Mid-Infrared Spectroscopy of Optically Faint Extragalactic 70 micron Sources

TL;DR

This study analyzes mid-infrared spectra of optically faint, luminous infrared galaxies at redshifts 0.35 to 1.9, revealing diverse spectral features and emphasizing the importance of 70 micron selection for understanding galaxy evolution.

Contribution

It provides the first mid-infrared spectral analysis of optically faint 70 micron sources, highlighting their spectral diversity and implications for galaxy evolution studies.

Findings

Most sources show PAH emission features.

Absorbed sources are at higher redshifts and luminosities.

70 micron selection captures luminous starbursts and obscured galaxies.

Abstract

We present mid-infrared spectra of sixteen optically faint sources with 70 micron fluxes in the range 19-38mJy. The sample spans a redshift range of 0.35<z<1.9, with most lying between 0.8<z<1.6, and has infrared luminosities of 10^{12} - 10^{13} solar luminosities. Ten of 16 objects show prominent polycyclic aromatic hydrocarbon (PAH) emission features; four of 16 show weak PAHs and strong silicate absorption, and two objects have no discernable spectral features. Compared to samples with 24 micron fluxes >10mJy, the 70\um sample has steeper IR continua and higher luminosities. The PAH dominated sources are among the brightest starbursts seen at any redshift, and reside in a redshift range where other selection methods turn up relatively few sources. The absorbed sources are at higher redshifts and have higher luminosities than the PAH dominated sources, and may show weaker luminosity evolution. We conclude that a 70 micron selection extending to ~20mJy, in combination with selections at mid-IR and far-IR wavelengths, is necessary to obtain a complete picture of the evolution of IR-luminous galaxies over 0<z<2.