Strong PAH Emission from z~2 ULIRGs

TL;DR

This study uses Spitzer spectroscopy to analyze 23 high-redshift ULIRGs, revealing strong PAH emissions indicative of star formation and confirming the effectiveness of the 1.6 micron stellar bump in selecting obscured star-forming galaxies at z~2.

Contribution

It demonstrates that the 1.6 micron stellar bump is an efficient method to identify obscured star-forming ULIRGs at z~2 using mid-infrared spectra.

Findings

20 out of 23 ULIRGs show PAH emission, indicating active star formation.

The redshift distribution peaks sharply at z=1.9 due to 24 micron detectability.

Sources have infrared luminosities comparable to submillimeter galaxies.

Abstract

Using the Infrared Spectrograph on board the Spitzer Space Telescope, we present low-resolution (64 < lambda / dlambda < 124), mid-infrared (20-38 micron) spectra of 23 high-redshift ULIRGs detected in the Bootes field of the NOAO Deep Wide-Field Survey. All of the sources were selected to have 1) fnu(24 micron) > 0.5 mJy; 2) R-[24] > 14 Vega mag; and 3) a prominent rest-frame 1.6 micron stellar photospheric feature redshifted into Spitzer's 3-8 micron IRAC bands. Of these, 20 show emission from polycyclic aromatic hydrocarbons (PAHs), usually interpreted as signatures of star formation. The PAH features indicate redshifts in the range 1.5 < z < 3.0, with a mean of <z>=1.96 and a dispersion of 0.30. Based on local templates, these sources have extremely large infrared luminosities, comparable to that of submillimeter galaxies. Our results confirm previous indications that the rest-frame 1.6 micron stellar bump can be efficiently used to select highly obscured starforming galaxies at z~2, and that the fraction of starburst-dominated ULIRGs increases to faint 24 micron flux densities. Using local templates, we find that the observed narrow redshift distribution is due to the fact that the 24 micron detectability of PAH-rich sources peaks sharply at z = 1.9. We can analogously explain the broader redshift distribution of Spitzer-detected AGN-dominated ULIRGs based on the shapes of their SEDs. Finally, we conclude that z~2 sources with a detectable 1.6 micron stellar opacity feature lack sufficient AGN emission to veil the 7.7 micron PAH band.