[Paper Review] Giant Radio Galaxies as a probe of the cosmological evolution of the IGM, I. Preliminary deep detections and low-resolution spectroscopy with the SALT

J. Machalski, D. Kozieł‐Wierzbowska+1 more|ArXiv.org|Oct 24, 2007

Galaxies: Formation, Evolution, Phenomena3 citations

TL;DR

This study uses deep imaging and low-resolution spectroscopy with the Southern African Large Telescope (SALT) to search for high-redshift giant radio galaxies (GRGs) with lobe energy densities below 10⁻¹⁴ J m⁻³, aiming to probe the cosmological evolution of the intergalactic medium (IGM) pressure. Despite detecting 21 GRG candidates with projected sizes >1 Mpc, only two have redshifts above 0.4 and energy densities below the threshold, though one candidate (J1420−0545) may exceed 4.8 Mpc in size, suggesting it could be the largest known GRG.

ABSTRACT

A problem of the cosmological evolution of the IGM is recalled and a necessity to find distant (z>0.5) giant radio galaxies (GRGs) with the lobe energy densities lower than about 10^{-14} J m^{-3} to solve this problem is emphasized. Therefore we undertake a search for such GRGs on the southern sky hemisphere using the SALT. In this paper we present a selected sample of the GRG candidates and the first deep detections of distant host galaxies, as well as the low-resolution spectra of the galaxies identified on the DSS frames. The data collected during the Performance Verification (P-V) phase show that 21 of 35 galaxies with the spectroscopic redshift have the projected linear size greater than 1 Mpc (for H_{0}=71 km\s\Mpc). However their redshifts do not exceed the value of 0.4 and the energy density in only two of them is less than 10^{-14} J m^{-3}. A photometric redshift estimate of one of them (J1420-0545) suggests a linear extent larger than 4.8 Mpc, i.e. a larger than that of 3C236, the largest GRG known up to now.

Motivation & Objective

• To identify distant giant radio galaxies (GRGs) at z > 0.5 with lobe energy densities < 10⁻¹⁴ J m⁻³ to test the predicted (1+z)⁵ evolution of intergalactic medium (IGM) pressure.
• To overcome limitations of flux-limited and surface-brightness-selected samples by targeting low-luminosity, high-redshift GRGs with extended, diffuse radio structures.
• To use deep SALT imaging and low-resolution spectroscopy to detect faint host galaxies and measure redshifts for GRG candidates in the southern sky.
• To analyze the morphology and asymmetry of GRG lobes to infer environmental density gradients and IGM inhomogeneities on Mpc scales.
• To assess whether equipartition-based internal lobe pressure estimates reliably reflect true IGM pressure, especially in low-luminosity, high-redshift sources.

Proposed method

• Conducted deep imaging and low-resolution spectroscopy using the SALT telescope during its Performance Verification phase to detect faint, high-redshift GRG host galaxies.
• Identified GRG candidates via radio morphology in deep optical and radio data, focusing on sources with projected linear sizes >1 Mpc.
• Measured spectroscopic redshifts for 35 galaxies, with photometric redshift estimates for one candidate (J1420−0545) based on its faint magnitude (R > 20.5).
• Calculated lobe energy densities using the minimum energy condition, assuming equipartition between relativistic particles and magnetic fields.
• Analyzed lobe asymmetry via the arm separation ratio (Rθ) and central flux ratio (fc) to infer environmental density gradients.
• Used the Hubble constant H₀ = 71 km s⁻¹ Mpc⁻¹ to derive linear sizes from angular sizes and redshifts.

Experimental results

Research questions

• RQ1 Do giant radio galaxies at z > 0.5 with lobe energy densities below 10⁻¹⁴ J m⁻³ exist, as required to test the (1+z)⁵ evolution of IGM pressure?
• RQ2 Can deep SALT observations detect faint, high-redshift GRG host galaxies with R > 20.5 mag that remain optically unassociated in DSS images?
• RQ3 To what extent do the morphological asymmetries in GRG lobes reflect environmental density inhomogeneities rather than source orientation?
• RQ4 How reliable are equipartition-based internal lobe pressure estimates for low-luminosity, high-redshift GRGs in the absence of X-ray data?
• RQ5 Is there observational evidence for the predicted cosmological evolution of IGM pressure in the form pIGM(z) ∝ (1+z)⁵?

Key findings

• Of 35 galaxies with spectroscopic redshifts, 21 have projected linear sizes exceeding 1 Mpc, but none have redshifts above 0.4.
• Only two sources in the sample have lobe energy densities below 10⁻¹⁴ J m⁻³, falling short of the target threshold for probing IGM pressure evolution.
• Photometric redshift estimation for J1420−0545 suggests a projected linear size greater than 4.8 Mpc, potentially making it the largest known GRG.
• The median arm separation ratio (Rθ) of 1.25 ± 0.03 in the sample supports the hypothesis that GRG asymmetry arises from environmental density gradients rather than source inclination.
• The observed lobe asymmetry and low luminosity of the most extended sources suggest they may evolve in low-density intergalactic regions, possibly void-like environments.
• The lack of X-ray data for GRG lobes limits confidence in equipartition-based pressure estimates, but the method remains the best available for such distant, diffuse sources.