Polycyclic aromatic hydrocarbon selected galaxies^*

¹ Astronomisches Institut Ruhr–Universität Bochum, Universitätsstraße 150, 44801 Bochum, Germany e-mail: haas@astro.rub.de
² Department of Physics, University of California, Santa Barbara, CA 93106, USA
³ European Southern Observatory, Karl–Schwarzschild–Str. 2, 85748 Garching, Germany
⁴ Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany

Received: 16 June 2009
Accepted: 5 August 2009

Abstract

Context. This is the fourth in a series of papers based on the ISOCAM Parallel Survey at 6.7 μm. 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.

Aims. 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.

Methods. Combining the ISOCAM Parallel Survey at 6.7 μm 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 μm photometry.

Results. 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 , as well as H and Ca II 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 ( K, ) than for starburst galaxies like M 82 ( K). For most galaxies the monocromatic luminosity peaks at about equal height at optical, 6.7 μm (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 10⁸–10⁹  is inferred.

Conclusions. At moderate distance (), 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 24 μm surveys are dominated by powerful ongoing starbursts with high starforming efficiency.