Trimming down the Willman 1 dSph

Michael H. Siegel, Matthew D. Shetrone, Michael Irwin

Research output: Contribution to journalArticlepeer-review

21 Scopus citations


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 deg2 field centered on the overdensity. High-resolution Hobby-Eberly Terlescope (HET) spectra of two previously identified Willman 1 red giant branch (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 α-element abundance ([α/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 deg2, Willman 1 does not have a single RR Lyrae star, a single blue horizontal branch (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.

Original languageEnglish (US)
Pages (from-to)2084-2094
Number of pages11
JournalAstronomical Journal
Issue number6
StatePublished - Jun 1 2008

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


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