Spitzer Mid-Infrared Spectroscopy of Infrared Luminous Galaxies at z~2 III: Far-IR to Radio Properties and Optical Spectral Diagnostics

TL;DR

This study analyzes the multi-wavelength properties of z~2 infrared luminous galaxies, revealing the prevalence of obscured quasars and the varying contributions of AGN and starburst activity through spectral diagnostics and photometry.

Contribution

It provides a comprehensive multi-wavelength analysis of ULIRGs at z~2, quantifying AGN and starburst contributions and identifying obscured quasars using optical, IR, and radio diagnostics.

Findings

Strong-PAH sources have higher far-IR fluxes than weak-PAH sources.

Weak-PAH sources tend to have >70% AGN contribution.

57% of z>1.5 sources host obscured quasars.

Abstract

We present the far-IR, millimeter, and radio photometry as well as optical and near-IR spectroscopy of a sample of 48 z~1-3 Spitzer-selected ULIRGs with IRS mid-IR spectra. Our goals are to compute their bolometric emission, and to determine both the presence and relative strength of their AGN and starburst components. We find that strong-PAH sources tend to have higher 160um and 1.2mm fluxes than weak-PAH sources. The depth of the 9.7um silicate feature does not affect MAMBO detectability. We fit the far-IR SEDs of our sample and find an average <L_{IR}>~7x10^{12}Lsun for our z>1.5 sources. Spectral decomposition suggests that strong-PAH sources typically have ~20-30% AGN fractions. Weak-PAH sources by contrast tend to have >~70% AGN fractions, with a few sources having comparable contributions of AGN and starbursts. The optical line diagnostics support the presence of AGN in the bulk of the weak-PAH sources. With one exception, our sources are narrow-line sources, show no obvious correspondence between the optical extinction and the silicate feature depth, and, in two cases, show evidence for outflows. Radio AGN are present in both strong-PAH and weak-PAH sources. This is supported by our sample's far-IR-to-radio ratios (q) being consistently below the average value of 2.34 for local star-forming galaxies. We use survival analysis to include the lower-limits given by the radio-undetected sources, arriving at <q>=2.07+/-0.01 for our z>1.5 sample. In total, radio and, where available, optical line diagnostics support the presence of AGN in 57% of the z>1.5 sources, independent of IR-based diagnostics. For higher-z sources, the AGN luminosities alone are estimated to be >10^{12}Lsun, which, supported by the [OIII] luminosities, implies that the bulk of our sources host obscured quasars.