[Paper Review] Dying radio galaxies in the LOFAR Lockman Hole

M. Brienza, E. K. Mahony+3 more|arXiv (Cornell University)|Mar 6, 2016

Galaxies: Formation, Evolution, Phenomena3 references3 citations

TL;DR

This study uses deep 150-MHz LOFAR observations of the Lockman Hole to systematically search for dying radio galaxies—sources in the fading phase after jet activity ceases. By combining multi-frequency data and applying both morphological and spectral selection criteria, the authors find that 6–8% of the radio source population exhibits steep spectra consistent with dying sources, primarily due to spectral aging, offering critical constraints on luminosity evolution models.

ABSTRACT

After the jets have switched off, radio galaxies undergo a fading phase which is often named the dying phase. The luminosity evolution of the remnant plasma during this period is still poorly constrained because of the paucity of objects detected. Using the new 150-MHz deep LOFAR observations of the well-known extragalactic field the Lockman Hole, we performed a systematic search of dying radio sources aiming to provide good statistics on their detection and properties. To avoid selection biases towards any specific class of dying sources we used both morphological and spectral selection criteria. To do this we combined the LOFAR data with publicly available surveys at other frequencies as well as dedicated deep observations. Our preliminary results, show that the fraction of candidate dying radio sources is < 6-8% of the entire radio source population and is dominated by steep spectrum sources. By comparing these observational results with statistical modelling of the radio sky population we will be able to constrain the main mechanisms contributing to the dying radio galaxy luminosity evolution.

Motivation & Objective

• To systematically identify dying radio galaxies in the LOFAR Lockman Hole field using low-frequency, high-sensitivity observations.
• To overcome selection biases by employing both morphological and spectral selection criteria for comprehensive detection.
• To quantify the fraction and properties of dying radio sources to constrain luminosity evolution mechanisms.
• To use multi-wavelength data from 150 MHz to 1.4 GHz to characterize spectral indices and identify candidates via curvature and steepness.
• To develop a framework for future large-scale surveys across the northern sky using automated source detection and modeling.

Proposed method

• Conducted deep 150-MHz observations with LOFAR's High-Band Antennas over 10 hours, achieving an rms of 150 µJy/beam and a beam size of 18.6 × 14.7 arcsec.
• Cross-matched the LOFAR 150-MHz source catalogue with 1.4-GHz Westerbork and NVSS surveys to compute spectral indices using Fν ∝ ν−α.
• Applied a high-resolution (18.6 × 14.7 arcsec) and low-resolution (45 arcsec) imaging strategy to compare source detection and spectral properties across resolutions.
• Used spectral curvature criterion SPC = α_high − α_low (with α_low = α^150_327 and α_high = α^327_1400) to identify sources with spectral breaks indicating jet shutdown.
• Performed statistical simulations of radio source populations based on evolution models to compare with observed data and constrain physical mechanisms.
• Utilized automated source extraction via PyBDSM and BRATS survey module for scalability across large sky areas.

Experimental results

Research questions

• RQ1 What fraction of the radio source population in the Lockman Hole can be classified as dying radio galaxies based on spectral and morphological criteria?
• RQ2 How do the spectral properties of candidate dying sources—particularly steep spectra and curvature—differ between high- and low-resolution data?
• RQ3 What is the role of low-frequency sensitivity in detecting low-surface-brightness remnant plasma from faded radio galaxies?
• RQ4 How do observed candidate fractions compare with predictions from radio source evolution models, and what does this imply about luminosity evolution mechanisms?
• RQ5 To what extent can multi-frequency cross-matching with surveys from 327 MHz to 1.4 GHz improve the identification of dying sources via spectral curvature?

Key findings

• The fraction of candidate dying radio sources in the Lockman Hole is estimated at 6–8% of the total radio source population, based on spectral steepness and morphological criteria.
• Approximately 7% of sources in the high-resolution catalogue (18.6 × 14.7 arcsec) have spectral indices steeper than 1.2, consistent with spectral aging in dying sources.
• In the low-resolution catalogue (45 arcsec), 5.8% of sources have spectral indices >1.2, indicating comparable detection rates despite lower resolution and sensitivity.
• 25% of resolved sources (size ≥26 arcsec) in the high-resolution sample show steep spectra, while 35% of resolved sources (size ≥64 arcsec) in the low-resolution sample are steep, suggesting better detection of extended structures at lower resolution.
• Only four sources were identified as potential candidates via the spectral curvature criterion (SPC ≥ 0.5), highlighting limitations due to sensitivity and lack of data above 1.4 GHz.
• The consistency between high- and low-resolution results, despite a 4.5× sensitivity difference, supports the robustness of the spectral index selection method for identifying dying sources.