Galaxies 09: Assembly, Gas Content and Star Formation History of Galaxies

Jason Melbourne
California Institute of Technology

Near-IR Spectroscopy Reveals Buried AGN Within $z$=2 Dust Obscured Galaxies (DOGs)

Spitzer mid-IR imaging has revealed a class of extremely Dust Obscured Galaxies (DOGs) with a redshift distribution centered about $z=2$. Defined by very red optical to IR colors, $ f_{24\mu m}/ f_R > 1000$ (e.g. redder than Arp 220 at every redshift), DOGs are extreme even for ultra-luminous infrared galaxies. With clustering strengths similar to low redshift massive ellipticals, DOGs may represent an early, formative stage of the most massive galaxies in the universe. Optical-to-IR spectral energy distributions (SEDs) and mid-IR spectroscopy suggest two classes. DOGs with power-law SEDs show strong silicate features in their mid-IR spectra, typical of AGN activity. DOGs with a mid-IR "bump" in their SEDs exhibit strong PAH features in mid-IR spectroscopy, suggesting star formation. We recently obtained Palomar Triplespec near-IR spectroscopy of both power-law and "bump" DOGs. The power-law DOGs show very broad $H\alpha$ lines ($FWHM > 1000$ km/s) redshifted into the near-IR bands, further proof of a strong AGN power source. The "bump" DOGs however, show much narrower $H\alpha$ ($FWHM \sim 300$ km s$^{-1}$). Integral field spectroscopy obtained with OSIRIS and the Keck laser guide star adaptive optics system, shows that $H\alpha$ flux in the power-law DOGs is extremely concentrated with little to no emission arising from a more extended narrow-line region. This may indicate that star formation has already begun to shut down in the AGN dominated DOGs.

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