Astronomy and Astrophysics – Astrophysics
Scientific paper
2008-03-17
Astronomy and Astrophysics
Astrophysics
10 figures, accepted to Astronomical Journal
Scientific paper
10.1088/0004-6256/135/6/2084
Willman 1 is a small low surface-brightness object identified in the Sloan Digital Sky Survey and tentatively classified as a very low luminosity dSph galaxy. Further study has supported this classification while hinting that it may be undergoing disruption by the Milky Way potential. In an effort to better constrain the nature of Willman 1, we present a comprehensive analysis of the brightest stars in a 0.6 square degree field centered on the overdensity. High-resolution HET spectra of two previously identified Willman 1 RGB stars show that one is a metal-rich foreground dwarf while the other is a metal-poor giant. The one RGB star that we confirm as a member of Willman 1 has a low metallicity ([Fe/H]=-2.2) and a surprisingly low alpha-element abundance ([alpha/Fe]=-0.11). Washington+DDO51 photometry indicates that 2-5 of the seven brightest Willman 1 stars identified in previous studies are actually dwarf stars, including some of the more metal-rich stars that have been used to argue both for an abundance spread and a more metal-rich stellar population than galaxies of similar luminosity. The remaining stars are too blue or too faint for photometric classification. The Washington+DDO51 photometry identifies three potential RGB stars in the field but HET spectra show that they are background halo stars. Time series photometry identifies one apparent variable star in the field, but it is unlikely to be associated with Willman 1. Our wide-field survey indicates that over 0.6 square degrees, Willman 1 does not have a single RR Lyrae star, a single BHB star or a single RGB star beyond its tidal radius. While our results confirm that Willman 1 is most likely a low-luminosity metal-poor dSph galaxy, the possibility remains that it is a tidally disrupted metal-poor globular cluster.
Irwin Mike
Shetrone Mathew D.
Siegel Michael Hiram
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