Stochastic `Beads on a String' in the Accretion Tail of Arp 285

Abstract

We present Spitzer infrared, GALEX UV, and SDSS and SARA optical images of the peculiar interacting galaxy pair Arp 285 (NGC 2856/4), and compare with a new numerical model of the interaction. We estimate the ages of clumps of star formation in these galaxies using population synthesis models, carefully considering the uncertainties on these ages. This system contains a striking example of `beads on a string': a series of star formation complexes ~1 kpc apart. These `beads' are found in a tail-like feature that is perpendicular to the disk of NGC 2856, which implies that it was formed from material accreted from the companion NGC 2854. The extreme blueness of the optical/UV colors and redness of the mid-infrared colors implies very young stellar ages (~4 - 20 Myrs) for these star forming regions. Spectral decomposition of these `beads' shows excess emission above the modeled stellar continuum in the 3.6 micron and 4.5 micron bands, indicating either contributions from interstellar matter to these fluxes or a second older stellar population. These clumps have -12.0 < M(B) < -10.6, thus they are less luminous than most dwarf galaxies. Our model suggests that bridge material falling into the potential of the companion overshoots the companion. The gas then piles up at apo-galacticon before falling back onto the companion, and star formation occurs in the pile-up. A luminous (M(B) ~ -13.6) extended (FWHM ~ 1.3 kpc) `bright spot' is visible at the northwestern edge of the NGC 2856 disk, with an intermediate stellar population (400 - 1500 Myrs). Our model suggests that this feature is part of a expanding ripple-like `arc' created by an off-center ring-galaxy-like collision between the two disks.