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Silicates in ultraluminous infrared galaxies

Sirocky, M. M. and Levenson, N. A. and Elitzur, M. and Spoon, H. W. W. and Armus, L. (2008) Silicates in ultraluminous infrared galaxies. Astrophysical Journal, 678 (2). pp. 729-743. ISSN 0004-637X.

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We analyze the mid-infrared (MIR) spectra of ultraluminous infrared galaxies (ULIRGs) observed with the Spitzer Space Telescope's Infrared Spectrograph. Dust emission dominates the MIR spectra of ULIRGs, and the reprocessed radiation that emerges is independent of the underlying heating spectrum. Instead, the resulting emission depends sensitively on the geometric distribution of the dust, which we diagnose with comparisons of numerical simulations of radiative transfer. Quantifying the silicate emission and absorption features that appear near 10 and 18 μm requires a reliable determination of the continuum, and we demonstrate that including a measurement of the continuum at intermediate wavelength (between the features) produces accurate results at all optical depths. With high-quality spectra, we successfully use the silicate features to constrain the dust chemistry. The observations of the ULIRGs and local sight lines require dust that has a relatively high 18 μm/10 μm absorption ratio of the silicate features (around 0.5). Specifically, the cold dust of Ossenkopf et al. is consistent with the observations, while other dust models are not. We use the silicate feature strengths to identify two families of ULIRGs, in which the dust distributions are fundamentally different. Optical spectral classifications are related to these families. In ULIRGs that harbor an active galactic nucleus, the spectrally broad lines are detected only when the nuclear surroundings are clumpy. In contrast, the sources of lower ionization optical spectra are deeply embedded in smooth distributions of optically thick dust.

Item Type:Article
Related URLs:
URLURL TypeDescription
Levenson, N. A.0000-0003-4209-639X
Elitzur, M.0000-0001-8143-3550
Spoon, H. W. W.0000-0002-8712-369X
Armus, L.0000-0003-3498-2973
Additional Information:© 2008 The American Astronomical Society. Received 2007 October 30, accepted for publication 2008 January 29. We thank G. Sloan for stimulating discussions about Galactic dust chemistry and J. Bernard-Salas, V. Lebouteiller, and D. Whelan for communicating results and providing reduced spectra in advance of publications. This work is based in part on observations made with the Spitzer Space Telescope and has made use of the NASA/ IPAC Extragalactic Database, both of which are operated by the Jet Propulsion Laboratory, California Institute of Technology, under contracts with NASA. N. A. L. acknowledges work supported by the NSF under grant 0237291. M. E. acknowledges support from NSF AST-0507421 and NASA NNG05GC38G.
Funding AgencyGrant Number
Subject Keywords:dust: extinction; galaxies: active; galaxies: nuclei; infrared: galaxies; radiative transfer
Issue or Number:2
Record Number:CaltechAUTHORS:20090514-194614043
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Official Citation:M. M. Sirocky, N. A. Levenson, M. Elitzur, H. W. W. Spoon, and L. Armus 2008 ApJ 678 729-743 doi: 10.1086/586727
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:14229
Deposited By: Joy Painter
Deposited On:26 Aug 2009 21:16
Last Modified:04 Nov 2019 02:24

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