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Active Galactic Nuclei: from Central Engine to Host Galaxy
ASP Conference Series, Vol. xxx, 2003
S. Collin, F. Combes, and I. Shlosman
Which giant ellipticals can host radio-jets?: The UGC
FR I radio galaxy sample
Gijs Verdoes Kleijn, Ste Baum, Roeland van der Marel & Chris O'Dea
STScI, 3700 San Martin Drive, Baltimore, MD 21218, USA
Jacob Noel-Storr & Jacqueline van Gorkom
Columbia University, Astronomy Department., 550 W 120th St., New
York, NY 10027, USA
Tim de Zeeuw
Leiden Observatory, Postbus 9513, 2300 RA, Leiden, The Netherlands
Abstract. To address the question in the title, a brief discussion is
presented of results from HST optical observations for the `UGC FR I
sample' of nearby galaxies with radio jets. In particular, the central dust
distributions are discussed which trace the potential fuel reservoir for
nuclear activity. The results warrant further testing of the scenario that
many ellipticals go through a radio-jet phase.
Nuclear dust and ionized gas is detected in the cores of about 50% of nearby
early-type galaxies (e.g., Ho et al. 1997; Tran et al. 2001). The detections of black
holes in nearby elliptical galaxies suggest that many and perhaps all of them host
central black holes (e.g., Kormendy & Gebhardt 2001). This is supported by
the presence of nuclear activity in many giant ellipticals (e.g., Ho et al. 1997).
Interestingly, only in a few is this activity accompanied by kpc-scale radio jets.
Here we focus on the question why this small subset is capable of hosting large-
scale radio jets.
Ellipticals with and without radio jets
From the ground it is known that hosts of ellipticals with and without radio-jets
are very similar (e.g., Ledlow & Owen 1995; Govoni et al. 2000). Here we zoom
in on the nuclei of a complete sample of ellipticals with kpc-scale radio-jets, the
UGC FR I sample, using the Hubble Space Telescope (HST) (see Verdoes Kleijn
et al. 1999). Dust, with a size ranging from  100pc to a few kpc, is detected
in 19 sample members with various morphologies. The dust properties of the
UGC FR I galaxies are compared to those of a sample of 52 UGC galaxies in
the HST imaging archive, the UGC non-FR I sample (Verdoes Kleijn 2002a).
These galaxies do not have radio-jets (but can host weak active nuclei) and have
similar absolute magnitude and distance. Consistent with previous studies, dust
is detected in  50% of the UGC non-FR I galaxies. Apart from this dierence
in detection rate, the distribution over dust morphology, size and orientation in
111

112 Gijs Verdoes Kleijn
both samples is very similar (see Fig. 1). This indicates that the presence of
radio-jets is not in uenced by the dust properties and vice versa. Together with
the similarities in stellar hosts, this suggests a scenario in which the decisive trig-
ger for radio-jet formation resides at the  10pc scale. A detailed comparison
of the stellar hosts at HST resolution will provide a further test of this scenario.
Fig.1 From top to bottom: galaxy distance,
absolute blue magnitude and position an-
gle dierence between galaxy major axis and
dust as a function of dust size for UGC FR I
galaxies (lled symbols) and UGC non-FR I
galaxies (open symbols). Circles, triangles,
squares and stars indicate dust disks, lanes,
compact and irregular dust respectively (see
Fig. 1). The small dots indicate galaxies
without dust detection plotted at a ducial
dust size of 20pc. The morphology, size and
orientation of the potential black hole 'fuel'
reservoir, as traced by the dust, is very sim-
ilar for ellipticals with and without radio
jets. For both galaxy classes, dust struc-
tures with sizes < 500pc align within 30 ф
with the galaxy major axis, while large dust
lanes align with the galaxy minor axis.
Further details about the UGC FR I sample nuclei can be found in Noel-Storr
et al. (2001), Xu et al. (2000) and Verdoes Kleijn et al. (2002b,c).
Conclusions
The presence of central gas and dust is a necessary but not suфcient condition
for radio-jet formation. The discussed results suggest that the decisive trigger
of radio-jets in nearby ellipticals resides at  10pc from the nucleus. The nature
of this trigger and whether it can operate in some or all ellipticals remains
unknown. The detection of tiny radio-jets in some ellipticals (e.g., Nagar et
al. 2001) together with similarities in the nuclear activity of ellipticals with and
without radio-jets (e.g., Verdoes Kleijn et al. 2002) justify further steps to test
if most/all ellipticals go through a radio-jet phase.
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