Giant radio galaxies in the LOFAR deep fields: old and lonely?

Abstract

The study of radio galaxies (RGs) offers valuable insights into the complex dynamics between active galactic nuclei (AGN), host galaxies, and the surrounding medium. Among the diverse population of RGs, giant radio galaxies (GRGs) have a linear extent larger than 700 kpc and they are relatively rare compared to smaller RGs. Furthermore, the contributing factors to their exceptional size are still not very well known. In this doctoral thesis, I focus on unraveling the intricate interplay between different aspects, such as the properties of the host galaxy, the surrounding environment, the age of the radio source, and their collective role in shaping the size of GRGs. As a primary observational tool, I used the LOw-Frequency Array (LOFAR), which combines high sensitivity and angular resolution with a wide field of view and unique low-frequency capabilities. Throughout my doctoral studies, I constructed a complete sample of GRGs through a meticulous visual inspection of the LOFAR deep fields (Boo ̈tes, ELAIS-N1, and Lockman Hole), complemented by deep optical and infrared surveys for accurate host galaxy identification. The final sample counts about 1600 RGs of which 280 are GRGs. The sample containing smaller RGs was used as a control sample to compare the radio properties, the properties of the host galaxy and the environment between GRGs and smaller RGs. Host galaxy properties were found to be similar between GRGs and RGs, both hosting a central black hole undergoing radiatively inefficient accretion. Additionally, stellar masses of GRG and RG host galaxies exhibited similarities. Intriguingly, 41% GRG host galaxies displayed a moderate/high star formation rate, > 10 M /yr, in contrast to 20% of RG hosts exceeding this threshold. Integrated flux densities and radio luminosities were also determined for a subset of RGs and GRGs through available survey images at 50, 150, 610, and 1400 MHz to compute integrated spectral indices. I found that larger sources exhibit steeper integrated spectral indices, suggesting that GRGs are the late-time versions of RGs. Furthermore, the environmental analysis I implemented utilised a catalogue of photometric and spectroscopic redshifts of galaxies from the Legacy survey. I employed the number of neighbouring galaxies within a 10 Mpc radius from the host galaxy as a proxy for environmental density around RGs. This analysis exposed distinct density differences, with GRGs statistically inhabiting sparser environments compared to their smaller counterparts. Particularly noteworthy is the fact that only 3% of GRGs are located within a 3D comoving distance of 5 Mpc from a galaxy cluster. The compilation of a list of 447 RGs enabled an analysis of the alignment of the RG jets in the ELAIS-N1 field, which was reported by previous studies. Of the RGs used for this analysis, 95% possess either photometric or spectroscopic redshifts, facilitating a three-dimensional (3D) analysis. The spatial distribution of major axis position angles of RGs in the ELAIS-N1 field is presented, accompanied by the results of multiple statistical tests aimed at discerning the presence of any systematic alignment. The analysis reveals that the distribution of position angles is consistent with being uniform. Two apparent peaks around position angles of 50 and 140 degrees are deemed spurious, as evidenced by a rigorous 3D analysis, dispelling any indication of intrinsic alignment. In conclusion, my results do not support a 2D or 3D alignment of RGs on scales of smaller than 4 deg. In summary, in this study, I present a multi-wavelength analysis of the RGs in the LOFAR deep fields and study how the environment and the radio and optical properties of RGs affect their evolution. In the near future, a similar study extended to the full LOFAR Two-metre Sky Survey holds the potential to provide the necessary confirmation of these results.