Star Formation and the Interstellar Medium In Nearby Tidal Streams
  (SAINTS): Spitzer Mid-infrared Spectroscopy and Imaging of Intergalactic
  Star-forming Objects
  A spectroscopic analysis of 10 intergalactic star forming objects (ISFOs) and
a photometric analysis of 67 ISFOs in a sample of 14 interacting systems is
presented. The majority of the ISFOs have relative polycyclic aromatic
hydrocarbon (PAH) band strengths similar to those of nearby spiral and
starburst galaxies. In contrast to what is observed in blue compact dwarfs
(BCDs) and local giant HII regions in the Milky Way (NGC 3603) and the
Magellanic Clouds (30 Doradus and N 66), the relative PAH band strengths in
ISFOs correspond to models with a significant PAH ion fraction (<50%) and
bright emission from large PAHs (~100 carbon atoms). The [NeIII]/[NeII] and
[SIV]/[SIII] line flux ratios indicate moderate levels of excitation with an
interstellar radiation field that is harder than the majority of the Spitzer
Infrared Nearby Galaxies Survey and starburst galaxies, but softer than BCDs
and local giant HII regions. The ISFO neon line flux ratios are consistent with
a burst of star formation < 6 million years ago. Most of the ISFOs have
~million solar masses of warm molecular hydrogen with a likely origin in
photo-dissociation regions (PDRs). Infrared Array Camera photometry shows the
ISFOs to be bright at 8 um, with one third having [4.5] - [8.0] > 3.7, i.e.,
enhanced non-stellar emission, most likely due to PAHs, relative to normal
spirals, dwarf irregulars and BCD galaxies. The relative strength of the 8 um
emission compared to that at 3.6 um or 24 um separates ISFOs from dwarf
galaxies in Spitzer two color diagrams. The infrared power in two thirds of the
ISFOs is dominated by emission from grains in a diffuse interstellar medium.
One in six ISFOs have significant emission from PDRs, contributing ~30 % - 60 %
of the total power. ISFOs are young knots of intense star formation.
