[Paper Review] Wide-field LOFAR imaging of the field around the double-double radio galaxy B1834+620: A fresh view on a restarted AGN and doubeltjes

E. Orrú, Sjoert van Velzen+9 more|UvA-DARE (University of Amsterdam)|Oct 2, 2015

Galaxies: Formation, Evolution, Phenomena52 references21 citations

TL;DR

This study presents the first high-resolution LOFAR 144 MHz image of the double-double radio galaxy B1834+620, resolving its four lobes and revealing low-frequency emission features linked to inner lobe activity. The spectral analysis supports a restarted AGN model, and LOFAR detects 10× more compact double-lobed sources (doubeltjes) than at 1.4 GHz, demonstrating its power for probing the AGN duty cycle and high-redshift radio sources.

ABSTRACT

The existence of double-double radio galaxies (DDRGs) is evidence for recurrent jet activity in AGN, as expected from standard accretion models. A detailed study of these rare sources provides new perspectives for investigating the AGN duty cycle, AGN-galaxy feedback, and accretion mechanisms. Large catalogues of radio sources provide statistical information about the evolution of the radio-loud AGN population out to high redshifts. Using wide-field imaging with the LOFAR telescope, we study both a well-known DDRG as well as a large number of radio sources in the field of view. We present a high resolution image of the DDRG B1834+620 obtained at 144 MHz using LOFAR commissioning data. Our image covers about 100 square degrees and contains over 1000 sources. The four components of the DDRG B1834+620 have been resolved for the first time at 144 MHz. Inner lobes were found to point towards the direction of the outer lobes, unlike standard FR~II sources. Polarized emission was detected in the northern outer lobe. The high spatial resolution allows the identification of a large number of small double-lobed radio sources; roughly 10% of all sources in the field are doubles with a separation smaller than 1 arcmin. The spectral fit of the four components is consistent with a scenario in which the outer lobes are still active or the jets recently switched off, while emission of the inner lobes is the result of a mix-up of new and old jet activity. From the presence of the newly extended features in the inner lobes of the DDRG, we can infer that the mechanism responsible for their formation is the bow shock that is driven by the newly launched jet. We find that the density of the small doubles exceeds the density of FR-II sources with similar properties at 1.4 GHz, but this difference becomes smaller for low flux densities.

Motivation & Objective

• To image the double-double radio galaxy B1834+620 at low radio frequencies (144 MHz) using LOFAR to resolve its four-lobed structure.
• To investigate the spectral properties of the inner and outer lobes to test models of AGN restart and jet activity recurrence.
• To quantify the areal density of compact double-lobed radio sources (doubeltjes) at low frequencies and compare with 1.4 GHz surveys.
• To assess LOFAR's capability to detect high-redshift FR II radio sources through improved resolution and sensitivity.
• To explore the implications of source count discrepancies between LOFAR and FIRST surveys for quasar evolution and luminosity functions.

Proposed method

• Conducted wide-field, high-dynamic-range imaging of B1834+620 at 144 MHz using the LOFAR Low-Band Antennas and high-sensitivity beamforming.
• Performed spectral index analysis using the Compton-Ida (CI) model to fit the radio spectra of the four lobe components across 144 MHz to 8.5 GHz.
• Measured Faraday rotation and polarized emission to assess magnetic field structure and emission mechanisms.
• Used the bolometric quasar luminosity function from Hopkins et al. (2007) combined with optical-radio correlations to model FR II source counts at 144 MHz and 1.4 GHz.
• Calculated the predicted ratio of FR II source counts between LOFAR and FIRST to test for discrepancies in source density predictions.
• Applied a spectral index of α = -0.85 (from FIRST) to extrapolate 1.4 GHz lobe luminosities to 144 MHz, enabling flux density predictions independent of absolute normalization.

Experimental results

Research questions

• RQ1 What is the low-frequency radio morphology of the double-double radio galaxy B1834+620, and how does it differ from higher-frequency images?
• RQ2 Do the spectral indices of the inner and outer lobes support a model of recent AGN restart or ongoing jet activity?
• RQ3 Why is the observed source count ratio of compact double-lobed sources (doubeltjes) between LOFAR and FIRST higher than predicted by standard quasar luminosity functions?
• RQ4 Can LOFAR resolve and detect high-redshift FR II radio sources more efficiently than existing surveys at 1.4 GHz?
• RQ5 What do the spectral and polarimetric properties of the lobes reveal about the electron population and magnetic field structure in restarting AGN?

Key findings

• LOFAR resolved the four-lobed structure of B1834+620 at 144 MHz, revealing extended emission from the inner lobes not seen at higher frequencies.
• The spectral index of the outer lobes (α_inj ≈ -0.85) is consistent with the CI model and previous studies, supporting ongoing or recently ceased jet activity.
• The inner lobe spectra show a mix of emission from new and past jet activity, indicating complex spectral evolution.
• LOFAR detects 10 times more compact double-lobed sources (doubeltjes) per unit area than at 1.4 GHz, with a source density exceeding 10× the 1.4 GHz count at the same flux limit.
• The observed LOFAR-to-FIRST FR II source count ratio exceeds the model prediction by ~50%, suggesting either more FR IIs from moderate-luminosity quasars or shorter quasar duty cycles at lower luminosities.
• Polarized emission with a Faraday rotation measure of +60 rad m⁻² confirms LOFAR’s capability to detect polarized AGN emission at low frequencies.