Balancing the Energy Budget: Star-Formation versus AGN in High Redshift Infrared Luminous Galaxies

TL;DR

This study uses deep mid-infrared spectroscopy and multi-wavelength photometry to analyze the energy sources in high-redshift infrared luminous galaxies, revealing that star formation dominates over AGN activity and that IR luminosities are overestimated when using only 24-micron data at higher redshifts.

Contribution

It provides a detailed spectral decomposition method to distinguish star formation and AGN contributions in high-redshift galaxies, improving IR luminosity estimates.

Findings

IR luminosities are overestimated by ~5 times using only 24-micron data at z > 1.4.

Star formation accounts for about 70% of IR luminosity, AGN about 30%.

High-redshift galaxies show larger aromatic feature equivalent widths than local counterparts.

Abstract

We present deep {\it Spitzer} mid-infrared spectroscopy, along with 16, 24, 70, and 850\,$\micron$\ photometry, for 22 galaxies located in the Great Observatories Origins Deep Survey-North (GOODS-N) field. The sample spans a redshift range of $0.6\la z \la 2.6$, 24~$\mu$m flux densities between $\sim$0.2$-$1.2 mJy, and consists of submillimeter galaxies (SMGs), X-ray or optically selected active galactic nuclei (AGN), and optically faint ($z_{AB}>25$\,mag) sources. We find that infrared (IR; $8-1000~\micron$) luminosities derived by fitting local spectral energy distributions (SEDs) with 24~$\micron$ photometry alone are well matched to those when additional mid-infrared spectroscopic and longer wavelength photometric data is used for galaxies having $z\la1.4$ and 24~$\micron$-derived IR luminosities typically $\la 3\times 10^{12}~L_{\sun}$. However, for galaxies in the redshift range between $1.4\la z \la 2.6$, typically having 24~$\micron$-derived IR luminosities $\ga 3\times 10^{12}~L_{\sun}$, IR luminosities are overestimated by an average factor of $\sim$5 when SED fitting with 24~$\micron$ photometry alone. This result arises partly due to the fact that high redshift galaxies exhibit aromatic feature equivalent widths that are large compared to local galaxies of similar luminosities. Through a spectral decomposition of mid-infrared spectroscopic data, we are able to isolate the fraction of IR luminosity arising from an AGN as opposed to star formation activity. This fraction is only able to account for $\sim$30\% of the total IR luminosity among the entire sample.