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Spitzer Mid-Infrared Spectroscopy of Infrared Luminous Galaxies at z~2. II. Diagnostics

Sajina, Anna and Yan, Lin and Armus, Lee and Choi, Philip and Fadda, Dario and Helou, George and Spoon, Henrik (2007) Spitzer Mid-Infrared Spectroscopy of Infrared Luminous Galaxies at z~2. II. Diagnostics. Astrophysical Journal, 664 (2). pp. 713-737. ISSN 0004-637X.

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We present mid-IR spectral decomposition of a sample of 48 Spitzer-selected ULIRGs spanning z ~ 1-3 and likely L_(IR) ~ 10^(12)-10^(13) L_⊙. Our study aims at quantifying the star formation and active galactic nucleus (AGN) processes in these sources that recent results suggest have evolved strongly between the observed epoch and today. To do this, we study the mid-IR contribution of PAH emission, continuum, and extinction. About 3/4 of our sample are continuum (i.e., AGN) dominated sources, but ~60% of these show PAH emission, suggesting the presence of star formation activity. These sources have redder mid-IR colors than typical optically selected quasars. About 25% of our sample have strong PAH emission, but none are likely to be pure starbursts as reflected in their relatively high 5 μm hot dust continua. However, their steep 30-14 μm slopes suggest that star formation might dominate the total infrared luminosity. Six of our z ~ 2 sources have EW_(6.2) ≳ 0.3 μm and L_(14 μm) ≳ 10^(12) L_⊙ (implying L_(IR) ≳ 10^(13) L_⊙). At these luminosities, such high EW_(6.2) ULIRGs do not exist in the local universe. We find a median optical depth at 9.7 μm of <τ_(9.7 μm)> = 1.4. This is consistent with local IRAS-selected ULIRGs but differs from early results on SCUBA-selected z ~ 2 ULIRGs. Similar to local ULIRGs, about 25% of our sample shows extreme obscuration (τ_(9.7 μm) ≳ 3), suggesting buried nuclei. In general, we find that our sources are similar to local ULIRGs but are an order of magnitude more luminous. It is not clear whether our z ~ 2 ULIRGs are simply scaled-up versions of local ULIRGs or subject to fundamentally different physical processes.

Item Type:Article
Related URLs:
URLURL TypeDescription
Sajina, Anna0000-0002-1917-1200
Yan, Lin0000-0003-1710-9339
Armus, Lee0000-0003-3498-2973
Fadda, Dario0000-0002-3698-7076
Helou, George0000-0003-3367-3415
Spoon, Henrik0000-0002-8712-369X
Additional Information:© 2007 The American Astronomical Society. Received 2007 January 26; accepted 2007 April 12. We would like to thank Vassilis Charmandaris, Guilaine Lagache, and Daniel Stern for useful discussions. Many thanks to Kalliopi Dasyra for help with PG QSO data and useful discussions. We would like to thank the anonymous referee for helpful suggestions that have improved the presentation of this paper. This work is based on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under contract with NASA. Support for this work was provided by NASA through an award issued by JPL/Caltech. A. S. acknowledges support by NASA through grant 09865 from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract BAS5-26555.
Group:Infrared Processing and Analysis Center (IPAC)
Funding AgencyGrant Number
Subject Keywords:galaxies : active; galaxies : high-redshift; infrared : galaxies
Issue or Number:2
Record Number:CaltechAUTHORS:20091117-080409038
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Official Citation:Spitzer Mid-Infrared Spectroscopy of Infrared Luminous Galaxies at z~2. II. Diagnostics Anna Sajina, Lin Yan, Lee Armus, Philip Choi, Dario Fadda, George Helou, and Henrik Spoon 2007 ApJ 664 713-737 doi: 10.1086/519446
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:16720
Deposited By: Tony Diaz
Deposited On:25 Nov 2009 21:52
Last Modified:07 Nov 2019 15:08

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