HST NICMOS imaging of z~2, 24 micron-selected Ultraluminous Infrared Galaxies

TL;DR

This study uses HST NICMOS imaging to analyze the morphology, interactions, and properties of z~2 ULIRGs, revealing differences from local ULIRGs in merger types and structural features.

Contribution

First detailed NICMOS imaging analysis of z~2 ULIRGs showing their interaction states and structural differences from local counterparts.

Findings

At least 17 of 33 ULIRGs show interactions.

Most are minor mergers, few are major mergers.

High surface brightness and compactness linked to AGN or obscuration.

Abstract

We present Hubble Space Telescope NICMOS H-band imaging of 33 Ultraluminous Infrared Galaxies (ULIRGs) at z~2 that were selected from the 24 micron catalog of the Spitzer Extragalactic First Look Survey. The images reveal that at least 17 of the 33 objects are associated with interactions. Up to one fifth of the sources in our sample could be minor mergers whereas only 2 systems are merging binaries with luminosity ratio <=3:1, which is characteristic of local ULIRGs. The rest-frame optical luminosities of the sources are of the order 10^10-10^11 L_sun and their effective radii range from 1.4 to 4.9 kpc. The most compact sources are either those with a strong active nucleus continuum or those with a heavy obscuration in the mid-infrared regime, as determined from Spitzer Infra-Red Spectrograph data. The luminosity of the 7.7 micron feature produced by polycyclic aromatic hydrocarbon molecules varies significantly among compact systems whereas it is typically large for extended systems. A bulge-to-disk decomposition performed for the 6 brightest (m_H<20) sources in our sample indicates that they are best fit by disk-like profiles with small or negligible bulges, unlike the bulge-dominated remnants of local ULIRGs. Our results provide evidence that the interactions associated with ultraluminous infrared activity at z~2 can differ from those at z~0.