R. Michael Rich

Discovery of Extended Blue Horizontal Branches in Two Metal-rich Globular Clusters*

We have used WFPC2 to construct B, V color-magnitude diagrams of four metal-rich globular clusters, NGC 104 (47 Tuc), NGC 5927, NGC 6388, and NGC 6441. All four clusters have well populated red horizontal branches (RHB), as expected for their metallicity. However, NGC 6388 and 6441 also exhibit a prominent blue horizontal-branch (BHB) extension, including stars reaching as faint in V as the turnoff luminosity. This discovery demonstrates directly for the first time that a major population of hot horizontal-branch (HB) stars can exist in old, metal-rich systems. This may have important implications for the interpretation of the integrated spectra of elliptical galaxies. The cause of the phenomenon remains uncertain. We examine the possibility that NGC 6388 and 6441 are older than the other clusters, but a simple difference in age may not be sufficient to produce the observed distributions along the HB. The high central densities in NGC 6388 and 6441 suggest that the existence of the BHB tails might be caused by stellar interactions in the dense cores of these clusters, which we calculate to have two of the highest collision rates among globular clusters in the Galaxy. Tidal collisions might act in various ways to enhance loss of envelope mass and therefore populate the blue side of the HB. However, the relative frequency of tidal collisions does not seem large enough (compared to that of the clusters with pure RHBs) to account for such a drastic difference in HB morphology. While a combination of an age difference and dynamical interactions may help, prima facie the lack of a radial gradient in the BHB/RHB star ratio seems to argue against dynamical effects playing a role.

Suppression of star formation in early-type galaxies by feedback from supermassive black holes

Detailed high-resolution observations of the innermost regions of nearby galaxies have revealed the presence of supermassive black holes. These black holes may interact with their host galaxies by means of ‘feedback’ in the form of energy and material jets; this feedback affects the evolution of the host and gives rise to observed relations between the black hole and the host. Here we report observations of the ultraviolet emissions of massive early-type galaxies. We derive an empirical relation for a critical black-hole mass (as a function of velocity dispersion) above which the outflows from these black holes suppress star formation in their hosts by heating and expelling all available cold gas. Supermassive black holes are negligible in mass compared to their hosts but nevertheless seem to play a critical role in the star formation history of galaxies.

Microlensing by the galactic bar

We compute the optical depth and duration distribution of microlensing events towrd Baades window in a model composed of a Galactic disk and a bar. The bar model is a self-consistent dynamical model built out of individual orbits that has been populated to be consistent with the COBE maps of the Galaxy and kinematic observations of the Galactic bulge. We find that most of the lenses are in the bulge with a line-of-sight distance 6.25 kpc (adopting R(sub 0) = 8 kpc). The microlensing optical depth of a 2 x 10(exp 10) solar mass bar plus a truncated disk is (2.2 +/- 0.45) x 10(exp -6), consistent with the large optical depth (3.2 +/- 1.2) x 10(exp -6) found by Udalski et al. (1994). This model optical depth is enhanced over the predictions of axisymmetric models by Kiraga & Paczynski (1994) by slightly more than a factor of 2, since the bar is elongated along the line of sight. The large Einstein radius and small transverse velocity dispersion also predict a longer event duration in the self-consistent bar model than in the Kiraga-Paczynski model. The event rate and duration distribution also depend on the lower mass cutoff of the lens mass function. With a 0.1 solar mass cutoff, five to seven events (depending on the contribution of disk lenses) with a logarithmic mean duration of 20 days are expected for the Optical Gravitational Lensing Experiment (OGLE) according to our model, while Udalski et al. (1994) observed nine events with durations from 8 to 62 days. On the other hand, if most of the lenses are brown dwarfs, our model predicts too many short-duration events. A Kolmogorov-Smirnov test finds only 7% probability for the model with 0.01 solar mass cutoff to be consistent with current data.

Peculiar Multimodality on the Horizontal Branch of the Globular Cluster NGC 2808

We present distributions of colors of stars along the horizontal branch (HB) of the globular cluster NGC 2808, from Hubble Space Telescope Wide Field Planetary Camera 2 imaging in B, V, and an ultraviolet filter (F218W). This clusters HB is already known to be strongly bimodal, with approximately equal-sized HB populations widely separated in the color-magnitude diagram. Our images reveal a long blue tail with two gaps, for a total of four nearly distinct HB groups. These gaps are very narrow, corresponding to envelope-mass differences of only ~0.01 M?. This remarkable multimodality may be a signature of mass-loss processes, subtle composition variations, or dynamical effects; we briefly summarize the possibilities. The existence of narrow gaps between distinct clumps on the HB presents a challenge for models that attempt to explain HB bimodality or other peculiar HB structures.

A consistent microlensing model for the Galactic bar

We compute a microlensing map for the Galactic bar. The predicted event rate and event duration distribution are consistent with the 55 events recently reported by the MACHO and OGLE collaborations. Most of the events are due to lensing by stars in the near end of the bar_ Lens mass functions with about 30-60 per cent of the lens mass as brown dwarfs are rejected at 20-60 levels_ To make our model useful for other workers, we tabulate the optical depth of the bar and average event duration (scaled to I-M0 lenses) on a grid of galactic coordinates. The distance and the proper motions of the lens and the source are derived from a consistent dynamical model of the stellar bar, which has originally been built to fit data on the stellar light and stellar/gas kinematics of the bar. We explore several alternative models, and we find that our standard model best matches the observations.

Nuclear Properties of Kinematically Distinct Cores

We present WFPC2 F450W, F555W, and F814W photometry for three early-type galaxies with kinematically distinct cores (KDCs), namely NGC 2434, NGC 7192, and IC 4889. For these three objects we present the B, V, and I nuclear surface brightness, isophotal, B-V and V-I profiles and derive the deprojected luminosity densities. The three KDCs have nuclear cusp slopes γV in agreement with the γV versus absolute magnitude (MV) relation defined by kinematically normal galaxies and by other KDCs. NGC 7192 contains an unresolved blue nuclear spike of V 22.2, B-V 0, and V-I 0.7. We use these new data, and the WFPC2 photometry for 15 KDCs presented in Carollo et al., to investigate several open questions related to the nuclear properties of this class of galaxies. We find the following: 1. If there is a direct effect of environment on the nuclear properties of KDCs, this is similar to that acting on kinematically normal galaxies. Dynamically hot systems preferentially have low nuclear densities and shallow cusps when in clusters and high nuclear densities and steep cusps when in the field. The highest nuclear densities are reached in the field. 2. Within the limits imposed by the angular resolution of WFPC2 data and by the presence of nuclear dust, KDCs have symmetric nuclei. This result is independent of environment, form of the nuclear light profiles, nuclear morphology, and presence of an unresolved, likely nonthermal, central source. 3. The known Mg2 index-enhanced KDCs might not be all nuclear disks embedded in anisotropic stellar bodies. Three galaxies, namely NGC 2434, NGC 7192, and NGC 7626, do not show any photometric signature for a nuclear disk associated with their Mg2-rich KDCs. The Mg2-rich KDCs may arise from a large variety of morphological (and dynamical) structures, possibly indicating a variety of star formation and dynamical histories. 4. The delta-delta correlation between the residuals of the γV versus MV relation, and Mg2 versus MV relation, is approximately at the 3 σ level, which implies a residual dependence of the cusp slope γV on the Mg2 line strength: the higher the Mg enrichment, the shallower the nuclear cusp slope. 5. The Mg2-enhanced KDCs do not occupy any special location in the correlation between γV and Mg2 (correlation that worsens significantly if for these KDCs one considers the Mg2 value extrapolated from the outer galactic regions). They also do not occupy any special location in the delta-delta correlation above. Therefore, our data suggest that the processes that made the KDCs in the host galaxies (on scales of ≈ 1 kpc) also rebuilt their central structures (i.e., the nuclear cusps on scales 100 pc). Finally, our work adds further support to the previous findings (Forbes et al.; Carollo et al.) that KDCs show little or no evidence for any photometric difference compared to other early-type systems of similar luminosity.

WFPC2 Observations of Star Clusters in the Magellanic Clouds. I. The Large Magellanic Cloud Globular Cluster Hodge 11

We present our analysis of Hubble Space Telescope Wide Field Planetary Camera 2 observations in F555W (broadband V ) and F450W (broadband B) of the globular cluster Hodge 11 in the Large Magellanic Cloud galaxy. The resulting V vs. B − V color-magnitude diagram reaches 2.4 mag below the main-sequence turnoff (which is at VTO = 22.65 ±0.10 mag or M TO V = 4.00 ±0.16 mag). Comparing the fiducial sequence of Hodge 11 with that of the Galactic globular cluster M92, we conclude that, within the accuracy of our photometry, the age of Hodge 11 is identical to that of M92 with a relative age-difference uncertainty ranging from 10% to 21%. Provided that Hodge 11 has always been a part of the Large Magellanic Cloud and was not stripped from the halo of the Milky Way or absorbed from a cannibalized dwarf spheroidal galaxy, then the oldest stars in the Large Magellanic Clouds and the Milky Way appear to have the same age.

K giants in baade's window. I. velocity and line-strength measurements

This is the first in a series of papers in which we analyze medium--resolution spectra of over 400 K and M giants in Baades Window. Our sample was selected from the proper motion study of Spaenhauer et al. [AJ, 103, 297 (1992)]. We have measured radial velocities for most of the sample, as well as line--strength indices on the system of Faber et al. [ApJS, 57, 711 (1985)]. We analyze the random and systematic errors in velocities and line strengths, and show that the bright (V < 16.0) stars in our sample are predominantly foreground disk stars along the line--of--sight toward Baades Window. We find that most of the bulge K giants have stronger Mg absorption at a given color than do stars in the solar neighborhood. If the K giants in our sample are moderately old, we suggest that on average they may have [Mg/Fe] ~ +0.3, consistent with the results of recent high--resolution spectroscopy in Baades Window.

Hubble Space Telescope Observations of NGC 121: First Detection of Blue Stragglers in an Extragalactic Globular Cluster*

We have resolved stars in the core of the old SMC globular cluster NGC 121 with images from the refurbished Hubble Space Telescope and Wide Field Planetary Camera 2. Our photometry reaches 1.5 mag below the main-sequence turnoff, with negligible field contamination. A blue straggler star (BSS) sequence of 42 candidates is clearly seen—the first such detection in an extragalactic globular cluster. The BSSs are more centrally concentrated than the subgiant, red giant, and horizontal branch stars. Various blending and completeness tests demonstrate that at least 23 of our candidates are genuine BSSs. The BSSs extend about 1.8 mag brighter than the main-sequence turnoff, in accord with similar sequences in Galactic globular clusters. One BSS candidate is 2.4 mag brighter than the turnoff, as luminous as the brightest BSS seen in Galactic globulars. The cluster red horizontal branch is clearly detected and well populated, with a modest blueward extension. The relaxation times of the cluster stars and the BSSs are such that most BSSs are in energy equipartition with the other stars. We find ΔV= 3.3 ± 0.1 mag, corresponding to a cluster age that is 2 Gyr younger than that of most Galactic globular clusters and of one of the oldest LMC globulars.

The Proper Motion of NGC 6522 in Baade's Window*

We have detected seven stars with a common proper motion that are located within 25 of the globular cluster NGC 6522 in the Baades window field of the Galactic bulge. We argue that these stars are members of the cluster, and derive a weighted mean proper motion and heliocentric radial velocity of l = 1.4 ± 0.2 mas yr-1, b = -6.2 ± 0.2 mas yr-1 and = -28.5 ± 6.5 km s-1. We rederive the distance to NGC 6522 [(0.91 ± 0.04)R0, where R0 is the Galactocentric distance] and metallicity ([Fe/H] = -1.28 ± 0.12), making use of recent revisions in the foreground extinction toward the cluster (AV = 1.42 ± 0.05). We find the spatial velocity of the cluster and conclude that the cluster stays close to the Galactic center, and may have experienced significant bulge/disk shocking during its lifetime.