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

TL;DR

This paper reports the detection of extragalactic Anomalous Microwave Emission (AME) in a compact, optically-faint region of NGC 4725, supported by multi-frequency radio observations and infrared counterparts, suggesting a nascent star-forming region with dust-related emission.

Contribution

The study presents the first detection of extragalactic AME in NGC 4725, demonstrating that electric dipole emission from ultra-small grains can explain the observed radio spectrum.

Findings

Detected a compact radio source peaking at 33 GHz in NGC 4725

Infrared counterparts suggest dust emission associated with the source

Models of spinning dust grains reproduce the radio spectrum

Abstract

We discuss the nature of a discrete, compact radio source (NGC 4725 B) located $\approx$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 $\mu$Jy radio source peaking at $\approx$33 GHz. While the source is not identified in $BVRI$ photometry, we detect counterparts in the mid-infrared $Spitzer$/IRAC bands (3.6, 4.5, 5.8, 8.0 $\mu$m) 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 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 ($\lesssim 3$ 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.