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

Abstract

We have obtained Hα and Paα 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 Hα/Paα line ratios are used to obtain extinction estimates. A sample of 1373 Hα emission regions is cataloged using an automated and uniform measurement algorithm. Their sizes are typically 10-100 pc. The luminosity function for the Hα emission regions is obtained over the range L_Hα=10³⁶ to 2×10³⁹ ergs s^-1. The luminosity function is fitted well by a power law with dN/dlnL~L^-1.01. The power law is significantly truncated, and no regions were found with observed L_Hα 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 (α=-1.01) than that found from ground-based imaging (α=-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-à-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 σ detections in both Paα and Hα, 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 Hα luminosity), and 3.0 mag (weighting by the extinction-corrected luminosity). On average, the observed Hα 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 Hα 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 Q_Lyc,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_solar (between 1 and 120 M_solar). The total star formation rate in M51 is estimated from the extinction-corrected Hα luminosities to be ~4.2 M_solar yr^-1 (assuming a Salpeter initial mass function between 1 and 120 M_solar), 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 (α=-1.01) implies N(M_cl)/dM_cl~M^-2.01_cl. 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_solar) have a mass such that the initial mass function is well sampled up to ~120 M_solar, 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)_solar, which happens for clusters >=750 M_solar. 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.