A New Detection of Extragalactic Anomalous Microwave Emission in a Compact, Optically Faint Region of NGC 4725

July 2018 • 2018ApJ...862...20M

Authors • Murphy, E. J. • Linden, S. T. • Dong, D. • Hensley, B. S. • Momjian, E. • Helou, G. • Evans, A. S.

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.

Links

• PREPRINT http://arxiv.org/abs/1805.05965
• NED https://ned.ipac.caltech.edu/uri/NED::InRefcode/2018ApJ...862...20M
• ELECTR https://doi.org/10.3847%2F1538-4357%2Faac5f5
• SIMBAD http://simbad.u-strasbg.fr/simbo.pl?bibcode=2018ApJ...862...20M
• PDF https://iopscience.iop.org/article/10.3847/1538-4357/aac5f5/pdf
• PDF https://stacks.iop.org/0004-637X/862/20/pdf
• DATA https://irsa.ipac.caltech.edu/bibdata/2018/M/2018ApJ...862...20M.html