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.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- July 2018
- DOI:
- 10.3847/1538-4357/aac5f5
- arXiv:
- arXiv:1805.05965
- Bibcode:
- 2018ApJ...862...20M
- Keywords:
-
- dust;
- extinction;
- galaxies: individual: NGC 4725;
- H ii regions;
- radio continuum: general;
- stars: formation;
- Astrophysics - Astrophysics of Galaxies
- E-Print:
- 10 pages, 4 figures, accepted for publication in ApJ