HST/ACS OBSERVATIONS OF RR LYRAE STARS IN SIX ULTRA-DEEP FIELDS OF M31

Abstract

We present HST/ACS observations of RR Lyrae variable stars in six ultra-deep fields of the Andromeda galaxy (M31), including parts of the halo, disk, and giant stellar stream. Past work on the RR Lyrae stars in M31 has focused on various aspects of the stellar populations that make up the galaxy's halo, including their distances and metallicities. This study builds upon this previous work by increasing the spatial coverage (something that has been lacking in previous studies) and by searching for these variable stars in constituents of the galaxy not yet explored. Besides the 55 RR Lyrae stars we found in our initial field located 11 kpc from the galactic nucleus, we find additional RR Lyrae stars in four of the remaining five ultra-deep fields as follows: 21 in the disk, 24 in the giant stellar stream, three in the halo field 21 kpc from the galactic nucleus, and five in one of the halo fields at 35 kpc. No RR Lyrae stars were found in the second halo field at 35 kpc. The RR Lyrae populations of these fields appear to be mostly of Oosterhoff I type, although the 11 kpc field appears to be intermediate or mixed. We will discuss the properties of these stars including period and reddening distributions. We calculate metallicities and distances for the stars in each of these fields using different methods and compare the results, to an extent that has not yet been done. We compare these methods not just on RR Lyrae stars in our M31 fields, but also on a data set of Milky Way field RR Lyrae stars.

An ancient metal-poor population in M32, and halo satellite accretion in M31, identified by RR Lyrae stars

“…This is in contrast to the singlepeaked distributions of the M32 field and all previous M31 fields published to date (e.g. S09; Fiorentino et al 2010;Jeffery et al 2011). Fig.…”

“…The work of Jeffery et al (2011) is a continuation of the work of Brown et al (2004) and presents high-quality light curves for RR Lyraes in five fields around M31. These include a halo field at 21 kpc and two halo fields at 35 kpc.…”

“…The study of RR Lyraes in M31 has a rich history dating back to the seminal work of Pritchet & van den Bergh (1987) and culminating in the most recent papers by Sarajedini et al (2009) and Jeffery et al (2011). Sarajedini et al (2009, hereafter S09) present Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) observations for two fields in the range of 4-6 kpc from the centre of M31.…”

“…In addition, there is a field coincident with one of the streams in the vicinity of M31 and one that covers a disc region about 26 kpc from the centre along the major axis of the galaxy. Besides identifying a number of RR Lyrae variables in these fields, Jeffery et al (2011) also compare three methods for the determination of RR Lyrae metallicities. We note in passing also that Bernard et al (2012) present RR Lyraes in two M31 fields along the major axis based on HST/ACS observations.…”

“…The points designated 'M32' and 'Control' are from the present work. The halo11, halo21, halo35a and halo35b fields are the minor axis fields taken from the work of Jeffery et al (2011), while the stream and disc points also come from Jeffery et al (2011). The warp and outer disc fields are from the paper by Bernard et al (2012).…”