A&A 418, 429-443 (2004)
DOI: 10.1051/0004-6361:20034486

Further clues to the nature of composite LINER/H II galaxies

M. E. Filho^{1, 2, 3}, F. Fraternali^{1, 4}, S. Markoff⁵, N. M. Nagar¹, P. D. Barthel¹, L. C. Ho⁶ and F. Yuan<sup>7

1</sup>  Kapteyn Astronomical Institute, PO Box 800, 9700 AV Groningen, The Netherlands
²  Istituto di Radioastronomia, CNR, via P. Gobetti, 101, 40129 Bologna, Italy
³  Centro de Astrofísica da Universidade do Porto, Rua das Estrelas, 4150 - 762 Porto, Portugal
⁴  ASTRON, PO Box 2, 7990 AA Dwingeloo, The Netherlands
⁵  Massachusetts Institute of Technology, Center for Space Research, 77 Massachusetts Av., R. NE80 - 6035, Cambridge, Massachusetts 02139, USA
⁶  The Observatories of the Carnegie Institution of Washington, 813 Santa Barbara Street, Pasadena, California 91101, USA
⁷  Purdue University, Department of Physics, 525 Northwestern Av., West Lafayette, Indiana, 47907 - 2036, USA

(Received 9 October 2003 / Accepted 14 January 2004 )

Abstract
We have analyzed new, archival and published high resolution radio and X-ray observations of a sample of composite LINER/H II galaxies known to exhibit AGN-like properties. Five of the 16 AGN candidates have milliarcsecond-scale detections and are found to display a compact, flat spectrum, high brightness temperature radio core, four of which also exhibit extended radio emission. Five of the eight AGN candidates with available high resolution X-ray observations were found to possess a hard X-ray nuclear source, two of which have no milliarcsecond radio detection. The combined high resolution radio and X-ray data yield a 50% detection rate of low luminosity AGN among the AGN candidates, which translates into a 12% detection rate for the entire composite LINER/H II sample. In the sources where the AGN has been unambiguously detected, the ionizing power of the AGN is not sufficient to generate the observed emission lines, unless the hard X-rays are heavily obscured. We attempt to apply a canonical advection-dominated accretion flow (ADAF) and jet model to the sample sources in order to explain the observed radio and X-ray emission. While ADAFs may be responsible for the observed emission in submillijansky radio cores like NGC 7331, they do not appear consistent with the radio emission observed in the milliarcsecond-scale radio detected cores; the latter sources are more likely to have an energetically important contribution from a radio-emitting jet.

Key words: galaxies: active -- galaxies: nuclei -- radiation mechanisms: general

Offprint request: M. E. Filho, mfilho@ira.cnr.it

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