Studying the stellar populations in the outskirts of spiral galaxies can provide important constraints on their structure, formation, and evolution. To that end, we present VI photometry obtained with the Advanced Camera for Surveys for three fields located ∼ 20 − 30 in projected distance southeast of M33’s nucleus (corresponding to ∼ 4 − 6 visual scale lengths or ∼ 9 − 13 kpc in deprojected ra...

This paper is the fourth in a series presenting spectrophotometry of 51 globular cluster candidates, that were detected by Mochejska et al. in the nearby galaxy M33 using the data collected by the DIRECT project. The frames of M33 in this study were taken as part of the BATC Multicolor Sky Survey. We obtained the spectral energy distributions of these candidates in 13 intermediate-band filters....

We present a new determination of the metallicity gradient in M33, based on Keck LRIS measurements of oxygen abundances using the temperature-sensitive emission line [O iii] k4363 in 61 H ii regions. These data approximately triple the sample of direct oxygen abundances in M33. We find a central abundance of 12þ log (O/H) 1⁄4 8:36 0:04 and a slope of 0:027 0:012 dex kpc , in agreement with infr...

We present panchromatic resolved stellar photometry for 22 million stars in the Local Group dwarf spiral Triangulum (M33), derived from Hubble Space Telescope (HST) observations with Advanced Camera Surveys (ACS) optical (F475W, F814W), and Wide Field 3 (WFC3) near ultraviolet (F275W, F336W) near-infrared (F110W, F160W) bands. The large, contiguous survey area covers $\sim$14 square kpc extends...

Early studies of molecular gas in M33 used the CO J=1-0 transition to map out the total gas content in the inner kiloparsec of the galaxy and to study the properties of the giant molecular cloud population. In this review, I discuss recent detailed studies of individual molecular clouds in M33 in higher rotational transitions of CO, CO, and the fine structure line of atomic carbon. These data r...

We present the first interferometric 12 CO (J = 1 → 0) map of the entire Hα disk of M33. The 13 ′′ diameter synthesized beam corresponds to a linear resolution of 50 pc, sufficient to distinguish individual giant molecular clouds (GMCs). From these data we generated a catalog of 148 GMCs with an expectation that no more than 15 of the sources are spurious. The catalog is complete down to GMC ma...

We present first results from a Keck/DEIMOS spectroscopic survey of red giant branch (RGB) stars in M33. The radial velocity distributions of the stars in our fields are well described by three Gaussian components, corresponding to a candidate halo component with an uncorrected radial velocity dispersion of σ ≃ 50 km s, a candidate disk component with a dispersion σ ≃ 16 km s, and a third compo...

The two nearby galaxies, M31 and M33, are stepping stones for most of our current efforts to understand the evolving universe at large scales. We are undertaking a long term project, called DIRECT, to improve the direct distance estimate to M31 and M33. The massive photometry we have obtained as part of our project over the past 3 years provides us with very good light curves for known and new ...

I review the observational constraints on the star formation histories in the spheroids of M33 and M31, the other two spiral galaxies in the Local Group. M33 does not possess a traditional bulge; instead, it has a small nuclear region hosting stars with a wide range of ages. The star formation history of the M33 halo is poorly constrained, but composite spectra of its halo globular clusters imp...