2009A&A...507..713H

This is the fourth in a series of papers based on the ISOCAM Parallel Survey at 6.7µm. While the first three papers have been devoted to active galactic nuclei (AGN), here we report on emission-line galaxies without AGN signatures in their optical spectra. 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 6.7µm 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 3.6-160µm photometry. The Spitzer mid-infrared 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 H_δ 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 (10⁹L_☉-2x10¹²L_☉), but in general the dust is colder (T≲25 K, β=2) than for starburst galaxies like M82 (T≃40K). For most galaxies the monocromatic luminosity (ν.L_ν) peaks at about equal height at optical, 6.7µm (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 10⁸-10⁹M_☉ is inferred. At moderate distance (z∼0.1), PAH selected galaxies turn out to be a quite heterogeneous population of dust-rich, partly infrared-luminous galaxies but mostly cool with a range of post-starburst signatures and starforming activity which appears to be rather modest relative to the entire gas content (derived from the dust mass and assuming a standard gas/dust ratio). Our results on PAH selected galaxies question the often expressed interpretation that the majority of high redshift galaxies detected in 15 and 24µm surveys are dominated by powerful ongoing starbursts with high starforming efficiency.