Star cluster populations in the outer disks of nearby galaxies

Abstract

We present a Large Binocular Telescope imaging study that characterizes the star cluster component of nearby galaxy outer disks (beyond the optical radius R 25). Expanding on the pilot project of Herbert-Fort et al., we present deep (27.5 mag V-band point-source limiting magnitude) U- and V-band imaging of six galaxies: IC 4182, NGC3351, NGC4736, NGC4826, NGC5474, and NGC6503. We find that the outer disk of each galaxy is populated with marginally resolved star clusters with masses 103 M and ages up to 1Gyr (masses and ages are limited by the depth of our imaging and uncertainties are large given how photometry can be strongly affected by the presence or absence of a few stars in such low-mass systems), and that they are typically found out to at least 2 R 25 but sometimes as far as 3-4 R 25 - even beyond the apparent H I disk. The mean rate of cluster formation for 1 R 25 ≤ R ≤ 1.5 R 25 is at least one every 2.5Myr and the clusters are spatially correlated with the H I, most strongly with higher density gas near the periphery of the optical disk and with lower density neutral gas at the H I disk periphery. We hypothesize that the clusters near the edge of the optical disk are formed in the extension of spiral structure from the inner disk and are a fairly consistent phenomenon and that the clusters formed at the periphery of the H I disk are the result of accretion episodes. © 2012. The American Astronomical Society. All rights reserved..