Anomalous Silicate Dust Emission in the Type 1 Liner Nucleus of M81
Abstract
We report the detection and successful modeling of the unusual 9.7 μm Si-O stretching silicate emission feature in the type 1 (i.e., face-on) LINER nucleus of M81. Using the Infrared Spectrograph (IRS) instrument on Spitzer, we determine the feature in the central 230 pc of M81 to be in strong emission, with a peak at ~10.5 μm. This feature is strikingly different in character from the absorption feature of the galactic interstellar medium, and from the silicate absorption or weak emission features typical of galaxies with active star formation. We successfully model the high signal-to-noise ratio IRS spectra with porous silicate dust using laboratory-acquired mineral spectra. We find that the most probable fit uses micron-sized, porous grains of amorphous silicate and amorphous carbon. In addition to silicate dust, there is weak polycyclic aromatic hydrocarbon (PAH) emission present (particularly at 11.3 μm, arising from the C-H out-of-plane bending vibration of relatively large PAHs of ~500-1000 C atoms) whose character reflects the low-excitation active galactic nucleus environment, with some evidence that small PAHs of ~100-200 C atoms (responsible for the 7.7 μm C-C stretching band) in the immediate vicinity of the nucleus have been preferentially destroyed. Analysis of the infrared fine structure lines confirms the LINER character of the M81 nucleus. Four of the infrared H_2 rotational lines are detected and fit to an excitation temperature of T ~ 800 K. Spectral maps of the central 230 pc in the [Ne II] 12.8 μm line, the H_2 17 μm line, and the 11.3 μm PAH C-H bending feature reveal arc- or spiral-like structures extending from the core. We also report on epochal photometric and spectroscopic observations of M81, whose nuclear intensity varies in time across the spectrum due to what is thought to be inefficient, sub-Eddington accretion onto its central black hole. We find that, contrary to the implications of earlier photometry, the nucleus has not varied over a period of two years at these infrared wavelengths to a precision of about 1%.
Additional Information
© 2010 The American Astronomical Society. Received 2009 August 31; accepted 2010 April 16; published 2010 May 19. We thank Ciska Kemper-Markwick for her advice on and assistance with dust modeling, and Achim Tappe for help with CUBISM in general and IRS calibration issues in particular. We thank Nick Devereux for providing the data for the Hα image in Figure 2, and for his helpful comments. We also thank Lei Hao, Ciska Kemper-Markwick, Ralf Siebenmorgen, Henrik Spoon, Eckhard Sturm, Doug Whittet, and Martin Haas for providing us with the observational data of 3C273, GC Sgr A∗, IRAS 08572+3915, NGC 3998, Cygni OB2 #12, and 3CR galaxies. We thank the anonymous referee for his/her thoughtful comments, which helped to improve this paper. H.A.S. and M.L.N.A. acknowledge partial support from NASA Grant NAG5-10654. A.L., M.K., and M.P.L. are supported in part by Spitzer Theory Programs, a Herschel Theory Program, and NSF grant AST 07-07866. This work is based in part on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under NASA contract 1407. Support for the IRAC instrument was provided by NASA through Contract Number 960541 issued by JPL. CUBISM is a product of the SSC and the IRS team.Attached Files
Published - Smith2010p10353Astrophys_J.pdf
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Additional details
- Eprint ID
- 18719
- Resolver ID
- CaltechAUTHORS:20100617-095250290
- NAG5-10654
- NASA
- Spitzer Theory Programs
- Herschel Theory Program
- AST 07-07866
- NSF
- Created
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2010-07-15Created from EPrint's datestamp field
- Updated
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2021-11-08Created from EPrint's last_modified field