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

Murphy, E. J. and Hensley, B. S. and Linden, S. T. and Draine, B. T. and Dong, D. and Momjian, E. and Helou, G. and Evans, A. S. (2020) Where's the Dust?: The Deepening Anomaly of Microwave Emission in NGC 4725 B. Astrophysical Journal Letters, 905 (2). Art. No. L23. ISSN 2041-8205. doi:10.3847/2041-8213/abc7c8.

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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 ¹²CO ( 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 (≾10⁵ M_⊙) 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.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Murphy, E. J.0000-0001-7089-7325
Hensley, B. S.0000-0001-7449-4638
Linden, S. T.0000-0002-1000-6081
Draine, B. T.0000-0002-0846-936X
Dong, D.0000-0001-9584-2531
Momjian, E.0000-0003-3168-5922
Helou, G.0000-0003-3367-3415
Evans, A. S.0000-0003-2638-1334
Additional Information:© 2020. The American Astronomical Society. Received 2020 October 12; revised 2020 November 3; accepted 2020 November 5; published 2020 December 17. We would like to thank the anonymous referee for comments that helped to improve the content and presentation of this Letter. E. J. M. thanks J. J. Condon for useful discussions that helped improve the paper. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. This paper makes use of the following ALMA data: ADS/JAO.ALMA#2018.1.01826.S. ALMA is a partnership of ESO (representing its member states), NSF (USA), and NINS (Japan), together with NRC (Canada), MOST and ASIAA (Taiwan), and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO, and NAOJ. Software: APLpy (Robitaille & Bressert 2012; Robitaille 2019), CASA (v4.6.0, v4.7.0, v5.3.0, v5.4.0, v5.6.1 McMullin et al. 2007), corner (Foreman-Mackey 2016), emcee (Foreman-Mackey et al. 2013), Matplotlib (Hunter 2007), NumPy (van der Walt et al. 2011).
Group:Infrared Processing and Analysis Center (IPAC)
Subject Keywords:Radio continuum emission ; Interstellar emissions ; Dust continuum emission ; H II regions; Star-forming regions ; Star formation ; Galaxies ; Disk galaxies
Issue or Number:2
Classification Code: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)
Record Number:CaltechAUTHORS:20201218-153311222
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Official Citation:E. J. Murphy et al 2020 ApJL 905 L23
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:107209
Deposited By: George Porter
Deposited On:18 Dec 2020 23:51
Last Modified:16 Nov 2021 19:00

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