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VLA Detection of RRLs from the radio nucleus of NGC 253 : Ionization by a weak AGN, an obscured SSC or a compact SNR ? | | Niruj R. Mohan
; K. R. Anantharamaiah
; W. M. Goss
; | | Date: |
3 Apr 2002 | | Subject: | astro-ph | | Affiliation: | 1 and 2), K. R. Anantharamaiah , W. M. Goss ( RRI, India, IISc, India, NRAO, USA | | Abstract: | We have imaged the H92alpha and H75alpha 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 towards the nucleus are extremely wide, with FWHM of ~200 km /s. Modeling the RRL and radio continuum data for the radio nucleus shows that the lines arise in gas whose density is ~10^4 ç and mass is few thousand Msun, which requires an ionizing flux of (6-20)x10^{51} photons /s. We consider a SNR expanding in a dense medium, a star cluster and also an AGN as potential ionizing sources. Based on dynamical arguments, we rule out an SNR as a viable ionizing source. A star cluster model was considered and the dynamics of the ionized gas in a stellar-wind driven structure was investigated. Such a model is consistent with the properties of the ionized gas only for a cluster younger than ~10^5 years. 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 was 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 a inner ADAF disk and an outer thin disk is suggested, which could explain the radio, UV and the X-ray luminosities of the nucleus. | | Source: | arXiv, astro-ph/0204051 | | Services: | Forum | Review | PDF | Favorites | |
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