A&A 447, 97-112 (2006)
The host galaxy/AGN connection in nearby early-type galaxies
Is there a miniature radio-galaxy in every "core" galaxy?B. Balmaverde1 and A. Capetti2 1
Universitá di Torino, Via Giuria 1, 10125, Torino, Italy
INAF - Osservatorio Astronomico di Torino, Strada
Osservatorio 20, 10025 Pino Torinese, Italy
(Received 11 August 2005 / Accepted 20 September 2005)
This is the second of a series of three papers exploring the
connection between the multiwavelength
properties of AGN in nearby early-type galaxies
and the characteristics of their hosts.
We selected two samples with 5 GHz
VLA radio flux measurements down to 1 mJy, reaching levels of
radio luminosity as low as 1036 erg s-1.
In Paper I
we presented a study of the surface brightness profiles
for the 65 objects with available archival HST images
out of the 116 radio-detected galaxies. We classified early-type galaxies into
"core" and "power-law" galaxies,
discriminating on the basis of the slope of their nuclear brightness
profiles, following the Nukers scheme.
Here we focus on the 29 core galaxies (hereafter CoreG).
We used HST and Chandra data to isolate their optical and X-ray
nuclear emission. The CoreG invariably host radio-loud
nuclei, with an average radio-loudness parameter of
The optical and X-ray nuclear luminosities correlate with the
radio-core power, smoothly extending the analogous correlations
already found for low luminosity radio-galaxies (LLRG) toward even lower
power, by a factor of
1000, covering a combined range of 6
orders of magnitude. This supports the interpretation of a common
non-thermal origin of the nuclear emission also for CoreG.
The luminosities of the nuclear sources,
most likely dominated by jet emission,
set firm upper limits, as low as
the optical and X-ray band, on any emission from the accretion process.
The similarity of CoreG and LLRG when considering the distributions
host galaxies luminosities and black hole masses,
as well as of the surface brightness profiles,
indicates that they are drawn from the same
population of early-type galaxies. LLRG represent only the tip
of the iceberg associated with (relatively) high activity levels,
with CoreG forming the bulk of the population.
We do not find any relationship between radio-power and black hole
mass. A minimum black hole mass of
apparently associated with the radio-loud nuclei in both CoreG and LLRG,
but this effect must be tested on a sample of less luminous
galaxies, likely to host smaller black holes.
In the unifying model for BL Lacs and radio-galaxies,
represent the counterparts of the large population of low luminosity
BL Lac now emerging from the surveys at low radio flux
limits. This suggests the presence of relativistic jets
also in these quasi-quiescent early-type "core" galaxies. Key words:
galaxies: active --
galaxies: nuclei --
galaxies: elliptical and lenticular, cD --
galaxies: jets --
galaxies: BL Lacertae objects: general
© ESO 2006