Stochastic “Beads on a String” in the Accretion Tail of Arp 285

Abstract

We present Spitzer infrared, Galaxy Evolution Explorer UV, and Sloan Digitized Sky Survey and Southeastern Association for Research in Astronomy 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 Myr) for these star-forming regions. Spectral decomposition of these "beads" shows excess emission above the modeled stellar continuum in the 3.6 µm and 4.5 µm 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 apogalacticon before falling back onto the companion, and star formation occurs in the pile-up. There was a time delay of ∼500 Myr between the point of closest approach between the two galaxies and the initiation of star formation in this feature. 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 Myr). 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.

NGC 6845: metallicity gradients and star formation in a complex compact group

“…The internal gaseous colour excess measured through the Balmer decrement for the star-forming regions located in the NGC 6845 group have values ranging from E(B-V)=0.00 to 0.77. These values are in agreement with the results found by other authors for star-forming regions in interacting galaxies (see Smith et al 2008).…”

“…This effect is shown by a black dashed line in Fig. 5, which has been taken from Smith et al (2008). For comparison, in Fig.…”

“…The ages for the star-forming regions in NGC 6845 are in agreement with the results found by other authors, for similar regions. For example, Smith et al (2008) studied the star-forming regions in the pair of galaxies NGC 2856/4. These authors found ages in the range of ∼4-20 Myrs.…”

Star Formation in Interacting Galaxies

“…3 and Smith, et al 2008). Like the beads phenomenon, SF occurs in a line of clumps in this region, but simulations suggest that this is random, not systematic.…”

“…5. Smith, et al 2008). The northern galaxy has a rather faint "beads-on-a-string" plume, which models indicate may be a pileup zone of material transferred from the southern galaxy.…”