High-mass, OB star formation in M51:: Hubble Space Telescope Hα and Pαα imaging

We have obtained H alpha and P alpha alpha 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 0.1-0.2 pixels provide 4.6-9 pc resolution in M51, and the H alpha /P alpha line ratios are used to obtain extinction estimates. A sample of 1373 H Phi emission regions is cataloged using an automated and uniform measurement algorithm. Their sizes are typically 10-100 pc. The luminosity function for the H alpha emission regions is obtained over the range L(H alpha) = 10(36) to 2 x 10(39) ergs s(-1). The luminosity function is fitted well by a power law with dN/d1n L(proportional to) L(-1.01). The power law is significantly truncated, and no regions were found with observed L(H alpha) above 2 x 10(39) 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 10(40) ergs s(-1). The logarithmically binned luminosity function is also somewhat steeper (alpha = -1.01) than that found from ground-based imaging (alpha = -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 less than or equal to2, or 96 pc. To analyze the variations of H II region properties vis-a-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 greater than or equal to5 sigma detections in both Pa alpha and H alpha, the observed line ratios indicate visual extinctions in the range A(V) = 0-6 mag. The mean extinction was A(V) = 3.1 mag (weighting each region equally), 2.4 mag (weighting each by the observed H alpha luminosity), and 3.0 mag (weighting by the extinction-corrected luminosity). On average, the observed H alpha luminosities should be increased by a factor of similar to 10, implying comparable increases in global OB star cluster luminosities and star formation rates. The full range of extinction-corrected H alpha luminosities is between 10(37) and 2 x 10(40) ergs s(-1). The most luminous regions have sizes greater than or equal to 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 (less than or equal to 50 pc) to which an H II region might conceivably expand within the similar to3 x 10(6) yr lifetime of the OB stars. It is also consistent with the observed correlation (L proportional to D(2)) between the measured luminosities and sizes of the H II regions. We therefore generated a subsample of 1101 regions with sizes less than or equal to 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 x 10(37) and 10(39) 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 less than or equal to 10(39) ergs s(-1), implying Q(Lyc,up) similar or equal to 7 x 10(50) 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 less than or equal to 5000 M. (between 1 and 120 M .). The total star formation rate in M51 is estimated from the extinction-corrected H alpha luminosities to be similar to4.2 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 x 10(9) 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 (alpha = -1.01) implies N(M(cl))/dM(cl) proportional to M(cl)(-2.01). 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 (similar to 1000 M.) have a mass such that the initial mass function is well sampled up to similar to 120 but this cluster mass is less than or equal to1% 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 similar to 500-1000 (L/M)., which happens for clusters greater than or equal to 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.