The Bologna Complete Sample: radio and gamma-ray data

To this aim, it is important to define and observe a sample that is free from selection effects looking at radiogalaxies in low radio frequencies survey. In fact, sources in low-frequency samples are dominated by their extended and unbeamed (isotropic) emission, rather than the beamed compact emission that dominates in high-frequency studies. Low frequency surveys are therefore unbiased with respect to the orientation of the nuclear relativistic jet. With this purpose in mind, we initiated a project to investigate a complete sample of radio galaxies selected from the B2 Catalogue of Radio Sources and the Third Cambridge Revised Catalogue (3CR) ( [4,6]), with no selection constraint on the nuclear properties. We named this sample “the Bologna Complete Sample” (BCS).

In the original sample, 95 radio sources from the B2 and 3CR catalogues were present, but because of the rejection of one source ( [7,10]), we redefined the complete sample to be 94 sources. We selected the sources to be stronger than a flux density limit of 0.25 Jy at 408 MHz for the B2 sources and greater than 10 Jy at 178 MHz for the 3CR sources ( [2]). We also applied the following criteria: 1) declination > 10 • ; 2)Galactic latitude |b| > 15 • ; 3)redshift z<0.1. As our main goals was to properly study the central engine of this sample, we asked and obtained high resolution observations with VLBI technique ([6, 7, 10]) for all sources not yet analysed at this spatial scale.

The Large Area Telescope (LAT) on board Fermi, with its large field of view and unprecedented sensitivity, is now putting us in the condition of a better understanding of the extragalactic gamma-ray source population. In anticipation of the launch of Fermi, large projects in the radio band have been undertaken (e.g. [3,8]). The results of these projects can now be exploited to gain insights into the radio properties of this population and into the relation between radio and gamma-ray properties. With these latter purposes, we decided to compare Fermi and results in radio band for our complete sample of nearby radiogalaxies. In the following, we describe our study and we report its most important conclusions.

Up to now, at parsec scale and in radio band, we analysed 76 sources. Our main results are:

• The detection rate is high: only 3 sources out of 76 (4%) have not been detected, even though we observed sources with an arcsecond core flux density as low as 5 mJy at 5 GHz. This result confirms the presence of compact radio nuclei at the center of radio galaxies.

• As expected in sources with relativistic parsecscale jets, the one-sided jet morphology is the predominant structure present in our VLBI images, however 22% of the observed sources show evidence of a two-sided structure. This result is in agreement with a random orientation and a high jet velocity ( β ∼ 0.9).

• We find two sources (4C26.42, [9] and 3C 310, Fig. 1) with a Z-shaped structure on the parsecscale suggesting the presence of low velocity jets in these peculiar radio sources.

• In 8 sources, the low core dominance suggests that the nuclear activity is now in a low activity state. The dominance of the extended emission implies a greater activity of the core in the past. However in these sources a parsec-scale core and even jets are present. In this scenario the nuclear activity may be in a low or high state but is not completely quiescent. This result is in eConf C110509 agreement with the evidence that a few sources show evidence of a recurring or re-starting activity. This point can be better addressed when observations are available for the full sample so that we can discuss the time-scale of the recurring activity.

• In most cases, the parsec and the kiloparsec scale jet structures are aligned and the main jet is always on the same side with respect to the nuclear emission. This confirms the idea that the large bends present in some BL Lacs sources are amplified by the small jet orientation angle with respect to the line-of-sight.

• In 62% of the sources, there is good agreement between the arcsecond-scale and the VLBI correlated flux density. For the other 38% of the sources, at the milliarcsecond scale more than 30% of the arcsecond core flux density is missing. This suggests the presence of variability, or of a significant sub-kiloparsec-scale structure, which will be better investigated with the EVLA at high frequency or with the e-MERLIN array.

We search in the 1 Year Fermi catalog (1FGL, ??) available information of the gamma-ray emission for all the BCS sources. We found that:

• Among the BCS,there are 3 sources in 1FGL catalog :2 BL Lacs (Mkn 421 and Mkn 501) plus M87;

• Thanks to radio data, we also estimated the core dominance (CD). According to [? ] and references therein, a correlation is present between the core

…(Full text truncated)…