Starbursting in Spiral Arms and ULIRGs

We present results from HST Hα and Pα imaging of HII regions in M51 which are used to define the properties of OB star clusters in the ‘starburst’ regions of spiral arms. These data indicate an observed Hα luminosity function which truncates on the high end at L_(Hα) = 10^(39) erg sec^(−1). This is far below the high end seen in ground-based imaging — we believe this is due to the fact that lower resolution ground-based imaging often blends multiple centers of OB star formation and the highest luminosity regions were really complexes rather single OB star clusters. On the other hand, based on the observed Pα/Hα ratios, we find the typical extinctions are high (2–3 mag) and when extinction corrections are applied to the observed Hα, the intrinsic Hα luminosities get up to ~ 10^(40) erg sec^(−1). This upper limit to the luminosity function corresponds to a cluster mass of ~few x 10^3 M_⊙ (for a salpeter IMF between 1 and 120 M_⊙; it is approximately at the point at which the IMF is first populated to ~ 100 M_⊙. We suggest that this limiting cluster mass may be understood physically if accretion (and thus cluster formation) in cloud cores is terminated when radiation pressure on the surrounding dust exceeds the self-gravity of the star cluster — this occurs when the highest mass stars are formed. In ULIRGs, the augmented star formation is probably due to the collision of massive clouds since, often, large numbers of bright clusters may be found in the overlap regions of the colliding galaxies. In the extreme object Arp 220, high resolution NICMOS and mm-wave interferometric imaging indicates double nuclei separated by ~ 350 pc, each of which is embedded in a massive gas and dust disk. Although the geometry in Arp 220 is clearly different than a spherical cloud core forming a single oB star cluster, the star formation may also be self-regulated at ~ 10^3 M_⊙ yr^(−1) by the radiation pressure arising from the starburst. The disks will swell vertically and self-regulate their star formation