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.