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Connecting far-infrared and radio morphologies of disk galaxies: Cosmic-ray electron diffusion after star formation episodes

Murphy, E. J. and Helou, G. and Kenney, J. D. P. and Armus, L. and Braun, R. (2008) Connecting far-infrared and radio morphologies of disk galaxies: Cosmic-ray electron diffusion after star formation episodes. Astrophysical Journal, 678 (2). pp. 828-850. ISSN 0004-637X. doi:10.1086/587123.

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We present results on the ISM properties of 29 galaxies based on a comparison of Spitzer far-infrared and Westerbork Synthesis Radio Telescope radio continuum imagery. Of these 29 galaxies, 18 are close enough to resolve at ≾ to 1 kpc scales at 70 µm and 22 cm. We extend the approach of our earlier work of smoothing infrared images to approximate cosmic-ray (CR) electron spreading and thus largely reproduce the appearance of radio images. Using a wavelet analysis, we decompose each 70 µm image into one component containing the star-forming structures and a second one for the diffuse disk. The components are smoothed separately, and their combination compared to a free-free corrected 22 cm radio image; the scale lengths are then varied to best match the radio and smoothed infrared images. We find that late-type spirals having high amounts of ongoing star formation benefit most from the two-component method. We also find that the disk component dominates for galaxies having low star formation activity, whereas the structure component dominates at high star formation activity. We propose that this result arises from an age effect rather than from differences in CR electron diffusion due to varying ISM parameters. The bulk of the CR electron population in actively star-forming galaxies is significantly younger than that in less active galaxies due to recent episodes of enhanced star formation; these galaxies are observed within ~10^8 yr since the onset of the most recent star formation episode. The sample irregulars have anomalously low best-fit scale lengths for their surface brightnesses compared to the rest of the sample spirals, which we attribute to enhanced CR electron escape.

Item Type:Article
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URLURL TypeDescription
Murphy, E. J.0000-0001-7089-7325
Helou, G.0000-0003-3367-3415
Armus, L.0000-0003-3498-2973
Additional Information:© 2008 The American Astronomical Society. Received 2007 July 4; accepted 2008 January 30. We wish to thank the referee, Don Cox, for his insightful comments, which have greatly improved the paper. E. J. M. is indebted to Eric Slezak for his highly constructive comments and critical remarks that helped guide the technical aspects of the wavelet decomposition. E. J. M. would also like to thank other members of the Spitzer Infrared Nearby Galaxies Survey (SINGS) team for their invaluable contributions to the work presented here, especially those of R. C. Kennicutt Jr., D. Calzetti, K. D. Gordon, C. W. Engelbracht, and G. Bendo. As part of the Spitzer Space Telescope Legacy Science Program, support was provided by NASA through contract 1224769 issued by the Jet Propulsion Laboratory, California Institute of Technology under NASA contract 1407. E. J. M. also acknowledges support for this work provided by the Spitzer Science Center Visiting Graduate Student program. This research has made use of the NASA/IPAC Extragalactic Database, which is operated by JPL/Caltech, under contract with NASA. This publication makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and IPAC/Caltech, funded by NASA and the National Science Foundation.
Group:Infrared Processing and Analysis Center (IPAC)
Funding AgencyGrant Number
Spitzer Science Center Visiting Graduate Student ProgramUNSPECIFIED
Subject Keywords:cosmic rays; infrared: galaxies; radio continuum: galaxies
Issue or Number:2
Record Number:CaltechAUTHORS:MURapj08
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:13396
Deposited By: Tony Diaz
Deposited On:07 May 2009 19:26
Last Modified:08 Nov 2021 22:37

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