Session 13 - Galaxies & Globular Clusters.
Oral session, Monday, June 09
North Main Hall F/G,

## [13.05] Globular Cluster Formation From Giant Molecular Clouds in Colliding Galaxies

L. Ozernoy (GMU and GSFC/NASA)

Galaxy collisions whenever they occur face-on should be accompanied by collisions between giant molecular clouds (GMCs) located in the central few kpc of the galaxies. The cloud-cloud collisions trigger, during the time when the central parts of galaxies cross each other, gravitational instability in the bulk of the GMCs. This process, as I show, can result in the formation of many so called super star clusters' (SSCs).

The proposed model explains the basic observational features of SSCs: (i) appearance in irregular and spiral galaxies of all types; (ii) independence of the formation process on metallicity of gas; (iii) a more or less simultaneous formation of \sim10^5-10^6 stars in each GMC before the gas as a building material is blown out. An unambiguous signature of this model is that the ages of globular clusters in a given galaxy would reveal a distribution in the form of discrete peaks (each related to the respected collision of the parent galaxy) rather than be distributed continuously.

The majority of galaxy collisions are grazing and therefore they do not involve the bulk of then-available GMCs. Although the ram pressure of the intercloud gas is strong enough to induce some star formation in the GMCs of grazingly colliding galaxies, this process differs substantially from the head-on collision case. It is also worth noting the difference with a collisionally-induced star formation in the central part of the Milky Way Galaxy^1, which results from the difference in velocities of the colliding GMCs. Finally, I discuss a conjecture that the GMC collision mechanism could also be responsible for globular cluster (then CSS) formation prior to, or in parallel with, the galaxy formation by collisions of protogalactic clouds^2.

\renewcommand \baselinestretch0.2 \footnotesize ^1Ozernoy, L.M., 1997, Starbursts in Our Home Galaxy", in Star Formation, Near and \hbox Far (Eds. S.S. Holt amp; L.G. Mundy). AIP Conf. Proc. (in press) ^2Ozernoy, L.M., 1989, Primeval and Rejuvenated' Galaxies", in Ann. New York \hbox Acad. Sci. 571, 219