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We present a radio and optical analysis of a sample of Low Luminosity
Compact (LLC) objects, selected from FIRST survey and observed with MERLIN
at L-band and C-band. About 70% of the observed LLC sources are galaxies and
all of them are nearby objects with redshifts z in the range 0.04 -
0.9. Most
of them have been resolved and about 30% of them have weak extended emission
and disturbed structures when compared with the observations of higher
luminosity CSS sources.We suggest that some of the sources with the breaking
up structures or one-sided morphology are candidates for compact faders
(Fig.1a). We studied correlation between radio power and linear size, and
redshift with a larger sample that included also published samples of
compact objects and large scale FRIIs and FRIs (see Kunert-Bajraszewska et
al,2010a for details). The Luminosity-Size diagram (Fig.1b) shows an
evolutionary scheme of radio-loud AGNs.The selection criteria used for the
new sample resulted in approximately one third of the LLC sources having a
value of the 1.4 GHz radio luminosity comparable to FRIs. Their luminosities
are definitely lower than CSS sources from last existing samples (Fanti et
al.2001 and Marecki et al.2003). We suggest that many of them might be
short-lived objects, and their radio emission may be disrupted several times
before becoming FRIIs.
The optical analysis of the LLC sources were made based on the available
SDSS images and spectra (Kunert-Bajraszewska et al., 2010b). We have
classified the sources as high and low
excitation galaxies (HEG and LEG, respectively). We have compared the [OIII]
luminosity with the radio properties for LLC sources, and expanded the
sample with other CSS, GPS sources and FRI and FRII objects. The whole
sample shows that, for a given size or radio luminosity, HEG sources are
brighter than LEG in the [OIII] line by a factor of 10. The LLC objects
follow the same correlation between [OIII] luminosity and radio power, as
the rest of the sample, although the LLC objects have lower values of [OIII]
luminosity than the more powerful CSS sources.
Based on the analysis above, we propose a scenario where the differences in
the nature of LEG and HEG (accretion mode or black hole spin) are already
visible in the CSS phase of AGN evolution and determine the evolution of the
source: i.e. CSSLEG evolve to FRLEG,
CSSHEG evolve to FRHEG. The main
evolution scenario (GPS-CSS-FRII) was proposed years ago. However, once the
HEG/LEG division is included, these sources seem to evolve in parallel
(Fig.1c):
GPSLEG-CSSLEG-FRLEG and
GPSHEG-CSSHEG-FRHEG.
Concerning LEG, it is still not clear if CSSLEG would evolve
directly to
FRILEG or go through a FRIILEG phase before the
FRILEG. As discussed in
Kunert-Bajraszewska et al., 2010a, there should also exist a group of
short-lived CSS objects with lower radio luminosities. These short-lived
CSSs could probably show the low [OIII] luminosities seen in FRIs.
The above results have been published in
Kunert-Bajraszewska et al.(2010a), MNRAS, 408, 2261 and Kunert-Bajraszewska
et al.(2010b), MNRAS, 408, 2279
Fig.1:
a) 0821+321 - a candidate for a compact fader, b) Luminosity-size
diagram for AGNs; c) [OIII] luminosity - radio luminosity diagram for AGNs
classified as HEG and LEG. Squares indicate CSS sources from samples of
Labiano et al.(2007), Laing et al.(1983), Willott et al.(1999), Fanti et
al.(2001) and Marecki et al.(2003). The diamonds indicate GPS and HFP
objects from sample Labiano et al.(2007). The circles indicate FRI and FRII
objects from samples of Laing et al.(1983), Buttiglione et al.(2009a,b,
2010) and Willot et al.(1999). The triangles indicate the current sample
of LLC sources.
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