Edward J. Groth

Planetary Camera observations of NGC 1275 - Discovery of a central population of compact massive blue star clusters

We have discovered a population of bright blue pointlike sources within 5 kpc of the nucleus of NGC 1275 using HST Planetary Camera observations. The typical object has M_v~- 12 to - 14 (H_0 = 75 km s^(-1) Mpc^(-1); the brightest has M_v~-16. They are all blue, with V- R≾0.3. The color distribution and lack of excess Ha emission are consistent with nearly all being continuum sources. Many of the sources are unresolved even with the HST and consequently have sizes of ≾ 15 pc. We suggest that these are young star clusters that will evolve to look like globular clusters. They are bluer than any clusters seen in the Milky Way or M87, and brighter than the blue clusters seen in the LMC. We derive ages of several hundred million years or less and corresponding masses of 10^5-10^8 M_☉. The existence of these young clusters may be connected with a current or previous interaction with another galaxy, with the cooling flow in NGC 1275, or with some combination. Structure is detected in the underlying galaxy light that is suggestive of a merge between NGC 1275 and a second galaxy some 10^8 yr ago. If this merger triggered star formation, it would naturally account for the observed uniformity of cluster colors. Steady-state star formation in the x-ray cooling flow would imply a wider range in cluster age and color than is seen, unless the clusters disrupt. An interaction with the projected high-velocity, infalling system cannot explain the observations because this system has not yet reached the center of NGC 1275 where the clusters are concentrated, and because it has a total interaction time that is far too short for either the observed cluster lifetimes or the dynamical lifetime of structure in the galaxy. If the presence of recently formed protoglobulars around NGC 1275 is related to a previous merger, this would remove an important objection to the merger hypothesis for elliptical galaxy origins, provided that adequate gas is available in the merger for their formation.

The DEEP Groth Strip Galaxy Redshift Survey. III. Redshift Catalog and Properties of Galaxies

The Deep Extragalactic Evolutionary Probe (DEEP) is a series of spectroscopic surveys of faint galaxies, targeted at understanding the properties and clustering of galaxies at redshifts z ~ 1. We present the redshift catalog of the DEEP1 Groth Strip pilot phase of this project, a Keck LRIS survey of faint galaxies in the Groth Survey Strip imaged with HST WFPC2. The redshift catalog and data, including reduced spectra, are made publicly available through a Web-accessible database. The catalog contains 658 secure galaxy redshifts with a median z = 0.65. The distribution of these galaxies shows large-scale structure walls to z ~ 1. We find a bimodal distribution in the galaxy color-magnitude diagram that persists to the same distance. A similar color division has been seen locally by the SDSS and to z ~ 1 by the COMBO-17 survey. The HST imaging allows us to measure structural properties of the galaxies, and we find that the color division corresponds generally to a structural division. Most red galaxies, ~75%, are centrally concentrated, with a red bulge or spheroidal stellar component, while blue galaxies usually have exponential profiles. However, there are two subclasses of red galaxies that are not bulge dominated: edge-on disks and a second category that we term diffuse red galaxies (DIFRGs). Comparison to a local sample drawn from the RC3 suggests that distant edge-on disks are similar in appearance and frequency to those at low redshift, but analogs of DIFRGs are rare among local red galaxies. DIFRGs have significant emission lines, indicating that they are reddened mainly by dust rather than age. The DIFRGs in our sample are all at z > 0.64, suggesting that DIFRGs are more prevalent at high redshifts; they may be related to the dusty or irregular extremely red objects beyond z > 1.2 that have been found in deep K-selected surveys. We measure the color evolution of both red and blue galaxies by comparing our U - B colors to those from the RC3. For red galaxies, we find a reddening of only 0.11 mag from z ~ 0.8 to now, about half the color evolution measured by COMBO-17. Larger, more carefully defined samples with better colors are needed to improve this measurement. Reconciling evolution in color, luminosity, mass, morphology, and star formation rates will be an active topic of future research.

The Luminosity Function and Initial Mass Function in the Galactic Bulge

We present deep photometry obtained with the Hubble Space Telescope in a field in Baades window in the Galactic bulge. We derive a luminosity function down to I ~ 24.3, or V ~ 27.5, corresponding to M ~ 0.3 M_☉. The luminosity function from the turnoff down to this level appears remarkably similar to that observed in the solar neighborhood. We derive a mass function using both an empirical local mass-luminosity relation and a mass-luminosity relation from recent stellar model calculations, allowing for the presence of binaries and photometric errors. The mass function has a power-law form with dN/dM ∝ M^(-2.2) for M ≳ 0.7 M_☉. However, we find strong evidence for a break in the mass function slope around 0.5–0.7 M_☉, with a significantly shallower slope at lower masses. The value of the slope for the low masses depends on the assumed binary fraction and the accuracy of our completeness correction. This mass function should directly reflect the initial mass function.

MEASUREMENT OF COSMIC SHEAR WITH THE SPACE TELESCOPE IMAGING SPECTROGRAPH

Weak lensing by large-scale structure allows a direct measure of the dark matter distribution. We have used parallel images taken with the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope to measure weak lensing, or cosmic shear. We measure the shapes of 26,036 galaxies in 1292 STIS fields and measure the shear variance at a scale of 051. The charge transfer efficiency (CTE) of STIS has degraded over time and introduces a spurious ellipticity into galaxy shapes during the readout process. We correct for this effect as a function of signal-to-noise ratio and CCD position. We further show that the detected cosmic shear signal is nearly constant in time over the approximately 4 yr of observation. We detect cosmic shear at the 5.1 σ level, and our measurement of the shear variance is consistent with theoretical predictions in a ΛCDM universe. This provides a measure of the normalization of the mass power spectrum σ8 = 0.46 0.18. The 1 σ error includes noise, cosmic variance, systematics, and the redshift uncertainty of the source galaxies. This is consistent with previous cosmic shear measurements, but tends to favor those with a high value of σ8. It is also consistent with the recent determination of σ8 from the Wilkinson Microwave Anisotropy Probe (WMAP) experiment.

Titan: Evidence for seasonal change—A comparison of Hubble space telescope and voyager images

Abstract Images of Titan were obtained by the Hubble Space Telescope (HST) on 26 August 1990. Comparison with Voyager 1 and Voyager 2 images obtained 10 and 9 years earlier shows that the seasonal hemispheric brightness asymmetry has reversed near 440 and 550 nm wavelengths, with the northern hemisphere now being brighter. An additional, noisy HST image at 889 nm wavelength, for which there are no analogous Voyager data, suggests that the southern hemisphere may have been brighter than the northern at that wavelength in 1990.

Wide field camera observations of Baade's Window

We have observed a field in Baades Window using the Wide Field Camera of the Hubble Space Telescope (HST) and obtain V- and /-band photometry down to V~22.5. These data go several magnitudes fainter than previously obtained from the ground. The location of the break in the luminosity function suggests that there are a significant number of intermediate age ( < 10 Gyr) stars in the Galactic bulge. This conclusion rests on the assumptions that the extinction towards our field is similar to that seen in other parts of Baades Window, that the distance to the bulge is approximately 8 kpc, and that we can determine fairly accurate zero points for the HST photometry. Changes in any one of these assumptions could increase the inferred age, but a conspiracy oflower reddening, a shorter distance to the bulge, and/or photometric zero-point errors would be needed to imply a population entirely older than 10 Gyr. We infer an initial mass function slope for the main-sequence stars, and find that it is consistent with that measured in the solar neighborhood; unfortunately, the slope is poorly constrained because we sample only a narrow range of stellar mass and because of uncertainties in the observed luminosity function at the faint end.

The Intermediate-Mass Population in the Core of the R136 Star Cluster*

R136, the compact, luminous star cluster at the center of the 30 Doradus nebula, represents an extreme mode of star formation. Yet it is unique among these kinds of objects in that it can be resolved into individual stars and the star formation process probed through the stellar products. In a previous paper we reported on a study of the intermediate stellar mass population in R136 and derived a stellar initial mass function (IMF) for intermediate-mass stars in the region 0.5 to 4.7 pc from the center of the cluster. We have now obtained Hubble Space Telescope images designed to probe the stellar population in the core ( < 0.5 pc) of the cluster. We have measured the IMF for stars 5-15 M☉ at radii 0.11-0.34 pc, and for 4-15 M☉ at 0.34-0.46 pc. Beyond 0.46 pc we measure stars in mass bins from 2.8-15 M☉, as before. The slope of the stellar IMF beyond 0.5 pc is found to be -1.0 ± 0.1, which is within a few sigma of the value measured previously for the same annulus. The slopes of the IMFs become shallower at smaller radii, but the uncertainties do not rule out an IMF that is constant with cluster radius. The lower stellar mass limit is ≤5 M☉ in the cluster core. Thus, R136 is found to be similar to less concentrated OB associations even into 0.1 pc from the center of the cluster.

Hubble Space Telescope Planetary Camera observations of Arp 220

Planetary Camera images of peculiar galaxy Arp 220 taken with V, R, and I band filters reveal a very luminous object near the position of the western radio continuum source, assumed to be the major nucleus, ans seven lesser objects within 2 sec of this position. The most luminous object is formally coincident with the radio source to within the errors of Hubble Space Telescope (HST) pointing accuracy, but we have found an alternate, more compelling alignment of maps in which the eastern radio source coincides with one of the lesser objects and the OH radio sources reside near the surfaces of other optical objects. The proposed centering places the most luminous object 150 pc (0.4 sec) away from the western radio source. We explore the possibilities that the objects are either holes in the dense dust distribution, dusty clouds reflecting a hidden bright nucleus, or associations of bright young stars. We favor the interpretation that at least the brightest two objects are massive young star associations with luminosities 10(exp 9) to 10(exp 11) solar luminosity, but highly extinguished by intervening dust. These massive associations should fall into the nucleus on a time scale of 10(exp 8) yr. About 10% of the enigmatic far-IR flux arises from the observed objects. In addition, if the diffuse starlight out to a radius of 8 sec is dominated by stars with typical ages of order 10(exp 8) yr (the time since the alleged merger of two galaxies), as indicated by the blue colors at larger radius, then the lower limit to the reradiation of diffuse starlight contributes 3 x 10(exp 11) solar luminosity to the far-infrared flux, or greater than or equal to 25% of the total far-IR flux. Three additional bright objects (M(sub V) approximately equals -13) located about 6 sec from the core are likely young globular clusters, but any of these could be recently exploded supernovae instead. The expected supernovae rate, if the dominant energy source is young stars, is about one per month for the region where the intense far-infrared flux originates. Also, individual giant dust clouds are visible in these images. Their typical size is 300 pc (1 sec).

The DEEP Groth Strip Survey. I. The Sample

The Deep Extragalactic Exploratory Probe (DEEP) is a multiphase research program dedicated to the study of the formation and evolution of galaxies and of large-scale structure in the distant universe. This paper describes the first five-year phase, denoted DEEP1. A series of 10 DEEP1 papers will discuss a range of scientific topics (e.g., the study of photometric and spectral properties of a general distant galaxy survey, the evolution observed in galaxy populations of varied morphologies). The observational basis for these studies is the Groth Survey Strip field, a 127 arcmin2 region that has been observed with the Hubble Space Telescope (HST) in both broad I-band and V-band optical filters and with the Low Resolution Imaging Spectrograph on the Keck Telescopes. Catalogs of photometric and structural parameters have been constructed for 11,547 galaxies and stars at magnitudes brighter than 29, and spectroscopy has been conducted for a magnitude-color weighted subsample of 818 objects. We evaluate three independent techniques for constructing an imaging catalog for the field from the HST data and discuss the depth and sampling of the resultant catalogs. The selection of the spectroscopic subsample is discussed, and we describe the multifaceted approach taken to prioritizing objects of interest for a variety of scientific subprograms. A series of Monte Carlo simulations then demonstrates that the spectroscopic subsample can be adequately modeled as a simple function of magnitude and color cuts in the imaging catalog.

Imaging of the gravitational lens system PG 1115+080 with the Hubble Space Telescope

This paper is the first of a series presenting observations of gravitational lenses and lens candidates, taken with the Wide Field/Planetary Camera (WFPC) of the Hubble Space Telescope (HST). We have resolved the gravitational lens system PG 1115 + 080 into four point sources and a red, extended object that is presumably the lens galaxy; we present accurate relative intensities, colors, and positions of the four images, and lower accuracy intensity and position of the lens galaxy, all at the epoch 1991.2. Comparison with earlier data shows no compelling evidence for relative intensity variations between the QSO components having so far been observed. The new data agree with earlier conclusions that the system is rather simple, and can be produced by the single observed galaxy. The absence of asymmetry in the HST images implies that the emitting region of the quasar itself has an angular radius smaller than about 10 milliarcsec (100 pc for H_0=50, q_0=0.5).