Nearby radio loud AGN and the Unified Model

The statistical study of the parsec scale properties of radio sources is crucial to get information on the nature of the central engine and to provide the foundations of the current unified theories, suggesting that the appearance of active galactic nuclei depends strongly on orientation. We started a project to observe at sub-arcsec resolution a complete sample of 94 nearby (z<0.1) radio galaxies, the Bologna Complete Sample, which is not affected by any selection effect on the jet velocity and orientation with respect to the line of sight. Up to now, we published our parsec scale analysis of 77/94 sources. Here, we describe the last VLBA observations at 5 GHz and EVN data at 18 cm obtained for the 17 remaining faintest radio core (<5 mJy at 5 GHz in VLA images) BCS sources and we report our preliminary results on the whole complete sample.Comment: 4 pages, 1 figure; accepted for publication in the proceeding of the "12th European VLBI Network Symposium and Users Meeting - EVN 2014" (7-10 October 2014, Cagliari, Italy); published online in Proceedings of Science, PoS(EVN 2014)09

Twin SMBH candidates in the BCG of RBS 797

The radio-loud BCG at the center of the cool core cluster RBS 797 is known to exhibit a misalignment of its 5 GHz radio emission observed at different VLA resolutions, with the innermost kpc-scale jets being almost orthogonal to the radio lobes which extends for tens of kpc filling the X-ray cavities seen by Chandra. The different radio directions may be caused by rapid jet reorientation due to interaction with a secondary supermassive black hole (SMBH), or to the presence of two AGN, probably in a merging phase, which are emitting radio jets in different directions. We present the results of new 5 GHz observations performed with the EVN in May 2013. In particular, we detected two compact radio components, with a projected separation of 77 pc. We discuss two possible scenarios for the origin and nature of the EVN double source, showing that both interpretations are consistent with the presence of a SMBH binary system in the BCG of RBS 797.Comment: 4 pages, 1 figure; accepted for publication in the proceeding of the "12th European VLBI Network Symposium and Users Meeting - EVN 2014" (7-10 October 2014, Cagliari, Italy); published online in Proceedings of Science, PoS(EVN 2014)08

e-EVN monitoring of M87

M87 is a privileged laboratory for a detailed study of the properties of jets, owing to its proximity (D=16.7 Mpc, 1 mas = 0.080 pc), its massive black hole (~6.0 x 10^9M) and its conspicuous emission at radio wavelengths and above. We started on November 2009 a monitoring program with the e-EVN at 5 GHz, in correspondence of the season of Very High Energy (VHE) observations. Indeed, two episodes of VHE activity have been reported in February and April 2010. We present here the main results of these multi-epoch observations: the inner jet and HST-1 are both detected and resolved in our datasets. We study the apparent velocity of HST-1, which seems to be increasing since 2005, and the flux density variability in the inner jet. All in all, the radio counterpart to this year’s VHE event seems to be different from the ones in 2005 and 2008, opening new scenario for the radio-high energy connection

EVN Observations of HESS J1943+213: Evidence for an Extreme TeV BL Lac Object

We report on the 1.6 GHz (18 cm) VLBI observations of the unresolved, steady TeV source HESS J1943+213 located in the Galactic plane, performed with the European VLBI Network (EVN) in 2014. Our new observations with a nearly full EVN array provide the deepest image of HESS J1943+213 at the highest resolution ever achieved, enabling us to resolve the long-standing issues of the source identification. The milliarcsecond-scale structure of HESS J1943+213 has a clear asymmetric morphology, consisting of a compact core and a diffuse jet-like tail. This is broadly consistent with the previous e-EVN observations of the source performed in 2011, and re-analyzed in this work. The core component is characterized by the brightness temperature of $\gtrsim1.8 \times 10^9$ K, which is typical for low-luminosity blazars in general. Overall, radio properties of HESS J1943+213 are consistent with the source classification as an "extreme high-frequency-peaked BL Lac object". Remarkably, we note that since HESS J1943+213 does not reveal any optical or infrared signatures of the AGN activity, it would never be recognized and identified as a BL Lac object, if not its location close to the Galactic plane where the High Energy Stereoscopic System has surveyed, and the follow-up dedicated X-ray and radio studies triggered by the source detection in the TeV range. Our results suggest therefore a presence of an unrecognized, possibly very numerous population of particularly extreme HBLs, and simultaneously demonstrate that the low-frequency VLBI observations with high-angular resolution are indispensable for a proper identification of such objects.Comment: 8 pages, 3 figures, accepted to ApJ

Evidence for nuclear radio jet and its structure down to <100 Schwarzschild radii in the center of the Sombrero galaxy (M 104, NGC 4594)

Abridged: We investigated the detailed radio structure of the nucleus of the Sombrero galaxy using high-resolution, quasi-simultaneous, multi-frequency, phase-referencing VLBA observations. We obtained the VLBI images toward this nucleus, with unprecedented sensitivities and resolutions, at the seven frequencies between 1.4 and 43 GHz, where those at 15, 24 and 43 GHz are the first clear VLBI detections. At 43 GHz, the nuclear structure was imaged on a linear scale under 100 Schwarzschild radii. For the first time, we have discovered the presence of the extended structure in this nucleus, which is directing from the radio core in two sides toward northwest/southeast directions. The nuclear structure shows a clear spatial gradient on the radio spectra, which is similar to that commonly seen in more luminous AGN with powerful relativistic jets. Moreover, the radio core shows a frequency-dependent size with an elongated shape, and the position of the core also tends to be frequency dependent. A set of these new findings provide evidence that the central engine of the Sombrero is powering radio jets. Based on the observed brightness ratio of jet-to-counter jet, core position shift and its comparison with a theoretical model, we constrained the following fundamental physical parameters for the M 104 jets: (1) the northern side is the approaching jet, whereas the southern side is receding: (2) the inclination angle of the jet is relatively close to our line-of-sight, probably less than ~25degrees: (3) the jet intrinsic velocity is highly sub-relativistic at a speed less than ~0.2c. The derived pole-on nature of the M 104 jet is in accordance with the previous argument that M 104 contains a true type II AGN, i.e., the broad line region of this nucleus is actually absent or intrinsically weak, if the plane of the presumed circumnuclear torus is perpendicular to the axis of the radio jets.Comment: 12 pages, 9 figures, 4 tables, accepted for publication in Ap

Low-luminosity radio-loud active galactic nuclei

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Molecular gas and nuclear activity in early-type galaxies: any link with radio-loudness?

Aims. We want to study the amount of molecular gas in a sample of nearby early-type galaxies (ETGs) which host low-luminosity Active Galactic Nuclei (AGN). We look for possible differences between the radio-loud (RL) and radio-quiet (RQ) AGN. Methods. We observed the CO(1-0) and CO(2-1) spectral lines with the IRAM 30m and NRO 45m telescopes for eight galaxies. They belong to a large sample of 37 local ETGs which host both RQ and RL AGN. We gather data from the literature for the entire sample. Results. We report the new detection of CO(1-0) emission in four galaxies (UGC0968, UGC5617, UGC6946, and UGC8355) and CO(2-1) emission in two of them (UGC0968 and UGC5617). The CO(2-1)/CO(1-0) ratio in these sources is $\sim0.7\pm0.2$. Considering both the new observations and the literature, the detection rate of CO in our sample is 55 $\pm$ 9%, with no statistically significant difference between the hosts of RL and RQ AGNs. For all the detected galaxies we converted the CO luminosities into the molecular masses, $M_{H_2}$, that range from 10$^{6.5}$ to 10$^{8.5}$ M$_{\odot}$, without any statistically significant differences between RL and RQ galaxies. This suggests that the amount of molecular gas does not likely set the radio-loudness of the AGN. Furthermore, despite the low statistical significance, the presence of a weak trend between the H$_{2}$ mass with various tracers of nuclear activity (mainly [O III] emission line nuclear power) cannot be excluded.Comment: Accepted for publication on A&A, 9 pages, 5 figure