HST WFPC2 Color-Magnitude Diagrams of Halo Globular Clusters in M33: Implications for the Early Formation History of the Local Group

Abstract

We have constructed color-magnitude diagrams (CMDs) for ten globular clusters in the halo of the nearby spiral galaxy M33 based on Hubble Space Telescope Wide-Field Planetary Camera 2 observations in the F555W (~V) and F814W (~I) filters. These data reveal the morphology of the HB and allow us to estimate the cluster metallicity using the shape and color of the red giant branch. The principal result we report herein is that 8 of the 10 clusters possess exclusively red HB morphologies yet their metallicities are as metal-poor as [Fe/H] ~ -1.6. Indeed, these 8 clusters present basically only giant branch clumps reminiscent of intermediate age star clusters in the Magellanic Clouds. In addition, two of the clusters form a second parameter pair which have similar metallicities but very disparate HB types. Under the assumption that cluster age is the global second parameter, the average age of halo globular clusters in M33 appears to be a few Gyr younger than halo clusters in the Milky Way. Using the observed properties of HB stars in M31 and M33 along with published main-sequence turnoff ages for the globular clusters in the Milky Way, LMC, SMC, and the Sagittarius dwarf spheroidal (Sgr), we attempt to sketch the early formation history of these galaxies. This indicates that the Milky Way, M31, M33, the LMC, and Sgr all experienced their first epoch of cluster formation soon after the Big Bang. Three to four Gyr later, the SMC began to form its first generation of clusters; the bulk of the M33 clusters formed later still. We note that the halo clusters in M33 formed over a much longer time period than those in the Milky Way and M31.

13 pages, 3 figures, to be published in the Astrophysical Journal Letters

References in corpus (9)

• WFPC2 Observations of Star Clusters in the Magellanic Clouds. II. The Oldest Star Clusters in the Small Magellanic Cloud
• The Relative Ages of Galactic Globular Clusters
• CCD Photometry of the Globular Cluster M5. I. The Color-Magnitude Diagram and Luminosity Functions
• The Relative Ages of Galactic Globular Clusters
• NGC 2419, M92, and the Age Gradient in the Galactic Halo
• The \mv vs \fe Calibration: I. HST Color-Magnitude Diagrams of 8 Globular Clusters in M 31
• Horizontal branch morphology in galactic globular clusters: dense environment is "a" second parameter
• Deep HST V- and I-Band Observations of Two Globular Clusters in the Halo of M31
• The Globular Cluster M54 and the Star Formation History of the Sagittarius Dwarf Galaxy

Cited by in corpus (33)

• RR Lyrae Variables in M33. I. Evidence For a Field Halo Population
• The Globular Cluster Systems of Five Nearby Spiral Galaxies: New Insights from Hubble Space Telescope Imaging
• A Population of Massive Globular Clusters in NGC 5128
• Deep CCD Surface Photometry of the Edge-On Spiral NGC 4244
• The Metallicity Dependence of RR Lyrae Absolute Magnitudes from Synthetic Horizontal-Branch Models
• Determination of the Distance to M33 Based on the Tip of the Red Giant Branch and the Red Clump
• Evolutionary Histories of Dwarf Galaxies in the Local Group
• The Globular Cluster Systems of the Sculptor Group
• The Stellar Populations in the Outer Regions of M33. I. Metallicity Distribution Function
• Structural Parameters and Dynamical Masses for Globular Clusters in M33
• The Outer Regions of the Nearby Sc Galaxies NGC 2403 and M33: Evidence for an Intermediate Age Population at Large Radii
• Determination of the Distance to M33 Based on Single Epoch I-band HST Observations of Cepheids
• Horizontal-Branch Models and the Second-Parameter Effect. IV. The Case of M3 and Palomar 3
• Globular cluster systems II: On the formation of old globular clusters and their sites of formation
• The elusive old population of the dwarf spheroidal galaxy Leo I
• The chemical composition of globular clusters in the Local Group
• The Disk and Extraplanar Environment of NGC 247
• The M33 Globular Cluster System with PAndAS Data: The Last Outer Halo Cluster?
• Detailed abundance analysis of globular clusters in the Local Group: NGC 147, NGC 6822, and Messier 33
• The Disk and Extraplanar Regions of NGC 2403
• Newly Identified Star Clusters in M33. I. Integrated Photometry and Color-Magnitude Diagrams
• SED, Metallicity and Age of Halo Globular Clusters in M33
• A Genuine Intermediate-Age Globular Cluster in M33
• The Disk and Extraplanar Regions of NGC 55
• SAO-6m Telescope Spectroscopic Observations of Globular Clusters in Nearby Galaxies
• Metallicity Estimates for Old Star Clusters in M33
• Newly Identified Star Clusters in M33. III. Structural Parameters
• Deriving physical parameters of unresolved star clusters IV. The M33 star cluster system
• New $ubvri$ photometry of 234 M33 star clusters
• Spectral Energy Distributions of M81 Globular Clusters in BATC Multicolor Survey
• Structural Parameters for 10 Halo Globular Clusters in M33
• Second Parameter Globulars and Dwarf Spheroidals around the Local Group Massive Galaxies: What Can They Evidence?
• Metallicity distributions of globular cluster systems in galaxies