arXiv:0909.4540v1 [astro-ph.CO] 24 Sep 2009 To be published in ApJ Preprint typeset using L A T E X style emulateapj v. 08/22/09 A SPECTROSCOPIC PORTRAIT OF ANDROMEDA’S GIANT SOUTHERN STREAM Karoline M. Gilbert 1,2 , Puragra Guhathakurta 2 , Priya Kollipara 2 , Rachael L. Beaton 3 , Marla C. Geha 4 , Jason S. Kalirai 5 , Evan N. Kirby 6,2 , Steven R. Majewski 3 , and Richard J. Patterson 3 To be published in ApJ ABSTRACT The giant southern stream (GSS) is the most prominent tidal debris feature in M31’s stellar halo and covers a significant fraction of its southern quadrant. The GSS is a complex structure composed of a relatively metal-rich, high surface-brightness “core” and a lower metallicity, lower surface bright- ness “envelope.” We present spectroscopy of red giant stars in six fields in the vicinity of M31’s GSS (including four new fields and improved spectroscopic reductions for two previously published fields) and one field on Stream C, an arc-like feature seen in star-count maps on M31’s southeast minor axis at R ∼ 60 kpc. These data are part of our on-going SPLASH survey of M31 using the DEIMOS instru- ment on the Keck II 10-m telescope. Several GSS-related findings and measurements are presented here. We present the innermost kinematical detection of the GSS core to date (R = 17 kpc). This field also contains the inner continuation of a second kinematically cold component that was originally seen in a GSS core field at R ∼ 21 kpc. The velocity gradients of the GSS and the second component in the combined data set are parallel over a range of ΔR = 7 kpc, suggesting that this may represent a bifurcation in the line-of-sight velocities of GSS stars. We present the first kinematical detection of substructure in the GSS envelope (S quadrant, R ∼ 58 kpc). Using kinematically identified samples, we show that the envelope debris has a ∼ 0.7 dex lower mean photometric metallicity and possibly higher intrinsic velocity dispersion than the GSS core. The GSS is also identified in the field of the M31 dwarf spheroidal satellite And I; the GSS in this field has a metallicity distribution identical to that of the GSS core. We confirm the previous finding of two kinematically cold components in Stream C, and measure intrinsic velocity dispersions of ∼ 10 and ∼ 4 km s −1 . This compilation of the kinematical (mean velocity, intrinsic velocity dispersion) and chemical properties of stars in the GSS core and envelope, coupled with published surface brightness measurements and wide-area star-count maps, should improve constraints on the orbit and internal structure of the dwarf satellite progenitor. Subject headings: galaxies: halo — galaxies: individual (M31) — stars: kinematics — techniques: spectroscopic 1. INTRODUCTION Galaxy mergers play a key role in galaxy formation and evolution (e.g., Searle & Zinn 1978; White & Rees 1978; Abraham et al. 1996; Springel et al. 2005). Observations have uncovered abundant evidence of the ubiquity of mi- nor mergers in the form of tidal streams. Those dis- covered in our own Milky Way (MW) include the Magel- lanic stream (Mathewson et al. 1974), Monoceros stream (Yanny et al. 2003; Rocha-Pinto et al. 2003), and Sagit- tarius Stream (Ibata et al. 1994; Majewski et al. 2003; Newberg et al. 2003). Detailed studies of tidal debris features can provide insight into statistical models of hi- erarchical structure formation, the structure and merger history of an individual galaxy, and the properties of dwarf satellite systems. Due to their sparse stellar den- Electronic address: kgilbert@astro.washington.edu 1 Department of Astronomy, University of Washington, Box 351580, Seattle, WA, 98195-1580. 2 UCO/Lick Observatory, Department of Astronomy & Astro- physics, University of California Santa Cruz, 1156 High Street, Santa Cruz, California 95064. 3 Department of Astronomy, University of Virginia, P.O. Box 400325, Charlottesville, VA 22904-4325. 4 Astronomy Department, Yale University, P.O. Box 208101, New Haven, CT 06520-8101. 5 Space Telescope Science Institute, 3700 San Martin Drive, Bal- timore, MD 21218. 6 Astronomy Department, California Institute of Technology, 1200 East California Blvd, Pasadena, CA 91125. sity and long dynamical times, stellar halos are ideal for investigating the merger history of an individual galaxy in detail as it is possible for higher density tidal debris features to remain identifiable in phase-space for billions of years. Observations of the Galaxy and theoretical arguments indicate that present-day dwarf satellite galaxies are dif- ferent from the earlier generations of dwarf galaxies that merged to form the bulk of the virialized stellar halo. Stars in the classical satellites of the Galaxy on aver- age have lower relative abundances of alpha elements than stars of similar metallicities in the Galaxy’s halo (Fuhrmann 1998; Shetrone et al. 2001, 2003; Venn et al. 2004; Geisler et al. 2007), although the relative abun- dances of alpha elements are similar in the lowest metal- licity stars (Kirby et al. 2009; Frebel et al. 2009). This has been shown to be a natural consequence of the hierar- chical formation framework: simulations of stellar halo formation indicate that the majority of stellar mass in the halo was contributed by massive satellite galaxies that were accreted and disrupted early on, while surviv- ing satellites are typically accreted much later (result- ing in more metal-enrichment via Type Ia supernovae) and are on average less massive (and experienced less efficient star formation) than the average halo building block (Robertson et al. 2005; Font et al. 2006a). Minor mergers caught in the process of disruption bridge the gap between the surviving and completely