Multi-frequency observations of the Radio Galaxy 4C 26.42 and Diffuse Extended Radio emission in the galaxy cluster A1795

Abstract

We have observed the galaxy cluster Abell 1795 at 610 M Hz and 235 M Hz using the Giant Metrewave Radio Telescope(GMRT). Abell 1795 (A1795) is a relaxed cluster at a redshift of 0.06 and hosts a bright galaxy, 4C 26.42 in the centre. The motivation behind this thesis work is two fold: to confirm the presence of a mini-halo that is claimed to be present in the core of the galaxy cluster A1795 and to analyse the spectrum of the Brightest Cluster Galaxy (BCG), 4C 26.42. It is interesting to study mini-halos (diffuse radio emission of extent ∼ 100 kpc) in galaxy clusters because the spectrum of mini-halo can give information on the properties of the intracluster medium and on the evolution of the galaxy cluster. Till date only 21 mini-halos have been detected. Hence, it becomes important to detect more mini-halos to understand their spectra, properties and origin. Similarly, the spectrum of a galaxy is studied to understand the energetics and the lifetime of the galaxy. BCGs are massive galaxies at the centre of galaxy clusters and are most likely to host Active Galactic Nuclei. It is therefore interesting to study the spectrum of BCGs. Mini-halos are diffuse radio sources in the centre of a relaxed galaxy cluster. This diffuse non-thermal radio emission is due to synchrotron radiation of the relativistic electrons in the intracluster medium. A possible detection of a mini-halo in this cluster at 1.4 GHz with a flux density of 85 ± 5 mJy has been claimed. To confirm it’s presence, we analysed GMRT observations of A1795 at 610 M Hz and 235 M Hz using Astronomical Image Processing System (AIPS). Radio spectrum of the form S ν ∝ ν −α where S ν is the flux density at frequency ν and α is the spectral index is considered. Given the flux density of the mini-halo at 1.4 GHz, a flux density of atleast 160 mJy at 610 M Hz is expected by considering α = 1. However, from a 2 by 2 region (∼ 100 kpc in linear scale, the typical size of a mini-halo) around the central galaxy in the image containing only diffuse emission, we detected a flux density of ∼ 18 mJy at 610 M Hz which is a factor of ∼ 10 less than expected. We therefore give an upper limit of 18 mJy for the detection of the mini-halo in this cluster.We analysed the Brightest Cluster Galaxy (BCG) 4C 26.42 in the galaxy cluster A1795 at 610 M Hz and 235 M Hz. Using the flux density values of this galaxy from literature,we obtained the spectrum from 74 M Hz to 43.34 GHz at 8 frequencies. The spectrum follows the least square fit: logS ν = −0.14(logν) 2 − 0.16(logν) + 4.8. In order to estimate the break frequency, we fit two curves of the form S ν ∝ ν −α . The spectral indices 1.4GHz 43.34GHz from the least square fits are: α 74M Hz ≡ α 1 = 0.83 ±0.05 and α 4.86GHz ≡ α 2 = 1.37 ±0.14. The break frequency is ∼ 2GHz. α 2 ∼ α 1 + 0.5 which is as expected from the spectral aging models before and after the break frequency. Assuming equipartition of energy between the particle energy density and the magnetic field energy, we estimated the magnetic field strength and the energy of the relativistic electrons to be ∼ 36 μG and ∼ 8 × 10 56 erg respectively. Using the break frequency and the magnetic field, the age of relativistic electrons in the galaxy is estimated to be ∼ 3.4 M yr. The spectral indices of the various components of this galaxy have previously been studied at the parsec scale. In this thesis, we discuss the spectrum of the galaxy as a whole (at the kilo parsec scale).