Star clusters in M33 and NGC 6822.

摘要

We undertake the investigation of cluster systems in two Local Group galaxies, M33 and NGC 6822.; At a distance of ∼800 kpc, M33 is the only nearby, late-type spiral galaxy, and as such provides an important link between the cluster populations of the large spirals (the Milky Way and M31) and the numerous dwarf galaxies in the Local Group. In twenty Hubble Space Telescope WFPC2 fields in M33 (imaged in U, B, and V) sixty clusters were detected, of which 49 were not previously identified. Luminosities and colors were measured, and compared with stellar evolutionary models to extract cluster ages and masses. Quantitative cluster properties were used to construct the star formation history of M33. We find a continuous range of cluster ages from 4 x 106 to ∼1010 years, cluster masses between 4 x 102 M⊙ and 3 x 105 M⊙ , and core radii from less than 0.2 parsecs to ∼2 parsecs. The results indicate that compact clusters with masses >103 M⊙ have formed over M33's lifetime. There is a peak in cluster formation, corresponding to the most recent epoch (107-10 8 years ago) of star formation in the M33 "bulge". Examination of twelve archive HST WFPC2 fields imaged in V and I resulted in the detection of 72 additional clusters (59 were previously unknown), and yielded 20 new globular cluster candidates. However, there is some indication that many of these have ages 3-- 9 Gyrs, several Gyrs younger than typical Galactic globular clusters. Most of the other clusters have properties similar to the "populous" clusters found in the Magellanic Clouds. Ground based, integrated spectra of M33 clusters (from APO and WIYN) confirm photometric age estimates.; Surface brightness profiles were compared with single mass King models to provide the first quantitative size estimates for M33 clusters. M33 clusters are more compact on average than LMC and Galactic globulars, and the first candidate core collapse object in this galaxy was discovered.; Radial velocity measurements for 113 clusters from follow up ground based (WIYN) spectra demonstrated that velocity dispersion increases with age. Young M33 clusters rotate with the HI disk. The old cluster population meanwhile, exhibits a bimodal velocity dispersion distribution, correlated with cluster position in M33. Additionally, halo candidates have a large age spread (∼10--15 Gyrs) indicating that either (1) the M33 halo formed over timescales ∼2--3 times as long as the Milky Way spheroid, or (2) some halo clusters may have been accreted.; We also extracted photometric and spectroscopic observations for a smaller sample of clusters in NGC 6822, a relatively isolated, Magellanic-type dwarf irregular galaxy in the Local Group. HST photometry reveals three new clusters. We find that NGC 6822 has formed massive (104 M⊙ --106 M⊙ ) compact star clusters over its lifetime, similar to the populous clusters in M33 and the Magellanic Clouds, and consistent with GC masses. The metallicity of two clusters (from integrated CTIO spectra) indicates that NGC 6822 is more metal poor than the LMC and SMC. (Abstract shortened by UMI.)