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Detections of Water Ice, Hydrocarbons, and 3.3 μm PAH in z ~ 2 ULIRGs

Sajina, Anna and Spoon, Henrik and Yan, Lin and Imanishi, Masatoshi and Fadda, Dario and Elitzur, Moshe (2009) Detections of Water Ice, Hydrocarbons, and 3.3 μm PAH in z ~ 2 ULIRGs. Astrophysical Journal, 703 (1). pp. 270-284. ISSN 0004-637X.

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We present the first detections of the 3 μm water ice and 3.4 μm amorphous hydrocarbon (HAC) absorption features in z ~ 2 ULIRGs. These are based on deep rest-frame 2-8 μm Spitzer Infrared Spectrograph spectra of 11 sources selected for their appreciable silicate absorption. The HAC-to-silicate ratio for our z ~ 2 sources is typically higher by a factor of 2-5 than that observed in the Milky Way. This HAC "excess" suggests compact nuclei with steep temperature gradients as opposed to predominantly host obscuration. Beside the above molecular absorption features, we detect the 3.3 μm polycyclic aromatic hydrocarbon (PAH) emission feature in one of our sources with three more individual spectra showing evidence for it. Stacking analysis suggests that water ice, hydrocarbons, and PAH are likely present in the bulk of this sample even when not individually detected. The most unexpected result of our study is the lack of clear detections of the 4.67 μm CO gas absorption feature. Only three of the sources show tentative signs of this feature at significantly lower levels than has been observed in local ULIRGs. Overall we find that the closest local analogs to our sources, in terms of 3-4 μm color, HAC-to-silicate and ice-to-silicate ratios, as well as low PAH equivalent widths, are sources dominated by deeply obscured nuclei. Such sources form only a small fraction of ULIRGs locally and are commonly believed to be dominated by buried active galactic nuclei (AGNs). Our sample suggests that, in an absolute number, such buried AGNs are at least an order of magnitude more common at z ~ 2 than today. The presence of PAH suggests that significant levels of star formation are present even if the obscured AGNs typically dominate the power budget.

Item Type:Article
Related URLs:
URLURL TypeDescription
Sajina, Anna0000-0002-1917-1200
Spoon, Henrik0000-0002-8712-369X
Yan, Lin0000-0003-1710-9339
Imanishi, Masatoshi0000-0001-6186-8792
Fadda, Dario0000-0002-3698-7076
Elitzur, Moshe0000-0001-8143-3550
Additional Information:© 2009 American Astronomical Society. Print publication: Issue 1 (2009 September 20); received 2009 April 30; accepted for publication 2009 July 31; published 2009 August 26. We thank the anonymous referee for their thoughtful comments, which have improved this paper. We also wish to thank Brian Siana and Kalliopi Dasyra for helpful discussions. 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 a contract with NASA. Support for this work was provided by NASA through an award issued by JPL/Caltech.
Group:Infrared Processing and Analysis Center (IPAC)
Funding AgencyGrant Number
Subject Keywords:galaxies: high-redshift; galaxies: ISM; infrared: galaxies; quasars: absorption lines
Issue or Number:1
Record Number:CaltechAUTHORS:20090928-123624774
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Official Citation:Detections of Water Ice, Hydrocarbons, and 3.3 μm PAH in z ~ 2 ULIRGs Anna Sajina, Henrik Spoon, Lin Yan, Masatoshi Imanishi, Dario Fadda, and Moshe Elitzur 2009 ApJ 703 270-284 doi: 10.1088/0004-637X/703/1/270
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:16077
Deposited By: Jason Perez
Deposited On:02 Oct 2009 21:49
Last Modified:04 Nov 2019 01:44

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