Detection of the 3.3um PAH feature as well as water ice and HAC absorption in z~2 ULIRGs

TL;DR

This study analyzes high-quality mid-infrared spectra of z~2 ULIRGs to detect PAH features and molecular absorption, revealing differences from local ULIRGs and identifying the highest redshift 3.3um PAH detection.

Contribution

It provides the first high signal-to-noise spectra of z~2 ULIRGs focusing on PAH and molecular absorption features, with the discovery of the highest redshift 3.3um PAH detection.

Findings

Detection of 3.4um HAC in four sources

Detection of 3.05um water ice in three sources

Highest redshift 3.3um PAH detection reported

Abstract

We present preliminary results from the highest available signal-to-noise rest-frame 2-8um spectra of z~2 ULIRGs. Our 10 targets are selected for their deep silicate absorption features based on previous shallower IRS spectra. The goal of this follow-up program is: 1) allow for a more accurate analysis of inner/hot dust continuum, 2) detecting the 3.3um and 6.2um PAH features, and 3) detecting molecular absorption features such as due to water ice and hydrocarbons (HACs). We find that the 3.4um HAC absorption feature is observed in four sources, while the 3.05um water ice feature is observed in three of the sources. The HAC detectability is higher and ice detectability lower than expected from local ULIRGs, but consistent with a more AGN-dominated sample such as this one. Where ice is detected, the ice-to-silicate ratio is somewhat lower than many local ULIRGs implying on average thinner ice mantles. One source shows the, to our knowledge, highest redshift reported detection of the 3.3um PAH feature (along with a previously detected 6.2um feature). The strength of the 3.3um feature is as expected for a starburst-dominated ULIRG.