William A. Baum

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.

Globular Cluster Systems in Brightest Cluster Galaxies: Bimodal Metallicity Distributions and the Nature of the High-Luminosity Clusters

We present new (B, I) photometry for the globular cluster systems in eight brightest cluster galaxies (BCGs), obtained with the ACS/WFC camera on the Hubble Space Telescope. In the very rich cluster systems that reside within these giant galaxies, we find that all have strongly bimodal color distributions that are clearly resolved by the metallicity-sensitive (B - I) index. Furthermore, the mean colors and internal color range of the blue subpopulation are remarkably similar from one galaxy to the next, to well within the ±0.02-0.03 mag uncertainties in the foreground reddenings and photometric zero points. By contrast, the mean color and internal color range for the red subpopulation differ from one galaxy to the next by twice as much as the blue population. All the BCGs show population gradients, with much higher relative numbers of red clusters within 5 kpc of their centers, consistent with their having formed at later times than the blue, metal-poor population. A striking new feature of the color distributions emerging from our data is that for the brightest clusters (MI < -10.5) the color distribution becomes broad and less obviously bimodal. This effect was first noticed by Ostrov et al. and Dirsch et al. for the Fornax giant NGC 1399; our data suggest that it may be a characteristic of many BCGs and perhaps other large galaxies. Our data indicate that the blue (metal-poor) clusters brighter than MI -10 become progressively redder with increasing luminosity, following a mass/metallicity scaling relation Z ~ M0.55. A basically similar relation has been found for M87 by Strader et al. (2005). We argue that these GCS characteristics are consistent with a hierarchical-merging galaxy formation picture in which the metal-poor clusters formed in protogalactic clouds or dense starburst complexes with gas masses in the range 107-1010 M☉, but where the more massive clusters on average formed in bigger clouds with deeper potential wells where more preenrichment could occur.

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.

Preliminary results from the Viking orbiter imaging experiment

During its first 30 orbits around Mars, the Viking orbiter took approximately 1000 photographic frames of the surface of Mars with resolutions that ranged from 100 meters to a little more than 1 kilometer. Most were of potential landing sites in Chryse Planitia and Cydonia and near Capri Chasma. Contiguous high-resolution coverage in these areas has led to an increased understanding of surface processes, particularly cratering, fluvial, and mass-wasting phenomena. Most of the surfaces examined appear relatively old, channel features abound, and a variety of features suggestive of permafrost have been identified. The ejecta patterns around large craters imply that fluid flow of ejecta occurred after ballistic deposition. Variable features in the photographed area appear to have changed little since observed 5 years ago from Mariner 9. A variety of atmospheric phenomena were observed, including diffuse morning hazes, both stationary and moving discrete white clouds, and wave clouds covering extensive areas.

Saturn's E ring

Abstract The tenuous E ring of Saturn is found to commence abruptly at 3 Saturn radii, to peak sharply in the vicinity of the orbit of the satellite Enceladus (about 4 radii), and to spread out thinly to more than 8 radii. This distribution strongly suggests it to be associated with Enceladus and perhaps to be material ejected from Enceladus. The spread of E-ring material above and below the ring plane is greater in its tenuous outskirts than in its denser inner region, suggesting that the E ring may be at an early stage in its evolution. Thus far, our analysis reveals only a marginal variation of the ring with time or Enceladus azimuth. In this paper we describe the special instrumentation used for photometric observations of the E ring, and we present some of the data obtained in March 1980. In Paper II we shall derive the three-dimensional distribution of material in the E ring and discuss its cosmogonic implications.

Detection of the Tip of the Red Giant Branch in NGC 3379 (M105) in the Leo I Group Using the Hubble Space Telescope

We report the detection of individually resolved stars in the elliptical galaxy NGC 3379, a luminous member of the Leo I Group. The bright end of the stellar luminosity function has a logarithmic slope that is consistent with these stars being Population II red giants. An abrupt discontinuity in the apparent luminosity function at I = 26.30 ± 0.09 mag is identified with the tip of the first-ascent red giant branch (TRGB). Adopting MI(TRGB) = -4.0 ± 0.1 mag gives a distance modulus of 30.30 mag ± 0.14 (random errors) ± 0.23 (systematic errors) corresponding to a linear distance to NGC 3379 of 11.5 ± 1.6 Mpc. The TRGB distance compares very well with the Cepheid distance of 11.9 ± 0.9 Mpc (30.37 ± 0.16 mag) to another group member M96 (=NGC 3368). The distance to NGC 3379 can be used in turn to calibrate the zero points of four other distance indicators: surface brightness fluctuations, planetary nebula luminosity functions, globular cluster luminosity functions and the Dn - σ method. We apply two approaches to measuring the Hubble constant: (1) using a simple Virgocentric infall model and (2) stepping out from Leo I to the Coma cluster using the previously measured relative distance between the two clusters. These give values of the Hubble constant in the range H0 = 60-68 km s-1 Mpc-1, each having a 20% uncertainty. The largest systematic errors could potentially increase this value to H0 = 74 ± 14 km s-1 Mpc-1.

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.

Hubble Space Telescope observations of the 1990 equatorial disturbance on Saturn - Images, albedos, and limb darkening

In September 1990 a major equatorial eruption on Saturn produced a disturbance that spread in longitude until it completely girdled the planet. We report here on 150 images recorded in six passbands with the Wide Field/Planetary Camera (WF/PC) aboard the Hubble Space Telescope (HST) on 17 and 18 November 1990. For comparison, we used HST-WF/PC observations of Saturn obtained in three colors on 26 August 1990 before the onset of the disturbance, and in six colors on 5 and 6 June 1991 when almost no evidence of the disturbance remained. At both of those times, the equatorial belt was “normal” in appearance. Four of the passbands (with mean wavelengths of 336, 435, 546, and 716 nm) were selected for photometric analysis, and a patch of the B ring near the central meridian was used for photometric calibration. Using deconvolved images from all three epochs of observation, we measured reflectivities (I/F) of the disk along parallels of latitude as a function of longitudinal distance from the central meridian and also along the central meridian as a function of latitude from 0° to 90°. The longitudinal measurements cover essentially the whole visible disk; they were made at 1° intervals of planetographic latitude from 0° to 80°, and the results are expressed in terms of Minnaert coefficients k and Minnaert albedos (I/F)_0. We find that the cloud particles associated with the disturbance must differ in character from those that normally make up the visible cloud deck on Saturn. They were brighter and bluer, they had greater limb darkening, and their limb darkening was spectrally more neutral. The mutual relationship of those properties is such that features which stand out strongly near the meridian fade to invisibility when near the limb.

The Globular Cluster Systems in the Coma Ellipticals. IV: WFPC2 Photometry for Five Giant Ellipticals

We analyze photometric data in V and I for the globular cluster (GC) systems in five of the giant ellipticals in the Coma Cluster: NGC 4874, 4881, 4889, 4926, and IC 4051. All of the raw data, from the Hubble Space Telescope Wide Field Planetary Camera 2 Archive, are analyzed in a homogeneous way so that their five cluster systems can be strictly intercompared. We find that the GC luminosity functions are quite similar to one another and reinforce the common nature of the mass distribution of old, massive star clusters in gE galaxies. The globular cluster luminosity function (GCLF) turnover derived from a composite sample of more than 9,000 GCs appears at V = 27.71 ? 0.07 (MV = ?7.3), and our data reach about half a magnitude fainter than the turnover. We find that both a simple Gaussian curve and an evolved Schechter function fit the bright half of the GCLF equally well, though the Coma GCLF is broader and has a higher cutoff mass (Mc ~ 3 ? 106 M ?) than in any of the Virgo giants. These five Coma members exhibit a huge range in GC specific frequency, from a low of SN 0.7 for NGC 4881 up to 12 for IC 4051 and NGC 4874. No single formation scenario appears able to account for these differences in otherwise similar galaxies and may require carefully prescribed differences in their merger history, gas-free versus gas-rich progenitors, GC formation efficiency, initial density of environment, or tidal harassment within the Coma potential well. The supergiant cD galaxy NGC 4874 has the richest globular cluster system known, probably holding more than 30,000 clusters; its true extent is not yet determined and may extend well out into the Coma potential well. For the three biggest GC systems (NGC 4874, 4889, IC 4051), analysis of the (V ? I) color distributions shows that all three populations are dominated by red, metal-rich clusters. Their metallicity distributions also may all have the normal bimodal form, with the two sequences at mean colors V ? I(blue) 0.98 and V ? I(red) 1.15. These values fall along the previously established correlations of mean color with galaxy luminosity. However, the color distributions and relative numbers of metal-rich clusters show intriguing counterexamples to a trend established by Peng and colleagnes for the Virgo galaxies. For the brightest Virgo ellipticals, they find that the red GCs make up only ~30% of the cluster population, whereas in our similarly luminous Coma galaxies they make up more than half. At the very highest density and most massive regimes represented by the Coma supergiants, formation of metal-rich clusters seems to have been especially favored.

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).