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**TITLE**
ASP Conference Series, Vol. **VOLUME**, **PUBLICATION YEAR**
**EDITORS**
The Nuclei of Nearby Radio­Loud Ellipticals
G. A. Verdoes Kleijn, P. T. de Zeeuw
Leiden Observatory, Postbus 9513, 2300 RA, Leiden, The Netherlands
S. A. Baum, C. P. O'Dea, R. P. van der Marel, C. Xu
Space Telescope Science Institute, 3700 San Martin Drive, Baltimore,
MD 21218, USA
C. M. Carollo, J. Noel­Storr
Columbia University, Dept. of Astronomy, New York, NY 10027, USA
Abstract. We have observed a complete sample of 21 nearby (D <
70h -1 Mpc) Fanaro# & Riley Type I galaxies with HST/WFPC2 and de­
tected dust disks and lanes in 19 of them. The radio jets are roughly
perpendicular to the dust which is used to constrain the Doppler boost­
ing factors of the radio jet and cores. The VLBA core flux correlates
with the central H#+[NII] flux which might indicate that the VLBA core
is dominated by an isotropic component. Twelve galaxies show nuclear
optical sources. We discuss various possible origins for this emission.
1. Introduction
The Fanaro# & Riley Type I (FR I) radio galaxies in the nearby (z < 0.03)
universe can be characterized as early­type galaxies with jets emanating from
an AGN which is powered by a black hole (BH). These AGN display only weak
nuclear optical line and continuum emission. The FR I stellar hosts and their
unresolved cores bear resemblance to both normal early­type galaxies with radio
cores, which constitute a considerable fraction of the nearby early­type popu­
lation (# 50% at MB = -22, cf. Sadler 1997) and to early­type galaxies with
unresolved blue optical spikes (Carollo et al. 1997). On the other hand, the
FR I galaxies appear to be scaled­down versions of powerful radio galaxies and
quasars in the distant universe which have strong nuclear optical line and con­
tinuum emission. For a physical understanding of the connection between active
and normal galaxies, it is important to determine how these low­luminosity ac­
tive nuclei and their jets form and evolve. Relevant questions are then: where
did the accreted matter come from, how does this accretion trigger jet forma­
tion, and what is the origin of the optical AGN luminosity? We are using HST to
study the centers of a complete sample of FR I radio galaxies and to isolate weak
optical nuclear activity from the stellar background (Verdoes Kleijn et al. 1999).
Here we discuss further interpretation of the WFPC2 data in combination with
VLBA radio data presented by Xu et al. (2000).
1

2 Verdoes Kleijn et al.
Figure 1. Left and middle: V ­band images of two sample galaxies.
NGC 193 has a dust lane and NGC 383 has a dust disk. Both galaxies
have a blue nuclear optical source. Right: the axis ratio of the dust
disk plotted versus the range of the angle # between the sides of the
dust disk symmetry axis and the radio­jet axis, that are closest to the
direction of the line of sight. Horizontal bars indicate the allowed range
for # and the solid dots indicate the median #.
2. Orientation of Dust and Radio Jets
We detected dust in the centres of 19 galaxies (Fig. 1). The dust extent ranges
from a few hundred pc to a few kpc. In eleven galaxies, the dust morphology
is smooth and elliptical (a `disk'), while in eight galaxies it is filamentary with
wisps and bends (a `lane'). Lanes are roughly perpendicular to the radio jets:
the position angle di#erence #PA is in the range 68 # -90 # . Processes that cause
such a preferred orientation are discussed in e.g., Quillen & Bower (1999). By
contrast, all disks closely align with the galaxy major axis. The #PA of the disk
major axis and radio jet is in the range range 23 # -90 # . One can assume that (i)
dust disks are circular and (ii) the brightest side of the radio jet is approaching
the observer. The allowed range of the intrinsic angle # between the sides of
both the dust disk symmetry axis and the radio jet axis that are closest to the
direction of the line of sight, can then be computed. By definition, the radio jet
inclination is in the range 0 # -90 # , but # can take a value in the range 0 # -180 # .
The right panel in Fig. 1 shows that the allowed range of # for each dust disk is
constrained such that the dust disk symmetry axis and the radio jet axis tend
to `align' (i.e., # < 90 # ). Further analysis indicates that the angle with the line
of sight for jet and disk are expected to di#er typically by only # 10 # - 20 # . A
similar result was derived by Capetti & Celotti (1999) for a small pilot sample.
3. Doppler Boosting of Radio Jets and Cores
Xu et al. (2000) report the surface brightness ratio S of VLBA jet and counter jet
at 1670 MHz for the sample galaxies. If the jets are intrinsically symmetric and
ejected in opposite directions, S depends on jet inclination and velocity (e.g.,
Urry & Padovani 1995). As discussed in the previous section, the dust disk
inclination is a reasonable estimator for the jet inclination and can constrain the

The Nuclei of Nearby Radio­Loud Ellipticals 3
Figure 2. Left: the VLBA jet to counter jet surface brightness ra­
tio S versus dust disk inclination. The arrows indicate lower limits.
The curves describe S for an isotropic continous jet with no preferred
direction of the magnetic field, a radio spectral index # = 0.75 and
# = 1.1 (solid line), # = 2.0 (dotted line) and # = 10.0 (dashed line).
The horizontal error bars indicate the uncertainty in the jet inclination.
Middle: H#+[NII] flux in the central 1 ## versus VLBA core flux (1670
MHz). Right: flux of the nuclear optical sources (I­band) detections
and upper limits versus VLBA core flux.
jet velocity. Fig. 2 plots S versus jet inclination together with model predictions
for constant jet velocity v, assuming an isotropic continuous jet with no preferred
direction of the magnetic field and a radio spectral index # = 0.75 (f # # # -# )
typical for jets. The observed values of S cannot constrain v very well given that
most are lower limits (i.e., the counter jet is not observed) and given the # 20 #
uncertainty of the jet inclination. For NGC 315 we can constrain the Lorentz
factor to be # > 10.
The VLBA radio­core flux (unresolved at the parsec scale) correlates tightly
with central H#+[NII] flux (Fig. 2). Interestingly, a similar correlation is found
for radio­core galaxies (Ho 1999). If the H#+[NII] flux is emitted isotropically
and correlates tightly with intrinsic VLBA core flux, the observed scatter in the
correlation might be due to Doppler boosting. The low scatter in the correlation
then constrains # < 2. Indeed, no dependence of the VLBA core flux on dust
disk inclination is found. Thus it seems that the VLBA core flux is dominated
by a relatively isotropic component instead of a relativistic jet. This isotropic
component might have uncollimated relativistic motion.
4. Nuclear Optical Sources
Twelve galaxies show blue nuclear optical sources (NOS) unresolved with WFPC2,
corresponding to sizes of tens of parsec or less. The right panel in Fig. 2 shows
NOS flux versus VLBA radio core flux. The observed correlation is significant at
the 99.999% level (generalized Kendall's Tau test). This agrees with results by
Chiaberge, Capetti, & Celotti (1999). Radio core emission is generally assumed
to be self­absorped synchrotron emission. The correlation might indicate that
the NOS is also synchrotron emission. The radio­to­optical spectral index varies

4 Verdoes Kleijn et al.
between 0.43 and 0.85. These values are consistent with those found for galaxies
in our sample with extended optical jets: 3C 66B, 3C 31 and M87 (Butcher, van
Breugel & Miley 1980; Biretta, Stern, & Harris 1991). The slope of the log­log
correlation, s = 1.04 ± 0.24, although not well determined, is consistent with a
power­law spectral energy distribution (SED) from radio to optical wavelengths.
Alternatively, the NOS might be emission from the accretion disk and/or
flow. For example, Di Matteo et al. (2000) obtain a reasonable fit to the nuclear
radio to X­ray SED of M87 using an ADAF model with matter outflow. However,
the models that fit the observed X­ray SED underpredict the nuclear optical
emission by a factor of # 4. Furthermore, Di Matteo et al. note evidence for a
contribution to the flux by the synchrotron jet at radio and millimeter fluxes.
If the accretion disk is inside an optically thick torus, the NOS detection rate
implies an opening angle # 130 # . Broad emission­line regions are commonly
detected in powerful AGN but typically not detected in FR Is. The large opening
angle would then suggest BLRs are generally not present in FR I galaxies
Finally, the NOS might be produced by a nuclear starburst. The high NOS
detection rate would require a continuous starburst on time scales on the order
of the radio­source lifetime, typically estimated to be 10 7-8 yr (cf. Chiaberge,
Capetti, & Celotti 1999). However, detailed studies of the optical nuclear spec­
tra of M87 and M84 indicate their NOS are indeed not produced by a starburst
but by non­thermal AGN emission ( Kormendy 1992; van der Marel 1994; Bower
et al. 2000).
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submitted (astro­ph/0009124)