Harvey B. Richer

The remnants of galaxy formation from a panoramic survey of the region around M31.

In hierarchical cosmological models, galaxies grow in mass through the continual accretion of smaller ones. The tidal disruption of these systems is expected to result in loosely bound stars surrounding the galaxy, at distances that reach 10–100 times the radius of the central disk. The number, luminosity and morphology of the relics of this process provide significant clues to galaxy formation history, but obtaining a comprehensive survey of these components is difficult because of their intrinsic faintness and vast extent. Here we report a panoramic survey of the Andromeda galaxy (M31). We detect stars and coherent structures that are almost certainly remnants of dwarf galaxies destroyed by the tidal field of M31. An improved census of their surviving counterparts implies that three-quarters of M31’s satellites brighter than Mv = -6 await discovery. The brightest companion, Triangulum (M33), is surrounded by a stellar structure that provides persuasive evidence for a recent encounter with M31. This panorama of galaxy structure directly confirms the basic tenets of the hierarchical galaxy formation model and reveals the shared history of M31 and M33 in the unceasing build-up of galaxies.

BINARIES IN GLOBULAR-CLUSTERS

Binary stars in a globular cluster (hereafter, GC) may be primordial (i.e. formed along with the cluster), or the result of cluster dynamics. “Dynamical” binaries can result from conservative three-body encounters (e.g. Spitzer, 1987) if a third star can carry away enough kinetic energy to leave two others bound, or from dissipative two-body encounters, if two stars happen to pass within a few stellar radii of one other (Fabian, Pringle, & Rees, 1975). Such non-primordial systems are likely to be found primarily in evolved GC cores, both because conditions are more favorable for making them there, and because of mass segregation. Knowledge of the formation process allows reasonable estimates to be made of their mass and energy distributions. The initial spatial, mass, and energy distributions of primordial binaries, on the other hand, are largely unknown.

The Initial-Final Mass Relation: Direct Constraints at the Low-Mass End* **

The initial-final mass relation represents a mapping between the mass of a white dwarf remnant and the mass that the hydrogen-burning main-sequence star that created it once had. The empirical relation thus far has been constrained using a sample of ~40 stars in young open clusters, ranging in initial mass from ~2.75 to 7 -->M?, and shows a general trend that connects higher mass main-sequence stars with higher mass white dwarfs. In this paper, we present CFHT CFH12K photometric and Keck LRIS multiobject spectroscopic observations of a sample of 22 white dwarfs in two older open clusters, NGC 7789 ( -->t = 1.4 Gyr) and NGC 6819 ( -->t = 2.5 Gyr). We measure masses for the highest signal-to-noise ratio spectra by fitting the Balmer lines to atmosphere models and place the first direct constraints on the low-mass end of the initial-final mass relation. Our results indicate that the observed general trend at higher masses continues down to low masses, with -->Minitial = 1.6 M? main-sequence stars forming -->Mfinal = 0.54 M? white dwarfs. When added to our new data from the very old cluster NGC 6791, the relation is extended down to -->Minitial = 1.16 M? (corresponding to -->Mfinal = 0.53 M?). This extension of the relation represents a fourfold increase in the total number of hydrogen-burning stars for which the integrated mass loss can now be calculated from empirical data, assuming a Salpeter initial mass function. The new leverage at the low-mass end is used to derive a purely empirical initial-final mass relation. The sample of white dwarfs in these clusters also shows several interesting systems that we discuss further: a DB (helium) white dwarf, a magnetic white dwarf, a DAB (mixed hydrogen/helium atmosphere or a double degenerate DA+DB) white dwarf(s), and two possible equal-mass DA double degenerate binary systems.

The White Dwarf Cooling Sequence of the Globular Cluster Messier 4

We present the white dwarf sequence of the globular cluster M4, based on a 123 orbit Hubble Space Telescope exposure, with a limiting magnitude of V ~ 30 and I ~ 28. The white dwarf luminosity function rises sharply for I > 25.5, consistent with the behavior expected for a burst population. The white dwarfs of M4 extend to approximately 2.5 mag fainter than the peak of the local Galactic disk white dwarf luminosity function. This demonstrates a clear and significant age difference between the Galactic disk and the halo globular cluster M4. Using the same standard white dwarf models to fit each luminosity function yields ages of 7.3 ± 1.5 Gyr for the disk and 12.7 ± 0.7 Gyr for M4 (2 σ statistical errors).

A Young White Dwarf Companion to Pulsar B1620-26: Evidence for Early Planet Formation

The pulsar B1620-26 has two companions, one of stellar mass and one of planetary mass. We detected the stellar companion with the use of Hubble Space Telescope observations. The color and magnitude of the stellar companion indicate that it is an undermassive white dwarf (0.34 ± 0.04 solar mass) of age 480 × 106 ± 140 × 106 years. This places a constraint on the recent history of this triple system and supports a scenario in which the current configuration arose through a dynamical exchange interaction in the cluster core. This implies that planets may be relatively common in low-metallicity globular clusters and that planet formation is more widespread and has happened earlier than previously believed.

White Dwarfs in Globular Clusters: Hubble Space Telescope Observations of M4*

Using WFPC2 on the Hubble Space Telescope, we have isolated a sample of 258 white dwarfs (WDs) in the Galactic globular cluster M4. Fields at three radial distances from the cluster center were observed, and sizable WD populations were found in all three. The location of these WDs in the color-magnitude diagram, their mean mass of 0.51(±0.03) M☉, and their luminosity function confirm basic tenets of stellar evolution theory and support the results from current WD cooling theory. The WDs are used to extend the cluster main-sequence mass function upward to stars that have already completed their nuclear evolution. The WD/red dwarf binary frequency in M4 is investigated and is found to be at most a few percent of all the main-sequence stars. The most ancient WDs found are ~9 Gyr old, a level that is set solely by the photometric limits of our data. Even though this is less than the age of M4, we discuss how these cooling WDs can eventually be used to check the turnoff ages of globular clusters and hence constrain the age of the universe.

Stellar Evolution in NGC 6791: Mass Loss on the Red Giant Branch and the Formation of Low-Mass White Dwarfs* **

We present the first detailed study of the properties (temperatures, gravities, and masses) of the NGC 6791 white dwarf population. This unique stellar system is both one of the oldest (8 Gyr) and most metal-rich ([Fe/H] ~ +0.4) open clusters in our Galaxy and has a color-magnitude diagram (CMD) that exhibits both a red giant clump and a much hotter extreme horizontal branch. Fitting the Balmer lines of the white dwarfs in the cluster using Keck/LRIS spectra suggests that most of these stars are undermassive, M = 0.43 ± 0.06 M☉, and therefore could not have formed from canonical stellar evolution involving the helium flash at the tip of the red giant branch. We show that at least 40% of NGC 6791s evolved stars must have lost enough mass on the red giant branch to avoid the flash and therefore did not convert helium into carbon-oxygen in their core. Such increased mass loss in the evolution of the progenitors of these stars is consistent with the presence of the extreme horizontal branch in the CMD. This unique stellar evolutionary channel also naturally explains the recent finding of a very young age (2.4 Gyr) for NGC 6791 from white dwarf cooling theory; helium-core white dwarfs in this cluster will cool ~3 times slower than carbon-oxygen-core stars, and therefore the corrected white dwarf cooling age is in fact 7 Gyr, consistent with the well-measured main-sequence turnoff age. These results provide direct empirical evidence that mass loss is much more efficient in high-metallicity environments and therefore may be critical in interpreting the ultraviolet upturn in elliptical galaxies.

A Trio of New Local Group Galaxies with Extreme Properties

We report on the discovery of three new dwarf galaxies in the Local Group. These galaxies are found in new CFHT/MegaPrime g,i imaging of the southwestern quadrant of M31, extending our extant survey area to include the majority of the southern hemisphere of M31s halo out to 150 kpc. All these galaxies have stellar populations which appear typical of dwarf spheroidal (dSph) systems. The first of these galaxies, Andromeda XVIII, is the most distant Local Group dwarf discovered in recent years, at ~1.4 Mpc from the Milky Way (~600 kpc from M31). The second galaxy, Andromeda XIX, a satellite of M31, is the most extended dwarf galaxy known in the Local Group, with a half-light radius of rh ~ 1.7 kpc. This is approximately an order of magnitude larger than the typical half-light radius of many Milky Way dSphs, and reinforces the difference in scale sizes seen between the Milky Way and M31 dSphs (such that the M31 dwarfs are generally more extended than their Milky Way counterparts). The third galaxy, Andromeda XX, is one of the faintest galaxies so far discovered in the vicinity of M31, with an absolute magnitude of order MV ~ − 6.3. Andromeda XVIII, XIX, and XX highlight different aspects of, and raise important questions regarding, the formation and evolution of galaxies at the extreme faint end of the luminosity function. These findings indicate that we have not yet sampled the full parameter space occupied by dwarf galaxies, although this is an essential prerequisite for successfully and consistently linking these systems to the predicted cosmological dark matter substructure.

The galaxy hosts and large-scale environments of short-hard (gamma)-ray bursts

The rapid succession of discoveries of short-duration hard-spectrum gamma-ray bursts (GRBs) has led to unprecedented insights into the energetics of the explosion and nature of the progenitors. Yet short of the detection of a smoking gun, such as a burst of coincident gravitational radiation or a Li-Paczynski minisupernova, it is unlikely that a definitive claim can be made for the progenitors. As was the case with long-duration soft-spectrum GRBs, however, the expectation is that a systematic study of the hosts and locations of short GRBs could begin to yield fundamental clues as to their nature. We present an aggregate study of the host galaxies of short-duration hard-spectrum GRBs. In particular, we present the Gemini-North and Keck discovery spectra of the galaxies that hosted three short GRBs and a moderate-resolution (R ≈ 6000) spectrum of a fourth host. We find that these short-hard GRBs originate in a variety of low-redshift (z < 1) environments that differ substantially from those of long-soft GRBs, both on individual galaxy scales and on galaxy-cluster scales. Specifically, three of the bursts are found to be associated with old and massive galaxies with no current (<0.1 M☉ yr-1) or recent star formation. Two of these galaxies are located within a cluster environment. These observations support an origin from the merger of compact stellar remnants, such as double neutron stars or a neutron star-black hole binary. The fourth event, in contrast, occurred within a dwarf galaxy with a star formation rate exceeding 0.3 M☉ yr-1. Therefore, it appears that like supernovae of Type Ia, the progenitors of short-hard bursts are created in all galaxy types, suggesting a corresponding class with a wide distribution of delay times between formation and explosion.

Hubble Space Telescope observations of the white dwarf cooling sequence of M4

We investigate in detail the white dwarf cooling sequence of the globular cluster Messier 4. In particular, we study the influence of various systematic uncertainties, both observational and theoretical, on the determination of the cluster age from the white dwarf cooling sequence. These include uncertainties in the distance to the cluster and the extinction along the line of sight, as well as the white dwarf mass, envelope, and core compositions and the white dwarf-main-sequence mass relation. We find that fitting to the full two-dimensional color-magnitude diagram offers a more robust method for age determination than the traditional method of fitting the one-dimensional white dwarf luminosity function. After taking into account the various uncertainties, we find a best-fit age of 12.1 Gyr, with a 95% lower limit of 10.3 Gyr. We also perform fits using two other sets of cooling models from the literature. The models of Chabrier et al. yield an encouragingly similar result, although the models of Salaris et al. do not provide as good a fit. Our results support our previous determination of a delay between the formation of the Galactic halo and the onset of star formation in the Galactic disk.