University of Oulu

Aaron E. Watkins et al 2018 ApJL 858 L16.

Discovery of a vast ionized gas cloud in the M51 system

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Author: Watkins, Aaron E.1,2; Mihos, J. Christopher2; Bershady, Matthew3;
Organizations: 1Astronomy Research Unit, University of Oulu, FIN-90014, Finland
2Department of Astronomy, Case Western Reserve University, Cleveland, OH 44106, USA
3Department of Astronomy, University of Wisconsin, 475 North Charter Street, Madison, WI 53706, USA
Format: article
Version: published version
Access: open
Online Access: PDF Full Text (PDF, 3.2 MB)
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Language: English
Published: IOP Publishing, 2018
Publish Date: 2018-06-13


We present the discovery of a vast cloud of ionized gas (hereafter, the Cloud) 13′ (32 kpc) north of the interacting system M51. We detected this cloud via deep narrowband imaging with the Burrell Schmidt Telescope, where it appears as an extended, diffuse Hα-emitting feature with no embedded compact regions. The Cloud spans ∼10′ × 3′ (25 × 7.5 kpc) in size and has no stellar counterpart; comparisons with our previous deep broadband imaging show no detected continuum light to a limit of μlim,B ∼ 30 mag arcsec−2. WIYN⁴ SparsePak observations confirm the Cloud’s kinematic association with M51, and the high [N II]/Hα, [S II]/Hα, and [O I]/Hα line ratios that we measure imply a hard ionization source such as active galactic nuclei (AGN) photoionization or shock heating rather than photoionization due to young stars. Given the strong [N II] emission, we infer roughly solar metallicity for the Cloud, ruling out an origin due to infall of primordial gas. Instead, we favor models where the gas has been expelled from the inner regions of the M51 system due to tidal stripping or starburst/AGN winds and has been subsequently ionized either by shocks or a fading AGN. This latter scenario raises the intriguing possibility that M51 may be the nearest example of an AGN fossil nebula or light echo, akin to the famous “Hanny’s Voorwerp” in the IC 2497 system.

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Series: Astrophysical journal letters
ISSN: 2041-8205
ISSN-E: 2041-8213
ISSN-L: 2041-8205
Volume: 858
Issue: 2
Article number: L16
DOI: 10.3847/2041-8213/aabba1
Type of Publication: A1 Journal article – refereed
Field of Science: 115 Astronomy and space science
Funding: Support for this project was provided by NSF/AST-1108964 (JCM) and NSF/AST-1517006 (MAB).
Copyright information: © 2018. The American Astronomical Society. All rights reserved. Published in this repository with the kind permission of the publisher.