WILLMAN 1-A PROBABLE DWARF GALAXY WITH AN IRREGULAR KINEMATIC DISTRIBUTION

Journal Article · Sat Oct 15 00:00:00 EDT 2011 · Astronomical Journal (New York, N.Y. Online)

DOI:https://doi.org/10.1088/0004-6256/142/4/128· OSTI ID:21582827

Willman, Beth; Warres, Alex ^[1]; Geha, Maria; Ho, Nhung ^[2]; Strader, Jay ^[3]; Strigari, Louis E ^[4]; Simon, Joshua D ^[5]; Kirby, Evan ^[6]

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1. Departments of Physics and Astronomy, Haverford College, Haverford, PA 19041 (United States)
2. Astronomy Department, Yale University, New Haven, CT 06520 (United States)
3. Harvard-Smithsonian CfA, Cambridge, MA 02144 (United States)
4. Kavli Institute for Particle Astrophysics and Cosmology, Stanford University, Stanford, CA 94305 (United States)
5. Observatories of the Carnegie Institution of Washington, Pasadena, CA 91101 (United States)
6. California Institute of Technology, Pasadena, CA 91106 (United States)

We investigate the kinematic properties and stellar population of the Galactic satellite Willman 1 (Wil 1) by combining Keck/DEIMOS spectroscopy with Kitt Peak National Observatory mosaic camera imaging. Wil 1, also known as SDSS J1049+5103, is a nearby, ultra-low luminosity Milky Way companion. This object lies in a region of size-luminosity space (M{sub V} {approx} -2 mag, d {approx} 38 kpc, r{sub half} {approx} 20 pc) also occupied by the Galactic satellites Booetes II and Segue 1 and 2, but no other known old stellar system. We use kinematic and color-magnitude criteria to identify 45 stars as possible members of Wil 1. With a systemic velocity of v{sub helio} = -12.8 {+-} 1.0 km s{sup -1}, Wil 1 stars have velocities similar to those of foreground Milky Way stars. Informed by Monte Carlo simulations, we identify 5 of the 45 candidate member stars as likely foreground contaminants, with a small number possibly remaining at faint apparent magnitudes. These contaminants could have mimicked a large velocity dispersion and abundance spread in previous work. The significant spread in the [Fe/H] of the highly likely Wil 1 red giant branch members ([Fe/H] = -1.73 {+-} 0.12 and -2.65 {+-} 0.12) supports the scenario that Wil 1 is an ultra-low luminosity dwarf galaxy, or the remnants thereof, rather than a star cluster. However, Wil 1's innermost stars move with radial velocities offset by 8 km s{sup -1} from its outer stars and have a velocity dispersion consistent with 0 km s{sup -1}, suggesting that Wil 1 may not be in dynamical equilibrium. The combination of the foreground contamination and unusual kinematic distribution make it difficult to robustly determine the dark matter mass of Wil 1. As a result, X-ray or gamma-ray observations of Wil 1 that attempt to constrain models of particle dark matter using an equilibrium mass model are strongly affected by the systematics in the observations presented here. We conclude that, despite the unusual features in the Wil 1 kinematic distribution, evidence indicates that this object is, or at least once was, a dwarf galaxy.

• OSTI ID: 21582827
• Journal Information: Astronomical Journal (New York, N.Y. Online), Vol. 142, Issue 4; Other Information: DOI: 10.1088/0004-6256/142/4/128; ISSN 1538-3881
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTROPHYSICS
COSMOLOGY AND ASTRONOMY
ABUNDANCE
COLOR
COMPUTERIZED SIMULATION
GAMMA RADIATION
LUMINOSITY
MILKY WAY
MONTE CARLO METHOD
NONLUMINOUS MATTER
RADIAL VELOCITY
SATELLITES
SPECTROSCOPY
STAR CLUSTERS
STARS
X RADIATION
CALCULATION METHODS
ELECTROMAGNETIC RADIATION
GALAXIES
IONIZING RADIATIONS
MATTER
OPTICAL PROPERTIES
ORGANOLEPTIC PROPERTIES
PHYSICAL PROPERTIES
RADIATIONS
SIMULATION
VELOCITY