SAGAN
Search & Analysis of GRGs with Associated Nuclei
Giant radio galaxies (GRGs) are the single largest objects known in the Universe (see the review article by Dabhade et al. 2023 titled 'Decoding the extragalactic giant radio sources'). This class of radio galaxies has sizes greater than 0.7 Mpc! One megaparsec (Mpc) is equivalent to the size of ~33 Milkyway galaxies put together in a line.
To date, we have not been able to completely understand the growth and evolution of giant radio galaxies. In comparison to radio galaxies' population, GRGs are rare.
One of the main aims of project SAGAN is to make a uniform sample (complete database) of all known GRGs. The data available on these GRGs over a period of ~5 decades is highly non-uniform. Under the project SAGAN, efforts have been taken to make a complete compilation of known GRGs along with finding new ones from the existing radio and optical archives. The pilot results of this project were presented in Dabhade et al. 2017, followed by papers - SAGAN-I, SAGAN-II, and SAGAN-III.
The updated GRS catalogue of 657 sources can be found on Vizier [Link].
Objectives of the Project SAGAN : -
Create a complete and uniform database of GRGs from the literature spanning five decades using a single cosmological model with Ho = 67.8 km/s/Mpc, Omega _m = 0.308, and Omega_Lambda = 0.692 (flat LambdaCDM). See SAGAN-I.
Search for more GRGs from existing radio and optical/infrared survey data.
Using the newly created large database of GRGs, carry out multi-wavelength studies of the host AGNs of the GRGs. We intend to focus on some key physical properties such as the accretion rate of the black hole, excitation type, black hole mass, Eddington ratio, spin, host galaxy star formation rate (SFR), and high energy gamma-ray emission from jets. See SAGAN-I, SAGAN-II, and SAGAN-III.
Exploring effects of the environment on the morphology and growth of the GRGs.
Using Magneto-hydrodynamical (MHD) simulations to investigate the jet physics and the necessary conditions required for the collimation and stability of relativistic jets, propagating to megaparsec or larger physical distances from the host AGN.
# SAGAN also refers to the famous Scientist Carl Sagan who has inspired numerous people through his outstanding science outreach programs.
Articles written on our published work -
MNRAS OUP blog - " Large sample of giant radio galaxies discovered "
Nature highlights on LoTSS GRG paper- "LoTSS of giants "
Nature Astronomy Community on SAGAN- "Giant radio galaxies- the cosmic behemoths "
Research Papers
"Search and Analysis of Giant radio galaxies with Associated Nuclei (SAGAN) - I: New sample and multi-wavelength studies"- Dabhade et al, 2020b - A&A Journal - Arxiv link
Plot shows how different surveys were used over the years for finding GRGs and only in the past two decades the maximum number of GRGs were discovered and reported. In recent times, the LoFAR surveys and our SAGAN project using multiple surveys like NVSS +TGSS+FIRST+VLASS have found ~50% of the known population.
Since 1974 to 2020 (April), 820 GRGs have been reported in literature (including our SAGAN GRG Sample). The above pie chart shows the percentage share from LoTSS, SAGAN and the rest work from literature.
The plot shows the sky distribution of all the known GRGs from the year 1974 to 2020 along with our SAGAN GRG sample in Aitoff projection. The total number of GRGs plotted here is 820 (LoTSS:225 + SAGAN:162 + all others from literature: 433). The large clustering of GRGs seen in the northern region of the plot (right ascension 10h45m to 15h30m and declination 45 deg to 57 deg is the result of the finding of large sample of GRGs (225) from the LoTSS by us (Dabhade et al 2020a- A&A-Published). The colour of the points on the plot corresponds to their redshift indicated in the vertical colour bar on the right side of the plot. Updated catalogue of giants (up to March 2020) can be found here-lin
SAGAN-II : Molecular gas content of Giant Radio Galaxies*
*Based on observations carried out with IRAM-30m
Dabhade et al. 2020c. A&A journal | arXiv
Hosts of GRGs on the Kennicutt-Schmidt (KS) relation.
SAGAN-III : Properties of giant radio quasars
Mahato, Dabhade et al. 2022. Accepted for publication in A&A journal (22/11/21) | arXiv
For a review see - Dabhade, P., Saikia, D.J. & Mahato, M. Decoding the giant extragalactic radio sources. J Astrophys Astron 44, 13 (2023). https://doi.org/10.1007/s12036-022-09898-5
List of key GRG papers from literature can be found here.
Interesting articles on GRSs