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Astron. Astrophys. 355, 922-928 (2000)

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1. Introduction

Low ionization nuclear emission regions (LINERs, Heckman 1980) are galaxies with strong forbidden lines from low ionization states, compared with those from higher ionization states, typically [O II ] 3727 [FORMULA] [O III ] 5007. LINER phenomena are the most common activities of galaxies known in the local universe (Ho et al. 1997). If most LINERs are truely low luminosity active galactic nuclei (LLAGN) (Ho 1998), it will be important to understand the nature of AGN by investigating the role of LINERs. However, the origin of LINER phenomena is debated (Maoz et al. 1998; Lawrence 1998; Ho 1998; and the references therein). Maoz et al. (1998) claimed that the young stellar population may provide enough ionizing photons for the observed spectra of a significant fraction of LINERs, and it is also possible that LINERs may be a heterogeneous class. A good example is M 81, one of the nearest bright LINERs. Ho (1998) stressed the incontrovertible, nonstellar nature of M 81, though the stellar contribution cannot be ruled out (Maoz et al. 1998). Recently, Hameed & Devereux (1999) discussed the extended nuclear emission-line regions having a LINER spectrum in M 81, and argued that M 81 is likely a composite object. They also proposed that shock heating or UV-photons from post asymptotic giant branch stars are probably responsible for the extended LINER emission. Further evidence for the "composite" nature for M 81 is provided by the ROSAT HRI data (Colbert & Mushotzky 1998), in which the intensity ratio of the point-like component to the extended one is about 3/2 (Colbert, private communication).

It has been suggested (Condon et al. 1982) that the AGN activities can be distinguished from starbursts by using the FIR-radio correlation, which is more significant for starbursts than for AGN. The weak FIR-radio correlation of Seyferts may imply the starburst (SB)-dominated bolometric luminosity for Seyferts (Forbes & Norris 1998; also see, Norris et al. 1988). The hypothesis of a close connection between starbursts and Seyferts has been proposed by Terlevich and his collaborators (Terlevich & Melnick 1985; Terlevich et al. 1992). They claim that the radio-quiet AGN are powered by massive nuclear starburst in a metal-rich environment. Recently, Heckman et al. (1997) discovered a powerful nuclear starburst (in the Wolf-Rayet (WR) phase) in Seyfert galaxy Mrk 477. The bolometric luminosity of Mrk 477 in the central region might be dominated by the nuclear starburst. It was remarked by Maiolino et al. (1998) that Heckman et al's study would provide observational evidence partly supporting Terlevich's hypothesis.

Following these leads, we have explored the energetics of LINERs by using the FIR-radio correlation for Wolf-Rayet (WR) galaxies, suggesting different energy budgets of LINERs.

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© European Southern Observatory (ESO) 2000

Online publication: March 21, 2000
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