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Michael R. Corbin

Publications and source records attributed to Michael R. Corbin.

13 recordsLinked to original sources
arXiv2007

The Nearby and Extremely Metal-Poor Galaxy CGCG 269-049

We present Hubble Space Telescope (HST) and Spitzer Space Telescope images and photometry of the extremely metal-poor (Z = 0.03 Z_sol) blue dwarf galaxy CGCG 269-049. The HST images reveal a large population of red giant and asymptotic giant branch stars, ruling out the possibility that the galaxy has recently formed. From the magnitude of the tip of the red giant branch, we measure a distance to CGCG 269-049 of only 4.9 +/- 0.4 Mpc. The spectral energy distribution of the galaxy between ~3.6 - 70 microns is also best fitted by emission from predominantly ~10 Gyr old stars, with a component of thermal dust emission having a temperature of 52 +/- 10 K. The HST and Spitzer photometry indicate that more than 60% of CGCG 269-049's stellar mass consists of stars ~10 Gyr old, similar to other local blue dwarf galaxies. Our HST H-alpha image shows no evidence of a supernova-driven outflow that could be removing metals from the galaxy, nor do we find evidence that such outflows occurred in the past. Taken together with CGCG 269-049's large ratio of neutral hydrogen mass to stellar mass (~10), these results are consistent with recent simulations in which the metal deficiency of local dwarf galaxies results mainly from inefficient star formation, rather than youth or the escape of supernova ejecta.

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arXiv2006

Ultracompact Blue Dwarf Galaxies: Hubble Space Telescope Imaging and Stellar Population Analysis

We present deep Hubble Space Telescope Advanced Camera for Surveys / High Resolution Channel U, narrow-V, and I images of nine "ultracompact" blue dwarf galaxies (UCBDs) selected from the Sloan Digital Sky Survey. We define UCBDs as local (z < 0.01) star-forming galaxies having angular diameters < 6" and physical sizes < 1 kpc. They are also among the most metal-poor galaxies known, and are found to reside within voids. Both the HST images and the objects' optical spectra reveal that they are composites of young (~1 Myr) populations that dominate their light, and older (~10 Gyr) populations that dominate their stellar masses, which we estimate to be ~10^7 - 10^8 Msol. An intermediate-age population is also indicated in most cases. The objects are not as dynamically disturbed as the prototype UCBD, POX 186, but the structure of several of them suggests that their current starbursts have been triggered by the collisions/mergers of smaller clumps of stars. In one case, HS 0822+3542, the ACS/HRC images resolve the object into two small (~100 pc) components which appear to have recently collided, supporting this interpretation. In six of the objects much of their star formation is concentrated in Young Massive Star clusters. The evidence that the galaxies consist mainly of ~10 Gyr old stars establishes that they are not protogalaxies; their low metallicities are more likely to be the result of the escape of supernova ejecta, as opposed to youth. These results are consistent with recent galaxy formation simulations which predict that cosmic re-ionization at z ~ 6 significantly limited the subsequent star formation of dwarf galaxies in voids due to the photo-evaporation of baryons from their cold dark matter halos (Abridged).

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arXiv2005

Hubble Space Telescope Imaging of the Ultracompact Blue Dwarf Galaxy HS 0822+3542: An Assembling Galaxy in a Local Void?

We present deep U, narrow-V, and I-band images of the ultracompact blue dwarf galaxy HS 0822+3542, obtained with the Advanced Camera for Surveys / High Resolution Channel of the Hubble Space Telescope. This object is extremely metal-poor (12 + log(O/H) = 7.45) and resides in a nearby void. The images resolve it into two physically separate components that were previously described as star clusters in a single galaxy. The primary component is only \~100 pc in maximum extent, and consists of starburst region surrounded by a ring-like structure of relatively redder stars. The secondary component is ~50 pc in size and lies at a projected distance of ~80 pc away from the primary, and is also actively star-forming. We estimate masses ~10^7 M(sol) and ~10^6 M(sol) for the two components, based on their luminosities, with an associated dynamical timescale for the system of a few Myr. This timescale and the structure of the components suggests that a collision between them triggered their starbursts. The spectral energy distributions of both components can be fitted by the combination of recent (few Myr old) starburst and an evolved (several Gyr old) underlying stellar population, similar to larger blue compact dwarf galaxies. This indicates that despite its metal deficiency the object is not forming its first generation of stars. However, the small sizes and masses of the two components suggests that HS 0822+3542 represents a dwarf galaxy in the process of assembling from clumps of stars intermediate in size between globular clusters and objects previously classified as galaxies. Its relatively high ratio of neutral gas mass to stellar mass (~1) and high specific star formation rate, log(SFR/M(sol) = -9.2, suggests that it is still converting much of its gas to stars.

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arXiv2004

Iron Abundance Diagnostics in High-Redshift QSOs

The abundance of alpha-process elements such as magnesium and carbon relative to iron measured from the broad emission lines of QSOs can serve as a diagnostic of the star formation and chemical enrichment histories of their host galaxies. We investigate the relationship between Fe/Mg and Fe/C abundance ratios and the resulting Fe II / Mg II 2800A and Fe II / 1900A-blend flux ratios, both of which have been measured in QSOs out to redshifts of approximately six. Using a galactic chemical evolution model based on a starburst in a giant elliptical galaxy, we find that these flux ratios are good tracers of the chemical enrichment of the nuclei. However, the values of these ratios measured in objects at redshifts of approximately six suggest that iron enrichment has occurred more rapidly in these objects than predicted by the assumed elliptical starburst model, under currently favored cosmologies.

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arXiv2003

Iron Emission in z ~ 6 QSOs and its Possible Implications

We have obtained low-resolution near-infrared spectra of five SDSS QSOs at 5.3 < z < 6.3 using the NICMOS instrument of the Hubble Space Telescope. We find evidence of emission in the Fe II complex centered near 2500 A (rest) in all five objects. We estimate Fe II / Mg II 2800 A flux ratios comparable to those measured in QSOs at lower redshifts, which indicate metallicities near or above the solar level. We discuss the possible implications of this result assuming the iron enrichment to have been produced mainly in Type Ia supernovae.

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arXiv2003

Iron Emission in z~6 QSOs

We have obtained low-resolution near infrared spectra of three QSOs at 5.7 < z < 6.3 using the NICMOS instrument of the Hubble Space Telescope. The spectra cover the rest-frame ultraviolet emission of the objects between lambda_rest ~ 1600A - 2800A. The Fe II emission-line complex at 2500A is clearly detected in two of the objects, and possibly detected in the third. The strength of this complex and the ratio of its integrated flux to that of Mg II lambda 2800 are comparable to values measured for QSOs at lower redshifts, and are consistent with Fe/Mg abundance ratios near or above the solar value. There thus appears to be no evolution of QSO metallicity to z~6. Our results suggest that massive, chemically enriched galaxies formed within 1 Gyr of the Big Bang. If this chemical enrichment was produced by Type Ia supernovae, then the progenitor stars formed at z ~ 20 +/- 10, in agreement with recent estimates based on the cosmic microwave background. These results also support models of an evolutionary link between star formation, the growth of supermassive black holes and nuclear activity.

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arXiv2002

POX 186: A Dwarf Galaxy in the Process of Formation?

We present deep U, V and I band images of the "ultracompact" blue dwarf galaxy POX 186 obtained with the Planetary Camera 2 of the Hubble Space Telescope. We have also obtained a near-ultraviolet spectrum of the object with the Space Telescope Imaging Spectrograph, and combine this with a new ground-based optical spectrum. The images confirm the galaxy to be extremely small, with a maximum extent of only 300 pc, a luminosity ~ 10^-4 L*, and an estimated mass ~ 10^7 M(sun). Its morphology is highly asymmetric, with a tail of material on its western side that may be tidal in origin. The U-band image shows this tail to be part of a stream of material in which stars have recently formed. Most of the star formation in the galaxy is however concentrated in a central, compact (d ~ 10 - 15 pc) star cluster. The outer regions of the galaxy are significantly redder than the cluster, with V - I colors consistent with a population dominated by K and M stars. While these results rule out earlier speculation that POX 186 is a protogalaxy, its morphology, mass and active star formation suggest that it represents a recent (within ~ 10^8 yr) collision between two clumps of stars of sub-galactic size (~ 100 pc). POX 186 may thus be a very small dwarf galaxy that, dynamically speaking, is still in the process of formation. This interpretation is supported by the fact that it resides in a void, so its morphology cannot be explained as the result of an encounter with a more massive galaxy. Clumps of stars this small may represent the building blocks required by hierarchical models of galaxy formation, and these results also support the recent "downsizing" picture of galaxy formation in which the least massive objects are the last to form.

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arXiv2002

Hubble Space Telescope NICMOS Imaging of the Core of M87

We present broad-band 1.1, 1.6 and 2.2 micron images and a 2.37 micron narrow-band image of the inner 19" of the nearby radio galaxy M87, obtained with the Near Infrared Camera and Multi-Object Spectrometer of the Hubble Space Telescope (HST). The isophotes of the broad-band images are almost perfectly circular to within approximately 0.5" (~ 50 pc) of the active nucleus, and an r**1/4 law provides a good fit to the galaxy profile to within the same distance. This result agrees with predictions that the nuclear supermassive black hole will produce a nearly spherical distribution of the surrounding stars within a galaxy crossing time. A difference image formed from the 1.6 micron image and a V-band image obtained with the HST Wide Field Planetary Camera 2 does not show any clear evidence of a physically thick dusty torus around the nucleus, consistent with its lack of strong thermal infrared emission. The images and associated colors also confirm that the regions beyond the nucleus do not contain strongly concentrated dust,in contrast to many other radio galaxies. In combination with other recent observations, these results indicate that M87 represents the dynamically evolved product of past galaxy mergers, and suggest that its nucleus is in the final stages of activity.

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arXiv2001

NICMOS Imaging of the Cores of M 31 and M 32

We present 1.1, 1.6, and 2.2 micron images of the cores of the Local Group galaxies M 31 and M 32 obtained with the Near Infrared Camera and Multi-Object Spectrometer (NICMOS) instrument of the Hubble Space Telescope. These images are also compared to optical images of the galaxies obtained with the HST Wide Field Planetary Camera 2 (WFPC2). In M 31 the nucleus remains distinctly asymmetric in all of the infrared bands, with no evidence of concentrated dust, although the distinction between the two nuclei, P1 and P2, is not as strong as in the WFPC and WFPC2 images. This result is nonetheless consistent with the model of the nucleus as an eccentric stellar disk produced by the dynamical influence of a central supermassive black hole. Several individual M-giant stars are detected within ~ 3" of the nuclei, and may represent stars that have escaped from the disk and phase-mixed around the nuclear center of mass. In M 32 we also do not find strong evidence of dust, and the brightness profiles within the central ~ 1" of the infrared images can be fitted by a power law of the same form as that fitted to the optical images. The infrared color profiles of the central few arcseconds of M 32 show no strong gradients, and indicate a population dominated by K-giant stars. This is consistent with recent groud-based spectroscopy and photometry of the core region, which similarly show no strong gradients in age or metallicity within its stellar population.

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arXiv2000

A Multivariate Analysis of Galaxies in the Hubble Deep Field North

We use the ultraviolet and optical WFPC2 and near-infrared NICMOS images of the Hubble Deep Field North to measure and statistically compare an array of parameters for over 250 of the galaxies it contains. These parameters include redshift, rest-frame visible asymmetry and concentration, bolometric luminosity and extinction-corrected star formation rate. We find only one strong correlation, between bolometric luminosity and star formation rate, from which early-type galaxies noticeably deviate. When our asymmetry measurements are combined with those of a sample of nearby galaxies covering the full Hubble sequence, we find a weak correlation between redshift and rest-frame visible asymmetry, consistent with the qualitative evidence of galaxy morphological evolution from these and other deep Hubble Space Telescope images. The mean values of these asymmetry measurements show a monotonic increase with redshift interval over the range 0 < z < 2, increasing by a factor of approximately three. If this trend is real, it suggests that galaxy morphological evolution is a gradual process that is continuing through the present cosmological epoch. There is evidence that the dominant source of this evolution is the "minor" mergers of disk galaxies with smaller companions, which could also transform late-type spiral galaxies into early-type spiral galaxies. Interestingly, in contrast to local galaxies we find no correlations between star formation rate and either UV or visible asymmetry. This could arise if the star formation of high-redshift galaxies proceeds in episodes that are short (~ 100 Myr) relative to the time scales over which galaxy mergers produce strong asymmetries (~ 500 Myr), a result suggested by the high star formation rates of Lyman break galaxies.

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arXiv2000

A Color Analysis of the NICMOS Parallel Image Archive

We present a photometric analysis of all high Galactic lattitude (|b| > 20d) broad-band parallel images taken by the Near Infrared Camera and Multi-Object Spectrometer (NICMOS) instrument during its initial lifetime in HST Cycle 7. These images, taken through the F110W and F160W filters, reach a mean 3-sigma limiting magnitude of approximately 22 in both bands, and cover a total area of approximately 92 square arcminutes. The reddest of the 358 galaxies detected have F110W-F160W colors and F160W magnitudes consistent with being a combination of both dusty star-forming and evolved early-type galaxies at 1 < z < 2. The surface density of these galaxies is comparable to that of the population of Extremely Red Objects (EROs) discovered in ground-based surveys (~ 100 deg-2), suggesting that EROs also represent a combination of both galaxy types in this redshift range. Roughly 10% of the detected galaxies appear to be blue compact dwarf galaxies at z < 1, a result consistent with studies of the HST Medium Deep Survey Fields. The surface density of these objects down to magnitude of 22 in F160W is ~ 300 deg-2. None of the 681 point sources detected have F110W-F160W colors matching those expected for QSOs whose continua have been significantly reddened by internal dust. Our data limit the surface density of such QSOs to be < 50 deg-2 down the mean limiting magnitudes of the sample images. Since the surface density of QSOs detected on the basis of ultraviolet excess in optical surveys to comparable depth is ~ 100 deg-2, this argues against the suggestion that dust-reddened QSOs comprise the undetected majority of the QSO population. The F110W-F160W color can also be used to identify unreddened QSOs at z > 8, but we find no such candidates. This is consistent with the evidence that QSO space density declines sharply at z > 5.

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arXiv2000

On the Role of Minor Galaxy Mergers in the Formation of Active Galactic Nuclei

The large scale (~ 100 kpc) environments of Seyfert galaxies are not significantly different from those of non-Seyfert galaxies. In the context of the interaction model of the formation of active galactic nuclei (AGN), it has been proposed that AGN form via "minor mergers" of large disk galaxies with smaller companions. We test this hypothesis by comparing the nuclear spectra of 105 bright nearby galaxies with measurements of their R or r band morphological asymmetries at three successive radii. We find no significant differences in these asymmetries between the 13 Seyfert galaxies in the sample and galaxies having other nuclear spectral types (absorption, H II-region like, LINER), nor is there strong qualitative evidence that such mergers have occured among any of the Seyferts or LINERs. Thus either any minor mergers began > 1 Gyr ago and are essentially complete, or they did not occur at all, and AGN form independently of any type of interaction. Support for the latter interpretation is provided by the growing evidence that supermassive black holes exist in the cores of most elliptical and early-type spiral galaxies, which in turn suggests that nuclear activity represents a normal phase in the evolution of the bulges of massive galaxies. Galaxy mergers may increase the luminosity of Seyfert nuclei to the level of QSOs, which could explain why the latter objects appear to be found in rich environments and in interacting systems.

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