On the Extended Knotted Disks of Galaxies

The stellar disks of many spiral galaxies are twice as large as generallythought. We use archival data from the Galaxy Evolution Explorer mission(GALEX) to quantify the statistical properties of young stellar clusters in theouter, extended disks of a sample of eleven nearby galaxies. We find an excessof sources between 1.25 and 2 optical radii, R(25), for five of the galaxies,which statistically implies that at least a quarter of such galaxies have thiscluster population (90% confidence level), and no significant statisticalexcess in the sample as a whole beyond 2 optical radii, even though one galaxy(M 83) individually shows such an excess. Although the excess is typically mostpronounced for blue (FUV -NUV < 1, NUV < 25) sources, there is also an excessof sources with redder colors. Although from galaxy to galaxy the number ofsources varies significantly, on average, the galaxies with such sources have75 +- 10 blue sources at radii between 1.25 and 2 R(25). In addition, theradial distribution is consistent with the extended dust emission observed inthe far IR and with the properties of H-alpha sources, assuming a constantcluster formation rate over the last few hundred Myrs. All of these resultssuggest that the phenomenon of low-level star formation well outside theapparent optical edges of disks (R ~ R(25)) is common and long-lasting.Comment: 8 pages, accepted for publication in A