Spitzer Mid-Infrared Spectroscopy of 70um-Selected Distant Luminous Infrared Galaxies

TL;DR

This study presents Spitzer mid-infrared spectra of 11 distant, luminous infrared galaxies selected at 70um, revealing diverse features indicative of starburst activity and active galactic nuclei, with implications for understanding their energy sources.

Contribution

First Spitzer IRS spectra of 70um-selected distant luminous infrared galaxies, showing their spectral diversity and potential AGN dominance in some cases.

Findings

Seven galaxies show strong PAH emission indicating star formation.

Four galaxies exhibit silicate absorption with no PAH features, suggesting AGN dominance.

Infrared luminosities imply star formation rates up to 720 solar masses per year.

Abstract

We present mid-infrared spectroscopy obtained with the Spitzer Space Telescope of a sample of 11 optically faint, infrared luminous galaxies selected from a Spitzer MIPS 70um imaging survey of the NDWFS Bootes field. These are the first Spitzer IRS spectra presented of distant 70um-selected sources. All the galaxies lie at redshifts 0.3<z<1.3 and have very large infrared luminosities of L_IR~ 0.1-17 x 10^12 solar luminosities. Seven of the galaxies exhibit strong emission features attributed to polycyclic aromatic hydrocarbons (PAHs). The average IRS spectrum of these sources is characteristic of classical starburst galaxies, but with much larger infrared luminosities. The PAH luminosities of L(7.7) ~ 0.4 - 7 x 10^11 solar luminosities imply star formation rates of ~ 40 - 720 solar masses per year. Four of the galaxies show deep 9.7um silicate absorption features and no significant PAH emission features (6.2um equivalent widths < 0.03um). The large infrared luminosities and low f70/f24 flux density ratios suggests that these sources have AGN as the dominant origin of their large mid-infrared luminosities, although deeply embedded but luminous starbursts cannot be ruled out. If the absorbed sources are AGN-dominated, a significant fraction of all far-infrared bright, optically faint sources may be dominated by AGN.