On the extended knotted disks of galaxies

The stellar disks of many spiral galaxies are twice as large as generally thought. We use archival data from the Galaxy Evolution Explorer mission to quantify the statistical properties of young stellar clusters in the outer, extended disks of a sample of 11 nearby galaxies. We find an excess of sources between 1.25 and 2 optical radii, R₂₅, 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 2R₂₅, even though one galaxy (M83) 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 1.25R₂₅ and 2R ₂₅. In addition, the radial distribution is consistent with the extended dust emission observed in the far-IR and with the properties of Ha sources, assuming a constant cluster formation rate over the last few hundred megayears. 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.