Polycyclic aromatic hydrocarbon selected galaxies
  Polycyclic aromatic hydrocarbon (PAH) emission has been found in both
starbursts and modestly starforming galaxies, but the relation between
starforming activity and PAH luminosity is still a matter of debate. The
different correlation degrees could be caused by the variety of optical and
far-infrared sample selection criteria. In order to obtain a census of the
typical properties of PAH emitting galaxies, we here study moderately distant
galaxies which have been selected by their PAH emission. Combining the ISOCAM
Parallel Survey at 6.7 micron with 2MASS we have colour-selected a sample of
120 candidates for strong PAH emission. We obtained optical and mid-infrared
spectra of 75 and 19 sources, respectively, and analysed IRAS-ADDSCANs and
available Spitzer 3.6-160 micron photometry. The Spitzer spectra exhibit clear
PAH features and corroborate that our photometric selection criteria trace the
PAH emission of galaxies fairly well. The optical spectra show emission lines,
at median redshift z~0.1, as well as Hdelta and CaII absorption, indicating
ongoing starformation as well as post-starbursts. The mid- and far-infrared
spectral energy distributions (SEDs) provide evidence for a broad range of
far-infrared (FIR) luminosities, but in general the dust is colder than for
starburst galaxies like M82. For most galaxies the monocromatic luminosity
peaks at about equal height at optical, 6.7 micron (PAH) and FIR wavelengths.
In about 15% of the sources the FIR luminosity exceeds the optical and PAH
energy output by about a factor 5-10 despite the cool dust temperature; in
these galaxies a large dust mass of 10^8 - 10^9 M_sun is inferred.
