SCYON Abstract

Received on February 21 2003

Infrared Spectroscopy of Massive Stellar Clusters in Starbursts

AuthorsAndrea M. Gilbert
AffiliationUniversity of California, Berkeley
Max-Planck Institut für extraterrestrische Physik
To appear inPhD thesis, advisor James R. Graham (University of CA, Berkeley, conferred 19 Dec. 2002)


Star formation in starbursts produces massive, compact super star clusters (SSCs) whose physical properties are comparable with those of globular clusters (GCs). We examine the stellar populations of SSCs and their relation to GCs in two nearby prototypical starbursts, the young merger system NGC 4038/39 (the Antennae) and the dwarf irregular galaxy NGC 1569, which are examples of two widely varied environments in which SSCs form.

The Antennae have formed a large SSC population, and its youngest star-forming regions are embedded in a large reservoir of molecular gas and dust. Hidden from optical view but prominent in the infrared (IR) are the most powerful young SSCs, whose ionizing radiation powers giant compact HII regions and dissociates the surrounding molecular gas. The youngest SSCs drive supersonic outflows (viewed in broad Br-gamma emission) that efficiently entrain the local medium. These Emission-Line Clusters (ELCs) constitute at least 15% of the star-formation rate in the Antennae, assuming a Salpeter IMF (0.1 - 100 Msun), and their high star-formation efficiencies imply that they will evolve into bound SSCs. The ELC radial velocity field resembles that of the molecular gas, and its dispersion is typical of a galactic disk. Near-IR spectroscopy and spectral population synthesis modeling of the few SSCs in NGC 1569 reveals their supergiant-dominated stellar populations and permits measurement of their ages and internal velocity dispersions, which yield stellar masses and constraints on their initial mass functions (IMFs). The IMF in unresolved clusters can be probed only via measurement of cluster masses via high-resolution spectroscopy. This is critical in determining the connection between SSCs and GCs because GCs are old, harboring only low-mass stars, while the observed properties of SSCs are dominated by massive stars, and starbursts have been argued to form preferentially more massive stars. While a range of IMFs is inferred for SSCs in the Antennae and a few other systems, the brightest SSCs in NGC 1569 have normal IMFs, and thus they can evolve into GCs.