Radio Frequency Observations of the Nuclei of Galaxies (original) (raw)
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Astronomy and Astrophysics, 2007
We present high angular resolution radio snap-shot observations of seven nearby low-luminosity active galaxies (LLAGN) from the NUclei of GAlaxies (NUGA) survey. The observations were conducted with MERLIN and EVN/VLBI at 18 cm and 6 cm. At all observed angular resolutions and frequencies, we find indications for extended emission in about ∼ 40% of the sources, consistent with the decrease of flux with increasing angular resolution. The extended components resemble jet emission in a majority of cases, consistent with the optically thin synchrotron emission implied by their steep spectra. We consider the compact 6cm EVN/VLBI radio emission of our sources in the context of the "fundamental plane" that previous LLAGN studies identified within the threedimensional parameter space of radio luminosity, X-ray luminosity, and black hole mass. We demonstrate, using NGC 7217 and NGC 1068 as particular examples, that high-resolution, multi-epoch radio observations offer useful information about the origin of offsets from the fundamental plane.
AMIGA Project. Radio Continuum and Nuclear Activity in a Complete Sample of Isolated Galaxies
Astrophysics and Space Science Proceedings, 2010
The aim of the AMIGA project (Analysis of the interstellar medium in isolated galaxies) is building a reference sample of isolated galaxies to study the role of the environment in galactic evolution. AMIGA began in 2003 and nowadays involves more than 30 participants from 15 international institutions. Radio continuum emission in isolated spiral galaxies is coming from disk-dominated emission in spiral galaxies, in contrast to the results found in high-density environments where nuclear activity is more frequent. The radio continuum power is lower on average in our sample than in interacting galaxies or galaxies without an environment selection criterion. This confirms the relevance of our sample as a baseline to study the effects of the environment. Finally, we have studied the nuclear activity in isolated galaxies. We used different selection methods of isolated galaxies with active nucleus: 1) the far infrared colours give us a fraction of 7 % to 20 % of AGN candidates and 2) the rate of radio excess galaxies in the correlation of far infrared with radio continuum is less than 1%, which is the lowest rate found comparing with samples in other environments. This confirms the role of the environment as fundamental in the triggering of the radio nuclear activity.
The nuclear 10 micron emission of spiral galaxies
The Astrophysical Journal, 1994
We examine the 10µm emission of the central regions of 281 spiral galaxies, after having compiled all ground-based, small-aperture (∼5") broad-band photometric observations at λ∼10µm (N magnitudes) published in the literature. We evaluate the compactness of the ∼10µm emission of galaxy nuclei by comparing these small-beam measures with the large-beam IRAS 12µm fluxes. In the analysis of different subsets of objects, we apply survival analysis techniques in order to exploit the information contained in "censored" data (i.e., upper limits on the fluxes). Seyferts are found to contain the most powerful nuclear sources of mid-infrared emission, which in ∼1/3 of cases provide the bulk of the emission of the entire galaxy; thus, mid-infrared emission in the outer disc regions is not uncommon in Seyferts. The 10µm emission of Seyferts appears to be unrelated to their X-ray emission. HII region-like nuclei are stronger mid-infrared sources than normal nuclei and LINER nuclei (whose level of emission is not distinguishable from that of normal nuclei). Interacting objects have, on average, greater 10µm luminosities than non-interacting ones and exhibit more compact emission. Early-type spirals have stronger and more compact 10µm emission than late-type ones. Barred spirals are brighter at ∼10µm than unbarred systems, essentially because they more frequently contain HII region-like nuclei. The results of our detailed comparison between the behaviour of various categories of objects stress that the 10µm emission of spiral nuclei is closely linked to the (predominantly non-thermal synchrotron) radio emission.
NUclei of GAlaxies: V. Radio emission in 7 NUGA sources
2007
We present high angular resolution radio snap-shot observations of seven nearby low-luminosity active galaxies (LLAGN) from the NUclei of GAlaxies (NUGA) survey. The observations were conducted with MERLIN and EVN/VLBI at 18cm and 6cm. At all observed angular resolutions and frequencies, we find indications for extended emission in about ~40% of the sources, consistent with the decrease of flux with increasing angular resolution. The extended components resemble jet emission in a majority of cases, consistent with the optically thin synchrotron emission implied by their steep spectra. We consider the compact 6cm EVN/VLBI radio emission of our sources in the context of the ``fundamental plane'' that previous LLAGN studies identified within the three-dimensional parameter space of radio luminosity, X-ray luminosity, and black hole mass. We demonstrate, using NGC7217 and NGC1068 as particular examples, that high-resolution, multi-epoch radio observations offer useful informatio...
A radio census of nuclear activity in nearby galaxies
Astronomy and Astrophysics, 2006
In order to determine the incidence of black hole accretion-driven nuclear activity in nearby galaxies, as manifested by their radio emission, we have carried out a high-resolution Multi-Element Radio-Linked Interferometer Network (MERLIN) survey of LINERs and composite LINER/Hii galaxies from a complete magnitude-limited sample of bright nearby galaxies (Palomar sample) with unknown arcsecond-scale radio properties. There are fifteen radio detections, of which three are new subarcsecond-scale radio core detections, all being candidate AGN. The detected galaxies supplement the already known low-luminosity AGN -low-luminosity Seyferts, LINERs and composite LINER/Hii galaxies -in the Palomar sample. Combining all radio-detected Seyferts, LINERs and composite LINER/Hii galaxies (LTS sources), we obtain an overall radio detection rate of 54% (22% of all bright nearby galaxies) and we estimate that at least ∼50% (∼20% of all bright nearby galaxies) are true AGN. The radio powers of the LTS galaxies allow the construction of a local radio luminosity function. By comparing the luminosity function with those of selected moderate-redshift AGN, selected from the 2dF/NVSS survey, we find that LTS sources naturally extend the RLF of powerful AGN down to powers of about 10 times that of Sgr A*.
Core Radio and Optical Emission in the Nuclei of nearby FR I Radio Galaxies
The Astronomical Journal, 2002
In this paper we analyze the relation between radio, optical continuum and H+[N ii] emission from the cores of a sample of 21 nearby Fanaroff and Riley type I galaxies as observed with the VLBA and HST. The emission arises inside the inner tens of parsecs of the galaxies. Core radio emission is observed in 19/20 galaxies, optical core continuum emission is detected in 12/21 galaxies and H+[N ii] core emission is detected in 20/21 galaxies. We confirm the recently detected linear correlation between radio and optical core emission in FR I galaxies and show that both core emissions also correlate with central H+[N ii] emission. The tight correlations between radio, optical, and H+[N ii] core emission constrain the bulk Lorentz factor to $ 2-5 and . 2 for a continuous jet and a jet consisting of discrete blobs, respectively, assuming jet-viewing angles in the range 30 -90 . Radio and optical core emissions are likely to be synchrotron radiation from the inner jet, possibly with a significant contribution from emission by an accretion disk and/or flow. Elliptical galaxies with LINER nuclei without large-scale radio jets seem to follow the core emission correlations found in FR I galaxies. This suggests that the central engines could be very similar for the two classes of active galactic nuclei.
Nuclear radio emission in megaparsec-size radio galaxies
Astronomy and Astrophysics
We present high resolution radio observations of the nuclear radio emission in a sample of eight radio galaxies having Megaparsec sizes, using the European VLBI network at 18 cm. At a resolution of 25 mas we detect all the giant radio galaxies from our sample. Among them DA 240 and 1331−099 were imaged for an extended period. These observations reveal a twin-jet structure for DA 240 and one-sided core-jet morphology in 1331−099. The implications of our results in the context of the unification of quasars and powerful radio galaxies are discussed. Our results are consistent with the suggestion that powerful radio galaxies are the unbeamed counterparts of quasars.