SCYON Abstract

Received on September 6 2001

High Mass, OB Star Formation in M51 : HST Halpha and Palpha Imaging

AuthorsN. Z. Scoville1, M. Polletta1, 2, S. Ewald1, S. R. Stolovy1, R. Thompson3 & M. Rieke3
Affiliation1California Institute of Technology, Pasadena, CA 91125,
2Observatory of Geneva, Sauverny, Switzerland,
3Steward Observatory, University of Arizona, Tucson, AZ 85721
Accepted byAstronomical Journal


We have obtained Halpha and Palpha emission line images covering the central 3 - 4' of M51 using the WFPC2 and NICMOS cameras on HST to study the high-mass stellar population. The 0.1 - 0.2 arcsec/pixels provide 4.6 - 9 pc resolution in M51 and the Halpha/Palpha line ratios are used to obtain extinction estimates. A sample of 1373 Halpha emission regions is catalogued using an automated and uniform measurement algorithm. Their sizes are typically 10 - 100 pc. The luminosity function for the Halpha emission regions is obtained over the range LHalpha = 1036 to 2 x 1039 erg/s. The luminosity function is fit well by a power law with dN/dlnL ~ L-1.01). The power law is significantly truncated and no regions were found with observed LHalpha above 2 x 1039 erg/s (uncorrected for extinction). (The maximum seen in ground-based studies is approximately a factor of 5 higher, very likely due to blending of multiple regions.) The extinctions derived here increase the maximum intrinsic luminosity to above 1040 erg/s). The logarithmically binned luminosity function is also somewhat steeper (alpha = -1.01) than that found in ground-based imaging (alpha = -0.5 to -0.8) --- probably also a result of our resolving regions which were blended in the ground-based images. The 2-point correlation function for the HII regions exhibits strong clustering on scales < 2 arcsec or 96 pc.

To analyze the variations of HII region properties vis-a-vis the galactic structure, the spiral arm areas were defined independently from mm-CO and optical continuum imaging. Although the arms constitute only 25% of the disk surface area, they contain 45% of the catalogued HII regions. The luminosity function is somewhat flatter in spiral arm regions than in the interarm areas (-0.72 -> -0.95); however, this is very likely the result of increased blending of individual HII regions in the arms, which have higher surface density. No significant difference is seen in the sizes and electron densities of the HII regions in spiral arm and interarm regions. For 209 regions which have > 5 sigma detections in both Palpha and Halpha, the observed line ratios indicate visual extinctions in the range AV = 0 to 6 mag. The mean extinction is AV = 3.1 mag (weighting each region equally), 2.4 mag (weighting each by the observed Halpha luminosity) and 3.0 mag (weighting by the extinction-corrected luminosity). On average, the observed Halpha 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 Halpha luminosities is between 1039 - 2 x 1040 erg/s.

The most luminous regions have sizes > 100 pc, so it is very likely 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 HII region might conceivably expand within the ~ 3 x 106 yr lifetime of the OB stars. It is also consistent with observed correlation (L ~ D2) found between the measured luminosities and sizes of the HII regions. We therefore generated a subsample of 1101 regions with sizes < 50 pc which constitutes those regions which might conceivably be ionized by a single cluster. Their extinction-corrected luminosities range between 2 x 1037 and 1039 erg/s$, or between 2/3 of M42 (the Orion Nebula) and W49 (the most luminous Galactic radio HII region). The upper limit for individual clusters is therefore conservatively < 1039 erg/s, implying QLyCup ~ 7 x 1050 s-1 (with no corrections for dust absorption of the Lyman continuum or UV which escapes to the diffuse medium). This corresponds to cluster masses < 5000 MO (between 1 and 120 MO).

The total star formation rate in M51 is estimated from the extinction-corrected Halpha luminosities to be ~ 4.2 Msun/yr (assuming a Salpeter IMF between 1 and 120 Msun) and the cycling time from the neutral ISM into stars is 1.2 x 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 end of the luminosity function (alpha = -1.01) implies N(Mcl)/dMcl ~ Mcl-2.01. This implies that high mass star formation, cloud disruption due to OB stars and UV production is contributed by a large range of cluster masses with equal effects per logarithmic interval of cluster mass.

The high mass clusters (~ 1000 Msun) have a mass such that the IMF is well sampled up to ~ 120 Msun, but this cluster mass is < 1% of that available in a typical GMC. 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 ~ 500 -- 1000 Lsun/Msun which happens for clusters > 750 Msun. We have modelled the core collapse hydrodynamically and find that a second wave of star formation may propagate outwards in a radiatively compressed shell surrounding the core star cluster --- this triggered, secondary star formation may be the mechanism for formation of super star clusters (SSCs) seen in starburst galaxies.