Characterizing extragalactic anomalous microwave emission in NGC 6946 with CARMA
Using 1 cm and 3 mm observations from the Combined Array for Research in Millimeter-wave Astronomy and 2 mm observations from the Goddard IRAM Superconducting 2 Millimeter Observer observations, we follow up the first extragalactic detection of anomalous microwave emission (AME) reported by Murphy et al. in an extranuclear region (Enuc. 4) of the nearby face-on spiral galaxy NGC 6946. We find the spectral shape and peak frequency of AME in this region to be consistent with models of spinning dust emission. However, the strength of the emission far exceeds the Galactic AME emissivity given the abundance of polycyclic aromatic hydrocarbons (PAHs) in that region. Using our galaxy-wide 1 cm map (21 arcsec resolution), we identify a total of eight 21 arcsec × 21 arcsec regions in NGC 6946 that harbour AME at >95 per cent significance at levels comparable to that observed in Enuc. 4. The remainder of the galaxy has 1 cm emission consistent with or below the observed Galactic AME emissivity per PAH surface density. We probe relationships between the detected AME and dust surface density, PAH emission, and radiation field, though no environmental property emerges to delineate regions with strong versus weak or non-existent AME. On the basis of these data and other AME observations in the literature, we determine that the AME emissivity per unit dust mass is highly variable. We argue that the spinning dust hypothesis, which predicts the AME power to be approximately proportional to the PAH mass, is therefore incomplete.
© 2015 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. Accepted 2015 February 9. Received 2015 January 19; in original form 2014 November 25. First published online March 19, 2015. We thank John Carpenter for invaluable assistance reducing the CARMA data and Bruce Draine and the anonymous referee for extensive feedback that greatly improved the manuscript. We also thank Tim Brandt, Thiem Hoang, Chris Tibbs, and Chris White for helpful conversations. BH acknowledges support from the NSF Graduate Research Fellowship under Grant no. DGE-0646086 and NSF grant AST-1408723. The GISMO observations and JS were supported through NSF ATI grants 1020981 and 1106284.
Submitted - 1411.7030v1.pdf
Published - MNRAS-2015-Hensley-809-19.pdf