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[21.01] Stars in the Hubble Ultra Deep Field

N. Pirzkal, K. C. Sahu (STScI), A. J. Burgasser (Department of Astrophysics, Division of Physical Sciences, American Museum of Natural History), C, Xu, S. Malhotra, J. E. Rhoads (STScI), GRAPES Collaboration

We identify 46 unresolved objects down to i₇₇₅=29.5 (AB mag) in the Hubble Ultra Deep Field. This identification is based on a parameter S, which measures the deviation from the curve-of-growth of a point source. Extensive testing of this parameter is presented, including the effects of decreasing signal-to-noise and of the apparent motions of stars. Low resolution grism spectra of 29 objects brighter than i₇₇₅=27.5 mag identify 18 M and later stellar type dwarfs, 2 candidate L-dwarfs, 2 QSOs, and 5 white dwarfs. Using the observed population of M4 and later dwarfs, we derive a Galactic disk scale height of 400 ±100 pc for the M and L stellar population. The local white dwarf density is computed to be as low as (5.4 ±3.8) X 10^-3 stars/pc³ and as high as ( 1.9 ±0.6) X 10^-2 stars/pc³. Based on observations taken 73 days apart, no object in the field is observed to have a proper motion that is larger than 0.027''/year (3 sigma detection limit). We do not find any white dwarf with a proper motion that is large enough to unambiguously be placed in the Galactic halo. This is however consistent with an old halo white dwarf population, which would render them undetectable in this study.

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Bulletin of the American Astronomical Society, 36 5
© 2004. The American Astronomical Society.