arXiv subjects

M. Dickinson

Publications and source records attributed to M. Dickinson.

At least 199 records · Page 11Linked to original sources

arXiv2003

A Deep, Wide Field, Optical, and Near Infrared Catalog of a Large Area around the Hubble Deep Field North

We have conducted a deep multi-color imaging survey of 0.2 degrees^2 centered on the Hubble Deep Field North (HDF-N). We shall refer to this region as the Hawaii-HDF-N. Deep data were collected in U, B, V, R, I, and z' bands over the central 0.2 degrees^2 and in HK' over a smaller region covering the Chandra Deep Field North (CDF-N). The data were reduced to have accurate relative photometry and astrometry across the entire field to facilitate photometric redshifts and spectroscopic followup. We have compiled a catalog of 48,858 objects in the central 0.2 degrees^2 detected at 5 sigma significance in a 3" aperture in either R or z' band. Number counts and color-magnitude diagrams are presented and shown to be consistent with previous observations. Using color selection we have measured the density of objects at 3 5.5 using the Lyman break technique suffer from more contamination by low redshift objects than suggested by previous studies.

astro-ph↗

arXiv2003

The Evolution of Early-type Field Galaxies Selected from a NICMOS Map of the Hubble Deep Field North

The redshift distribution of well-defined samples of distant early-type galaxies offers a means to test the predictions of monolithic and hierarchical galaxy formation scenarios. NICMOS maps of the entire Hubble Deep Field North in the F110W and F160W filters, when combined with the available WFPC2 data, allow us to calculate photometric redshifts and determine the morphological appearance of galaxies at rest-frame optical wavelengths out to z ~ 2.5. Here we report results for two subsamples of early-type galaxies, defined primarily by their morphologies in the F160W band, which were selected from the NICMOS data down to H160_{AB} < 24.0. The observed redshift distributions of our two early-type samples do not match that predicted by a monolithic collapse model, which shows an overabundance at z > 1.5. A hierarchical formation model better matches the redshift distribution of the HDF-N early-types at z > 1.5, but still does not adequately describe the observed early-types. The hierarchical model predicts significantly bluer colors on average than the observed early-type colors, and underpredicts the observed number of early-types at z < 1. [abridged]

astro-ph↗

arXiv2003

The Rest-Frame UV Luminosity Density of Star-Forming Galaxies at Redshifts z>3.5

We have measured the rest--frame lambda~1500 Ang comoving specific luminosity density of star--forming galaxies at redshift 3.5<z<6.5 from deep images taken with the Hubble Space Telescope (HST)and the Advanced Camera for Surveys (ACS), obtained as part of the Great Observatories Origins Deep Survey (GOODS). We used color selection criteria to construct samples of star--forming galaxies at redshifts z~4, 5 and 6, identified by the signature of the 912 Ang Lyman continuum discontinuity and Lyman-alpha forest blanketing in their rest--frame UV colors (Lyman--break galaxies). The ACS samples cover ~0.09 square degree, and are also relatively deep, reaching between 0.2 and 0.5 L_3^*, depending on the redshift, where $L_3^*$ is the characteristic UV luminosity of Lyman--break galaxies at z~3. The specific luminosity density of Lyman--break galaxies appears to be nearly constant with redshift from z~3 to z~6, although the measure at z~6 remains relatively uncertain, because it depends on the accurate estimate of the faint counts of the z~6 sample. If Lyman--break galaxies are fair tracers of the cosmic star formation activity, our results suggest that at z~6 the universe was already producing stars as vigorously as it did near its maximum several Gyr later, at 1<~z<~3. Thus, the onset of large--scale star formation in the universe is to be sought at around z~6 or higher, namely at less than ~7% of the current cosmic age.

astro-ph↗

arXiv2003

Photometric Redshifts for Galaxies in the GOODS Southern Field

We use extensive multi-wavelength photometric data from the Great Observatories Origins Deep Survey (GOODS) to estimate photometric redshifts for a sample of 434 galaxies with spectroscopic redshifts in the Chandra Deep Field South. Using the Bayesian method, which incorporates redshift/magnitude priors, we estimate photometric redshifts for galaxies in the range 18 < R (AB) < 25.5, giving an rms scatter of 0.11. The outlier fraction is < 10%, with the outlier-clipped rms being 0.047. We examine the accuracy of photometric redshifts for several, special sub--classes of objects. The results for extremely red objects are more accurate than those for the sample as a whole, with rms of 0.051 and very few outliers (3%). Photometric redshifts for active galaxies, identified from their X-ray emission, have a dispersion of 0.104, with 10% outlier fraction, similar to that for normal galaxies. Employing a redshift/magnitude prior in this process seems to be crucial in improving the agreement between photometric and spectroscopic redshifts.

astro-ph↗

arXiv2003

Color-selected galaxies at z~6 in the Great Observatories Origins Deep Survey

We report early results on galaxies at z~6, selected from Hubble Space Telescope imaging for the Great Observatories Origins Deep Survey. Spectroscopy of one object with the Advanced Camera for Surveys grism and from the Keck and VLT observatories a shows a strong continuum break and asymmetric line emission, identified as Lya at z=5.83. We detect only five spatially extended, z~6 candidates with signal-to-noise ratios > 10, two of which have spectroscopic confirmation. This is many fewer than would be expected if galaxies at z=6 had the same luminosity function as those at z=3. There are many fainter candidates, but we expect substantial contamination from foreground interlopers and spurious detections. Our best estimates favor a z=6 galaxy population with fainter luminosities, higher space density, and similar co-moving ultraviolet emissivity to that at z=3, but this depends critically on counts at fluxes fainter than those reliably probed by the current data.

astro-ph↗

arXiv2003

Lyman Break Galaxies at Redshift z~3: Survey Description and Full Data Set

We present the basic data for a large ground-based spectroscopic survey for z~3 ``Lyman break galaxies'' (LBGs), photometrically selected using rest-UV colors from very deep images in 17 high Galactic latitude fields. The total survey covers an area of 0.38 square degrees, and includes 2347 photometrically-selected candidate LBGs to an apparent R_{AB} magnitude limit of 25.5. Approximately half of these objects have been observed spectroscopically using the Keck telescopes, yielding 940 redshifts with =2.96 +/- 0.29. We discuss the images, photometry, target selection, and the spectroscopic program in some detail, and present catalogs of the photometric and spectroscopic data, made available in electronic form. We discuss the general utility of conducting nearly-volume-limited redshift surveys in prescribed redshift intervals using judicious application of photometric pre-selection.

astro-ph↗

arXiv2002

Massive Ellipticals at High Redshift: NICMOS Imaging of Z~1 Radio Galaxies

We present deep, continuum images of eleven high-redshift (0.811 < z < 1.875) 3CR radio galaxies observed with NICMOS. Our images probe the rest-frame optical light where stars are expected to dominate the galaxy luminosity. The rest-frame UV light of eight of these galaxies demonstrates the well-known ``alignment effect''. Most of the radio galaxies have rounder, more symmetric morphologies at rest-frame optical wavelengths. Here we show the most direct evidence that in most cases the stellar hosts are normal elliptical galaxies with de Vaucouleurs law light profiles. For a few galaxies very faint traces of the UV-bright aligned component are also visible in the infrared images. We derive both the effective radius and surface-brightness for nine of eleven sample galaxies by fitting surface-brightness models to them. We find their sizes are similar to those of local FRII radio source hosts and are in general larger than other local galaxies. The derived host galaxy luminosities are very high and lie at the bright end of luminosity functions constructed at similar redshifts. The galaxies in our sample are also brighter than the rest-frame size--surface-brightness locus defined by the low-redshift sources. Passive evolution roughly aligns the z ~ 1 galaxies with the low-redshift samples. The optical host is sometimes centered on a local minimum in the rest-frame UV emission, suggesting the presence of substantial dust obscuration. We also see good evidence of nuclear point sources in three galaxies. Overall, our results are consistent with the hypothesis that these galaxies have already formed the bulk of their stars at redshifts greater than z >~ 2, and that the AGN phenomenon takes place within otherwise normal, perhaps passively evolving, galaxies. (abridged)

astro-ph↗

arXiv2002

Discovery of the host galaxy of HDF850.1, the brightest sub-mm source in the Hubble Deep Field

Despite extensive observational efforts, the brightest sub--mm source in the Hubble Deep Field, HDF850.1, has failed to yield a convincing optical/infrared identification almost 4 years after its discovery. This failure is all the more notable given the availability of supporting multi-frequency data of unparalleled depth, and sub-arcsec positional accuracy for the sub-mm/mm source. Consequently, HDF850.1 has become a test case of the possibility that the most violently star-forming objects in the universe are too red and/or distant to be seen in the deepest optical images. Here we report the discovery of the host galaxy of HDF850.1. This object has been revealed by careful analysis of a new, deep K-prime image of the HDF obtained with the Subaru 8.2-m telescope. Its reality is confirmed by a similar analysis of the HST NICMOS F160W image of the same region. This object is extremely faint (K=23.5), clumpy (on sub-arcsec scales) and very red (I-K > 5.2; H-K = 1.4 +- 0.35). The likelihood that it is the correct identification is strongly reinforced by a reanalysis of the combined MERLIN+VLA 1.4-GHz map of the field which yields a new radio detection of HDF850.1 only 0.1 arcsec from the new near-ir counterpart, and with sufficient positional accuracy to exclude all previously considered alternative optical candidates. We have calculated new confidence limits on the estimated redshift of HDF850.1 and find z = 4.1 +- 0.5. We also calculate that the flux density of HDF850.1 has been boosted by a factor of ~3 through lensing by the intervening elliptical 3-586.0, consistent with predictions that a small but significant fraction of blank-field sub-mm sources are lensed by foreground galaxies. We discuss the wider implications of these results for the sub-mm population and cosmic star-formation history.

astro-ph↗

arXiv2002

The Population of Faint Optically-Selected AGN at z~3

We discuss a sample of 29 AGN (16 narrow-lined and 13 broad-lined) discovered in a spectroscopic survey of ~1000 star-forming Lyman-break galaxies (LBGs) at z~3. Reaching apparent magnitudes of R_{AB}=25.5, the sample includes broad-lined AGN approximately 100 times less UV-luminous than most surveys to date covering similar redshifts, and the first statistical sample of UV/optically-selected narrow-lined AGN at high redshift. The fraction of objects in our survey with clear evidence for AGN activity is ~3%. A substantial fraction, perhaps even most, of these objects would not have been detected in even the deepest existing X-ray surveys. We argue that these AGN are plausibly hosted by the equivalent of LBGs. The UV luminosities of the broad-lined AGN in the sample are compatible with Eddington-limited accretion onto black holes that satisfy the locally determined M_{BH} versus M_{bulge} relation given estimates of the stellar masses of LBGs. The clustering properties of the AGN are compatible with their being hosted by objects similar to LBGs. The implied lifetime of the active AGN phase in LBGs, if it occurs some time during the active star-formation phase, is ~10^7 years.

astro-ph↗

arXiv2002

The Kinematic Connection Between QSO-Absorbing Gas and Galaxies at Intermediate Redshift

We present complementary data on 5 intermediate redshift (0.44 < z < 0.66) MgII absorbing galaxies, combining high spatial resolution imaging from HST, high--resolution QSO spectroscopy from Keck/HIRES, and galaxy kinematics from intermediate resolution spectroscopy using Keck/LRIS. These data allow a direct comparison of the kinematics of gas at large galactocentric impact parameters with the galaxy kinematics obtained from the faint galaxy spectroscopy. All 5 galaxies appear to be relatively normal spirals, with measured rotation curves yielding circular velocities in the range 100 < v_c < 260 km/s. We find that in 4 of the 5 cases examined, the velocities of all of the Mg II absorption components lie entirely to one side of the galaxy systemic redshift. These observations are consistent with rotation being dominant for the absorbing gas kinematics; however, the total range of velocities observed is inconsistent with simple disk rotation in every case. Simple kinematic models that simultaneously explain both the systemic offset of the absorbing material relative to the galaxy redshifts, and the total velocity width spanned by the absorption, require either extremely thick rotating gas layers, rotation velocities that vary with height above the extrapolation of the galactic plane, or both. In any case, our small sample suggests that rotating ``halo'' gas is a common feature of intermediate redshift spiral galaxies, and that the kinematic signature of rotation dominates over radial infall or outflow even for gas well away from the galactic plane. We discuss possible explanations for this behavior, and compare our observations to possible local analogs.

astro-ph↗

arXiv2001

The Rest-Frame Optical Properties of z~3 Galaxies

We present the results of a near-infrared imaging survey of z~3 Lyman Break Galaxies (LBGs). The survey covers a total of 30 arcmin^2 and includes 118 photometrically selected LBGs with K_s band measurements, 63 of which also have J band measurements, and 81 of which have spectroscopic redshifts. Using the distribution of optical {\cal R} magnitudes from previous work and {\cal R}-K_s colors for this sub-sample, we compute the rest-frame optical luminosity function of LBGs. At the brightest magnitudes, where it is fairly well constrained, this luminosity function strikingly exceeds locally determined optical luminosity functions. The V-band luminosity density of only the observed bright end of the z~3 LBG luminosity function already approaches that of all stars in the local universe. For the 81 galaxies with measured redshifts, we investigate the range of LBG stellar populations implied by the photometry which generally spans the range 900--5500 AA in the rest-frame. While there are only weak constraints on the parameters for most of the individual galaxies, there are strong trends in the sample as a whole. A unified scenario which accounts for the observed trends in bright LBGs is one in which a relatively short period of very rapid star-formation (hundreds of M_sun/yr) lasts for roughly 50--100 Myr, after which both the extinction and star-formation rate are considerably reduced and stars are formed at a more quiescent, but still rapid, rate for at least a few hundred Myr. In our sample, a considerable fraction (~20%) of the LBGs have best-fit star-formation ages ~> 1 Gyr, implied stellar masses of ~> 10^10 M_sun, and are still forming stars at \~30 M_sun/yr.

astro-ph↗

arXiv2001

The Galaxy Population of Cluster RXJ0848+4453 at z=1.27

We present a study of the galaxy population in the cluster RXJ0848+4453 at z=1.27, using deep HST NICMOS and WFPC2 images. We morphologically classify all galaxies to K_s=20.6 that are covered by the HST imaging, and determine photometric redshifts using deep ground based BRIzJK_s photometry. Of 22 likely cluster members with morphological classifications, eleven (50%) are classified as early-type galaxies, nine (41%) as spiral galaxies, and two (9%) as ``merger/peculiar''. At HST resolution the second brightest cluster galaxy is resolved into a spectacular merger between three red galaxies of similar luminosity, separated from each other by ~6 kpc, with an integrated magnitude K=17.6 (~3 L* at z=1.27). The two most luminous early-type galaxies also show evidence for recent or ongoing interactions. Mergers and interactions between galaxies are possible because RXJ0848+4453 is not yet relaxed. The fraction of early-type galaxies in our sample is similar to that in clusters at 0.5<z<1, and consistent with a gradual decrease of the number of early-type galaxies in clusters from z=0 to z=1.3. We find evidence that the color-magnitude relation of the early-type galaxies is less steep than in the nearby Coma cluster. This may indicate that the brightest early-type galaxies have young stellar populations at z=1.27, but is also consistent with predictions of single age ``monolithic'' models with a galactic wind. The scatter in the color-magnitude relation is ~0.04 in rest frame U-V, similar to that in clusters at 0<z<1. Taken together, these results show that luminous early-type galaxies exist in clusters at z~1.3, but that their number density may be smaller than in the local Universe. Additional observations are needed to determine whether the brightest early-type galaxies harbor young stellar populations.

astro-ph↗

arXiv2000

The Fraction of High Redshift Galaxies in Deep Infrared Selected Field Galaxy Samples

Recent results on the incidence of red galaxies in a > 100 square arcminute field galaxy survey to K=20 and a K=22 survey of the Hubble Deep Field are presented. We argue that a simple photometric redshift indicator, based on J-K color and supported by spectroscopic results obtained with Keck, gives a reliable lower limit of ~25% for the fraction of z>1 galaxies in the 100 square arcminute survey. This fraction is substantially higher than found in previous smaller samples, and is at least as consistent with predictions for pure luminosity evolution as with those for hierarchical models. The same technique yields a very low fraction for the HDF, which appears to be unusually underabundant in red galaxies.

astro-ph↗

arXiv1999

Lyman Alpha Imaging of a Proto-Cluster Region at =3.09

We present very deep imaging observations, through a narrow-band filter tuned to Lyman alpha at , of a volume containing a significant over-density of galaxies at this redshift previously discovered in our survey for continuum-selected Lyman break galaxies (LBGs). The new observations are used in conjunction with our spectroscopic results on LBGs to compare the effectiveness of continuum and emission line searches for star forming galaxies at high redshift, and to extend the search for members of the structure at = 3.09 to much fainter continuum luminosities. We find that only 20-25 percent of all galaxies at a given UV continuum luminosity would be flagged as narrow-band excess objects subject to the typical limits W_Lya > 80 Angstroms in the observed frame. The density enhancement of strong Lyman alpha emitters in this field is consistent with that inferred from the analysis of the spectroscopic Lyman break galaxy sample in the same region (6+/-1), but extends to continuum luminosities up to 2 magnitudes fainter. There is no evidence for a significantly higher fraction of large Lyman alpha line equivalent widths at faint continuum luminosities. By combining the 24 spectroscopic members of the z=3.09 ``spike'' with the narrow-band candidates, we are able to produce a sample of 162 objects which are either known or likely members of this large structure. We have also discovered two extremely large and diffuse Lyman alpha emitting ``blobs'', which have physical extents >100/h kpc and Lyman alpha line fluxes 20-40 times larger than the typical line emitters at the same redshifts in the field. The possible nature of the ``blobs'' is discussed. (abstract abridged)

astro-ph↗

arXiv1999

NICMOS Imaging of High-Redshift Radio Galaxies

We have obtained near-infrared (1.6 micron) images of 11 powerful 3CR radio galaxies at redshifts 0.8 < z < 1.8 using NICMOS on board HST. The high angular resolution permits a detailed study of galaxy morphology in these systems at rest-frame optical wavelengths, where starlight dominates over the extended, aligned UV continuum. The NICMOS morphologies are mostly symmetric and are consistent with dynamically relaxed, elliptical host galaxies dominated by a red, mature stellar population. The aligned structures are sometimes faintly visible, and nuclear point sources may be present in a few cases which manifest the ``unveiled'' AGN that is obscured from view at optical wavelengths. Our observations are consistent with the hypothesis that the host galaxies of z ~ 1-2 radio galaxies are similar to modern-day gE galaxies. Their sizes are typical of gE galaxies but smaller than present-day cD and brightest cluster galaxies, and their surface brightnesses are higher, as expected given simple luminosity evolution.

astro-ph↗

arXiv1999

Lyman Break Galaxies at z>4 and the Evolution of the UV Luminosity Density at High Redshift

We present initial results of a survey for star-forming galaxies in the redshift range 3.8 < z < 4.5. This sample consists of a photometric catalog of 244 galaxies culled from a total solid angle of 0.23 square degrees to an apparent magnitude of I_{AB}=25.0. Spectroscopic redshifts in the range 3.61 < z < 4.81 have been obtained for 48 of these galaxies; their median redshift is =4.13. Selecting these galaxies in a manner entirely analogous to our large survey for Lyman break galaxies at smaller redshift (2.7 < z < 3.4) allows a relatively clean differential comparison between the populations and integrated luminosity density at these two cosmic epochs. Over the same range of UV luminosity, the spectroscopic properties of the galaxy samples at z~4 and z~3 are indistinguishable, as are the luminosity function shapes and the total integrated UV luminosity densities (rho_{UV}(z=3)/rho_{UV}(z=4) = 1.1 +/-0.3). We see no evidence at these bright magnitudes for the steep decline in the star formation density inferred from fainter photometric Lyman-break galaxies in the Hubble Deep Field (HDF). If the true luminosity density at z~4 is somewhat higher than implied by the HDF, as our ground-based sample suggests, then the emissivity of star formation as a function of redshift is essentially constant for all z>1 once internally consistent corrections for dust are made. This suggests that there is no obvious peak in star formation activity, and that the onset of substantial star formation in galaxies occurs at z > 4.5. [abridged abstract]

astro-ph↗

arXiv1998

Lyman Break Galaxies at z~3 and Beyond

We report on the status of large surveys of photometrically selected star forming galaxies at z~3 and z~4, with particular emphasis on both the advantages and the limitations of selecting objects using the ``Lyman break'' technique. Current results on the luminosity functions, luminosity densities, color distribution, star formation rates, clustering properties, and the differential evolution of the population as a function of redshift are summarized.

astro-ph↗

arXiv1998

The Host Galaxy of the Gamma--Ray Burst 971214

We report on Hubble Space Telescope (HST) observations of the host galaxy of GRB 971214, about four months after the burst. The redshift of the proposed host galaxy at z=3.418, combined with optical and radio observations of the burst afterglow, implies the extremely large isotropic energy release from the burst in gamma-rays of approximately 3x10^53 ergs, some two orders of magnitude higher than the previously commonly assumed numbers. The positional offset between the optical transient observed at the Keck telescope and the centroid of the proposed host galaxy in the HST image is 0.14 -+0.07 arcsec. We find no evidence in our deep HST image for a chance foreground galaxy superposed along the line of sight to the proposed host at z=3.418. The morphology and photometric properties of this galaxy, such as the total flux, morphology, radial surface profile and scale length, are typical as compared to other, spectroscopically confirmed z>3 galaxies.

astro-ph↗