High-Mass, OB Star Formation in M51: Hubble Space Telescope Hα and Paα Imaging

We have obtained H? and Pa? emission-line images covering the central 3'?4' of M51 using the WFPC2 and NICMOS instruments on the Hubble Space Telescope to study the high-mass stellar population. The 01?02 pixels provide 4.6?9 pc resolution in M51, and the H?/Pa? line ratios are used to obtain extinction estimates. A sample of 1373 H? emission regions is cataloged using an automated and uniform measurement algorithm. Their sizes are typically 10?100 pc. The luminosity function for the H? emission regions is obtained over the range LH? = 1036 to 2 ? 1039 ergs s-1. The luminosity function is fitted well by a power law with dN/d ln L ? L-1.01. The power law is significantly truncated, and no regions were found with observed LH? above 2 ? 1039 ergs s-1 (uncorrected for extinction). (The maximum seen in ground-based studies is approximately a factor of 5 higher, very likely because of the blending of multiple regions.) The extinctions derived here increase the maximum intrinsic luminosity to above 1040 ergs s-1. The logarithmically binned luminosity function is also somewhat steeper (? = -1.01) than that found from ground-based imaging (? = -0.5 to -0.8)?probably also a result of our resolving regions that were blended in the ground-based images. The two-point correlation function for the H II regions exhibits strong clustering on scales ?2'', or 96 pc. To analyze the variations of H II region properties vis-?-vis the galactic structure, the spiral arm areas were defined independently from millimeter-CO and optical continuum imaging. Although the arms constitute only 25% of the disk surface area, the arms contain 45% of the cataloged H II regions. The luminosity function is somewhat flatter in spiral arm regions than in the interarm areas (-0.72 to -0.95); however, this is very likely the result of increased blending of individual H II regions in the arms that have higher surface density. No significant difference is seen in the sizes and electron densities of the H II regions in spiral arm and interarm regions. For 209 regions that had ?5 ? detections in both Pa? and H?, the observed line ratios indicate visual extinctions in the range AV = 0?6 mag. The mean extinction was AV = 3.1 mag (weighting each region equally), 2.4 mag (weighting each by the observed H? luminosity), and 3.0 mag (weighting by the extinction-corrected luminosity). On average, the observed H? luminosities should be increased by a factor of ~10, implying comparable increases in global OB star cluster luminosities and star formation rates. The full range of extinction-corrected H? luminosities is between 1037 and 2 ? 1040 ergs s-1. The most luminous regions have sizes ?100 pc, so it is very likely that they are blends of multiple regions. This is clear based on their sizes, which are much larger than the maximum diameter (?50 pc) to which an H II region might conceivably expand within the ~3 ? 106 yr lifetime of the OB stars. It is also consistent with the observed correlation (L ? D2) between the measured luminosities and sizes of the H II regions. We therefore generated a subsample of 1101 regions with sizes ?50 pc, which is made up of those regions that might conceivably be ionized by a single cluster. Their extinction-corrected luminosities range between 2 ? 1037 and 1039 ergs s-1, or between two-thirds of M42 (the Orion Nebula) and W49 (the most luminous Galactic radio H II region). The upper limit for individual clusters is therefore conservatively ?1039 ergs s-1, implying QLyc,up 7 ? 1050 s-1 (with no corrections for dust absorption of the Lyman continuum or UV that escapes to the diffuse medium). This corresponds to cluster masses ?5000 M? (between 1 and 120 M?). The total star formation rate in M51 is estimated from the extinction-corrected H? luminosities to be ~4.2 M? yr-1 (assuming a Salpeter initial mass function between 1 and 120 M?), and the cycling time from the neutral interstellar medium into these stars is 1.2 ? 109 yr. We develop a simple model for the UV output from OB star clusters as a function of the cluster mass and age in order to interpret constraints provided by the observed luminosity functions. The power-law index at the high-luminosity end of the luminosity function (? = -1.01) implies N(Mcl)/dMcl ? M. This implies that high-mass star formation, cloud disruption due to OB stars, and UV production are contributed to by a large range of cluster masses with equal effects per logarithmic interval of cluster mass. The high-mass clusters (~1000 M?) have a mass such that the initial mass function is well sampled up to ~120 M?, but this cluster mass is ?1% of that available in a typical giant molecular cloud. We suggest that OB star formation in a cloud core region is terminated at the point that radiation pressure on the surrounding dust exceeds the self-gravity of the core star cluster and that this is what limits the maximum mass of standard OB star clusters. This occurs at a stellar luminosity-to-mass ratio of ~500?1000 (L/M)?, which happens for clusters ?750 M?. We have modeled the core collapse hydrodynamically and have found that a second wave of star formation may propagate outward in a radiatively compressed shell surrounding the core star cluster?this triggered, secondary star formation may be the mechanism for formation of the super?star clusters seen in starburst galaxies.