The stellar disks of many spiral galaxies are twice as large as generally
thought. We use archival data from the Galaxy Evolution Explorer mission
(GALEX) to quantify the statistical properties of young stellar clusters in the
outer, extended disks of a sample of eleven nearby galaxies. We find an excess
of 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 this
cluster population (90% confidence level), and no significant statistical
excess 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 most
pronounced for blue (FUV -NUV < 1, NUV < 25) sources, there is also an excess
of sources with redder colors. Although from galaxy to galaxy the number of
sources varies significantly, on average, the galaxies with such sources have
75 +- 10 blue sources at radii between 1.25 and 2 R(25). In addition, the
radial distribution is consistent with the extended dust emission observed in
the far IR and with the properties of H-alpha sources, assuming a constant
cluster formation rate over the last few hundred Myrs. All of these results
suggest that the phenomenon of low-level star formation well outside the
apparent optical edges of disks (R ~ R(25)) is common and long-lasting.Comment: 8 pages, accepted for publication in A
