Extended Radio Galaxies Research Articles

Giant radio galaxies in the LOFAR deep fields

The reason why some radio galaxies (RGs) grow to form so-called giant radio galaxies (GRGs) with sizes $>$ 700 kpc, is still unknown. In this study, we compare the radio, optical and environmental properties of GRGs with those of a control sample of smaller RGs we found in the three LOw-Frequency ARray (LOFAR) deep fields, namely the Bo\"otes, ELAIS-N1, Lockman Hole, for a total area of approx 95 deg$^2$. We inspected the LOFAR deep fields and created a catalogue of 1609 extended radio galaxies (ERGs). By visual inspection, we identified their host galaxies and spectroscopically or photometrically classified 280 of these as GRGs. We studied their properties, such as their accretion state, stellar mass and star formation rate (SFR) using deep optical and infrared survey data. Moreover, we explored the environment in terms of the surface number density of neighbouring galaxies within these surveys. Integrated flux densities and radio luminosities were also determined for a subset of ERGs through available survey images at 50, 150, 610, and 1400 MHz to compute integrated spectral indices. Considering the fraction of GRGs displaying an FRII morphology alongside the host galaxy properties, we suggest that GRGs consistently possess sufficient power to overcome jet frustration caused by the interstellar medium. Moreover, clear differences emerge in the environmental densities between GRGs and smaller RGs, using the number of neighbouring galaxies within 10 Mpc from the host galaxy as a proxy. GRGs preferentially reside in sparser environments compared to their smaller counterparts. In particular, only 3.6<!PCT!> of the GRGs reside within a 3D comoving distance of 5 Mpc from a previously reported galaxy cluster. We found that larger sources exhibit steeper integrated spectral indices, suggesting that GRGs are late-stage versions of RGs. These results suggest that GRGs are amongst the oldest radio sources with the most stable nuclear activity that reside in sparse environments.

RadioGalaxyNET: Dataset and novel computer vision algorithms for the detection of extended radio galaxies and infrared hosts

Abstract Creating radio galaxy catalogues from next-generation deep surveys requires automated identification of associated components of extended sources and their corresponding infrared hosts. In this paper, we introduce RadioGalaxyNET, a multimodal dataset, and a suite of novel computer vision algorithms designed to automate the detection and localization of multi-component extended radio galaxies and their corresponding infrared hosts. The dataset comprises 4 155 instances of galaxies in 2 800 images with both radio and infrared channels. Each instance provides information about the extended radio galaxy class, its corresponding bounding box encompassing all components, the pixel-level segmentation mask, and the keypoint position of its corresponding infrared host galaxy. RadioGalaxyNET is the first dataset to include images from the highly sensitive Australian Square Kilometre Array Pathfinder (ASKAP) radio telescope, corresponding infrared images, and instance-level annotations for galaxy detection. We benchmark several object detection algorithms on the dataset and propose a novel multimodal approach to simultaneously detect radio galaxies and the positions of infrared hosts.

Discovery of 178 Giant Radio Galaxies in 1059 deg2 of the Rapid ASKAP Continuum Survey at 888 MHz

We report the results of a visual inspection of images of the Rapid ASKAP Continuum Survey (RACS) in search of extended radio galaxies (ERG) that reach or exceed linear sizes on the order of one Megaparsec. We searched a contiguous area of 1059 deg2 from RAJ = 20h20m to 06h20m, and −50∘<DecJ<−40∘, which is covered by deep multi-band optical images of the Dark Energy Survey (DES) and in which previously only three ERGs larger than 1 Mpc had been reported. For over 1800 radio galaxy candidates inspected, our search in optical and infrared images resulted in hosts for 1440 ERG, for which spectroscopic and photometric redshifts from various references were used to convert their largest angular size (LAS) to projected linear size (LLS). This resulted in 178 newly discovered giant radio sources (GRS) with LLS >1 Mpc, of which 18 exceed 2 Mpc and the largest one is 3.4 Mpc. Their redshifts range from 0.02 to ∼2.0, but only 10 of the 178 new GRS have spectroscopic redshifts. For the 146 host galaxies, the median r-band magnitude and redshift are 20.9 and 0.64, while for the 32 quasars or candidates these are 19.7 and 0.75. Merging the six most recent large compilations of GRS results in 458 GRS larger than 1 Mpc, so we were able to increase this number by ∼39% to 636.

Tables of physical and morphological properties of nearby extended radio galaxies

It is brought the physical and morphological data of 267 nearby radio galaxies identified with elliptical galaxies brighter than 18th magnitude (sample 1) and for 280 extragalactic radio sources with known position angles between the integrated intrinsic radio polarization and radio axes (sample 2).

Investigations of the extended radio galaxy NGC 6251 and the galaxies in the environment

The environment of one of the most extended radio galaxies, NGC 6251, has been studied. The surface density of the environment galaxies in the region with the radius 300 arcmin around a radio galaxy NGC 6251 is 0.0315 in one arcmin2. In the distances less than 180 arcmin, the density of galaxies decreases, which is most likely due to the strong radio loudness and the interaction of radio galaxy with its surroundings.

RECURRENT RADIO OUTBURSTS AT THE CENTER OF THE NGC 1407 GALAXY GROUP

We present deep Giant Metrewave Radio Telescope (GMRT) radio observations at 240, 330 and 610 MHz of the complex radio source at the center of the NGC1407 galaxy group. Previous GMRT observations at 240 MHz revealed faint, diffuse emission enclosing the central twin-jet radio galaxy. This has been interpreted as an indication of two possible radio outbursts occurring at different times. Both the inner double and diffuse component are detected in the new GMRT images at high levels of significance. Combining the GMRT observations with archival Very Large Array data at 1.4 and 4.9 GHz, we derive the total spectrum of both components. The inner double has a spectral index \alpha=0.7, typical for active, extended radio galaxies, whereas the spectrum of the large-scale emission is very steep, with \alpha=1.8 between 240 MHz and 1.4 GHz. The radiative age of the large-scale component is very long, ~300 Myr, compared to ~30 Myr estimated for the central double, confirming that the diffuse component was generated during a former cycle of activity of the central galaxy. The current activity have so far released an energy which is nearly one order of magnitude lower than that associated with the former outburst. The group X-ray emission in the Chandra and XMM-Newton images and extended radio emission show a similar swept-back morphology. We speculate that the two structures are both affected by the motion of the group core, perhaps due to the core sloshing in response to a recent encounter with the nearby elliptical galaxy NGC1400.

Ultra-high energy cosmic rays detected by Auger and AGASA

The origin and composition of Ultra-High Energy Cosmic Ray Events (UHECRs) are under debate. Here we improve constraints on the source population(s) and compositions of UHECRs by accounting for UHECR deflections within existing Galactic magnetic field models (GMFs). We used Monte Carlo simulations for UHECRs detected by the Pierre Auger Observatory and AGASA in order to determine their outside-the-Galaxy arrival directions, and compared these with Galactic and extragalactic sources. The simulations, which used UHECR compositions from protons to Iron and seven models of the ordered GMF, include uncertainties in the GMF and a turbulent magnetic field. The correlation between UHECRs and nearby extended radiogalaxies (Nagar & Matulich 2008) remains valid, even strengthened, within several GMF models. Both the nearest radiogalaxy CenA, and the nearest radio-extended BL Lac, CGCG 413-019, are likely sources of multiple UHECRs. The correlation appears to be linked to the presence of the extended radio source rather than a tracer of an underlying population. It is possible, but unlikely, that all UHECRs originate in the nearby radiogalaxy CenA. For light UHECRs about a third of UHECRs can be "matched" to nearby galaxies with extended radio jets. The remaining UHECRs could also be explained as originating in extended radiogalaxies if one has at least one of: a large UHECR mean free path, a high cluster and/or intergalactic magnetic field, a heavy composition for two-thirds of the detected UHECRs. Several UHECRs have trajectories which pass close to Galactic magnetars and/or microquasars. If extended radiogalaxies are, or trace, UHECR sources, the most consistent models for the ordered GMF are the BS-S and BS-A models; the GMF models of Sun et al. 2008 are acceptable if a dipole component is added.

Ultra-high energy cosmic rays detected by the Pierre Auger observatory

(abridged) The Pierre Auger Collaboration has reported 27 Ultra-High Energy Cosmic Ray Events (UHECRs) with energies above 56 EeV and well determined arrival directions as of 2007 August 31. They find that the arrival directions are not isotropic, but instead appear correlated with the positions of nearby AGNs. Our aim was to determine the sources of these UHECRs by comparing their arrival directions with more comprehensive source catalogs. Four (eight) of the 27 UHECRs with energy >56EeV detected by the Pierre Auger Observatory have arrival directions within 1.5deg (3.5deg) of the extended (>180kpc) radio structures of nearby radiogalaxies or the single nearby BLLac with extended radio structure. Conversely the radio structures of three (six) of all ten nearest extended radiogalaxies are within 1.5deg (3.5deg) of a UHECR; three of the remaining four radiogalaxies are in directions with lower exposure times. This correlation between nearby extended radiogalaxies and a subset of UHECRs is significant at the 99.9% level. This is the first direct observational proof that radio galaxies are a significant source of UHECRs. For the remaining ~20 UHECRs, an isotropic distribution cannot be ruled out at high significance. The correlation found by the Auger Collaboration between the 27 UHECRs and AGNs in the Veron-Cetty & Veron catalog at D < 71Mpc has a much lower significance when one considers only the ~20 UHECRs not `matched' to nearby extended radiogalaxies. No correlation is seen between UHECRs and supernovae, supernova remnants, nearby galaxies, or nearby groups and clusters of galaxies. The primary difference between the UHECR detections at the Pierre Auger Observatory and previous experiments may thus be that the Southern Hemisphere is more privileged with respect to nearby extended radiogalaxies.

Study of Extended Radio Galaxies at 151.6 MHz using the Mauritius Radio Telescope

The MRT survey will be by far one of the most extensive survey at low frequencies. This survey will provide a moderately deep radio catalog reaching a source density of about 2 × 104 sr-1over the southern sky with an angular resolution of 4' × 4' and a limiting flux density of 70 mJy (1 σ) at 151 MHz. The availability of zero spacing and short baselines in the MRT array will make it sensitive to the background temperature and to large scale features in the sky. In addition to this feature, the low frequency operation makes a study of continuum emission from large radio sources by MRT to have several interesting and important implications in the study of radio galaxies. This paper discusses the parameter space of radio galaxies which can be explored using the MRT. Images of a few extended radio galaxies are also presented.

Evidence for a Circumnuclear Torus in Edge-On Radio Galaxies

We have used VLBI HI absorption imaging to search for evidence for a circumnuclear atomic structure in five radio galaxies. Of these, we have found that two, (PKS 2322-123 and 1946+708), are very symmetric on parsec scales and exhibit strong evidence for a circumnuclear torus, confirming the prediction of the unified scheme. The two nearby extended radio galaxies NGC 5128 and NGC 315, on the other hand, have asymmetric core-jet morphologies on parsec scales, and their HI absorption profiles appear to arise from gas in the host galaxy which is not associated with the AGN at all. This is also consistent with the unified scheme of AGN. One remaining source, NGC 3894, also has symmetric jets, but has a very complicated HI structure which changes on parsec scales. This source requires further study.

ASCA Observations of the Gigahertz-peaked Spectrum Radio Galaxies 1345+125 and 2352+495

We present ASCA SIS and GIS observations of two GHz peaked spectrum (GPS) radio galaxies, PKS 1345+125 (4C12.50, z = 0.122) and 2352+495 (z = 0.237). In the energy band 2–10 keV, 1345+125 is detected with about 2 × 1043 ergs s-1, while 2352+495 is not detected with an upper limit < 7 × 1042 ergs s-1. This is the first reported detection of X-ray emission from a GPS radio galaxy. These measurements do not violate predictions of Heinz et al.'s recent evolving source model for radio sources, although deeper X-ray observations are needed for a strong test of the model. The host galaxy of 1345+125 contains two nuclei separated by 2'' (4 kpc) which are not resolved by the ASCA observations. We suggest that the X-rays are emitted by the western of the two nuclei, which Gilmore & Shaw have shown has a NLRG/Seyfert-2 optical spectrum and which Evans et al. have recently identified with the GPS radio source. We obtain model dependent estimates for the 1345+125 radio source age of about 3 × 104 yr and expansion velocity of approximately 0.02c. We show that in samples of Seyfert 1's and powerful extended radio galaxies, optical emission-line luminosities are of a strength consistent with photoionization by the embedded active galactic nucleus (AGN; estimated using ASCA observations), and 1345+125 fits this picture assuming the X-ray emission is from the western nucleus. In contrast, the extrapolated limits on the nuclear ionizing continuum in 2352+495 are too low to account for the luminosity of the narrow emission-line region. If this is confirmed by further observations, it would favor a model in which some fraction of the emission lines are powered by shocks induced by the radio source or in which some of the AGN photons that ionize the gas are obscured from our view.

Coronal Faraday Rotation Observations: Measurements and Limits on Plasma Inhomogeneities

We report Faraday rotation measurements of the extended radio galaxy J0039)0319 (4C)03.01) seen through the solar corona when the source was at an average distance of 8.6 from the center of the R _ Sun. Nearly continuous polarimetric observations were made over an 11 hour period on 1997 March 28 with the NRAO Very Large Array at frequencies of 1465 and 1635 MHz. The observations were made near solar minimum conditions. Observations of radio galaxies have two advantages with respect to spacecraft transmitter signals. (1) The j2 dependence of the polarization position angle expected of Faraday rotation can be veri—ed. (2) Observations of spatially extended radio galaxies have the potential of directly measuring the propagation speed of coronal MHD irregularities. With the use of observations made when the source was far from the Sun, we measure an average rotation measure of )6.2 ^ 1.0 rad m~2 attributable to the corona. A rotation-measure time series was obtained for the most polarized component of the source. This rotation-measure time series showed slow variations during the observing session, with a total change of about 3 rad m~2. This variation is attributed to large-scale gradients and static plasma structures in the corona. We also obtain a weak detection of rotation-measure —uctuations on timescales of 15 minutes to 1 hour, which may be due to coronal waves. This —uctuating Alfvec n component of the coronal rotation measure has an rms value 0.40 rad m~2, comparable to previously reported detections. This measurement is then used to place model-dependent upper limits to the Alfvec n wave —ux at the coronal base. Depending on the precise geometry of the solar wind —ow from the coronal base to 8.6 the inferred wave —ux at the coronal base ranges from 2.4 ) 104 to 2.3 ) 105 R _ , ergs s~1 cm~2. These values range from slightly below to more than an order of magnitude below the wave —ux needed to heat and accelerate the solar wind to its observed values. Our results corroborate an increasing body of observational evidence indicating that long-wavelength MHD waves are not responsible for the heating and acceleration of the solar wind. Subject headings: galaxies: individual (J0039)0319) ¨ polarizationradio continuum: galaxies ¨ solar windSun: corona

Central Engines of Active Galactic Nuclei: Properties of Collimated Outflows and Applications for Cosmology

Powerful extended radio galaxies in the 3CR survey are observed out to redshifts of about 2. For redshifts greater than about 0.3, the average lobe-lobe size of the sources decreases monotonically with redshift for all reasonable cosmological parameter choices. This suggests that the characteristic time for which an AGN produces a highly collimated outflow that powers the radio emission is shorter for the high-redshift sources than it is for low-redshift sources. The fact that the characteristic time for which an AGN produces a collimated outflow decreases with redshift implies that the physical mechanism that gives rise to the large scale jets is not Eddington limited. Higher redshift sources have higher radio powers due the flux limit of the sample, and sources with higher radio powers have higher beam powers relative to lower redshift sources. Both the beam power and active lifetime of the AGN are controlled by processes occurring at the heart of the AGN, and may be related for non-Eddington limited processes. The relation between the active lifetime, $t_*$, and the beam power, $L_j$, of powerful extended radio galaxies is investigated here and found to be $t_*\proptoL_j^{-2/3}$. A comparison of the redshift evolution of the characteristic source sizes with that of the average lobe-lobe size for the 3CR sample of radio galaxies could be used to constrain cosmology.

Dependence of Linear Sizes and Spectral Indices of Extended Radio Galaxies on Redshift and Radio Luminosity

Apart from the well known evolution in the space density of powerful radio sources, the maximum linear sizes and the spectral indices of extended radio sources could also evolve with epoch and it is important to investigate their epoch-dependence in order to understand the cosmological evolution of radio sources. Evidence for size evolution (in the sense that sources were smaller at earlier epochs) has been presented both from the angular size – redshift relation (θ – z) for quasars and from the angular size – flux density relation (θ – S) for all radio sources. Doubts have sometimes been expressed, however, mainly because it has been difficult to investigate the epoch dependence independent of a possible dependence on radio luminosity. Reasonably reliable estimates of the distances of radio sources have generally been limited to a relatively small range in flux density (S), so that high redshift sources are also of high luminosity. An inverse correlation between linear size (l) and radio luminosity (P) is therefore hard to distinguish from a genuine evolution with epoch. A similar situation appears to hold for the spectral indices (α) of extended radio sources, which have generally been assumed to get steeper at higher radio luminosities (eg. Laing and Peacock 1980) but a dependence on epoch cannot at present be ruled out (Katgert-Merkelijn et al. 1980).

Cores in Extended Quasars

The Nuclear Radio cores of several nearby extended radio galaxies (e.g. M87, 3C236) consist not only of optically thick (&lt; 1 pc) components, but also of emission on somewhat larger scale. As extended radio sources associated with quasars have on average stronger and more luminous radio cores (see e.g. Miley, 1980, Ann. Rev. Astron. Astrophys. 18, 165), we have started a project to study the properties of these quasar cores.