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RR Lyrae Variables in M33. I. Evidence For a Field Halo Population
We present observations of RR Lyrae variables in the Local Group late-type
spiral galaxy M33. Using the Advanced Camera for Surveys on the Hubble Space
Telescope, we have identified 64 ab-type RR Lyraes in M33. We have estimated
reddenings for these stars based on their minimum light V-I colors and
metallicities based on their periods. From the distributions of these
properties, we conclude that the RR Lyraes belong to two populations - one
associated with the halo of M33 and the other with its disk. Given that RR
Lyraes are produced by populations older than ~10 Gyr, this suggests that not
only does the field halo of M33 contain an old component, but so does its disk.
This is one of the best pieces of evidence for the existence of a halo field
component in M33. Using a relation between RR Lyrae absolute magnitude and
metallicity (Mv(RR) = 0.23[Fe/H] + 0.93), we estimate a mean distance modulus
of = 24.67 +/- 0.08 for M33. This places M33 approximately 70 kpc
beyond M31 in line-of-sight distance.Comment: 20 pages, 17 figures, accepted for publication in The Astronomical
Journa
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