Collaborative Research: M31 Satellites Past and Present

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Collaborative Research: M31 Satellites Past and PresentAST 1010039, Puragra Guhathakurta, University of California-Santa Cruz (lead)AST-1009882, Steven R. Majewski, University of Virginia Main CampusAST-1009973, James Bullock, University of California-IrvineThis is a comprehensive study of the formation and evolution of the dwarf spheroidal (dSph) satellites of Andromeda (M31). These satellites within the Local Group include the least luminous galaxies known and they can be explored to test ideas about star formation in the smallest dark matter halos and at the earliest times.Extensive data sets on the resolved stellar populations of M31 satellites are combined with state-of-the-art galaxy models in order to learn how the dynamical -, merger -, assembly -, star formation -, and chemical enrichment histories of the current and past M31 satellite systems fit together in the context of the general paradigm of hierarchical galaxy formation. The data for M31 are compared to the satellites and halo population of the Milky Way galaxy for which were previously studied by the investigators. The Milky Way and M31 are both spiral galaxies but have different populations of satellite galaxies for which an explanations needs to be found. The additional data for M31 are important for evaluating solutions to the ?missing satellites problem? in Lambda-Cold Dark Matter (CDM) simulations, where the number of subhalos is predicted to be higher than the number of observed satellites. So far the Milky Way is the only system for which the comparison to models can be made which will change when the results of this study are available.The three principal investigators have a standing collaboration and they have initiated the SPLASH (Spectroscopic and Photometric Landscape of Andromeda's Stellar Halo) survey with previous support from the NSF. The methods and techniques used here are well established in this collaboration. The SPLASH survey uses the DEep Imaging Multi-Object Spectrograph (DEIMOS) on the Keck II telescope and the 4-m telescope with Mosaic CCD wide field Imager at Kitt Peak National Observatory. The survey extends to study red giant branch stars in more satellite galaxies of the Andromeda system. The group has already 139 DEIMOS masks (about 20,000 spectra) and spectroscopy of M31 red giant candidates selected from the imaging data. The goal is to observe another 135 DEIMOS masks to obtain spectra of more than 100 member red giant stars in each of the known and any newly found dSph satellites of M31. Further spectra are thought for about 3,000 stars in M33, a low-luminosity disk satellite interacting with M31; and about 1,000 stars in the NE shelf which is tidal debris from a past M31 satellite associated with the Giant Southern Stream in M31.One part of the work combines spectral and photometric diagnostics to isolate M31 red giants efficiently. Further there are measurements of precision stellar velocities to derive velocity dispersions in dwarf galaxies and tidal streams, which are both kinematically cold subsystems. Measurements of elemental abundances from moderate resolution co-added spectra tare used o constrain the chemical enrichment/star formation histories of M31 satellites. Theoretical models are used to determine the dark matter mass of each dwarf satellite galaxy in M31 to place the observations within a cosmological framework. This will give more data to decipher how the evolution of the Milky Way and M31 can be described in the context of Lambda-CDM cosmology.This work provides another data set for a galaxy system to test Lambda-CDM cosmologies. This will have a broad impact on other fields. The DEIMOS-derived chemical abundances are of interest for stellar population studies community. The new analytical methods developed here provide a foundation for similar studies of resolved stars in other stellar systems, including more distant galaxies to be targeted with the next generation of telescopes.Undergraduate students are participating in the research activities and graduate students work on parts of this project for their theses. This will help to train the next generation of scientists. The researchers make efforts to speak in K-12 classrooms and to reach out to underrepresented high-school and undergraduate students. Research results about the well-known large spiral galaxy Andromeda are well suited to be incorporated into public lectures and presentations which are given by the team members on a regular basis.