Star clusters in M 33. IV. A new survey from deep HST images

Abstract

We have detected 102 star clusters in M 33, from 35 deep Hubble Space Telescope (HST) WFPC2 fields taken from our program and from the HST archive. Twenty-eight fields have V and I band imaging, and an additional seven fields are imaged in a single V filter. Eighty-two of the clusters were previously unknown. Integrated photometry reveals that 25 clusters have (V-I) colors typical of those found in Galactic globular clusters (only ten of these objects were previously known). Our discovery of 15 new globular cluster candidates increases previous estimates of the old cluster population in M 33 by ~ 60%. An additional eleven objects (only two were previously known) have colors expected for intermediate age clusters. The new clusters have a range of ages from 6 million years to >15 Gyrs, and masses between 10² M_sun-10⁶ M_sun, although these parameters are estimated from only one color. The number of new clusters detected more than doubles our previous sample (Papers I and II), and extends coverage to older objects and new off-spiral arm locations in M 33. The luminosity function for old M 33 clusters shows a peak at M_V ~ -7.0, nearly half a magnitude fainter than found in the Galactic and M 31 globular cluster populations. The luminosity function for intermediate age objects shows no turnover down to M_V ~ -6.6. We estimate the total number of globular clusters in M 33 to be 75+/-14. This gives a specific frequency, S_N, of 2.07+/-0.39 - significantly higher than found for other late-type spiral galaxies, but in the range found for ellipticals. Table 2 is only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5), or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/366/498