The Missing Link in Star Cluster Evolution

Abstract

The currently most popular models for the dynamical evolution of star clusters predict that the power-law cluster luminosity functions (CLFs) of young star cluster systems will be transformed rapidly into the universal Gaussian CLFs of old Milky Way-type "globular" cluster systems. Here, we provide the first evidence for a turnover in the intermediate-age ∼1 Gyr old CLF in the center of the nearby starburst galaxy M82, which very closely matches the universal CLFs of old Milky Way-type globular cluster systems. Thiors provides an important test of both cluster disruption theories and hierarchical galaxy formation models. It also lends strong support to the scenario that these young cluster systems may eventually evolve into old Milky Way-type globular cluster systems. M82's proximity, its shortest known cluster disruption timescale of any galaxy, and its well-defined peak of cluster formation make it an ideal candidate to probe the evolution of its star cluster system to fainter luminosities, and thus lower masses, than has been possible for any galaxy before.

Gemini Spectroscopy andHSTImaging of the Stellar Cluster Population in Region B of M82

“…The spectra were obtained with the Space Telescope Imaging Spectrograph (STIS), and, although the spectra are of low quality, the derived ages of 350 AE 100 Myr are much lower than the photometrically based ages of $0.7Y6 Gyr (Smith et al 2006;de Grijs et al 2003b) for the same clusters. This discrepancy hints at the possibility that region B may be younger than expected and that the ages derived from the photometry are too high.…”

“…De Grijs et al (2003a) therefore suggest that because M82-B is of intermediate age, it provides the ''missing evolutionary link'' between the power-law CLFs found for YMCs and the Gaussian CLFs of globular clusters. However, for an environment as turbulent as that of M82, one would expect a timescale of several Gyr in order for preferential disruption to produce this effect (e.g., Goudfrooij et al [2004Goudfrooij et al [ , 2007, in which such a timescale is calculated for NGC 1316 and NGC 3610, respectively).…”

“…They identified 113 clusters and, by estimating ages and extinctions from BVI photometry, they found that a concentrated episode of cluster formation occurred 400Y1000 Myr ago, with a peak at 600 Myr. Subsequent to this analysis, de Grijs et al (2003b) rederived the ages and extinctions by combining BVI and JH photometry; they find that the peak of cluster formation occurred at 1.10 Gyr, with an age spread of 500Y1500 Myr. De Grijs et al (2003a) used these new ages to derive the cluster luminosity function (CLF) for a fiducial age of 1.0 Gyr and find that it has a Gaussian shape, similar to those of globular clusters (GCs), rather than the standard power-law CLF found for populations of young massive clusters ( YMCs; e.g., Larsen 2004;Gieles et al 2006a).…”

“…Subsequent to this analysis, de Grijs et al (2003b) rederived the ages and extinctions by combining BVI and JH photometry; they find that the peak of cluster formation occurred at 1.10 Gyr, with an age spread of 500Y1500 Myr. De Grijs et al (2003a) used these new ages to derive the cluster luminosity function (CLF) for a fiducial age of 1.0 Gyr and find that it has a Gaussian shape, similar to those of globular clusters (GCs), rather than the standard power-law CLF found for populations of young massive clusters ( YMCs; e.g., Larsen 2004;Gieles et al 2006a). This result is surprising, but it supports theoretical models advocating that an initial power-law distribution of cluster masses will be transformed into a Gaussian distribution because low-mass clusters will be preferentially disrupted (e.g., Fall & Zhang 2001;Vesperini & Zepf 2003;Gieles et al 2006b).…”

Star and Cluster Formation in Extreme Environments

“…We recently reported the first discovery of an approximately lognormal CLF (and CMF) for the star clusters in M82's fossil starburst region "B", formed roughly simultaneously in a pronounced burst of cluster formation (de Grijs et al 2003b; see also Goudfrooij et al 2004). This provides the very first sufficiently deep CLF (and CMF) for a star cluster population at intermediate age (of ∼ 1 Gyr), which thus serves as an important benchmark for theories of the evolution of star cluster systems.…”