Modes of Star Formation along the Hubble Sequence … and beyond Richard de Grijs University of Sheffield, UK Terschelling, 7 July 2005

Violent star (cluster!) formation in M82

M82 B (continued) (de Grijs et al., 2003, MNRAS, 340, 197; de Grijs et al., 2003, ApJ, 583, L17) Star cluster formation declined rapidly after the burst episode … … but star formation continued in M82 B until Myr ago Decoupling between the star and cluster formation modes

1. This is reminiscent of the LMC… (Rich et al., 2001, AJ, 122, 842) (movie courtesy Knut Olsen)

(Holtzman et al., 1999, AJ, 118, 2262) (bar field) (outer disk field)

2. … and also of NGC 5253: (Tremonti et al., 2001, ApJ, 555, 322) The clusters show strong O-star signatures (P Cygni profiles) not seen in the field: does star formation operate differently in the field than in the clusters?

Pixel-by-pixel analysis of the “Mice” and “Tadpole” galaxies (de Grijs et al., 2003, New Astron., 8, 155) (following Eskridge et al., 2003, ApJ, 586, 923)

Pixel colour-colour and colour- magnitude diagrams

Cluster formation in the tidal tails?

Massive star clusters in the Mice and Tadpole galaxies In both systems we find ~40 bright young star clusters, mostly in the tidal tails and spiral arms Overall, more than 35% of the active star formation occurs in star clusters In the tidal tails and spiral arms ~70% of the F475W flux originates in clusters or star- forming regions (~40% in F814W)

What about “normal” galaxies? (Kennicutt, 1998, ARA&A, 36, 189)

The Schmidt law Starbursts occupy a continuation of the Schmidt law for normal galaxies Local gas surface density seems the key driver! (Kennicutt, 1998)

1.The global SFRs of disks and (nuclear) starbursts correlate with the local gas density, over large ranges 2.This suggests that star formation is driven by the local gas surface density 3.The higher SFRs in starbursts might be due to external drivers: more efficient gas compression, to allow for rapid SF

(Elmegreen & Efremov, 1997, ApJ, 480, 235) In order to produce a massive star cluster, for a given ambient pressure, the star- formation efficiency needs to be high!

Global star formation efficiencies Quiescent spiral galaxies0.1 – 3% Irregular galaxies0.1 – 3% Dwarf starburst galaxies0.1 – 3% Starbursts in major interactions10 – 50% ULIRGs (on pc scales)>30% (massive merging gas-rich galaxies)(up to ~90%) SFE = Mass(stars) / Mass(gas)

Young massive star clusters in context: survival chances to old age (de Grijs et al., 2005, MNRAS, in press: astro-ph/ )

Intermediate-age clusters in M82: Possible proto-globular clusters? (de Grijs, Parmentier & Lamers, 2005, MNRAS, subm.) (Lamers et al., 2005, A&A, 429, 173)

The bottom line… The star formation rate in a large variety of galaxies seems to be driven by the local gas surface density Star cluster formation is a major mode of SF in extreme environments, i.e., galaxy interactions! Galaxy interactions lead to increased pressure (and turbulence) as well as higher SF efficiencies, and thus to higher-mass cluster formation The initial mass distribution of the intermediate-age clusters in M82 is more likely log-normal than a power law. These clusters are potential proto-globular clusters: globular cluster formation appears to be continuing until today! (at least in M82…)