Active Galaxies and Clusters
(C. H. Ishwara-Chandra, Dharam Vir Lal, Ruta Kale, Preeti Kharb, Ramananda Santra, Sanna Gulati, janhavi Baghel, Arpan Pal, Former members: Sravani Vaddi, D. J. Saikia, J. N. H. S. Aditya, Biny Sebastian, Satish Sonkamble, Silpa Sasikumar)
Active Galactic NucleiActive 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 polarization perspective on the radio outflow in III Zw 2
The origin of radio emission in radio-quiet and radio-intermediate active galactic nuclei (AGNs) is still a matter of debate. Primary contenders include low-power jets, winds, star-formation and coronal emission. Recent works have demonstrated the use of radio polarization as an efficient tool to distinguish between jets and winds based on the differences in their polarization signatures. Silpa et al. have carried out a polarization study of a radio-intermediate quasar, III Zw 2, with the upgraded Giant Metrewave Radio Telescope (uGMRT) at 685 MHz and the Karl G. Jansky Very Large Array (VLA) at 5 and 34 GHz. Silpa et al. detect a composite jet + 'wind' radio outflow in III Zw 2. This comprises a collimated jet/jet spine with poloidal inferred magnetic fields embedded inside a broader magnetized 'wind' with toroidal inferred magnetic fields. This ‘wind’ component could be a magnetized accretion disc wind or the outer layers of a broadened jet (like a jet sheath) or a combination of both. The current data cannot differentiate between these possibilities. The curved jet terminates in a bow-shock-like radio structure with inferred magnetic fields aligned with the lobe edges. Silpa et al. also detect a kpc-scale lobe emission to the south that is misaligned with the primary lobes in the uGMRT images. The spectral indices and the electron lifetimes in the misaligned lobe are similar to values in the primary lobe, suggesting that the misaligned lobe is not a relic. Silpa et al. propose that changing spectral states of the accretion disc, and the subsequent intermittent/'sputtering' behaviour of the outflow, along with the close interplay between the jet and ‘wind’ could explain the radio-intermediate nature of III Zw 2.
Interplay of jets, winds and emission-line gas in type 2 quasars
Active Galactic Nuclei (AGNs) are believed to regulate galaxy growth by injecting energy into the surrounding gas which has the effect of either heating and/or expelling the star-forming material or facilitating localized star-formation. While this so-called ``feedback'' (in the form of jets/winds) is believed to be a fundamental process of galaxy formation from a theoretical point of view, there are many outstanding questions from an observational point of view. Silpa et al. have carried out a combined radio polarization and emission-line study of five type 2 radio-quiet quasars at z < 0.2 with the Karl G. Jansky Very Large Array (VLA) B-array at 5 GHz and Hubble Space Telescope (HST) [OIII] observations. This work aims to establish the jet/wind origin of the radio emission from these sources and look for signatures of jet-medium interaction in them from a polarization perspective. Polarization is detected in 4 out of 5 sources in the current data, but not in J1010+1413. The morphological, spectral, and polarization properties suggest a jet origin for the radio emission in J0945+1737, J1000+1242, J1010+1413, and J1430+1339 whereas the current data cannot fully discern the origin of radio emission (jet or wind) in J1356+1026. These five sources are known to exhibit a close association between the radio structures and the ionized gas morphology and kinematics. Silpa et al. find an anticorrelation between various polarized knots in the radio and [OIII] emission in these sources, similar to that observed in some radio-loud AGNs in the literature. This suggests that the radio emission is likely to be depolarized by the emission-line gas. The anti-correlation between polarization and ionized gas could be interpreted as an effect of the jet/wind-gas interaction, and a possible source of AGN feedback. By modelling the depolarization effects, Silpa et al. estimate the size of the emission-line gas clouds to be ~(2.8 +/- 1.7) x 10^−5 parsec and the amount of thermal material mixed with the synchrotron plasma to be ~(1.01 +/- 0.08) x 10^6 solar masses in the lobe of J0945+1737 (which exhibits the most prominent polarization signature in its lobe). This work demonstrates that the interplay of jets/winds and emission-line gas is most likely responsible for the nature of radio outflows in radio-quiet AGNs.
NGC 4869 in the Coma Cluster: Twist, Wrap, Overlap, and Bend
The author presents a detailed analysis of deep upgraded Giant Metrewave Radio Telescope (uGMRT) images of the head-tail radio galaxy NGC 4869 in the Coma cluster. The uGMRT images have an angular resolution of ~6.3 arcsecs and ~2.2 arcsecs, at frequencies of 250-500 MHz and 1050-1450 MHz, respectively. The author also used archival GMRT data to image the source, with angular resolutions ranging from 4.9 arcsecs to 21.8 arcseconds at 610 MHz, 325 MHz, 240 MHz, and 150 MHz. The uGMRT images show that the radio morphology of NGC 4869 consists of five distinct regions, with the clear presence of a pinch at a distance of 38.8 kpc, and a ridge at a distance of ~94.2 kpc from the head of the radio galaxy. The sharp bend by ~ 70 degrees at ~97 kpc from the head is possibly due to projection effects. There is possibly re-acceleration of the synchrotron electrons and perhaps also magnetic field regeneration in the ~2.8 - 96.1 kpc region of the jet. The author reports a steep-spectrum sheath layer enveloping a flat-spectrum spine, hinting at a transverse velocity structure with a fast-moving spine surrounded by a slow-moving sheath layer. He also derives the lifetimes of the radiating electrons and equipartition parameters. The figure shows the uGMRT 250-500 MHz (green) and 1050-1450 MHz (red) images of NGC 4869, overlaid on a Chandra X-ray image. The red arrows indicate the location of the onset of flaring, i.e. the surface brightness edge. The two radio jets emanating from the apex of the host galaxy initially travel in opposite directions. As the galaxy plows through the dense intracluster gas, these jets form a trail behind the host galaxy due to interaction with the intracluster medium, forming a conical shaped feature centered on the nucleus. Subsequently, the two jets twist, wrap, overlap and eventually bend. The radio spectra show progressive spectral steepening with distance from the head (i.e. the radio core), due to synchrotron cooling. A plausible explanation for the characteristic feature, the ridge of emission perpendicular to the direction of tail, is the flaring of a straight, collimated radio jet as it crosses a surface brightness edge (due to Kelvin-Helmholtz instabilities).
Upgraded GMRT Observations of the Coma Cluster of Galaxies: The Observations
The author used the upgraded Giant Metrewave Radio Telescope (uGMRT) to map the Coma galaxy cluster at two frequencies, covering 250-500 MHz and 1050-1450 MHz. Coma is the nearest large galaxy cluster to us, and shows a complex dynamical state in its X-ray emission. The high resolution (~6.3 arcsec and 2.2 arcsec, respectively) and high sensitivity (RMS noise of 21 microJy/Beam and 12.7 microJy/Beam, respectively) of the uGMRT images allow the radio structure to be determined for a large number of radio sources in the cluster, of both compact and
extended morphologies. The author presents images and spectral index measurements for a subset of the 32 brightest sources of the cluster. He finds the steepening of the low-frequency radio spectra to be consistent with synchrotron cooling in the majority of sources. The median spectral index is -0.78, suggesting that ~60% of the sources have steep spectra. The deep uGMRT images presented here will enable detailed studies of the spectral properties, ages, and structures of individual radio galaxies within
the cluster. The figure shows the 250-500 MHz uGMRT image, with 6.3'' resolution and an RMS noise of 21 microJy/Beam; the image reveals a wide variety of radio morphologies for the detected radio sources, including a few new candidate extreme radio relics.
Tracing the evolution of ultraluminous infrared galaxies into radio galaxies with low frequency radio observations
Ultraluminous infrared galaxies (ULIRGs) are gas rich merger remnants that are extremely luminous at infrared wavelengths. They represent the final stage of the merging process of two comparable mass, gas-rich galaxies that finally evolve into elliptical galaxies, and, in some cases, quasars. Nandi et al. observed 13 ULIRGs that have optically-identified characteristics of active galactic nuclei (AGNs) with the Giant Metrewave Radio Telescope (GMRT). The main goal is to study ULIRGs at low frequencies and identify any signatures of core-jet structures or extensions. This can help determine whether there is an underlying evolutionary connection between ULIRGs and young radio sources like Gigahertz Peaked Spectrum (GPS) sources, Compact Steep Spectrum (CSS) sources, and compact symmetric objects (CSOs). The authors find that ULIRGs can have signatures of outflows at low frequencies. They examined the radio spectral energy distribution of this sample and studied their optical spectra. The integrated radio spectra of 5 ULIRGs have low frequency turnovers, similar to those of young radio sources. A spectral ageing analysis shows that the ULIRGs are younger than the extended large radio sources or remnant radio sources. Archival high frequency radio data revealed classical double structure for 3 sources (see figure), while 4 sources show double-peaked emission lines, the latter likely to arise due to either dual AGNs or outflows. The estimated spectral age, spectral shape, and radio morphology of these ULIRGs indicates that they are young radio sources and possible progenitors of radio galaxies.
Double-peaked Lines, Dual VLBI Components, and Precessing Jets in J1328+2752
Nandi et al. use low-frequency Giant Metrewave Radio Telescope (GMRT) observations and Very Large Array Faint Images of the Radio Sky at Twenty centimeters (FIRST) images to identify a radio galaxy, J1328+2752, with symmetric helical jets. The Sloan Digital Sky Survey (SDSS) spectrum of the galaxy shows that the central component has double-peaked line profiles with different emission strengths. The authors use the BPT diagnostic diagram to distinguish the different classes of ionization, to find that the two components of the double-peaked emission lines may come from two active galactic nuclei (AGNs) that underwent a merger. Large-scale radio jets with a rotationally-symmetric helical modulation are also an indirect indicator of black hole binaries. However, the confirmation of such binaries typically requires multiple signatures at different wavelengths. The authors carried out very long baseline interferometry (VLBI) 5 GHz imaging and kinematic precession modeling of this radio galaxy. The VLBI image reveals a core-jet structure (component A with sub-components 1 and 2 in the upper panel of the figure) and another single component (B) separated in projection by ∼ 6 parsec. The estimated binary separation obtained from the double-peaked lines matched exactly with the VLBI data. The precession helices generated by the kinematic model match well with the GMRT and FIRST images at 325, 610 and 1400 M Hz (lower panel of the figure). The model indicates that either the jet precession is induced by torques in the primary accretion disc due to the secondary black hole in a non-coplanar orbit around the primary, or the jet may forced to precess under the Bardeen-Petterson effect. The authors also studied the host galaxy properties using SDSS i- and r-band data, finding that a combination of Sersic and exponential profiles are required to fit the optical light distribution of the galaxy. The disk component dominates beyond 2.5 kpc, whereas the inner portion is bulge-dominated. The extended disk-like sub-structure may represent a gas-rich, unequal-mass merger. The optical emission lines, the helical kpc-scale jets, the parsec-scale VLBI image, and the kinematic model all support the binary black holes scenario in this source.
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;