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 \sim 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 ( \sim 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 \mu m and 4.5 \mu m 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 .
There was a time delay of \sim 500 Myrs between the point of closest approach between the two galaxies and the initiation of star formation in this feature .
A luminous ( M _ { B } \sim - 13.6 ) extended ( FWHM \sim 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 .