Star Formation in Collision Debris: Insights from the modeling of their
  Spectral Energy Distribution
  During galaxy-galaxy interactions, massive gas clouds can be injected into
the intergalactic medium which in turn become gravitationally bound, collapse
and form stars, star clusters or even dwarf galaxies. The objects resulting
from this process are both "pristine", as they are forming their first
generation of stars, and chemically evolved because the metallicity inherited
from their parent galaxies is high. Such characteristics make them particularly
interesting laboratories to study star formation. After having investigated
their star-forming properties, we use photospheric, nebular and dust modeling
to analyze here their spectral energy distribution (SED) from the
far-ultraviolet to the mid-infrared regime for a sample of 7 star-forming
regions. Our analysis confirms that the intergalactic star forming regions in
Stephan's Quintet, around Arp 105, and NGC 5291, appear devoid of stellar
populations older than 10^9 years. We also find an excess of light in the
near-infrared regime (from 2 to 4.5 microns) which cannot be attributed to
stellar photospheric or nebular contributions. This excess is correlated with
the star formation rate intensity suggesting that it is probably due to
emission by very small grains fluctuating in temperature as well as the
polycyclic aromatic hydrocarbons (PAH) line at 3.3 micron. Comparing the
attenuation via the Balmer decrement to the mid-infrared emission allows us to
check the reliability of the attenuation estimate. It suggests the presence of
embedded star forming regions in NGC 5291 and NGC 7252. Overall the SED of
star-forming regions in collision debris (and Tidal Dwarf Galaxies) resemble
more that of dusty star-forming regions in galactic disks than to that of
typical star-forming dwarf galaxies.
