Rodrigo A. Ibata

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.

Galactic Indigestion: Numerical Simulations of the Milky Way's Closest Neighbor

Are dwarf spheroidal galaxies dark matter dominated? We present N-body simulations of the interaction between the Milky Way and its closest companion, the Sagittarius dwarf spheroidal galaxy, constrained by new kinematic, distance, and surface density observations (detailed in a companion paper). It is shown that there is no possible self-consistent solution to the present existence of the Sagittarius dwarf if its distribution of luminous matter traces the underlying distribution of mass. The luminous component of the dwarf galaxy must therefore be shielded within a small dark matter halo. Although at present we are unable to construct a fully self-consistent model that includes both the stellar and dark matter components, it is shown numerically that it is possible that a pure dark matter model, approximating the dark matter halo deduced for the Sagittarius dwarf from analytical arguments, may indeed survive the Galactic tides. The orbit of the Sagittarius dwarf around the Milky Way is considered, taking into account the perturbative effects of the Magellanic Clouds. It is shown that at the present time, the orbital period must be short, ~0.7 Gyr; the initial orbital period for a 109 M☉ model will have been ~1 Gyr. It is found that a close encounter with the Magellanic Clouds may have occurred, although the chances of such an interaction affecting the orbit of the Sagittarius dwarf are negligible.

The White Dwarf Cooling Age of M67

A deep imaging survey covering the entire 23 diameter of the old open cluster M67 to V=25 has been carried out using the mosaic imager (UHCam) on the Canada-France-Hawaii Telescope. The cluster color-magnitude diagram (CMD) can be traced from stars on its giant branch at MV=+1 down through main-sequence stars at least as faint as MV=13.5. Stars this low in luminosity have masses below 0.15 M☉. A modest white dwarf (WD) cooling sequence is also observed commencing slightly fainter than MV=10 and, after correction for background galaxy and stellar field contamination, terminating near MV=14.6. The observed WDs follow quite closely a theoretical cooling sequence for 0.7 M☉ pure carbon core WDs with hydrogen-rich atmospheres (DA WDs). The cooling time to an MV of 14.6 for such WDs is 4.3 Gyr, which we take as the WD cooling age of the cluster. A fit of a set of isochrones to the cluster CMD indicates a turnoff age of 4.0 Gyr. The excellent agreement between these results suggests that ages derived from white dwarf cooling should be considered as reliable as those from other dating techniques. The WDs currently contribute about 9% of the total cluster mass, but the number seen appears to be somewhat low when compared with the number of giants observed in the cluster.

Hubble Space Telescope Observations of White Dwarfs in the Globular Cluster M4

With the Wide Field Planetary Camera 2 (WFPC2) on the Hubble Space Telescope, we have discovered in M4 (NGC 6121, C1620-264) the first extensive sequence of cooling white dwarfs seen in a globular cluster. Adopting a distance modulus of (m - M)V = 12.65 and a reddening of E(B - V) = 0.37, we show that the sequence, which extends over 9 < MU < 13, is comprised of white dwarfs of mass ~0.5 M☉. The total mass loss from the present turnoff to the white dwarf sequence is 0.31 M☉, and the intrinsic dispersion in the mean mass appears to be <0.05 M☉. Both the location of the white dwarf cooling sequence in the cluster color-magnitude diagram and the cumulative luminosity function attest to the basic correctness and completeness of the physics in theoretical models for the upper three magnitudes of the observed white dwarf cooling sequence. To test the theory in globular clusters at cooling ages beyond ~3 × 108 yr will require deeper and more complete data.

HUBBLE SPACE TELESCOPE Photometry of the Globular Cluster M4

This paper presents a detailed description of the acquisition and processing of a large body of imaging data for three fields in the globular cluster M4 taken with the Wide Field and Planetary Camera 2 aboard the Hubble Space Telescope. Analysis with the ALLFRAME package yielded the deepest photometry yet obtained for this cluster. The resulting data set for 4708 stars (positions and calibrated photometry in V, I, and, in two fields, U) spanning approximately six cluster core radii is presented. The scientific analysis is deferred to three companion papers, which investigate the significant white dwarf population discovered and the main-sequence population.

A panoramic view of the Southern quadrant of the Andromeda galaxy outer halo

How clumpy are galactic halos? Recent observations around both the Milky Way and the Andromeda galaxy (M31) have revealed numerous faint stellar streams and dwarf galaxies, leading to the belief that more of these may yet remain undetected. In this contribution, we present the map produced from the Megacam/CFHT survey that our group has undertaken in the outer halo of M31 and that, for the first time, gives a deep panoramic view of a significant region of the outer halo of a spiral galaxy. This panoramic survey, which covers ∼ 60 sq. deg. of the southern quadrant of the M31 halo, extends the WFC/INT survey of the inner halo (Ferguson et al. 2002) from a projected distance of ∼ 50 to ∼ 150 kpc. It is deep enough to cover three magnitudes below the tip of the red giant branch of stellar populations at the distance of M31. The survey reveals: • Three faint dwarf galaxies with absolute magnitudes in the range −7.3 < MV < −6.4 and the most remote M31 globular cluster at a projected distance of ∼ 120 kpc from M31 (see Martin et al. 2006 for more details). • That the giant stream of Ibata et al. (2001) covers a much wider area than previously expected from shallower surveys, has an apocenter at 125 ± 25 kpc from M31 and is probably due to the accretion of a small disk galaxy. • A new stellar stream or shell approximately perpendicular to the minor axis of M31 at a projected distance of 120 kpc and with a metallicity of [Fe/H] ∼ −1.5 (assuming it is at the distance of M31). • A new stellar stream along the ma jor axis of M31 that extends to at least 100 kpc from M31 with [Fe/H] ∼ −1.3 (once again assuming it is at the distance of M31). • Regions void of any stellar structure brighter than 34− 35mag/arcsec at a distance of 100 to 130 kpc from M31. The survey shows that the outer halo of the Andromeda galaxy is very structured, in qualitative agreement with recent cosmological simulations (e.g. Bullock & Johnston, 2005).