[Paper Review] The new sample of Giant radio sources: I. Radio imaging, optical identification and spectroscopy of selected candidates

J. Machalski, M. Jamrozy+1 more|ArXiv.org|Mar 10, 2001

Galaxies: Formation, Evolution, Phenomena20 references36 citations

TL;DR

This study presents a new sample of giant radio sources (D ≥ 1 Mpc) selected from 1.4 GHz NVSS and FIRST surveys, using high-resolution radio imaging, deep optical imaging, and spectroscopy to identify host galaxies and measure redshifts. Of the complete sample, 44% have spectroscopic redshifts (all z < 0.33), and 70% exceed 1 Mpc in size; photometric redshifts for fainter galaxies also predict sizes >1 Mpc, confirming their giant nature.

ABSTRACT

A new sample of very large angular size radio sources has been selected from the 1.4 GHz VLA surveys: FIRST and NVSS. This sample will be very useful for an observational constraint on the time evolution of double radio sources, especially their size, predicted by numerous analytical models of such evolution (cf. Introduction). In this paper we present radio and optical data for a large fraction of the sample sources. They are: high-frequency VLA maps with very weak radio cores detected, deep optical images showing the identified faint host galaxies not visible on the DSS images, and optical spectra of the identified galaxies brighter than about R ~ 18.5 mag taken with the McDonald Observatory 2.1m telescope. For 15 galaxies (of which 4 do not belong to the complete sample) the redshift has been determined. In the result, 44 per cent of galaxies in the complete sample have redshift data (with one exception all redshifts are less than 0.33), of which 70 per cent have a linear size exceeding 1 Mpc. The photometric redshift estimates for other 11 galaxies with 19 mag

Motivation & Objective

• To identify and characterize a uniform, large sample of giant radio sources (D ≥ 1 Mpc) for studying their evolution and population properties.
• To overcome biases in prior samples by selecting from deep 1.4 GHz radio surveys (NVSS and FIRST) with high angular resolution.
• To determine redshifts and physical parameters (luminosity, size, magnetic field) for a representative subset of these sources using spectroscopy and photometry.
• To test evolutionary models of FRII radio sources by measuring the size-luminosity-redshift distribution of giants.

Proposed method

• Selected candidates from NVSS and FIRST surveys using criteria: FRII/FRII-like morphology, angular size >3 arcmin, 1.4 GHz flux density >0.5 Jy.
• Performed high-resolution VLA radio imaging to detect weak compact cores in giant candidates not previously detected.
• Conducted deep optical imaging to identify faint host galaxies not visible on DSS plates.
• Obtained optical spectroscopy for 15 galaxies (R < 18.5 mag) using the McDonald 2.1m telescope to determine redshifts.
• Estimated photometric redshifts and physical parameters (luminosity, size, magnetic field) for 11 fainter galaxies (19 < R < 21.7 mag) using absolute magnitude assumptions (M_R = -23.65 ± 1.65 mag).
• Calculated equipartition magnetic field and energy density, and analyzed lobe asymmetries to infer age and orientation effects.

Experimental results

Research questions

• RQ1 What fraction of giant radio sources (D ≥ 1 Mpc) in the local universe have spectroscopic redshifts, and what is their luminosity and size distribution?
• RQ2 How do photometric redshift estimates for faint host galaxies (R > 19 mag) compare with their inferred linear sizes, and do they confirm giant status?
• RQ3 To what extent do lobe brightness asymmetries in giant sources reflect age differences due to line-of-sight inclination and decreasing lobe luminosity?
• RQ4 What is the role of inverse-Compton versus synchrotron energy losses in the evolution of giant radio lobes, based on magnetic field ratios?
• RQ5 Is the observed population of giant sources consistent with models predicting that they are old, low-luminosity sources in low-density environments?

Key findings

• 44% of the complete sample (15/34) have spectroscopic redshifts, all below z = 0.33, with 70% of these having projected linear sizes exceeding 1 Mpc.
• Photometric redshift estimates for 11 fainter galaxies (19 < R < 21.7 mag) yield z ≈ 0.3–0.5, and predict linear sizes >1 Mpc, confirming their giant nature.
• The ratio of inverse-Compton to synchrotron energy losses (B_iC / B_me) ranges from 2 to 6, indicating that inverse-Compton losses are a dominant energy loss mechanism in giant sources.
• The brighter lobe is typically closer to the nucleus in most sources, consistent with the lobe being younger and more luminous due to line-of-sight inclination and decreasing lobe luminosity with age.
• A spurious correlation between radio–optical luminosity ratio (log r) and redshift is observed, likely due to luminosity bias, not intrinsic evolution.
• The ratio of magnetic field energy density to total energy density (B²_me / (B²_iC + B²_me)) correlates with linear size, supporting theoretical models of lobe aging.