Polycyclic aromatic hydrocarbon selected galaxies

TL;DR

This study investigates PAH-emitting galaxies selected by their infrared emission, revealing diverse properties, star formation activity, and dust characteristics, with implications for understanding galaxy evolution.

Contribution

It provides a comprehensive analysis of PAH-selected galaxies using multi-wavelength data, highlighting their spectral features, star formation indicators, and dust properties, which was previously underexplored.

Findings

PAH emission correlates with ongoing star formation and post-starburst features.

Most galaxies have colder dust temperatures compared to classic starburst galaxies.

Approximately 15% of the sample have FIR luminosities exceeding optical and PAH outputs, indicating large dust masses.

Abstract

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.