Joydeep Bagchi - Academia.edu (original) (raw)
Papers by Joydeep Bagchi
The Astrophysical Journal, 2015
Monthly Notices of the Royal Astronomical Society, 2020
In this work, we investigate the dynamical state of the galaxy cluster Abell 2631, a massive stru... more In this work, we investigate the dynamical state of the galaxy cluster Abell 2631, a massive structure located at the core of the Saraswati supercluster. To do this, we first solve a tension found in the literature regarding the weak-lensing mass determination of the cluster. We do this through a comprehensive weak-lensing analysis, exploring the power of the combination of shear and magnification data sets. We find M200rmwl=8.7−2.9+2.5times1014M_{200}^{\rm wl} = 8.7_{-2.9}^{+2.5} \times 10^{14}M200rmwl=8.7−2.9+2.5times1014 M⊙. We also determined the mass based on the dynamics of spectroscopic members, corresponding to M200rmdy=12.2pm3.0times1014M_{200}^{\rm dy} = 12.2\pm 3.0 \times 10^{14}M200rmdy=12.2pm3.0times1014 M⊙, consistent within a 68 per cent CL with the weak-lensing estimate. The scenarios provided by the mass distribution and dynamics of galaxies are reconciled with those provided by X-ray observations in a scenario where A2631 is observed at a late stage of merging.
American Journal of Physics
We have designed a low-cost radio telescope system named the Bose Horn Antenna Radio Telescope (B... more We have designed a low-cost radio telescope system named the Bose Horn Antenna Radio Telescope (BHARAT) to detect the 21 cm hydrogen line emission from our Galaxy. The system is being used at the Radio Physics Laboratory (RPL) (Radio Physics Lab, IUCAA NCRA-TIFR, < http://www.iucaa.in/∼rpl/ >, < http://www.ncra.tifr.res.in/ncra/rpl/ >), Inter-University Centre for Astronomy and Astrophysics (IUCAA), India, for laboratory sessions and training students and teachers. It is also a part of the laboratory curriculum at several universities and colleges. Here, we present the design of a highly efficient, easy to build, and cost-effective dual-mode conical horn used as a radio telescope and describe the calibration procedure. We also present some model observation data acquired using the telescope for facilitating easy incorporation of this experiment in the laboratory curriculum of undergraduate or post-graduate programs. We have named the antenna after Acharya (teacher or an ...
Abstract. Damped Lyman-α systems (DLAs) seen in the spectra of high-z QSOs allow us to probe the ... more Abstract. Damped Lyman-α systems (DLAs) seen in the spectra of high-z QSOs allow us to probe the physical conditions in protogalaxies. Our understanding of physical conditions in DLAs at high-z is primarily based on the absorption lines of H2 molecules and fine-structure transitions. Another important way of probing the thermal state of interstellar medium in these systems is by studying the 21-cm absorption in the spectra of background quasars. Here we report the main results of our GMRT survey to search for 21-cm absorption in a representative and unbiased sample of 35 DLA candidates at 1.10≤z≤1.45. Our sample of DLA candidates is drawn from the strong Mg ii systems in SDSS DR5 and has resulted in discovery of 9 new 21-cm absorbers. Prior to our survey only one 21-cm absorber was known in the redshift range: 0.7≤z≤2. This survey has allowed us to investigate the dependence of detectability of 21-cm absorption on the properties of UV absorption lines detected in SDSS spectra and es...
Proceedings of the International Astronomical Union, 2014
We have initiated the search and detailed study of large scale structures present in the universe... more We have initiated the search and detailed study of large scale structures present in the universe using galaxy redshift surveys. In this process, we take the volume-limited sample of galaxies from Sloan Digital Sky Survey III and find very large structures even beyond the redshift of 0.2. One of the structures is even greater than 600 Mpc which raises a question on the homogeneity scale of the universe. The shapes of voids-structures (adjacent to each other) seem to be correlated, which supports the physical existence of the observed structures. The other observational supports include galaxy clusters' and QSO distribution's correlation with the density peaks of the volume limited sample of galaxies.
Astronomy & Astrophysics
We present for the first time a study of peculiar giant radio galaxy (GRG) J223301+131502 using d... more We present for the first time a study of peculiar giant radio galaxy (GRG) J223301+131502 using deep multi-frequency radio observations from GMRT (323, 612, and 1300 MHz) and LOFAR (144 MHz) along with optical spectroscopic observations with the WHT 4.2m optical telescope. Our observations have firmly established its redshift of 0.09956 and unveiled its exceptional jet structure extending more than ∼200 kpc leading to a peculiar kink structure of ∼100 kpc. We measure the overall size of this GRG to be ∼1.83 Mpc; it exhibits lobes without any prominent hotspots and closely resembles a barbell. Our deep low-frequency radio maps clearly reveal the steep-spectrum diffuse emission from the lobes of the GRG. The magnetic field strength of ∼5 μG and spectral ages between about 110 to 200 mega years for the radio lobes were estimated using radio data from LOFAR 144 MHz observations and GMRT 323 and 612 MHz observations. We discuss the possible causes leading to the formation of the observed...
Monthly Notices of the Royal Astronomical Society, 2015
We present a deep Chandra observation of the extremely massive spiral galaxy 2MASX J23453268-0449... more We present a deep Chandra observation of the extremely massive spiral galaxy 2MASX J23453268-0449256, the first X-ray observation of this very rare system which features the largest known relativistic jets from a spiral galaxy. We detect extended X-ray emission from the hot halo surrounding the galaxy, reaching out to 80 kpc in radius. The hot halo is elongated along the plane of the spiral galaxy, and one possibility is that the powerful relativistic jets have disrupted the hot halo gas located perpendicular to the disc. Our calculations indicate that it is energetically feasible that the AGN feedback in this system could have uplifted or completely expelled a significant fraction of the gas in the 20-80-kpc radial range. We also detect extended emission which appears to be associated with the inner and outer southern radio lobes, and is possibly the result of inverse Compton emission. Using the observed X-ray and radio luminosity of the central AGN, the Fundamental Plane of Gultekin et al. predicts a black hole mass of 5 × 10 8 M , with a range of 1 × 10 8-3 × 10 9 M when the scatter in the Fundamental Plane relation is taken into account. This is consistent with the possibility that an exceptionally massive (>10 9 M) black hole lies at the centre of this galaxy, as suggested by the M BH-σ scaling relation, but a tighter constraint cannot be made.
We report the discovery of large-scale diffuse non-thermal radio emission in PLCK G287.0+32.9, an... more We report the discovery of large-scale diffuse non-thermal radio emission in PLCK G287.0+32.9, an exceptionally hot (T ∼ 13 keV), massive, and luminous galaxy cluster, strongly detected by the Planck satellite in a recent, all-sky blind search for new clusters through Sunyaev-Zel'dovich effect. Giant Metrewave Radio Telescope 150 MHz and Very Large Array 1.4 GHz radio data reveal a pair of giant (>1 Mpc) "arc" shaped peripheral radio-relics (signatures of shock waves) of unprecedented scale (linear separation ∼4.4 Mpc at redshift 0.39), located at distances from the cluster center which are about 0.7 and 1.3 of the cluster's virial radius. Another possible giant relic and a radio-halo is detected closer to the cluster center. These relic sources are unique "signposts" of extremely energetic mergers and shocks (both ongoing and past), that are assembling and heating up this very massive galaxy cluster. They are also a probe of the filamentary cosmic-web structure beyond the cluster virial radius. Optical imaging with the IUCAA 2m telescope and XMM-Newton X-ray data confirm a very rich galaxy cluster with a morphologically disturbed core region, suggesting a dynamically perturbed merging system.
giant radio jet of very unusual polarization in a single-lobed
arXiv: Cosmology and Nongalactic Astrophysics, 2016
Massive galaxy clusters are the most violent large scale structures undergoing merger events in t... more Massive galaxy clusters are the most violent large scale structures undergoing merger events in the Universe. Based upon their morphological properties in X-rays, they are classified as un-relaxed and relaxed clusters and often host (a fraction of them) different types of non-thermal radio emitting components, viz., haloes, mini-haloes, relics and phoenix within their Intra Cluster Medium (ICM). The radio haloes show steep (alpha = -1.2) and ultra steep (alpha < -1.5) spectral properties at low radio frequencies, giving important insights on the merger (pre or post) state of the cluster. Ultra steep spectrum radio halo emissions are rare and expected to be the dominating population to be discovered via LOFAR and SKA in the future. Further, the distribution of matter (morphological information), alignment of hot X-ray emitting gas from the ICM with the total mass (dark + baryonic matter) and the bright cluster galaxy (BCG) is generally used to study the dynamical state of the clus...
VizieR Online Data Catalog, Oct 1, 2020
We present Hubble Space Telescope (HST) observations of the nearby (z= 0.023), very massive, high... more We present Hubble Space Telescope (HST) observations of the nearby (z= 0.023), very massive, highly rotating (488.4±12.5 km s −1) hybrid (S0-Sa) galaxy UGC 12591, along with observations taken at other wavelengths from UV to FIR. Deep HST data of the galaxy in V, I and H bands is used to disentangle its bulge and disk components. The surface brightness distributions show a dominance of the bulge component over the disk with H-band bulge-to-disc luminosity ratio ∼ 69%. From spectral energy distribution (SED) fitting, we find an extremely low global star formation rate (SFR) of ∼ 0.1 − 0.2M y −1 , which is unusually low for this galaxy's huge stellar mass (M ★ ∼ 1.6 × 10 11 M), that implies a strong quenching of it's SFR. For at least past 10 8 years the galaxy has remained in the quiescent state as a sterile, 'red and dead' galaxy. The fraction of total infra-red dust luminosity contributed by the ISM dust is as high as ∼ 90%, with total dust luminosity L dust = (0.5 − 3.5) × 10 10 L , dominated by cold dust at equilibrium temperature T C ≈ 15K. UGC 12591 is found host to a super massive black hole (SMBH) of mass 6.18 × 10 8 M which at the present moment is quiescent, i.e., neither we see large (> 1kpc) radio jets nor is the black hole contributing significantly to the mid-IR SED, ruling out the presence of strong radiative feedback from a bright AGN. We have obtained a detailed census of all observable baryons in this galaxy. The star formation efficiency is very low (∼ 0.02 − 0.06) and within a virial radius the galaxy has a total observed baryonic mass of 6.87 × 10 11 M , amounting to a baryonic deficiency of ∼ 80% relative to the cosmological mean. A small fraction of these cosmic baryons reside in a warm/hot circum-galactic halo phase, while the majority of baryons are still not observable. We have discussed various astrophysical scenarios for explaining its unusual physical properties. Our work is a major step forward in understanding the assembly history of such extremely massive, isolated galaxies.
arXiv: Astrophysics of Galaxies, Nov 23, 2021
Giant radio quasars (GRQs) are radio-loud active galactic nuclei (AGNs), propelling megaparsec-sc... more Giant radio quasars (GRQs) are radio-loud active galactic nuclei (AGNs), propelling megaparsec-scale jets. In order to understand GRQs and their properties, we have compiled all known GRQs ("the GRQ catalogue"), and a subset of small (size < 700 kpc) radio quasars (SRQs) from the literature. In this process, we have found 10 new FR-II GRQs, in the redshift range of 0.66 < z < 1.72, which we include in the GRQ catalogue. Using the above samples, we have carried out a systematic comparative study of GRQs and SRQs, using optical and radio data. Our results show that the GRQs and SRQs statistically have similar spectral index and black hole mass distributions. However, SRQs have higher radio core power, core dominance factor, total radio power, jet kinetic power and Eddington ratio compared to GRQs. On the other hand, when compared to giant radio galaxies (GRGs), GRQs have higher black hole mass and Eddington ratio. The high core dominance factor of SRQs is an indicator of them lying closer to the line of sight than GRQs. We also find a correlation of the accretion disc luminosity with the radio core and jet power of GRQs, which provides evidence for disc-jet coupling. Lastly, we find the distributions of Eddington ratios of GRGs and GRQs to be bi-modal, similar to that found in small radio galaxies (SRGs) and SRQs, which indicate that size is not strongly dependent on the accretion state. Using all of these, we provide a basic model for the growth of SRQs to GRQs.
We report the discovery of a very unusual, highly asymmetric radio galaxy whose radio jet, the la... more We report the discovery of a very unusual, highly asymmetric radio galaxy whose radio jet, the largest yet detected, emits strongly polarized synchrotron radiation and can be traced all the way from the galactic nucleus to the hot spot located ~440 kpc away. This jet emanates from an extremely massive black hole (>10^9 solar mass) and forms a strikingly compact radio lobe. No radio lobe is detected on the side of the counterjet, even though it is similar to the main jet in brightness up to a scale of tens of kiloparsecs. Thus, contrary to the nearly universal trend, the brightness asymmetry in this radio galaxy increases with distance from the nucleus. With several unusual properties, including a predominantly toroidal magnetic field, this Fanaroff-Riley type II megajet is an exceptionally useful laboratory for testing the role of magnetic field in jet stabilization and radio lobe formation.
We present a detailed analysis of a young merging galaxy cluster (z=0.43), from Chandra X-ray and... more We present a detailed analysis of a young merging galaxy cluster (z=0.43), from Chandra X-ray and Hubble Space Telescope archival data. X-ray observations confirm that the X-ray emitting intra-cluster medium (ICM) in this system is among the hottest (average T=12.1 ± 0.6 keV) and most luminous known. Comparison of X-ray and optical images confirm that this system hosts two merging subclusters SC1 and SC2, separated by a projected distance of about 650 kpc. The subcluster SC2 is newly identified in this work, while another subcluster (SC0), previously thought to be part of this merging system, is shown to be possibly a foreground object. Apart from two subclusters, we find a tail-like structure in the X-ray image, extending to a projected distance of ∼1 Mpc, along the north-east direction of the eastern subcluster (SC1). From a surface brightness analysis, we detect two sharp surface brightness edges at ∼40 (∼320 kpc) and ∼80 (∼640 kpc) to the east of SC1. The inner edge appears to b...
Radio galaxy phenomenon is directly connected to mass accreting, spinning supermassive black hole... more Radio galaxy phenomenon is directly connected to mass accreting, spinning supermassive black holes found in the active galactic nuclei (AGN). It is still unclear how the collimated jets of relativistic plasma on hundreds to thousands of kpc scale form, and why nearly always they are launched from the nuclei of bulge dominated elliptical galaxies and not flat spirals. Here we present the discovery of giant radio source J2345-0449 (z=0.0755), a clear and extremely rare counter example where relativistic jets are ejected from a luminous and massive spiral galaxy on scale of ~1.6 Mpc, the largest known so far. Extreme physical properties observed for this bulgeless spiral host, such as its high optical and infra-red luminosity, large dynamical mass, rapid disk rotation, and episodic jet activity are possibly the results of its unusual formation history, which has also assembled, via gas accretion from a disk, its central black hole of mass >2 x 10^8 M_sun. The very high mid-IR lumino...
We report the discovery of large-scale diffuse radio emission from what appears to be a large-sca... more We report the discovery of large-scale diffuse radio emission from what appears to be a large-scale filamentary network of galaxies in the region of cluster ZwCl 2341.1+0000, and stretching over an area of at least 6 h^-1_50 Mpc in diameter. Multicolour CCD observations yield photometric redshifts indicating that a significant fraction of the optical galaxies in this region is at a redshift of z=0.3. This is supported by spectroscopic measurements of 4 galaxies in the SDSS survey at a mean z=0.27. We present VLA images at 20 cm (NVSS) and 90 cm wavelengths, showing the detailed radio structure of the filaments. Comparison with the VLA high resolution FIRST radio survey shows that the diffuse emission is not due to known individual point sources. The diffuse radio-emission has a spectral index α≲ -0.5, and is most likely synchrotron emission from relativistic charged particles in an inter-galactic magnetic field. Furthermore, this optical/radio structure is detected in X-rays by the ...
The Astrophysical Journal, 2015
Monthly Notices of the Royal Astronomical Society, 2020
In this work, we investigate the dynamical state of the galaxy cluster Abell 2631, a massive stru... more In this work, we investigate the dynamical state of the galaxy cluster Abell 2631, a massive structure located at the core of the Saraswati supercluster. To do this, we first solve a tension found in the literature regarding the weak-lensing mass determination of the cluster. We do this through a comprehensive weak-lensing analysis, exploring the power of the combination of shear and magnification data sets. We find M200rmwl=8.7−2.9+2.5times1014M_{200}^{\rm wl} = 8.7_{-2.9}^{+2.5} \times 10^{14}M200rmwl=8.7−2.9+2.5times1014 M⊙. We also determined the mass based on the dynamics of spectroscopic members, corresponding to M200rmdy=12.2pm3.0times1014M_{200}^{\rm dy} = 12.2\pm 3.0 \times 10^{14}M200rmdy=12.2pm3.0times1014 M⊙, consistent within a 68 per cent CL with the weak-lensing estimate. The scenarios provided by the mass distribution and dynamics of galaxies are reconciled with those provided by X-ray observations in a scenario where A2631 is observed at a late stage of merging.
American Journal of Physics
We have designed a low-cost radio telescope system named the Bose Horn Antenna Radio Telescope (B... more We have designed a low-cost radio telescope system named the Bose Horn Antenna Radio Telescope (BHARAT) to detect the 21 cm hydrogen line emission from our Galaxy. The system is being used at the Radio Physics Laboratory (RPL) (Radio Physics Lab, IUCAA NCRA-TIFR, < http://www.iucaa.in/∼rpl/ >, < http://www.ncra.tifr.res.in/ncra/rpl/ >), Inter-University Centre for Astronomy and Astrophysics (IUCAA), India, for laboratory sessions and training students and teachers. It is also a part of the laboratory curriculum at several universities and colleges. Here, we present the design of a highly efficient, easy to build, and cost-effective dual-mode conical horn used as a radio telescope and describe the calibration procedure. We also present some model observation data acquired using the telescope for facilitating easy incorporation of this experiment in the laboratory curriculum of undergraduate or post-graduate programs. We have named the antenna after Acharya (teacher or an ...
Abstract. Damped Lyman-α systems (DLAs) seen in the spectra of high-z QSOs allow us to probe the ... more Abstract. Damped Lyman-α systems (DLAs) seen in the spectra of high-z QSOs allow us to probe the physical conditions in protogalaxies. Our understanding of physical conditions in DLAs at high-z is primarily based on the absorption lines of H2 molecules and fine-structure transitions. Another important way of probing the thermal state of interstellar medium in these systems is by studying the 21-cm absorption in the spectra of background quasars. Here we report the main results of our GMRT survey to search for 21-cm absorption in a representative and unbiased sample of 35 DLA candidates at 1.10≤z≤1.45. Our sample of DLA candidates is drawn from the strong Mg ii systems in SDSS DR5 and has resulted in discovery of 9 new 21-cm absorbers. Prior to our survey only one 21-cm absorber was known in the redshift range: 0.7≤z≤2. This survey has allowed us to investigate the dependence of detectability of 21-cm absorption on the properties of UV absorption lines detected in SDSS spectra and es...
Proceedings of the International Astronomical Union, 2014
We have initiated the search and detailed study of large scale structures present in the universe... more We have initiated the search and detailed study of large scale structures present in the universe using galaxy redshift surveys. In this process, we take the volume-limited sample of galaxies from Sloan Digital Sky Survey III and find very large structures even beyond the redshift of 0.2. One of the structures is even greater than 600 Mpc which raises a question on the homogeneity scale of the universe. The shapes of voids-structures (adjacent to each other) seem to be correlated, which supports the physical existence of the observed structures. The other observational supports include galaxy clusters' and QSO distribution's correlation with the density peaks of the volume limited sample of galaxies.
Astronomy & Astrophysics
We present for the first time a study of peculiar giant radio galaxy (GRG) J223301+131502 using d... more We present for the first time a study of peculiar giant radio galaxy (GRG) J223301+131502 using deep multi-frequency radio observations from GMRT (323, 612, and 1300 MHz) and LOFAR (144 MHz) along with optical spectroscopic observations with the WHT 4.2m optical telescope. Our observations have firmly established its redshift of 0.09956 and unveiled its exceptional jet structure extending more than ∼200 kpc leading to a peculiar kink structure of ∼100 kpc. We measure the overall size of this GRG to be ∼1.83 Mpc; it exhibits lobes without any prominent hotspots and closely resembles a barbell. Our deep low-frequency radio maps clearly reveal the steep-spectrum diffuse emission from the lobes of the GRG. The magnetic field strength of ∼5 μG and spectral ages between about 110 to 200 mega years for the radio lobes were estimated using radio data from LOFAR 144 MHz observations and GMRT 323 and 612 MHz observations. We discuss the possible causes leading to the formation of the observed...
Monthly Notices of the Royal Astronomical Society, 2015
We present a deep Chandra observation of the extremely massive spiral galaxy 2MASX J23453268-0449... more We present a deep Chandra observation of the extremely massive spiral galaxy 2MASX J23453268-0449256, the first X-ray observation of this very rare system which features the largest known relativistic jets from a spiral galaxy. We detect extended X-ray emission from the hot halo surrounding the galaxy, reaching out to 80 kpc in radius. The hot halo is elongated along the plane of the spiral galaxy, and one possibility is that the powerful relativistic jets have disrupted the hot halo gas located perpendicular to the disc. Our calculations indicate that it is energetically feasible that the AGN feedback in this system could have uplifted or completely expelled a significant fraction of the gas in the 20-80-kpc radial range. We also detect extended emission which appears to be associated with the inner and outer southern radio lobes, and is possibly the result of inverse Compton emission. Using the observed X-ray and radio luminosity of the central AGN, the Fundamental Plane of Gultekin et al. predicts a black hole mass of 5 × 10 8 M , with a range of 1 × 10 8-3 × 10 9 M when the scatter in the Fundamental Plane relation is taken into account. This is consistent with the possibility that an exceptionally massive (>10 9 M) black hole lies at the centre of this galaxy, as suggested by the M BH-σ scaling relation, but a tighter constraint cannot be made.
We report the discovery of large-scale diffuse non-thermal radio emission in PLCK G287.0+32.9, an... more We report the discovery of large-scale diffuse non-thermal radio emission in PLCK G287.0+32.9, an exceptionally hot (T ∼ 13 keV), massive, and luminous galaxy cluster, strongly detected by the Planck satellite in a recent, all-sky blind search for new clusters through Sunyaev-Zel'dovich effect. Giant Metrewave Radio Telescope 150 MHz and Very Large Array 1.4 GHz radio data reveal a pair of giant (>1 Mpc) "arc" shaped peripheral radio-relics (signatures of shock waves) of unprecedented scale (linear separation ∼4.4 Mpc at redshift 0.39), located at distances from the cluster center which are about 0.7 and 1.3 of the cluster's virial radius. Another possible giant relic and a radio-halo is detected closer to the cluster center. These relic sources are unique "signposts" of extremely energetic mergers and shocks (both ongoing and past), that are assembling and heating up this very massive galaxy cluster. They are also a probe of the filamentary cosmic-web structure beyond the cluster virial radius. Optical imaging with the IUCAA 2m telescope and XMM-Newton X-ray data confirm a very rich galaxy cluster with a morphologically disturbed core region, suggesting a dynamically perturbed merging system.
giant radio jet of very unusual polarization in a single-lobed
arXiv: Cosmology and Nongalactic Astrophysics, 2016
Massive galaxy clusters are the most violent large scale structures undergoing merger events in t... more Massive galaxy clusters are the most violent large scale structures undergoing merger events in the Universe. Based upon their morphological properties in X-rays, they are classified as un-relaxed and relaxed clusters and often host (a fraction of them) different types of non-thermal radio emitting components, viz., haloes, mini-haloes, relics and phoenix within their Intra Cluster Medium (ICM). The radio haloes show steep (alpha = -1.2) and ultra steep (alpha < -1.5) spectral properties at low radio frequencies, giving important insights on the merger (pre or post) state of the cluster. Ultra steep spectrum radio halo emissions are rare and expected to be the dominating population to be discovered via LOFAR and SKA in the future. Further, the distribution of matter (morphological information), alignment of hot X-ray emitting gas from the ICM with the total mass (dark + baryonic matter) and the bright cluster galaxy (BCG) is generally used to study the dynamical state of the clus...
VizieR Online Data Catalog, Oct 1, 2020
We present Hubble Space Telescope (HST) observations of the nearby (z= 0.023), very massive, high... more We present Hubble Space Telescope (HST) observations of the nearby (z= 0.023), very massive, highly rotating (488.4±12.5 km s −1) hybrid (S0-Sa) galaxy UGC 12591, along with observations taken at other wavelengths from UV to FIR. Deep HST data of the galaxy in V, I and H bands is used to disentangle its bulge and disk components. The surface brightness distributions show a dominance of the bulge component over the disk with H-band bulge-to-disc luminosity ratio ∼ 69%. From spectral energy distribution (SED) fitting, we find an extremely low global star formation rate (SFR) of ∼ 0.1 − 0.2M y −1 , which is unusually low for this galaxy's huge stellar mass (M ★ ∼ 1.6 × 10 11 M), that implies a strong quenching of it's SFR. For at least past 10 8 years the galaxy has remained in the quiescent state as a sterile, 'red and dead' galaxy. The fraction of total infra-red dust luminosity contributed by the ISM dust is as high as ∼ 90%, with total dust luminosity L dust = (0.5 − 3.5) × 10 10 L , dominated by cold dust at equilibrium temperature T C ≈ 15K. UGC 12591 is found host to a super massive black hole (SMBH) of mass 6.18 × 10 8 M which at the present moment is quiescent, i.e., neither we see large (> 1kpc) radio jets nor is the black hole contributing significantly to the mid-IR SED, ruling out the presence of strong radiative feedback from a bright AGN. We have obtained a detailed census of all observable baryons in this galaxy. The star formation efficiency is very low (∼ 0.02 − 0.06) and within a virial radius the galaxy has a total observed baryonic mass of 6.87 × 10 11 M , amounting to a baryonic deficiency of ∼ 80% relative to the cosmological mean. A small fraction of these cosmic baryons reside in a warm/hot circum-galactic halo phase, while the majority of baryons are still not observable. We have discussed various astrophysical scenarios for explaining its unusual physical properties. Our work is a major step forward in understanding the assembly history of such extremely massive, isolated galaxies.
arXiv: Astrophysics of Galaxies, Nov 23, 2021
Giant radio quasars (GRQs) are radio-loud active galactic nuclei (AGNs), propelling megaparsec-sc... more Giant radio quasars (GRQs) are radio-loud active galactic nuclei (AGNs), propelling megaparsec-scale jets. In order to understand GRQs and their properties, we have compiled all known GRQs ("the GRQ catalogue"), and a subset of small (size < 700 kpc) radio quasars (SRQs) from the literature. In this process, we have found 10 new FR-II GRQs, in the redshift range of 0.66 < z < 1.72, which we include in the GRQ catalogue. Using the above samples, we have carried out a systematic comparative study of GRQs and SRQs, using optical and radio data. Our results show that the GRQs and SRQs statistically have similar spectral index and black hole mass distributions. However, SRQs have higher radio core power, core dominance factor, total radio power, jet kinetic power and Eddington ratio compared to GRQs. On the other hand, when compared to giant radio galaxies (GRGs), GRQs have higher black hole mass and Eddington ratio. The high core dominance factor of SRQs is an indicator of them lying closer to the line of sight than GRQs. We also find a correlation of the accretion disc luminosity with the radio core and jet power of GRQs, which provides evidence for disc-jet coupling. Lastly, we find the distributions of Eddington ratios of GRGs and GRQs to be bi-modal, similar to that found in small radio galaxies (SRGs) and SRQs, which indicate that size is not strongly dependent on the accretion state. Using all of these, we provide a basic model for the growth of SRQs to GRQs.
We report the discovery of a very unusual, highly asymmetric radio galaxy whose radio jet, the la... more We report the discovery of a very unusual, highly asymmetric radio galaxy whose radio jet, the largest yet detected, emits strongly polarized synchrotron radiation and can be traced all the way from the galactic nucleus to the hot spot located ~440 kpc away. This jet emanates from an extremely massive black hole (>10^9 solar mass) and forms a strikingly compact radio lobe. No radio lobe is detected on the side of the counterjet, even though it is similar to the main jet in brightness up to a scale of tens of kiloparsecs. Thus, contrary to the nearly universal trend, the brightness asymmetry in this radio galaxy increases with distance from the nucleus. With several unusual properties, including a predominantly toroidal magnetic field, this Fanaroff-Riley type II megajet is an exceptionally useful laboratory for testing the role of magnetic field in jet stabilization and radio lobe formation.
We present a detailed analysis of a young merging galaxy cluster (z=0.43), from Chandra X-ray and... more We present a detailed analysis of a young merging galaxy cluster (z=0.43), from Chandra X-ray and Hubble Space Telescope archival data. X-ray observations confirm that the X-ray emitting intra-cluster medium (ICM) in this system is among the hottest (average T=12.1 ± 0.6 keV) and most luminous known. Comparison of X-ray and optical images confirm that this system hosts two merging subclusters SC1 and SC2, separated by a projected distance of about 650 kpc. The subcluster SC2 is newly identified in this work, while another subcluster (SC0), previously thought to be part of this merging system, is shown to be possibly a foreground object. Apart from two subclusters, we find a tail-like structure in the X-ray image, extending to a projected distance of ∼1 Mpc, along the north-east direction of the eastern subcluster (SC1). From a surface brightness analysis, we detect two sharp surface brightness edges at ∼40 (∼320 kpc) and ∼80 (∼640 kpc) to the east of SC1. The inner edge appears to b...
Radio galaxy phenomenon is directly connected to mass accreting, spinning supermassive black hole... more Radio galaxy phenomenon is directly connected to mass accreting, spinning supermassive black holes found in the active galactic nuclei (AGN). It is still unclear how the collimated jets of relativistic plasma on hundreds to thousands of kpc scale form, and why nearly always they are launched from the nuclei of bulge dominated elliptical galaxies and not flat spirals. Here we present the discovery of giant radio source J2345-0449 (z=0.0755), a clear and extremely rare counter example where relativistic jets are ejected from a luminous and massive spiral galaxy on scale of ~1.6 Mpc, the largest known so far. Extreme physical properties observed for this bulgeless spiral host, such as its high optical and infra-red luminosity, large dynamical mass, rapid disk rotation, and episodic jet activity are possibly the results of its unusual formation history, which has also assembled, via gas accretion from a disk, its central black hole of mass >2 x 10^8 M_sun. The very high mid-IR lumino...
We report the discovery of large-scale diffuse radio emission from what appears to be a large-sca... more We report the discovery of large-scale diffuse radio emission from what appears to be a large-scale filamentary network of galaxies in the region of cluster ZwCl 2341.1+0000, and stretching over an area of at least 6 h^-1_50 Mpc in diameter. Multicolour CCD observations yield photometric redshifts indicating that a significant fraction of the optical galaxies in this region is at a redshift of z=0.3. This is supported by spectroscopic measurements of 4 galaxies in the SDSS survey at a mean z=0.27. We present VLA images at 20 cm (NVSS) and 90 cm wavelengths, showing the detailed radio structure of the filaments. Comparison with the VLA high resolution FIRST radio survey shows that the diffuse emission is not due to known individual point sources. The diffuse radio-emission has a spectral index α≲ -0.5, and is most likely synchrotron emission from relativistic charged particles in an inter-galactic magnetic field. Furthermore, this optical/radio structure is detected in X-rays by the ...