Title:	Very large array detection of radio recombination lines from the radio nucleus of NGC 253: Ionization by a weak active galactic nucleus, an obscured super star cluster, or a compact supernova remnant?
Authors:	Mohan, Niruj R. Anantharamaiah, K.R. Goss, W.M.
Keywords:	Galaxies: Individual: NGC Number: NGC 253, Galaxies: ISM, Galaxies: Nuclei, Galaxies: Starburst, Radio Lines: Galaxies
Issue Date:	1-Aug-2002
Publisher:	The University of Chicago Press for the American Astronomical Society
Citation:	Astrophysical Journal, 2002, Vol.574, p701-708
Abstract:	We have imaged the H92α and H75α radio recombination line (RRL) emissions from the starburst galaxy NGC 253 with a resolution of ~4 pc. The peak of the RRL emission at both frequencies coincides with the unresolved radio nucleus. Both lines observed toward the nucleus are extremely wide, with FWHMs of ~200 km s-1. Modeling the RRL and radio continuum data for the radio nucleus shows that the lines arise in gas whose density is ~104 cm-3 and mass is a few thousand Msolar, which requires an ionizing flux of (6-20)×1051 photons s-1. We consider a supernova remnant (SNR) expanding in a dense medium, a star cluster, and also an active galactic nucleus (AGN) as potential ionizing sources. Based on dynamical arguments, we rule out an SNR as a viable ionizing source. A star cluster model is considered, and the dynamics of the ionized gas in a stellar-wind driven structure are investigated. Such a model is only consistent with the properties of the ionized gas for a cluster younger than ~105 yr. The existence of such a young cluster at the nucleus seems improbable. The third model assumes the ionizing source to be an AGN at the nucleus. In this model, it is shown that the observed X-ray flux is too weak to account for the required ionizing photon flux. However, the ionization requirement can be explained if the accretion disk is assumed to have a big blue bump in its spectrum. Hence, we favor an AGN at the nucleus as the source responsible for ionizing the observed RRLs. A hybrid model consisting of an inner advection-dominated accretion flow disk and an outer thin disk is suggested, which could explain the radio, UV, and X-ray luminosities of the nucleus.
URI:	http://hdl.handle.net/2289/1459
ISSN:	0004-637X 1538-4357(Online)
Alternative Location:	http://dx.doi.org/10.1086/341004
Copyright:	(2002) by the American Astronomical Society.
Appears in Collections:	Research Papers (A&A)

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