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Register a new userTracing Ghost Cavities with Low Frequency Radio Observations
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Tracy Clarke
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We present X-ray and multi-frequency radio observations of the central radio sources in several X-ray cavity systems. We show that targeted radio observations are key to determining if the lobes are being actively fed by the central AGN. Low frequency observations provide a unique way to study both the lifecycle of the central radio source as well as its energy input into the ICM over several outburst episodes.
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We present radio observations of ultraluminous infrared galaxies (ULIRGs) using the Giant Metrewave Radio Telescope (GMRT) and combine them with archival multi-frequency observations to understand whether ULIRGs are the progenitors of the powerful radio loud galaxies in the local Universe. ULIRGs are characterized by large infrared luminosities ($L_{IR}>$10$^{12}$L$odot$), large dust masses ($sim10^{8}M_{odot}$) and vigorous star formation (star formation rates $sim$10-100 $M_{odot}~$yr$^{-1}$). Studies show that they represent the end stages of mergers of gas-rich spiral galaxies. Their luminosity can be due to both starburst activity and active galactic nuclei (AGN). We study a sample of 13 ULIRGs that have optically identified AGN characteristics with 1.28~GHz GMRT observations. Our aim is to resolve any core-jet structures or nuclear extensions and hence examine whether the ULIRGs are evolving into radio loud ellipticals. Our deep, low frequency observations show marginal extension for only one source. However, the integrated radio spectra of 9 ULIRGs show characteristics that are similar to that of GPS/CSS/CSO/young radio sources. The estimated spectral ages are 0.4 to 20 Myr and indicate that they are young radio sources and possible progenitors of radio galaxies. Hence, we conclude that although most ULIRGs do not show kpc scale extended radio emission associated with nuclear activity, their radio spectral energy distributions do show signatures of young radio galaxies.
We propose an experimental setup to search for Axion-like particles (ALPs) using two superconducting radio-frequency cavities. In this light-shining-through-wall setup the axion is sourced by two modes with large fields and nonzero $vec Ecdot vec B$ in an emitter cavity. In a nearby identical cavity only one of these modes, the spectator, is populated while the other is a quiet signal mode. Axions can up-convert off the spectator mode into signal photons. We discuss the physics reach of this setup finding potential to explore new ALP parameter space. Enhanced sensitivity can be achieved if high-level modes can be used, thanks to improved phase matching between the excited modes and the generated axion field. We also discuss the potential leakage noise effects and their mitigation, which is aided by O(GHz) separation between the spectator and signal frequencies.
A previous analysis of the Chandra X-ray image of the center of the cooling core cluster Abell 2597 showed two ``ghost holes in the X-ray emission to the west and northeast of the central radio galaxy PKS 2322-123. Previous radio observations did not detect any radio emission coming from the interior of the X-ray holes. We present new low frequency radio observations of Abell 2597. At 330 MHz, radio emission extends into the interior of the western ghost bubble, but not the northeast one. Our re-analysis of the archival Chandra data shows evidence for an X-ray tunnel (elongated region of reduced X-ray emission) extending from near the center of the cD out to the west ghost bubble. We also detect a smaller X-ray hole to the northeast of the center of the cD and closer than the outer ghost bubbles. Radio observations at 1.3 GHz show extensions to the west along the X-ray tunnel toward the west ghost bubble, to the northeast into the new X-ray hole, and to the northwest. All of these structures are much larger than the two inner radio lobes seen previously at 8 GHz. The X-ray tunnel suggests that the west ghost bubble is part of a continuous flow of radio plasma out from the active galactic nucleus, rather than a detached buoyant old radio lobe, and thus it may be an intermediate case between an active radio galaxy and a buoyant lobe.
We have observed seven nearby large angular sized galaxies at 0.33 GHz using GMRT with angular resolution of $sim10$ and sub-mJy sensitivity. Using archival higher frequency data at 1.4 or $sim$6 GHz, we have then determined their spatially resolved non-thermal spectrum. As a general trend, we find that the spectral indices are comparatively flat at the galaxy centres and gradually steepen with increasing galactocentric distances. Using archival far infrared (FIR) MIPS 70 ${mu} m$ data, we estimate the exponent of radio-FIR correlation. One of the galaxy (NGC 4826) was found to have an exponent of the correlation of $sim1.4$. Average exponent from 0.33 GHz data for the rest of the galaxies was 0.63$pm$0.06 and is significantly flatter than the exponent 0.78$pm$0.04 obtained using 1.4 GHz data. This indicates cosmic ray electron (CRe) propagation to have reduced the correlation between FIR and 0.33 GHz radio. Assuming a model of simple isotropic diffusion of CRe, we find that the scenario can explain the frequency dependent cosmic ray electron propagation length scales for only two galaxies. Invoking streaming instability could, however, explain the results for the majority of the remaining ones.
Diffuse radio emission from galaxy clusters in the form of radio halos and relics are tracers of the shocks and turbulence in the intra-cluster medium. The imprints of the physical processes that govern their origin and evolution can be found in their radio morphologies and spectra. The role of mildly relativistic population of electrons may be crucial for the acceleration mechanisms to work efficiently. Low frequency observations with telescopes that allow imaging of extended sources over a broad range of low frequencies ($< 2$ GHz) offer the best tools to study these sources. I will review the Giant Metrewave Radio Telescope (GMRT) observations in the past few years that have led to: i) statistical studies of large samples of galaxy clusters, ii) opening of the discovery space in low mass clusters and iii) tracing the spectra of seed relativistic electrons using the Upgraded GMRT.
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