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.
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.
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.
Recent Publications of the Group:
- Sambruna, R.M. et al. 2001, "A survey of extended radio jets in AGN
with Chandra and HST: First Results" , ApJ, 571, 206
- Pesce, J.E., Sambruna, R.M., et al. 2001,
"Detection of an X-ray Jet in 3C371 with Chandra",
ApJ, 556, L79
- Sambruna, R.M. et al. 2001, "Chandra
Observations of the X-ray Jet of 3C273", ApJ, 549, L161
- Tavecchio, F. et al. 2000, "The X-Ray Jet of PKS 0637-752: Inverse
Compton Radiation from the Cosmic Microwave Background?", ApJ,
Return to Main Page
This page last updated: July 3, 2002