What powers Weak-Line Radio Galaxies?



Weak-Line Radio Galaxies (WLRGs) are defined as powerful radio galaxies with underluminous [O III] emission lines and line ratios indicative of a low ionization state of the narrow-line region. Tadhunter et al. (1998) advance the hypothesis that the most likely explanation for the properties of WLRGs is that they harbor low luminosity AGNs. We conducted an ASCA study of a small sample of 6 WLRGs (Sambruna, Eracleous, & Mushotzky 1999), which gave us the first view of these systems at hard X-rays. We find that in five out of the six WLRGs observed, the X-ray spectrum can be decomposed into a hard X-ray component plus a soft thermal component with kT=1 keV. The hard component can be described by either a flat power law, with = 1.5 (and individual slopes as flat as = 1.3). The intrinsic luminosity of the hard component is L(2-10) keV= 10^40-10^42 ergs/s, 2 orders of magnitude fainter than in the other radio sources of our ASCA sample.

SEDs The [O II]/[O III] line ratios of WLRGs locate them in the LINER/H II region part of the diagnostic line-ratio diagrams of Filippenko (1996), raising the possibility that WLRGs represent the radio-loud analogs of LINERs. We are also conducting an extensive optical spectroscopy survey of all the WLRGs of the Tadhunter et al. sample, and we are finding that their line ratios locate them in various areas of the Fillippenko diagram. Some WLRGs have line ratios consistent with starbursts, other with Seyferts, other lie in between.

We know there is an AGN in WLRGs because they exhibit compact cores in their radio maps. But what powers the Low-Luminosity AGN in WLRGs? Their Spectral Energy Distributions (SEDs) are different from those of the more luminous Broad-Line Radio Galaxies (left figure), typically lacking emission in the UV-EUV band. We speculate that an ADAF lies at the very heart of these systems. From this perspective, WLRGs may provide a link between powerful radio galaxies and ellipticals.

SEDs We have started a systematic study of WLRGs at X-rays using Chandra and XMM (right figure) and at other wavelengths. The goal is to measure their spatial and spectral properties, quantifying the contribution of an unresolved jet to their bolometric emission, detect the Fe Kalpha line and resolve its profile to constrain models for the accretion flow in these systems.

This work is supported by NASA through the LTSA and ADP programs.

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