Where’s the Dust?: The Deepening Anomaly of Microwave Emission in NGC 4725 B

Abstract

We present new Atacama Large Millimeter Array (ALMA) observations toward NGC 4725 B, a discrete, compact, optically faint region within the star-forming disk of the nearby galaxy NGC 4725 that exhibits strong anomalous microwave emission (AME). These new ALMA data include continuum observations centered at 92, 133, 203, and 221 GHz accompanied by spectral observations of the 12 CO ( =  J 2 1) line. NGC 4725 B is detected in the continuum at all frequencies, although the detection at 203 GHz is marginal. While molecular gas is not detected at the exact location of NGC 4725 B, there is molecular gas in the immediate vicinity (i.e., 100 pc) along with associated diffuse 8 μm emission. When combined with existing Very Large Array continuum data at 1.5, 3, 5.5, 9, 14, 22, 33, and 44 GHz, the spectrum is best fit by a combination of AME, synchrotron, and free-free emission that is free-free absorbed below ∼6 GHz. Given the strength of the AME, there is surprisingly no indication of millimeter dust emission associated with NGC 4725 B on 6″ spatial scales at the sensitivity of the ALMA interferometric data. Based on the properties of the nearest molecular gas complex and the inferred star formation rate, NGC 4725 B is consistent with being an extremely young (∼3-5 Myr) massive (10 5 M e ) cluster that is undergoing active cluster feedback. However, the lack of millimeter thermal dust emission is difficult to reconcile with a spinning dust origin of the 30 GHz emission. On the other hand, modeling NGC 4725 B as a new class of background radio galaxy is also unsatisfactory. Unified Astronomy Thesaurus concepts: Radio continuum emission (1340); Interstellar emissions (840); Dust continuum emission (412); H II regions (694); Star forming regions (1565); Star formation (1569); Galaxies (573); Disk galaxies (391)

The mm-to-cm SED of spiral galaxies

“…While AME is almost ubiquitous in the MW, it has been detected with high significance in three extragalactic sources only. AME has been found with pointed observations in just one star-forming region in NGC 6946 [37] and one compact radio-source in NGC 4725 ( [38], even though, in this case, it does not appear to be associated with dust [39]). On the global SED, strong evidence for AME has been recently found in the Andromeda galaxy [11], with an AME component compatible with that in the MW.…”

QUIJOTE Scientific Results -- VII. Galactic AME sources in the QUIJOTE-MFI Northern Hemisphere Wide-Survey

“…Detection of AME has also been reported by Murphy et al (2018) in NGC 4725B using VLA data. In a following work Murphy et al (2020) discussed complementary ALMA observations on NGC 4725B that show discrepancy with expected thermal dust component making the interpretation of the results quite puzzling. In our study we sampled the AME component over several AME candidate regions in our Galaxy.…”

Characterizing Compact 15–33 GHz Radio Continuum Sources in Local U/LIRGs

“…For three "SF" regions, IC1623B_n4, NGC5257_e1, and IC2810_e1, α 15-33  0 after accounting for uncertainties, which is unexpected from optically thin thermal free-free emission. Given that all "SF" regions have brightness temperatures much lower than the optically thick starburst temperature of ∼10 4 K (see Section 4.2), a potential cause for the higher-than-expected 33 GHz continuum flux may be anomalous microwave emission from spinning dust particles in heavily obscured young starbursts (Murphy et al 2020). This possible explanation will require more high-resolution observations above and below 33 GHz to confirm.…”