Download Fig.4

Fig.2: 8.6GHz on 0.3-7keV X-ray
UNMASKING THE AGN IN
J2310-437
Fig.1: Combined IVU band image,
J2310-437 circled
Authors: A.Bliss, D.M.Worrall, M.Birkinshaw
(Bristol),
H.Tananbaum, S.Murray (Harvard-Smithsonian
C.f.A.)
INTRODUCTION
J2310-437 (Fig.1) is hosted by an elliptical galaxy of z=0.0886, at the centre of a cluster of Abell richness class 0 (Ref.1),
where Abell richness is the number of galaxies in a cluster that lie within the magnitude range m3 to m3+2 (m3 is the magnitude
of the third brightest member of the cluster). An Abell richness of class 0 indicates that there are 30-49 galaxies in the
cluster within this range (2). It is unusual in that it appears as a bright (1044 erg s-1) X-ray source (3), and radio mapping shows
a moderately bright object with a small jet (Fig.2). However, in the optical there is no unequivocal evidence for an AGN, and
the spectrum shows no emission lines. This ongoing project uses high-quality ESO NTT imaging to investigate the level of jet
activity in five optical bands and look for evidence of a compact core in these images, with a view to confirming the
interpretation of J2310-437 as an anomalous BL Lac object with a faint optical core and bright host galaxy. This is similar to
the optically dull, X-ray and radio-loud galaxy 3C 264 (4), raising the question whether these two sources are extreme BL Lac
objects or part of a distinct population.
REFERENCES
1. D. M. Worrall et al, ApJ, 1999
2. G. Abell, ApJS, 1958
3. S. S. Murray et al, in prep.
4. H. Tananbaum et al, ApJ, 1997
OPTICAL IMAGES
The imaging data was from the U, B, V, R
and I bands, measured over 3 nights
using the SUSI2 instrument on the ESO
NTT .
These images were reduced using IRAF,
and Fig.3 shows optical images as colour
scale with radio contours overlaid to
show the position and angle of the jet.
The B band (421.20nm) shows the best
jet in the optical, with a small hook seen
in the lower left quadrant. The jet is
seen less defined in the U band as a
diffuse fan.
Fig.3: Optical images with
radio (8.6GHz) contours
overlaid
OPTICAL SPECTRUM (Fig.4)
At this galaxy’s redshift of 0.086, the CaII break which is used to distinguish
between BL Lac objects and elliptical galaxies should be about 4350Å. However, the
short-wavelength cutoff on this spectrum is around 4700Å which is more typical of an
elliptical galaxy. There are also no Hß or [OIII] emission lines which rules out the
possible presence of a Seyfert galaxy or quasar (4).
Fig.5: Optical image
with galaxy
subtracted
Fig.4 (1)
POSSIBLE OPTICAL CORE?
Using the ELLIPSE command in IRAF the host galaxy was subtracted from the R band
(641.58nm) optical image. This left a bright object (shown in Fig.5) with what appears to be
a small-scale jet, corresponding to the radio contours. There is also an area which could be
interpreted as a bright disk although this may be a feature caused by ELLIPSE.
CONCLUSIONS AND FURTHER WORK
Results so far seem to support the interpretation of J2310-437 as an anomalous BL
Lac object; radio and X-ray luminous, whilst optically dull with no emission lines and a
spectrum typical of an elliptical galaxy. Galactic subtraction of the images will be continued
to better determine the presence of a disk and compact optical core.