Active Galaxies and Clusters

(C. H. Ishwara-Chandra, Dharam Vir Lal, Ruta Kale, Preeti Kharb, Biny Sebastian, Satish Sonkamble, Silpa Sasikumar, Former members: Sravani Vaddi, D. J. Saikia, J. N. H. S. Aditya)

Active Galactic Nuclei
Active Galactic Nuclei (AGNs) are galaxies where extremely energetic phenomena take place, driven by activity around the super-massive black holes at their centres. These result in the emission of enormous amounts of radiation in various wavebands, including the radio, making them observable out to very large distances in the Universe. The identification and detailed study of Compact Steep Spectrum and Gigahertz Peaked Spectrum sources, which constitute a significant fraction of such bright AGNs but are not well understood, are carried out at NCRA-TIFR. Research is also carried out on studies of giant radio sources, recurrent activity in radio galaxies, the interaction between radio plasma and the inter-cluster and intra-group media, radio halos and relics. Searches are also being carried out for radio galaxies at high redshifts, and efforts are under way to model their gaseous environments, at all redshifts.

Seyfert galaxies were first identified by Carl Seyfert in 1943 as those exhibiting star-like nuclei superimposed on the centers of spiral galaxies, along with peculiar emission line spectra showing the presence of strong narrow and broad emission lines. These are now understood to be a class of AGNs. Sensitive radio observations have detected kiloparsec-scale radio structures (KSRs) or lobes in a large fraction of Seyfert galaxies. The origin of these KSRs is still debated: some studies have suggested them to be driven by starburst superwinds from the nuclei of spiral galaxies, while others have favoured an AGN-driven origin. Several large samples of Seyfert galaxies have been, and are currently being observed at low radio frequencies with the GMRT. Low radio frequency observations are not only detecting radio emission from the spiral galaxy disks themselves, but also "relic" lobe emission from previous AGN activity cycles in some Seyfert galaxies. Radio spectral studies are providing important constraints on the contributions of stellar versus AGN activity. On the one hand, low radio frequencies detect a larger fraction of host galaxy emission in these Seyferts, which contaminates the spectral index values of lobes, the detection of ``relic'' lobe emission strongly favours the AGN-driven origin for KSRs. NCRA-TIFR astronomers are also studying these Seyfert galaxies with Very Long Baseline Interferometry, using telescopes like the Very Long Baseline Array in the USA, and MERLIN in the UK. We are probing parsec-scale AGN jet emission with these studies, in order to finally construct a comprehensive picture of outflows in Seyfert galaxies.

Clusters of Galaxies
Clusters of galaxies are gravitationally bound collections of hundreds to thousands of galaxies. The space between these galaxies is filled with diffuse, hot plasma (~ten million Kelvin), called the intra-cluster medium (ICM). The thousands of galaxies and the gas are held together in the gravitational potential of dark matter which makes about 80-85% of a cluster's total mass.  These are the most massive gravitationally-bound physical systems in the Universe.

The ICM is a soup of thermal gas, cosmic rays, and magnetic fields. The cosmic rays and magnetic fields are very difficult to detect, and hence the physics of how energy is exchanged between these components and the thermal gas is not well understood. Radio observations (e.g. with the GMRT) provide a unique probe for the relativistic electrons and magnetic fields in the ICM by allowing us to detect the radio emission from them. In the special case of clusters that are undergoing violent mergers with other clusters, the magnetic fields in the ICM are amplified, and shocks and turbulence are driven into the ICM. These shocks and turbulence can accelerate electrons to relativistic energies leading to the formation of sources called radio halos and radio relics. NCRA-TIFR astronomers use the GMRT and other radio and X-ray telscopes to uncover the physical processes that govern these phenomena. Indeed, a large GMRT survey provided the first estimate of the statistical occurrence of such radio sources in merging and non-merging clusters.

The brightest cluster galaxies (BCGs) that typically reside at the centres of galaxy clusters are the largest and most massive galaxies in the Universe. The BCGs are also known to be spectacular sources of radio emission triggered by the accretion of mass on to their central supermassive black holes (SMBHs). The radio mode feedback from the BCGs is considered to offset the cooling of cluster cores but the delicate balance between the two is still not well understood. NCRA-TIFR astronomers use the GMRT and other telescopes to study the triggering of the radio jets from the SMBHs and their feedback on the ICM.

The active galaxies and radio galaxies in galaxy clusters are of interest as they are responsible for depositing cosmic rays and magnetic fields in the ICM. NCRA-TIFR astronomers have used cluster galaxies to study their dense environments. When the extended radio emission is not being actively fed by the SMBH jets, the radio emission fades rapidly, causing the radio spectrum to steepen, becoming relatively brighter at low frequencies. Such remnants of radio galaxies are of interest in understanding the effects of shock compression and re-acceleration of electrons in the ICM. Radio continuum observations with the wide frequency bands of the upgraded GMRT are now being used to learn the physical phenomena of re-acceleration of electrons in the ICM.

Recent Results
A radio halo surrounding the Brightest Cluster Galaxy in RXCJ0232.2-4420: a mini-halo in transition?
Kale et al. report the discovery of a ``radio halo'', a diffuse radio source, in the galaxy cluster RXCJ0232.2-4420 (SPT-CL J0232-4421, z = 0.2836) using observations with the Giant Metrewave Radio Telescope. Diffuse radio sources associated with the intra-cluster medium - the medium that pervades the space between galaxies in a galaxy cluster- are direct probes of cosmic ray electrons and magnetic fields in the cluster. Although magnetic fields are believed to be ubiquitous in galaxy clusters, such radio sources are rare. The known sample of such sources has been broadly classified into radio halos that are >700 kpc-sized sources that occur in merging clusters and mini-halos that are only a couple of hundred kpc in size and occur in relaxed clusters. It has been proposed that mini-halos transition into radio halos when a relaxed system undergoes a merger; however, this transition has not been observed clearly. The newly-discovered source has an extent of 550 kpc x 800 kpc - a size in the radio halo category. However, it surrounds the Brightest Cluster Galaxy like a typical mini-halo. Kale et al. have compared the radio power of this source with that of known radio halos and mini-halos and found it to be consistent with both populations. In the X-ray bands, this cluster has been classified as a complex system - indicating a state that is neither a merger nor a completely relaxed state. Kale et al. hence propose that this system is among the rare class of transition systems between mini-halos and radio halos. The 3-color image shows the image of the galaxy cluster in radio waves (blue), X-rays (green) and visible light (red).
A VLA-GMRT look at 11 FR-II Quasars
Active Galactic Nuclei (AGNs) are a special class of galaxies that emit enormous amounts of energy from the nuclear region.  There are several variants of AGNs classified based on their observed properties.  Detailed observations of AGNs over two decades have suggested that AGNs are intrinsically similar objects, but may appear different due to different viewing angles; this idea is now known as the AGN unification scheme.  Vaddi et al. address the unification of radio-loud AGNs via statistical and spectral analysis approaches.  A sample of 11 steep-spectrum radio quasars and 13 Fanaroff-Riley-II radio galaxies that span similar luminosity and redshift ranges were used for this study. Matched resolution radio data for the quasars were obtained using the Jansky Very Large Array. The results are in general agreement with orientation-based AGN unification. However, the authors find that environmental effects cannot be ignored. The lack of correlation between the statistical orientation indicators such as misalignment angle and radio core prominence (see the figure), and the larger lobe distortions in quasars compared to radio galaxies suggest that additional intrinsic or environment effects are at play.
Curvature in the spectrum of a remnant radio galaxy with the uGMRT
The origin of cosmic rays in the intra-cluster medium (ICM) has been attributed to re-acceleration of charged particles in shocks and turbulence. For these re-acceleration mechanisms to work, it is expected that there will be reservoirs of seed cosmic ray electrons in the ICM. Radio galaxies with jets and lobes are strong candidates for providing these seeds. Dr. Kale and collaborators have used the unique broad band observing capabilities of the recently operational upgraded Giant Metrewave Radio Telescope (uGMRT) to study an enigmatic "dead radio galaxy" or a "remnant radio relic" in the galaxy cluster Abell 4038. They have shown that the spectrum of the source varies considerably across its extent and undergoes extreme changes from high to low frequencies that are quantified in a parameter called the "spectral curvature". The authors fine that the assumption that the spectra of seed particles are simple power-laws may not be correct, given the extreme spectral curvature measured using the uGMRT images. Their study has recently been accepted for publication in the Monthly Notices of the Royal Astronomical Society.
A fourth radio arc in Abell 2626
The supermassive black holes at the centres of active galaxies can lead to the formation of spectacular jets that are detectable in deep radio imaging studies. When such black holes are situated close to the centres of galaxy clusters, they experience a dense environment. The radio jets can be affected by the black hole itself and by the environment, leading to complex morphologies. A system of three concave arcs was earlier known towards the galaxy cluster Abell 2626. Kale & Gitti used the 610 MHz receivers of the GMRT to discover a fourth arc in the sytem, that completes an intriguing symmetric structure of four arcs around the central massive galaxy that itself has two active nuclei. The origin of the exotic source is as yet unknown, but may be a rare event of precessing jets from the double nuclei of the central galaxy or a similarly rare configuration of a gravitational lens. The image shows the GMRT radio image in blue, overlaid on X-ray (red) and optical (green) images.
A Giant Radio Galaxy at z ~ 0.57
Giant radio galaxies (GRGs) are radio galaxies whose linear extent is more than 1 Mpc. Most of the known GRGs are less than a billion light years away from us. The sharp decline in the number of GRGs at larger distances, i.e. higher redshifts, is a mystery because the number of normal radio sources is actually higher at high redshifts. We recently used the GMRT to carry out a deep 150 MHz study of a small region of the sky in the Lynx constellation, and discovered a large GRG, of size 7 million light years, at a distance of about 5 billion light years, i.e. a redshift of 0.57. We used the GMRT to carry out detailed imaging studies of the GRG, at 325 MHz, 610 MHz and 1420 MHz; the new data suggest that the object is probably a double-double radio galaxy. Further, the radio core of the galaxy shows an unusually steep spectrum, which may imply that there is yet another unresolved pair of lobes within the core, making this GRG a candidate triple-double radio galaxy. Further investigations of the central region of the GRG, to test if it is a re-started radio source, are now under way using the European Very Long Baseline Interferometry Network (EVN), which has the resolution to probe the central region very close to the supermassive black hole. The figure shows the GMRT 610 MHz image of the new GRG, overlaid on the optical SDSS gri-composite image. The optical host galaxy is shown separately in the rectangular box. The double-lobe structure on either side of the central core is clearly visible.
A candidate sub-parsec binary black hole in the Seyfert galaxy NGC 7674
Kharb, Lal & Merritt have used Very Long Baseline Array (VLBA) observations to discover only the second candidate sub-parsec binary black hole. The existence of such binary super-massive black holes (SMBHs) is predicted by models of hierarchical galaxy formation, but only a single such binary SMBH has been imaged until now. Kharb et al. used the VLBA to study the gas-rich interacting spiral galaxy NGC7674, which possesses a kpc-scale Z-shaped radio jet. The leading model for the formation of such Z-shaped sources postulates the presence of an uncoalesced binary SMBH, created during the infall of a satellite galaxy. Kharb et al. used the high angular resolution of the VLBA to image the central region of NGC7674 at radio frequencies between 2 and 15 GHz, resulting in the detection of two radio cores, separated by just 1 light year, at the highest observing frequency, 15 GHz. The inverted radio spectra of the two cores are consistent with their being accreting super-massive black holes!
Discovery of a radio relic in the low mass galaxy cluster PLCK G200.9-28.2
Kale et al. used the Giant Metrewave Radio Telescope (GMRT), the XMM-Newton X-ray Observatory, and the Jansky Very Large Array to discover a new radio relic in the galaxy cluster PLCKG200.9-28.2 at z~0.22. Such arc-like radio relics are usually found at the periphery of massive colliding clusters, and are extremely rare, arising in fewer than 5% of merging clusters. Despite their rarity, radio relics are an excellent tracer of the shocks that are expected to be driven in the diffuse intra-cluster medium by violent cluster collisions. Indeed, it is very difficult to even detect these shocks at other wavelengths. So far, radio relics have been found only in the vicinity of merging massive clusters. The new radio relic detected by Kale et al. is very interesting because it arises in a cluster of low mass, the lowest mass at which such a relic has ever been seen! This demonstrates that violent mergers in low-mass clusters are capable of producing strong shock waves in their diffuse media. In the adjoining figure, the 235 MHz emission imaged with the GMRT is shown in red and the X-ray emission imaged with the XMM-Newton satellite observatory is shown in blue. The elongated source seen in red is the new radio relic.
An AGN's rendezvous with a radio relic
van Weeren et al. used data from the Chandra X-ray Observatory, the Giant Metrewave Radio Telescope, the Jansky Very Large Array, and other telescopes to discover a cosmic event never seen before. Galaxy clusters contain multiple sources of radio emission, including active galactic nuclei (AGNs), radio halos and radio relics. A long-standing problem in studies of clusters is how low-Mach-number shocks can accelerate electrons efficiently to produce the observed radio relics. van Weeren et al. discovered, for the first time, a direct connection between a radio relic and an AGN (a radio galaxy) in the merging galaxy cluster Abell 3411-3412 by combining radio, X-ray and optical data. This discovery indicates that fossil relativistic electrons from AGNs are re-accelerated at cluster shocks. It also implies that radio galaxies play an important role in governing the non-thermal component of the intra-cluster medium in merging clusters. For the first time, two of the most powerful phenomena in the Universe have been clearly linked together in the same system. Image credits: X-ray: NASA/CXC/SAO/R. van Weeren et al; Optical: NAOJ/Subaru;