Astrophys. J., 862, 20-20 (2018/July-3)
A new detection of extragalactic anomalous microwave emission in a compact, optically faint region of NGC 4725.
MURPHY E.J., LINDEN S.T., DONG D., HENSLEY B.S., MOMJIAN E., HELOU G. and EVANS A.S.
Abstract (from CDS):
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.
© 2018. The American Astronomical Society. All rights reserved.
dust, extinction - galaxies: individual: NGC 4725 - H II regions - radio continuum: general - stars: formation
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