Denise Antoinette Hurley-Keller

M31's UNDISTURBED THIN DISK OF GLOBULAR CLUSTERS

We show that there is a subsystem of the M31 globular clusters with thin-disk kinematics. These clusters span the entire metallicity range of the M31 globular cluster system, in contrast to the (thick) disk globular clusters in the Milky Way, which are predominantly metal-rich. Disk globular clusters are found across the entire disk of M31 and form � 40% of the clusters projected on its disk. The existence of such a disk system suggests that there was a relatively large thin disk in place very early in M31’s history. Accurate measures of the ages of these clusters will constrain the epoch of disk formation in M31. There is currently no strong evidence for differences in age between Milky Way and M31 globular clusters. While age differences are subtle for old populations, it is unlikely that disk clusters with [Fe/H] around � 2.0 were formed after significant star formation began in the galaxy, as the protocluster gas would be enriched by supernova ejecta. Thus it is likely that M31 had a rather large disk in place at early epochs. The very existence of such a cold disk means that M31 has suffered no mergers with an object of 10% or more of the disk mass since the clusters were formed. This makes the suggestion of Brown et al. that M31 could have suffered an equal-mass merger 6–8 Gyr ago less viable. Subject headings: galaxies: evolution — galaxies: individual (M31) — galaxies: star clusters — globular clusters: general — Local Group

PLANETARY NEBULA KINEMATICS IN M31

We present kinematics of 135 planetary nebulae (PNs) in M31 from a survey covering 3.9 deg2 and extending out to 15 kpc from the southwest major axis and more than 20 kpc along the minor axis. The majority of our sample, even well outside the disk, shows significant rotational support (mean line-of-sight velocity 116 km s-1). We argue that these PNs belong to the outer part of M31s large R1/4 bulge. Only five PNs have velocities clearly inconsistent with this fast rotating bulge. All five may belong to tidal streams in M31s outer halo. One is projected on the Northern Spur and is counterrotating with respect to the disk there. Two are projected near the major axis at X = -10 kpc and have M32-like velocities; they could be debris from that galaxy. The remaining two halo PNs are located near the center of the galaxy, and their velocities follow the gradient found by Ibata and coworkers, implying that these PNs could belong to the Southern Stream. If M31 has a nonrotating, pressure-supported halo, we have yet to find it, and it must be a very minor component of the galaxy.

ANDROMEDA VIII: A NEW TIDALLY DISTORTED SATELLITE OF M31

We report the detection of a new satellite of M31, projected close to M32. Andromeda VIII is tidally distorted, with length ~10 kpc and width a few kiloparsecs. It contains 5-12 planetary nebulae and 1-3 globular clusters and has a velocity of -204 km s-1 with respect to M31, some 350 km s-1 away from M32s velocity. There are also ~4 × 105 M☉ of H I, well-separated from the disk, at the same position and velocity. The satellite has luminosity of 1.2-2.4 × 108 L☉ and a central surface brightness of the order of μV = 24. Both these values are typical of Local Group dwarf galaxies. Its surface brightness is some 6 mag brighter than any of the stellar streams found in the Milky Way or M31. The three associated globular clusters have reddening consistent with foreground reddening from the Milky Way only, making it likely that the satellite is in front of M31, unlike the giant tidal stream of Ibata et al., which is behind M31 in the southeast quadrant. However, the major axis of And VIII is aligned with the western edge of this giant stream, and we suggest that its unusual fan shape is caused by superposition of two streams, the westernmost of which was tidally stripped from And VIII.

Photometry Results for the Globular Clusters M10 and M12: Extinction Maps, Color-Magnitude Diagrams, and Variable Star Candidates

We report on photometry results of the equatorial globular clusters (GCs) M10 and M12. These two clusters are part of our sample of GCs, which we are probing for the existence of photometrically varying eclipsing binary stars. During the search for binaries in M10 and M12, we discovered the signature of differential reddening across the fields of the clusters. The effect is stronger for M10 than for M12. Using our previously described dereddening technique, we create differential extinction maps for the clusters, which dramatically improve the appearance of the color-magnitude diagrams (CMDs). Comparison of our maps with the dust emissivity maps of Schlegel, Finkbeiner, & Davis (SFD) shows good agreement in terms of spatial extinction features. Several methods of adding an EV-I zero point to our differential maps are presented, of which isochrone fitting proved to be the most successful. Our EV-I values fall within the range of widely varying literature values. More specifically, our reddening zero-point estimate for M12 agrees well with the SFD estimate, whereas the one for M10 falls below the SFD value. Our search for variable stars in the clusters produced a total of five variables: three in M10 and two in M12. The M10 variables include a binary system of the W Ursae Majoris (W UMa) type, a background RR Lyrae star, and an SX Phoenicis pulsator, none of which is physically associated with M10. M12s variables are two W UMa binaries, one of which is most likely a member of the cluster. We present the phased photometry light curves for the variable stars, estimate their distances, and show their locations in the fields and the CMDs of the GCs.

WFPC2 observations of LEO A: A Predominantly young galaxy within the local group

The unprecedented detail of the WFPC2 colour-magnitude diagrams of the resolved stellar population of Leo A presented here allows us to determine a new distance and an accurate star formation history for this extremely metal-poor Local Group dwarf irregular galaxy. From the position of the red clump, the helium-burning blue loops and the tip of the red giant branch, we obtain a distance modulus, m-M=24.2+/-0.2, or 690 +/- 60 kpc, which places Leo A firmly within the Local Group. Our interpretation of these features in the WFPC2 CMDs at this new distance based upon extremely low metallicity (Z=0.0004) theoretical stellar evolution models suggests that this galaxy is predominantly young, i.e. <2 Gyr old. A major episode of star formation 900 - 1500 Gyr ago can explain the red clump luminosity and also fits in with our interpretation of the number of anomalous Cepheid variable stars seen in this galaxy. We cannot rule out the presence of an older, underlying globular cluster age stellar population with these data. However, using the currently available stellar evolution models, it would appear that such an older population is limited to no more than 10% of the total star formation to have occured in this galaxy. Leo A provides a nearby laboratory for studying young metal poor stars and investigations of metal-poor galaxy evolution, such as is supposed to occur for larger systems at intermediate and high redshifts.