes were, on average, consistent with the 100-Myr mean SFRs of the SFHs to within the 1$σ$ scatter. Overall, the scatter was larger than the uncertainties in the SFRs and particularly evident among the smallest regions. The scatter was consistent with an even combination of discrete sampling of the initial mass function and high variability in the SFHs. This result demonstrates the importance of satisfying both the full-IMF and the constant-SFR assumptions for obtaining precise SFR estimates from FUV flux. Assuming a robust FUV extinction correction, we estimate that a factor of 2.5 uncertainty can be expected in FUV-based SFRs for regions smaller than $10^5$ pc$^2$, or a few hundred pc. We also examined ages and masses derived from UV flux under the common assumption that the regions are simple stellar populations (SSPs). The SFHs showed that most of the regions are not SSPs, and the age and mass estimates were correspondingly discrepant from the SFHs. For those regions with SSP-like SFHs, we found mean discrepancies of 10 Myr in age and a factor of 3 to 4 in mass. It was not possible to distinguish the SSP-like regions from the others based on integrated FUV flux.

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arXiv2014

Bulge Growth and Quenching since z = 2.5 in CANDELS/3D-HST

Exploiting the deep high-resolution imaging of all 5 CANDELS fields, and accurate redshift information provided by 3D-HST, we investigate the relation between structure and stellar populations for a mass-selected sample of 6764 galaxies above 10^10 Msun, spanning the redshift range 0.5 < z < 2.5. For the first time, we fit 2-dimensional models comprising a single Sersic fit and two-component (i.e., bulge + disk) decompositions not only to the H-band light distributions, but also to the stellar mass maps reconstructed from resolved stellar population modeling. We confirm that the increased bulge prominence among quiescent galaxies, as reported previously based on rest-optical observations, remains in place when considering the distributions of stellar mass. Moreover, we observe an increase of the typical Sersic index and bulge-to-total ratio (with median B/T reaching 40-50%) among star-forming galaxies above 10^11 Msun. Given that quenching for these most massive systems is likely to be imminent, our findings suggest that significant bulge growth precedes a departure from the star-forming main sequence. We demonstrate that the bulge mass (and ideally knowledge of the bulge and total mass) is a more reliable predictor of the star-forming versus quiescent state of a galaxy than the total stellar mass. The same trends are predicted by the state-of-the-art semi-analytic model by Somerville et al. In the latter, bulges and black holes grow hand in hand through merging and/or disk instabilities, and AGN-feedback shuts off star formation. Further observations will be required to pin down star formation quenching mechanisms, but our results imply they must be internal to the galaxies and closely associated with bulge growth.

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arXiv2014

A new distant Milky Way globular cluster in the Pan-STARRS1 3π survey

We present a new satellite in the outer halo of the Galaxy, the first Milky Way satellite found in the stacked photometric catalog of the Panoramic Survey Telescope and Rapid Response System 1 (Pan-STARRS1) Survey. From follow-up photometry obtained with WFI on the MPG/ESO 2.2m telescope, we argue that the object, located at a heliocentric distance of 145+/-17 kpc, is the most distant Milky Way globular cluster yet known. With a total magnitude of M_V=-4.3+/-0.2 and a half-light radius of 20+/-2 pc, it shares the properties of extended globular clusters found in the outer halo of our Galaxy and the Andromeda galaxy. The discovery of this distant cluster shows that the full spatial extent of the Milky Way globular cluster system has not yet been fully explored.

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arXiv2014

The Panchromatic Hubble Andromeda Treasury V: Ages and Masses of the Year 1 Stellar Clusters

We present ages and masses for 601 star clusters in M31 from the analysis of the six filter integrated light measurements from near ultraviolet to near infrared wavelengths, made as part of the Panchromatic Hubble Andromeda Treasury (PHAT). We derive the ages and masses using a probabilistic technique, which accounts for the effects of stochastic sampling of the stellar initial mass function. Tests on synthetic data show that this method, in conjunction with the exquisite sensitivity of the PHAT observations and their broad wavelength baseline, provides robust age and mass recovery for clusters ranging from $\sim 10^2 - 2 \times 10^6 M_\odot$. We find that the cluster age distribution is consistent with being uniform over the past $100$ Myr, which suggests a weak effect of cluster disruption within M31. The age distribution of older ($>100$ Myr) clusters fall towards old ages, consistent with a power-law decline of index $-1$, likely from a combination of fading and disruption of the clusters. We find that the mass distribution of the whole sample can be well-described by a single power-law with a spectral index of $-1.9 \pm 0.1$ over the range of $10^3-3 \times 10^5 M_\odot$. However, if we subdivide the sample by galactocentric radius, we find that the age distributions remain unchanged. However, the mass spectral index varies significantly, showing best fit values between $-2.2$ and $-1.8$, with the shallower slope in the highest star formation intensity regions. We explore the robustness of our study to potential systematics and conclude that the cluster mass function may vary with respect to environment.

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arXiv2014

Bulgeless Galaxies at Intermediate Redshift: Sample Selection, Colour Properties, and the Existence of Powerful AGN

We present a catalogue of bulgeless galaxies, which includes 19225 objects selected in four of the deepest, largest multi-wavelength datasets available -- COSMOS, AEGIS, GEMS and GOODS -- at intermediate redshift ($0.4 \leq z \leq 1.0$). The morphological classification was provided by the Advanced Camera for Surveys General Catalogue (ACS-GC), which used publicly available data obtained with the ACS instrument on the Hubble Space Telescope. Rest-frame photometric quantities were derived using kcorrect. We analyse the properties of the sample and the evolution of pure-disc systems with redshift. Very massive [$\log (M_\star/M_{\odot}) > 10.5$] bulgeless galaxies contribute to ~30% of the total galaxy population number density at $z \geq 0.7$, but their number density drops substantially with decreasing redshift. We show that only a negligible fraction of pure discs appear to be quiescent systems, and red sequence bulgeless galaxies show indications of dust-obscured star formation. X-ray catalogues were used to search for X-ray emission within our sample. After visual inspection and detailed parametric morphological fitting we identify 30 AGN that reside in galaxies without a classical bulge. The finding of such peculiar objects at intermediate redshift shows that while AGN growth in merger-free systems is a rare event (0.2% AGN hosts in this sample of bulgeless galaxies), it can indeed happen relatively early in the Universe history.

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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↗

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

Galaxy Zoo: Observing Secular Evolution Through Bars

In this paper, we use the Galaxy Zoo 2 dataset to study the behavior of bars in disk galaxies as a function of specific star formation rate (SSFR), and bulge prominence. Our sample consists of 13,295 disk galaxies, with an overall (strong) bar fraction of $23.6\pm 0.4\%$, of which 1,154 barred galaxies also have bar length measurements. These samples are the largest ever used to study the role of bars in galaxy evolution. We find that the likelihood of a galaxy hosting a bar is anti-correlated with SSFR, regardless of stellar mass or bulge prominence. We find that the trends of bar likelihood and bar length with bulge prominence are bimodal with SSFR. We interpret these observations using state-of-the-art simulations of bar evolution which include live halos and the effects of gas and star formation. We suggest our observed trends of bar likelihood with SSFR are driven by the gas fraction of the disks; a factor demonstrated to significantly retard both bar formation and evolution in models. We interpret the bimodal relationship between bulge prominence and bar properties as due to the complicated effects of classical bulges and central mass concentrations on bar evolution, and also to the growth of disky pseudobulges by bar evolution. These results represent empirical evidence for secular evolution driven by bars in disk galaxies. This work suggests that bars are not stagnant structures within disk galaxies, but are a critical evolutionary driver of their host galaxies in the local universe ($z<1$).

astro-ph.CO↗

arXiv2013

Evidence for an Interaction in the Nearest Starbursting Dwarf Irregular Galaxy IC 10

Using deep 21-cm HI data from the Green Bank Telescope we have detected an ~18.3 kpc-long gaseous extension associated with the starbursting dwarf galaxy IC 10. The newly-found feature stretches 1.3 deg to the northwest and has a large radial velocity gradient reaching to ~65 km/s lower than the IC 10 systemic velocity. A region of higher column density at the end of the extension that possesses a coherent velocity gradient (~10 km/s across ~26 arcmin) transverse to the extension suggests rotation and may be a satellite galaxy of IC 10. The HI mass of IC 10 is 9.5x10^7 (d/805 kpc)^2 Msun and the mass of the new extension is 7.1x10^5 (d/805 kpc)^2 Msun. An IC 10-M31 orbit using known radial velocity and proper motion values for IC 10 show that the HI extension is inconsistent with the trailing portion of the orbit so that an M31-tidal or ram pressure origin seems unlikely. We argue that the most plausible explanation for the new feature is that it is the result of a recent interaction (and possible late merger) with another dwarf galaxy. This interaction could not only have triggered the origin of the recent starburst in IC 10, but could also explain the existence of previously-found counter-rotating HI gas in the periphery of the IC 10 which was interpreted as originating from primordial gas infall.

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arXiv2013

Confirmation of Small Dynamical and Stellar Masses for Extreme Emission Line Galaxies at z~2

Spectroscopic observations from the Large Binocular Telescope and the Very Large Telescope reveal kinematically narrow lines (~50 km/s) for a sample of 14 Extreme Emission Line Galaxies (EELGs) at redshifts 1.4 < z < 2.3. These measurements imply that the total dynamical masses of these systems are low (< 3x10^9 M_sun). Their large [O III] 5007 equivalent widths (500-1100 Angstroms) and faint blue continuum emission imply young ages of 10-100 Myr and stellar masses of 10^8-10^9 M_sun, confirming the presence of a violent starburst. The dynamical masses represent the first such determinations for low-mass galaxies at z > 1. The stellar mass formed in this vigorous starburst phase represents a large fraction of the total (dynamical) mass, without a significantly massive underlying population of older stars. The occurrence of such intense events in shallow potentials strongly suggests that supernova-driven winds must be of critical importance in the subsequent evolution of these systems.

astro-ph.CO↗

arXiv2013

A Tidally-Stripped Stellar Component of the Magellanic Bridge

Deep photometry of the Small Magellanic Cloud (SMC) stellar periphery (R=4 deg, 4.2 kpc) is used to study its line-of-sight depth with red clump (RC) stars. The RC luminosity function is affected little by young (<1 Gyr) blue-loop stars in these regions because their main-sequence counterparts are not observed in the color magnitude diagrams. The SMC's eastern side is found to have a large line-of-sight depth (~23 kpc) while the western side has a much shallower depth (~10 kpc), consistent with previous photographic plate photometry results. We use a model SMC RC luminosity function to deconvolve the observed RC magnitudes and construct the density function in distance for our fields. Three of the eastern fields show a distance bimodality with one component at the "systemic" ~67 kpc SMC distance and a second component at ~55 kpc. Our data are not reproduced well by the various extant Magellanic Cloud and Stream simulations. However, the models predict that the known HI Magellanic Bridge (stretching from the SMC eastward towards the LMC) has a decreasing distance with angle from the SMC and should be seen in both the gaseous and stellar components. From comparison with these models we conclude that the most likely explanation for our newly identified ~55 kpc stellar structure in the eastern SMC is a stellar counterpart of the HI Magellanic Bridge that was tidally stripped from the SMC ~200 Myr ago during a close encounter with the LMC. This discovery has important implications for microlensing surveys of the SMC.

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arXiv2013

A New Approach to Detailed Structural Decomposition from the SPLASH and PHAT Surveys: Kicked-up Disk Stars in the Andromeda Galaxy?

We characterize the bulge, disk, and halo subcomponents in the Andromeda galaxy (M31) over the radial range 4 < R_proj < 225 kpc. The cospatial nature of these subcomponents renders them difficult to disentangle using surface brightness (SB) information alone, especially interior to ~20 kpc. Our new decomposition technique combines information from the luminosity function (LF) of over 1.5 million bright (20 < m_814W < 22) stars from the Panchromatic Hubble Andromeda Treasury (PHAT) survey, radial velocities of over 5000 red giant branch stars in the same magnitude range from the Spectroscopic and Photometric Landscape of Andromeda's Stellar Halo (SPLASH) survey, and integrated I-band SB profiles from various sources. We use an affine-invariant Markov chain Monte Carlo algorithm to fit an appropriate toy model to these three data sets. The bulge, disk, and halo SB profiles are modeled as a Sersic, exponential, and cored power-law, respectively, and the LFs are modeled as broken power-laws. We present probability distributions for each of 32 parameters describing the SB profiles and LFs of the three subcomponents. We find that the number of stars with a disk-like LF is ~5% larger than the the number with disk-like (dynamically cold) kinematics, suggesting that some stars born in the disk have been dynamically heated to the point that they are kinematically indistinguishable from halo members. This is the first kinematical evidence for a "kicked-up disk" halo population in M31. The fraction of kicked-up disk stars is consistent with that found in simulations. We also find evidence for a radially varying disk LF, consistent with a negative metallicity gradient in the stellar disk.

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arXiv2013

Perseus I: A distant satellite dwarf galaxy of Andromeda

We present the discovery of a new dwarf galaxy, Perseus I/Andromeda XXXIII, found in the vicinity of Andromeda (M31) in stacked imaging data from the Pan-STARRS1 3π survey. Located 27.9° away from M31, Perseus I has a heliocentric distance of 785 +/- 65 kpc, compatible with it being a satellite of M31 at 374 +14/-10 kpc from its host. The properties of Perseus I are typical for a reasonably bright dwarf galaxy (M_V = -10.3 +/- 0.7), with an exponential half-light radius of r_h = 1.7 +/- 0.4 arcminutes or r_h = 400 +105/-85 pc at this distance, and a moderate ellipticity (ε= 0.43 +0.15/-0.17). The late discovery of Perseus I is due to its fairly low surface brightness (μ_0=25.7 +1.0/-0.9 mag/arcsec^2), and to the previous lack of deep, high quality photometric data in this region. If confirmed to be a companion of M31, the location of Perseus I, far east from its host, could place interesting constraints on the bulk motion of the satellite system of M31.

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arXiv2013

The Progenitors of the Compact Early-Type Galaxies at High-Redshift

We use GOODS and CANDELS images to identify progenitors of massive (log M > 10 Msun) compact "early-type" galaxies (ETGs) at z~1.6. Since merging and accretion increase the size of the stellar component of galaxies, if the progenitors are among known star-forming galaxies, these must be compact themselves. We select candidate progenitors among compact Lyman-break galaxies at z~3 based on their mass, SFR and central stellar density and find that these account for a large fraction of, and possibly all, compact ETGs at z~1.6. We find that the average far-UV SED of the candidates is redder than that of the non-candidates, but the optical and mid-IR SED are the same, implying that the redder UV of the candidates is inconsistent with larger dust obscuration, and consistent with more evolved (aging) star-formation. This is in line with other evidence that compactness is a sensitive predictor of passivity among high-redshift massive galaxies. We also find that the light distribution of both the compact ETGs and their candidate progenitors does not show any extended "halos" surrounding the compact "core", both in individual images and in stacks. We argue that this is generally inconsistent with the morphology of merger remnants, even if gas-rich, as predicted by N-body simulations. This suggests that the compact ETGs formed via highly dissipative, mostly gaseous accretion of units whose stellar components are very small and undetected in the HST images, with their stellar mass assembling in-situ, and that they have not experienced any major merging until the epoch of observations at z~1.6.

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