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 Lyrae stars 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 Lyrae stars belong to two populations: one associated with the halo of M33 and the other associated with its disk. Given that RR Lyrae stars 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 ⟨(m - M)0⟩ = 24.67 ± 0.08 for M33. This places M33 approximately 70 kpc beyond M31 in line-of-sight distance.