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