Michael R. Corbin

The Hubble Ultra Deep Field

This paper presents the Hubble Ultra Deep Field (HUDF), a 1 million s exposure of an 11 arcmin2 region in the southern sky with the Advanced Camera for Surveys on the Hubble Space Telescope using Directors Discretionary Time. The exposure time was divided among four filters, F435W (B435), F606W (V606), F775W (i 775), and F850LP (z850), to give approximately uniform limiting magnitudes mAB ~ 29 for point sources. The image contains at least 10,000 objects, presented here as a catalog, the vast majority of which are galaxies. Visual inspection of the images shows few if any galaxies at redshifts greater than ~4 that resemble present-day spiral or elliptical galaxies. The image reinforces the conclusion from the original Hubble Deep Field that galaxies evolved strongly during the first few billion years in the infancy of the universe. Using the Lyman break dropout method to derive samples of galaxies at redshifts between 4 and 7, it is possible to study the apparent evolution of the galaxy luminosity function and number density. Examination of the catalog for dropout sources yields 504 B435 dropouts, 204 V 606 dropouts, and 54 i775 dropouts. The i775 dropouts are most likely galaxies at redshifts between 6 and 7. Using these samples, which are at different redshifts but derived from the same data, we find no evidence for a change in the characteristic luminosity of galaxies but some evidence for a decrease in their number densities between redshifts of 4 and 7. Assessing the factors needed to derive the luminosity function from the data suggests that there is considerable uncertainty in parameters from samples discovered with different instruments and derived using independent assumptions about the source populations. This assessment calls into question some of the strong conclusions of recently published work on distant galaxies. The ultraviolet luminosity density of these samples is dominated by galaxies fainter than the characteristic luminosity, and the HUDF reveals considerably more luminosity than shallower surveys. The apparent ultraviolet luminosity density of galaxies appears to decrease from redshifts of a few to redshifts greater than 6, although this decrease may be the result of faint-end incompleteness in the most distant samples. The highest redshift samples show that star formation was already vigorous at the earliest epochs at which galaxies have been observed, less than 1 billion years after the big bang.

Physical conditions in the Orion nebula and an assessment of its helium abundance

New long-slit spectrophotometric observations were obtained to redetermine the helium abundance of the Orion Nebula. The ionic ratio He(+)/H(+) is found to remain nearly constant at 0.088 {plus minus} 0.006. In the bright innermost region, the errors are largely systematic due to uncertainties in the reddening curve for Orion grains and deviations from case B emissivity. In the outer regions, the errors are dominated by statistical errors. Photoionization models are computed to determine the correction for the unobserved presence of neutral helium in regions where hydrogen is ionized. 114 refs.

INITIAL ON-ORBIT PERFORMANCE OF NICMOS

On 1997 February 13, Space Shuttle astronauts placed the Near-Infrared Camera and Multiobject Spectrometer (NICMOS) into the Hubble Space Telescope. Following installation, the servicing mission orbital verification program (SMOV) performed extensive testing of the instrument to verify that it can be operated and calibrated effectively. This program is essentially completed, and routine science observations have begun in most of the NICMOS modes of operation. This Letter describes the performance levels of NICMOS at this time.

Kinematics and ionization of extended gas in active galaxies. VI. The Seyfert 2 galaxy NGC 1386

The results of direct CCD imaging and long-slit medium-dispersion spectroscopy of the extended narrow-line-emitting gas in the nearby Seyfert 2 galaxy NGC 4388 are presented. The spectra are used to investigate the distribution, kinematics, and ionization structure of the excited gas in the regions surrounding the nucleus out to a distance of about 4.8 kpc. The data support earlier suggestions that the galaxy, which appears to lie near the core of the Virgo cluster, has been tidally distorted, possibly by the nearby cluster core galaxies M84 or IC 3303. Much of the highly ionized gas in the galaxy appears to be distributed in two wide cones originating from the nucleus and extending above and below the disk. Explanations are offered for the distribution of this gas and for the remainder of the gas above and below the disk. 47 references.

NGC 2264 IRS: Evidence for Triggered Star Formation

As part of its commissioning process the Near-Infrared Camera and Multiobject Spectrometer (NICMOS) obtained images of the infrared source NGC 2264 IRS, also known as Allens source, in three colors . In addition to the completely overexposed image of IRS there are six point objects, never previously observed, at projected separations of 26 to 49 from IRS. These sources are interpreted as near solar mass, pre-main-sequence stars that may be the result of triggered star formation. Regardless of the star formation method, it appears certain that this group of stars is a coeval formation of various mass stars. This makes it a very valuable data set for the study of pre-main-sequence evolution.

The Location of the Nucleus and the Morphology of Emission-Line Regions in NGC 1068

This Letter presents new Near-Infrared Camera and Multi-Object Spectrometer data on the location of the nucleus and the morphology of hydrogen and [Si VI] emission in NGC 1068. The peak of the emission at 2.2 μm is a strong point source that marks the location of the nucleus. The [Si VI] line emission region consists of two main components, a diffuse region of coronal emission to the north-northeast of the nucleus and a bright emission spot 16 from the nucleus along the direction of the radio jet. A similar but less intense emission spot also occurs in the hydrogen Paschen-α and Wide Field Planetary Camera 2 Hα images. The accurate determination of the nuclear position and its relation to the emission-line morphology produces a clearer picture of the nature of the interaction between the radio jet and its surroundings.

A comparison of C IV and H-beta broad emission-line profiles in low-redshift active galactic nuclei

Recently published IUE spectra of low-redshift QSOs and Seyfert 1 galaxies have permitted a comparison of the profiles of the C IV 1549A and H-beta broad emission lines. A linear correlation between the FWHM values of the profiles has been found in a sample of 19 objects and is verified to not be the result of separate correlations between these parameters and luminosity. Interpreted in terms of the geometry of the broad-line region, this result is contrary to models which predict low-ionization line emission from an optically thick, geometrically thin accretion disk surrounding the central object. Current evidence favors a model in which broad-line emission arises predominantly in a stratified ensemble of clouds moving radially relative to the central continuum source. 46 refs.

Comparisons of the emission-line properties of radio-loud and radio-quiet QSOs at high redshift

The emission-line properties of two sets of samples of high-redshift radio-loud and radio-quiet QSOs that are well matched in optical luminosity are compared. No strong differences are found between any of the fundamental parameters of the strong ultraviolet lines, in agreement with the basic result of Steidel & Sargent (1991, ApJ, 382). Additionally, no evidence is found for a difference in the strength of the «Fe II bump» at 2500 A between the samples, in contrast to the differences known to exist in the strengths of optical Fe II emission features. Comparison of the radio and optical luminosities among the radio-loud objects also reveals no strong correlation

The emission-line properties of steep radio spectrum quasars

Data on the emission-line properties of a sample of 45 intermediate redshift quasars with steep radio spectra are presented and compared with those of a sample of flat radio spectrum sources of comparable mean luminosity and redshift. The steep-spectrum sources have, on average, smaller widths of the C IV 1549 A and semiforbidden C III 1909 A line profiles, larger Mg II 2800 A equivalent widths, and smaller (He II and semiforbidden O III)/C IV intensity ratios than the flat-spectrum sources. The steep-spectrum sources have C IV profiles that tend to be asymmetric to the red, while those in the flat-spectrum sources are asymmetric to the blue. The Baldwin effect and the correlation between the Mg II-C IV and semiforbidden C III-C IV redshift differences reported elsewhere are also present both within the combined sample and among the steep-spectrum objects alone. These results support proposed unified models of steep and flat radio spectrum quasars. 56 refs.

Photometric Redshifts and Morphologies of Galaxies in the NICMOS Parallel Fields

We present positions, magnitudes, sizes, and morphological classi—cations for 111 galaxies discovered in the Hubble Space Telescope NICMOS camera 1 and camera 2 parallel —elds. We combine the magni- tudes measured in the JHK-analog —lters with those from deep ground-based images in V and/or R to measure photometric redshifts for 71 objects using Bruzual & Charlot population synthesis models. We —nd that these objects fall in the range z D 0.0¨2.7, with SzT + 0.8 and a mean luminosity SL T + 1.6L*. The NICMOS images reveal many of the galaxies to be ordered spirals and ellipticals similar to those in the local universe, with a high degree of symmetry and brightness pro—les that are well —tted by de Vaucouleurss r1@4 and exponential disk laws. However, we —nd a higher fraction (D14%) of morphologi- cally peculiar and/or interacting galaxies in the sample than is observed among local galaxies (D3%¨ 4%). This is consistent with the result from other deep HST images including the Hubble Deep Field and Hubble Medium Deep Survey —eld that the fraction of peculiar and interacting galaxies increases with redshift. As the NICMOS images of the sample galaxies cover their rest-frame near-infrared and optical emission, this result increases con—dence that such changes in morphology are genuine, as opposed to an eUect produced by viewing galaxies in the rest-frame ultraviolet. We also —nd that at least 26 of the sample galaxies appear to be members of (noninteracting) pairs or groups, based on their pro- mixity to one another and photometric redshifts. This is consistent with the results of recent ground- based optical surveys for faint galaxies covering larger areas and with the detection of galaxy groups and —laments at redshifts higher than those covered by the present sample.