Ted K. Wyder

The Hii Regions of M33. II. A Photometric Catalog of 1272 Newly Identified Emission Regions

We have identified a total of 1272 newly recognized emission regions in M33. Combined with the previously cataloged total of 1066 H ii regions and supernova remnants, this brings M33s total to 2338 emission regions. This paper provides photometry of the new objects in Hα, which is combined with data from previous catalogs to produce a global H ii region luminosity function (corrected for incompleteness) that reaches a faint luminosity limit of 2 × 1034 ergs s–1 and shows a broad maximum with a peak frequency at luminosities of 6 × 1035^{35} ergs s–1. We also plot the H ii region size distribution and comment on unusual morphologies.

THE LUMINOSITY FUNCTION AND SIZE DISTRIBUTION OF THE H II REGIONS IN M33

We present Hα fluxes and sizes for most of the H 2 regions in M33 listed in the catalog of Courtes et al. (1987). The observed bright end of the differential luminosity function is approximately a power law with exponent a = -2.40 ± 0.15, while the number counts turn over at an Hα luminosity of approximately 1036.4 erg s-1. After accounting for the effects of incompleteness in our sample, we find that the faint end of the corrected luminosity function remains almost constant instead of turning over. The cumulative size distribution approximately follows an exponential law with scale size D0 = 32 ± 1 pc. We briefly discuss the implications of our results for our understanding of recent high-mass star formation in M3.

Hubble Space Telescope Observations of the Hubble Clusters in NGC 6822: Ages and Structure1

We present Hubble Space Telescope observations of four cluster candidates in NGC 6822 originally identified by Hubble. One of them, Hubble IV, is an H ii region while Hubble VI, VII, and VIII are confirmed as rich star clusters. Based upon cluster color-magnitude diagrams from which the field star contamination has been subtracted, we derive ages of 70±10 Myr for Hubble VI and 1.5±0.2 Gyr for Hubble VIII. While the age is more uncertain in the case of Hubble VII, the available evidence argues that this cluster is probably similar to the old, metal-poor globular clusters in the Milky Way. Comparisons of the cluster ages and integrated magnitudes with models of cluster evolution imply that the two young clusters are an order of magnitude less massive than the older cluster Hubble VII. The radial profiles of all three clusters are reasonably well fit by empirical King models. The core radius and central surface brightness of Hubble VII are consistent with observations of Galactic globular clusters. The half-light radii of these clusters are comparable to values for the inner Milky Way globulars and are on average smaller than clusters in the Magellanic Clouds. These clusters apparently are exceptions to the trend noted by van den Bergh that larger clusters tend to form in environments with lower stellar densities.

C32, A Young Star Cluster in IC 1613

The Local Group irregular galaxy IC 1613 has remained an enigma for many years because of its apparent lack of star clusters. We report the successful search for clusters among several of the candidate objects identified many years ago on photographic plates. We have used a single Hubble Space Telescope WFPC2 pointing and a series of images obtained with the WIYN telescope under exceptional seeing conditions, examining a total of 23 of the previously published candidates. All but six of these objects were found to be either asterisms or background galaxies. Five of the six remaining candidates possibly are small, sparse clusters, and the sixth, C32, is an obvious cluster. It is a compact, young object, with an age of less than 10 million years and a total absolute magnitude of MV = -5.78 ± 0.16 within a radius of 13 pc.

M31 vs. M33: Different modes of star formation

Among the various differences in star-formation modes between various types of galaxies, the differences in HII region luminosity functions are conspicuous. A comparison of those of M31 and M33, using new data, illustrates these differences, which are best understood in terms of different stellar clustering mass functions for the galaxies.