The nature of star formation in distant ultraluminous infrared galaxies selected in a remarkably narrow redshift range

TL;DR

This study analyzes mid-infrared spectra of high-redshift ultraluminous infrared galaxies, revealing starburst characteristics and systematic differences from local ULIRGs, suggesting extended star formation and reduced dust obscuration.

Contribution

It introduces an effective selection method for distant IR-luminous star-forming galaxies in narrow redshift ranges and compares their spectral features to local counterparts.

Findings

Most galaxies exhibit PAH features indicating starburst activity.

Sample spectra show weaker silicate absorption than local ULIRGs.

Composite spectra resemble local starbursts with lower IR luminosities.

Abstract

We present mid-infrared spectra of thirty two high redshift ultraluminous infrared galaxies, selected via the stellar photospheric feature at rest-frame 1.6um, and an observed-frame 24um flux of >500muJy. Nearly all the sample reside in a redshift range of <z>=1.71+/-0.15, and have rest-frame 1-1000um luminosities of 10^12.9 - 10^13.8 Lsun. Most of the spectra exhibit prominent polycyclic aromatic hydrocarbon emission features, and weak silicate absorption, consistent with a starburst origin for the IR emission. Our selection method appears to be a straightforward and efficient way of finding distant, IR-luminous, star-forming galaxies in narrow redshift ranges. There is however evidence that the mid-IR spectra of our sample differ systematically from those of local ULIRGs; our sample have comparable PAH equivalent widths but weaker apparent silicate absorption, and (possibly) enhanced PAH 6.2um/7.7um and 6.2um/11.2um flux ratios. Furthermore, the composite mid-IR spectrum of our sample is almost identical to that of local starbursts with IR luminosities of 10^10-10^11 Lsun rather than that of local ULIRGs. These differences are consistent with a reduced dust column, which can plausibly be obtained via some combination of (1) star formation that is extended over spatial scales of 1-4Kpc, and (2) star formation in unusually gas-rich regions.