Discovery of a Dynamical Cold Point in the Heart of the Sagittarius dSph Galaxy with Observations from the APOGEE Project

Majewski, Steven R. and Hasselquist, Sten and Lokas, Ewa L. and Nidever, David L. and Frinchaboy, Peter M. and Garcia Perez, Ana E. and Johnston, Kathryn V. and Mészáros, Szabolcs and Shetrone, Matthew and Allende, Prieto Carlos and Beaton, Rachael L. and Beers, Timothy C. and Bizyaev, Dmitry and Cunha, Katia and Damke, Guillermo and Ebelke, Garrett and Eisenstein, Daniel J. and Hearty, Fred and Holtzman, Jon and Johnson, Jennifer A. and Law, David R. and Malanushenko, Viktor and Malanushenko, Elena and O'Connell, Robert W. and Oravetz, Daniel and Pan, Kaike and Schiavon, Ricardo P. and Schneider, Donald P. and Simmons, Audrey and Skrutskie, Michael F. (2013) Discovery of a Dynamical Cold Point in the Heart of the Sagittarius dSph Galaxy with Observations from the APOGEE Project. ASTROPHYSICAL JOURNAL LETTERS, 777 (1). ISSN 2041-8205

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Abstract

The dynamics of the core of the Sagittarius (Sgr) dwarf spheroidal (dSph) galaxy are explored using high-resolution (R∼22, 500), H-band, near-infrared spectra of over 1,000 giant stars in the central 3 deg2 of the system, of which 328 are identified as Sgr members. These data, among some of the earliest observations from the SDSS-III/Apache Point Observatory Galactic Evolution Experiment (APOGEE) and the largest published sample of high resolution Sgr dSph spectra to date, reveal a distinct gradient in the velocity dispersion of Sgr from 11-14 km s−1 for radii > 0.8◦ from center to a dynamical cold point of 8 km s−1 in the Sgr center — a trend differing from that found in previous kinematical analyses of Sgr over larger scales that suggest a more or less flat dispersion pro- file at these radii. Well-fitting mass models with either cored and cusped dark matter distributions can be found to match the kinematical results, although the cored profile succeeds with significantly more isotropic stellar orbits than re- quired for a cusped profile. It is unlikely that the cold point reflects an unusual mass distribution. The dispersion gradient may arise from variations in the mix- ture of populations with distinct kinematics within the dSph; this explanation is suggested (e.g., by detection of a metallicity gradient across similar radii), but not confirmed, by the present data. Despite these remaining uncertainties about their interpretation, these early test data (including some from instrument com- missioning) demonstrate APOGEE’s usefulness for precision dynamical studies, even for fields observed at extreme airmasses.

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