A New Detection of Extragalactic Anomalous Microwave Emission in a Compact, Optically Faint Region of NGC 4725
Abstract
We discuss the nature of a discrete, compact radio source (NGC 4725 B) located ≈1.9 kpc from the nucleus in the nearby star-forming galaxy NGC 4725, which we believe to be a new detection of extragalactic anomalous microwave emission (AME). Based on detections at 3, 15, 22, 33, and 44 GHz, NGC 4725 B is a microjansky radio source peaking at ≈33 GHz. While the source is not identified in optical (BVRI) photometry, we detect counterparts in the midinfrared Spitzer/IRAC bands (3.6, 4.5, 5.8, 8.0 μm) that appear to be associated with dust emission in the central region of NGC 4725. Consequently, we conclude that NGC 4725 B is a new detection of AME and is very similar to a recent detection of AME in an outer-disk star-forming region in NGC 6946. We find that models of electric dipole emission from rapidly rotating ultra-small grains are able to reproduce the radio spectrum for reasonable interstellar medium conditions. Given the lack of an optical counterpart and the shape of the radio spectrum, NGC 4725 B appears consistent with a nascent star-forming region in which young (≾ 3 Myr) massive stars are still highly enshrouded by their natal cocoons of gas and dust with insufficient supernovae occurring to produce a measurable amount of synchrotron emission.
Additional Information
© 2018. The American Astronomical Society. Received 2018 April 20; revised 2018 May 11; accepted 2018 May 15; published 2018 July 18. We would like to thank the anonymous referee for very useful comments that helped to improve the content and presentation of this paper. E.J.M. thanks J. J. Condon and B. T. Draine for useful discussions that helped improve the paper. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. This research was carried out in part at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration.Attached Files
Published - Murphy_2018_ApJ_862_20.pdf
Accepted Version - 1805.05965
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Additional details
- Eprint ID
- 88472
- Resolver ID
- CaltechAUTHORS:20180801-142726478
- NASA/JPL/Caltech
- Created
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2018-08-01Created from EPrint's datestamp field
- Updated
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2021-11-16Created from EPrint's last_modified field
- Caltech groups
- Infrared Processing and Analysis Center (IPAC)