2006ApJ...651L.101T

We examine faint infrared emission features detected in Spitzer Space Telescope images of M51, which are associated with atomic hydrogen in the outer disk and tidal tail at R≳R₂₅(4'.9, ∼14 kpc at d=9.6 Mpc). The infrared colors of these features are consistent with the colors of dust associated with star formation in the bright disk. However, the star formation efficiency (as a ratio of star formation rate to neutral gas mass) implied in the outer disk is lower than that in the bright disk of M51 by an order of magnitude, assuming a similar relationship between infrared emission and star formation rate in the inner and outer disks.