Extreme Starbursts and the Low Mass IMF

Abstract

The temperatures and densities of the interstellar medium (ISM) in starburst galaxies are greatly elevated compared to those in star forming giant molecular clouds (GMCs) in normal galaxies. I review the observed ISM properties in the prototype starburst Arp 200 and then discuss considerations for star formation in such starburst galaxy nuclei. We have also found a previously unrecognized observational constraint on the low mass star populations applicable to these starbursts. Using stellar population synthesis models from Starburst99 with instantaneous and constant starbursts, we identify and quantify spectral diagnostics for stellar populations. The characteristic age of the stellar population dominating the restframe optical (not the younger stars dominating the UV) can be estimated from the 4000Å break strength (D4000). We find that the presence of the 4000Å break requires a stellar population older than ~100 Myr (since OB stars would dominate the continuum short of 4000Å and they exhibit no break). Very importantly, we also find that the initial mass function (IMF) must extend down to a few solar masses if the 4000Å break is present. Thus, the detection of the 4000Å break implies an IMF extending to low mass. The strength of the 4000Å break can be used to constrain the IMF at low stellar masses, while the absence of the feature could be used to identify PopIII stars (which would have low metallicity and probably few low mass stars). The break is, in fact, observed in Arp 220 and in the z = 2 – 7 galaxy SEDs, casting doubt on recent suggestions of top heavy IMFs for high z galaxies and starbursts. The apparent invariance of the IMF is likely due to the fact that the fragmentation scale for stellar mass condensations occurs at very high density on scales of ~100 AU. At this point, the densities will be ~10^(11) per cc and the initial conditions in the galactic GMCs will have been long forgotten. This 100 AU scale corresponds to the point at which solar mass condensations become optical thick in the far infrared and thus separate quasi-statically from the overall collapse – it is also the typical scale of binary star separations, lending support to the notion that this is the scale for stellar mass fragmentation within collapsing cloud cores.

Files

Additional details