The Origin and Evolution of Giant Radio Galaxies

Giant radio galaxies are arguably the least understood of jetted active galactic nuclei (AGN). We propose that radio galaxies are the product of large mergers that do not involve radio galaxies or radio quasars, such as in merging spiral galaxies, while giant radio galaxies emerge from a merger involving a parent that in the not-too-distant past harbored a radio galaxy. Predictions following from this are an upper limit to the number fraction of giant radio galaxies to radio galaxies, lower average redshift for giant radio galaxies, a higher incidence of high excitation for giant radio galaxies compared with radio galaxies, and lower average prograde black hole spin values for giant radio galaxies compared to radio galaxies and to bright radio quiet quasars.

Is TOL 1326--379 a Prototype of $\gamma$-ray Emitting FR0 Radio Galaxy?

With the possible spacial association to the Fermi/LAT source 3FGL J1330.0--3818, TOL 1326--379 may be the first one that is identified as a $\gamma$-ray emitting Fanaroff--Riley type 0 radio galaxy (FR0 RG). We analyze the $\sim$12 yr Fermi/LAT observation data of this $\gamma$-ray source and examine its association to TOL 1326--379. We show that the $\gamma$-ray source (named as J1331.0--3818) is tentatively detected with a TS value of 28.7, 3FGL J1330.0--3818 is out of the 95\% containment of J1331.0--3818, and their positions are spatially separated with 0.2$\degr$. 4FGL J1331.3--3818 falls into the 68\% containment of J1331.0--3818, suggesting that our result agrees with that reported in the Fourth Fermi LAT Source Catalog. TOL 1326--379 is out of the 95\% containment of J1331.0--3818, and their positions are spatially separated with 0.4$\degr$, indicating that the association between J1331.0--3818 and TOL 1326--379 is quite ambiguous. However, we do not find other possible potential radio and X-ray counterparts within the circle centered at J1331.0--3818 with a radius of 0.4$\degr$. The spectral energy distribution (SED) of TOL 1326--379 shows a bimodal feature as seen in the $\gamma$-ray emitting RGs. We fit the SED with the one-zone leptonic model and find that the average energy spectrum of J1331.0--3818 agrees with the model prediction. Assuming that J1331.0--3818 is an unidentified $\gamma$-ray source, we derive the upper-limit of the $\gamma$-ray flux for TOL 1326--379. It is not tight enough to exclude this possibility. Based on these analysis results, we cautiously argue that the $\gamma$-ray source J1331.0--3818 is associated with TOL 1326--379 and its jet radiation physic is similar to those $\gamma$-ray emitting RGs.

Remnant Radio Galaxy Candidates of Small Angular Sizes

Remnant radio galaxies (RRGs), characterized by the cessation of AGN activity, represent a short-lived last phase of radio galaxy’s life-cycle. Hitherto, searches for RRGs, mainly based on the morphological criteria, have identified large angular size sources resulting into a bias towards the remnants of powerful FR-II radio galaxies. In this study we make the first attempt to perform a systematic search for RRGs of small angular sizes (<30′′) in the XMM−LSS field. By using spectral curvature criterion we discover 48 remnant candidates exhibiting strong spectral curvature i.e., α150MHz325MHz−α325MHz1.4GHz≥ 0.5. Spectral characteristics at higher frequency regime (>1.4 GHz) indicate that some of our remnant candidates can depict recurrent AGN activity with an active core. We place an upper limit on the remnant fraction (frem) to be 3.9%, which increases to 5.4% if flux cutoff limit of S150MHz≥ 10 mJy is considered. Our study unveils, hitherto unexplored, a new population of small-size (<200 kpc) remnant candidates that are often found to reside in less dense environments and at higher redshifts (z) > 1.0. We speculate that a relatively shorter active phase and/or low jet power can be plausible reasons for the small size of remnant candidates.

Weathering the Largest Storms in the Universe: Understanding environmental effects on extended radio emission in clusters

<p>This thesis presents an investigation of the habitat of extended radio sources, and the way in which the generation and properties of these radio sources are affected by environmental factors. We begin with a detailed structure analysis of the 0.3 deg² area of the MUSYC-ACES field, generated by applying a density-based clustering method, known as DBSCAN, to our spectroscopic and photometric samples of the field. As a result, we identify 62 over-dense regions across the field. Based on the properties of the detected structures, we classify 13 as clusters, of which 90% are associated with diffuse soft-band X-ray emission. This provides a strong and independent confirmation that both the clustering and classification methodologies are reliable for use in investigation of the environment of the radio sources in the Chandra Deep Field South (CDFS). Using an interpolation-based method followed by a new calibration technique of using clusters of similar mass as standard candles, we are able to estimate the local environmental richness for a desired region. This methodology is applied to a sample of AGNs and star forming galaxies in the CDFS to probe whether or not the radio luminosity of the different radio sources is correlated to their environments. As a result, we do not find a significant correlation between the radio luminosity and the environment of star-forming galaxies and radio-quiet AGNs, however, a weak positive dependency is spotted for radio-loud AGNs. This may indicate that over-populated environments trigger or enhance the radio activity processes in the AGNs. We find that star-forming galaxies, unlike radio-loud AGNs, tend to avoid overpopulated environments especially at low redshifts. However, radio-loud AGN are found in both poor and rich environments. As a result, we find neither of these radio sources suitable for tracing the over-dense regions of the Universe, unlike tailed radio galaxies. It is believed that tailed radio galaxies reside in the dense environments of clusters and groups, and therefore, may be the signatures of overdensities in large-scale structure. To evaluate the idea of using tailed radio galaxies as tracers of dense environments, a systematic study of these sources as a function of density is required. For this reason and by using the 1.4 GHz Australia Telescope Large Area Survey (ATLAS) data, we examined over four deg² area of the ATLAS-CDFS field, which includes the entire CDFS. We present a catalogue of 56 non-linear, extended, and low surface brightness sources including 45 tailed radio galaxies, two relic candidates, and a possible radio halo. We report the detection of the most distant tailed radio galaxy to date, at a redshift of 2.1688. In addition, despite the lack of deep spectroscopic data in the ATLAS field, we find two of the detected tailed radio galaxies are associated with clusters. We find three Head-Tail galaxy candidates in the CDFS field, all of which are located at high redshifts, where the magnitude constraint of our redshift sample prevents any structure detection. One of the primary objectives of this research is to investigate the association between the morphology of tailed radio galaxies and the physical characteristics of the surrounding environment. In order to understand the role of the variety of factors that influence the radio morphology, we constructed a simple model that generates the overall radio structure of the sources in different habitats. We report the results of the simulation of the wide-angle tail radio galaxy PKS J0334-3900, which shows that both the gravitation interactions and a cluster wind are required to generate the observed radio tails. As a result, we find the morphology of the tailed radio galaxies as an invaluable tool to probe environmental characteristics. In a supplementary study, we investigate the role of cluster dynamics on generation and alternation of extended radio sources. We present a comprehensive structure and sub-structure analysis of the Abell 3266 galaxy cluster. Based on the results of the sub-structure test, position and orientation of a radio relic candidate, and morphology of a prominent tailed radio galaxy in the cluster, we propose an ongoing merger scenario for this chaotic cluster environment. Furthermore, we verify our theory by an N-body simulation of a pre-merger cluster and an in-falling group. The results of the simulation supports our merger scenario by explaining both the orientation of the radio relic and the observed morphology of the tailed radio galaxy. While there is a weak correlation between the luminosity of radio-loud AGNs and environmental density, tailed radio galaxies make superior probes of over-dense regions. Thus, overall we find tailed radio galaxies can be used to trace overdensities out to z ~ 2 and probe the details of the environments in which they are found.</p>

Weathering the Largest Storms in the Universe: Understanding environmental effects on extended radio emission in clusters

<p>This thesis presents an investigation of the habitat of extended radio sources, and the way in which the generation and properties of these radio sources are affected by environmental factors. We begin with a detailed structure analysis of the 0.3 deg² area of the MUSYC-ACES field, generated by applying a density-based clustering method, known as DBSCAN, to our spectroscopic and photometric samples of the field. As a result, we identify 62 over-dense regions across the field. Based on the properties of the detected structures, we classify 13 as clusters, of which 90% are associated with diffuse soft-band X-ray emission. This provides a strong and independent confirmation that both the clustering and classification methodologies are reliable for use in investigation of the environment of the radio sources in the Chandra Deep Field South (CDFS). Using an interpolation-based method followed by a new calibration technique of using clusters of similar mass as standard candles, we are able to estimate the local environmental richness for a desired region. This methodology is applied to a sample of AGNs and star forming galaxies in the CDFS to probe whether or not the radio luminosity of the different radio sources is correlated to their environments. As a result, we do not find a significant correlation between the radio luminosity and the environment of star-forming galaxies and radio-quiet AGNs, however, a weak positive dependency is spotted for radio-loud AGNs. This may indicate that over-populated environments trigger or enhance the radio activity processes in the AGNs. We find that star-forming galaxies, unlike radio-loud AGNs, tend to avoid overpopulated environments especially at low redshifts. However, radio-loud AGN are found in both poor and rich environments. As a result, we find neither of these radio sources suitable for tracing the over-dense regions of the Universe, unlike tailed radio galaxies. It is believed that tailed radio galaxies reside in the dense environments of clusters and groups, and therefore, may be the signatures of overdensities in large-scale structure. To evaluate the idea of using tailed radio galaxies as tracers of dense environments, a systematic study of these sources as a function of density is required. For this reason and by using the 1.4 GHz Australia Telescope Large Area Survey (ATLAS) data, we examined over four deg² area of the ATLAS-CDFS field, which includes the entire CDFS. We present a catalogue of 56 non-linear, extended, and low surface brightness sources including 45 tailed radio galaxies, two relic candidates, and a possible radio halo. We report the detection of the most distant tailed radio galaxy to date, at a redshift of 2.1688. In addition, despite the lack of deep spectroscopic data in the ATLAS field, we find two of the detected tailed radio galaxies are associated with clusters. We find three Head-Tail galaxy candidates in the CDFS field, all of which are located at high redshifts, where the magnitude constraint of our redshift sample prevents any structure detection. One of the primary objectives of this research is to investigate the association between the morphology of tailed radio galaxies and the physical characteristics of the surrounding environment. In order to understand the role of the variety of factors that influence the radio morphology, we constructed a simple model that generates the overall radio structure of the sources in different habitats. We report the results of the simulation of the wide-angle tail radio galaxy PKS J0334-3900, which shows that both the gravitation interactions and a cluster wind are required to generate the observed radio tails. As a result, we find the morphology of the tailed radio galaxies as an invaluable tool to probe environmental characteristics. In a supplementary study, we investigate the role of cluster dynamics on generation and alternation of extended radio sources. We present a comprehensive structure and sub-structure analysis of the Abell 3266 galaxy cluster. Based on the results of the sub-structure test, position and orientation of a radio relic candidate, and morphology of a prominent tailed radio galaxy in the cluster, we propose an ongoing merger scenario for this chaotic cluster environment. Furthermore, we verify our theory by an N-body simulation of a pre-merger cluster and an in-falling group. The results of the simulation supports our merger scenario by explaining both the orientation of the radio relic and the observed morphology of the tailed radio galaxy. While there is a weak correlation between the luminosity of radio-loud AGNs and environmental density, tailed radio galaxies make superior probes of over-dense regions. Thus, overall we find tailed radio galaxies can be used to trace overdensities out to z ~ 2 and probe the details of the environments in which they are found.</p>

A Radio View of the Bullet Cluster from 100 MHz to 9 GHz

<p>We investigate a sample of 10 massive galaxy clusters for diffuse synchrotron emission. The shortlisted clusters are drawn from a sample of clusters observed with the South Pole Telescope (SPT) shown to have high Sunyaev-Zeldovich (SZ) signals.They are analysed for diffuse emission from the results of the Australia Telescope Compact Array (ATCA) archival data reduction. The focus then is on the cluster with the most prominent diffuse emission - the Bullet cluster. We used the Murchison Widefield Array Commissioning Survey (MWACS) data in conjunction with the ATCA images to derive the spectral behaviour of the Bullet cluster from 0.118 GHz to 8.896 GHz. In particular, we study the spectral properties of the known radio halo and radio relic. We search for spectral bending of this diffuse emission as seen in other clusters like the Coma cluster, A2256, A521 and A3256. We detect the radio relic at all frequencies in the cluster periphery. Polarised flux is detected for the relic at all frequencies except at 1.344 GHz and as expected the percentage polarisation increases with frequency. Our spectral index values of -1.08 ± 0.02 and -1.74 ± 0.22 for 2 regions of the radio relic agreed with the literature. We detect spectral flattening for a region in the radio relic at 4.532 GHz. This is a common spectral characteristic for a radio galaxy. This suggests that the source could be a recently dead radio galaxy. We discuss a scenario in which a dead radio galaxy supplying seed electrons for reacceleration and a merger process providing the required energy for the diffuse radio relic. We detect the radio halo at all frequencies and we derive a spectral index of -2.11±0.03 using our ATCA flux measurements. Our individual flux measurements at 1.344 and 2.1 GHz agree with the literature. However, we get a steeper ATCA spectral index value for the radio halo as compared to the existing value in the literature. We observe spectral flattening of the radio halo in the Bullet cluster at low frequencies between 0.180 GHz and 1.3 GHz. This is similar to the spectral property of the halo in clusters like the Coma cluster, A521 and A3256.</p>

A Radio View of the Bullet Cluster from 100 MHz to 9 GHz

<p>We investigate a sample of 10 massive galaxy clusters for diffuse synchrotron emission. The shortlisted clusters are drawn from a sample of clusters observed with the South Pole Telescope (SPT) shown to have high Sunyaev-Zeldovich (SZ) signals.They are analysed for diffuse emission from the results of the Australia Telescope Compact Array (ATCA) archival data reduction. The focus then is on the cluster with the most prominent diffuse emission - the Bullet cluster. We used the Murchison Widefield Array Commissioning Survey (MWACS) data in conjunction with the ATCA images to derive the spectral behaviour of the Bullet cluster from 0.118 GHz to 8.896 GHz. In particular, we study the spectral properties of the known radio halo and radio relic. We search for spectral bending of this diffuse emission as seen in other clusters like the Coma cluster, A2256, A521 and A3256. We detect the radio relic at all frequencies in the cluster periphery. Polarised flux is detected for the relic at all frequencies except at 1.344 GHz and as expected the percentage polarisation increases with frequency. Our spectral index values of -1.08 ± 0.02 and -1.74 ± 0.22 for 2 regions of the radio relic agreed with the literature. We detect spectral flattening for a region in the radio relic at 4.532 GHz. This is a common spectral characteristic for a radio galaxy. This suggests that the source could be a recently dead radio galaxy. We discuss a scenario in which a dead radio galaxy supplying seed electrons for reacceleration and a merger process providing the required energy for the diffuse radio relic. We detect the radio halo at all frequencies and we derive a spectral index of -2.11±0.03 using our ATCA flux measurements. Our individual flux measurements at 1.344 and 2.1 GHz agree with the literature. However, we get a steeper ATCA spectral index value for the radio halo as compared to the existing value in the literature. We observe spectral flattening of the radio halo in the Bullet cluster at low frequencies between 0.180 GHz and 1.3 GHz. This is similar to the spectral property of the halo in clusters like the Coma cluster, A521 and A3256.</p>

Radio and X-ray Observations of the Restarted Radio Galaxy in the Galaxy Cluster CL 0838+1948

We present VLA Low-band Ionosphere and Transient Experiment (VLITE) 338 MHz observations of the galaxy cluster CL 0838+1948. We combine the VLITE data with Giant Metrewave Radio Telescope 610 MHz observations and survey data. The central galaxy hosts a 250 kpc source whose emission is dominated by two large lobes at low frequencies. At higher frequencies, a pair of smaller lobes (∼30 kpc) is detected within the galaxy optical envelope. The observed morphology is consistent with a restarted radio galaxy. The outer lobes have a spectral index αout=1.6, indicating that they are old, whereas the inner lobes have αinn=0.6, typical for an active source. Spectral modeling confirms that the outer emission is a dying source whose nuclear activity switched off not more than 110 Myr ago. Using archival Chandra X-ray data, we compare the radio and hot gas emission. We find that the active radio source is contained within the innermost and X-ray brightest region, possibly a galactic corona. Alternatively, it could be the remnant of a larger cool core whose outer layers have been heated by the former epoch of activity that has generated the outer lobes.