High-resolution international LOFAR observations of 4C~43.15 -- Spectral ages and injection indices in a high-z radio galaxy

Abstract

Radio sources with steep spectra are preferentially associated with the most distant galaxies, the $\alpha -z$ relation, but the reason for this relation is an open question. The spatial distribution of spectra in high-z radio sources can be used to study this relation, and low-frequency observations are particularly important in understanding the particle acceleration and injection mechanisms. However, the small angular sizes of high-z sources together with the inherently low resolution of low-frequency radio telescopes until now has prevented high angular resolution low-frequency observations of distant objects. Here we present subarcsecond observations of a $z=2.4$ radio galaxy at frequencies between $121$ MHz and $166$ MHz. We measure the spatial distribution of spectra, and discuss the implications for models of the $\alpha -z$ relation. We targeted 4C 43.15 with the High Band Antennas (HBAs) of the \textit{International LOFAR Telescope} (ILT) with a range of baselines up to $1300\mathrm{km}$. At the central frequency of $143$ MHz we achieve an angular resolution of $\sim 0.3^{\prime \prime }$. By complementing our data with archival \textit{Very Large Array} (VLA) data we study the spectral index distribution across 4C 43.15 between $55\mathrm{MHz}$ and $8.4\mathrm{GHz}$ at resolutions of $0.4^{\prime \prime }$ and $0.9^{\prime \prime }$. With a magnetic field strength of $B=5.2$ nT and fitted injection indices of ${\alpha }_{\mathrm{inj}}^{\mathrm{north}}=-0.8$ and ${\alpha }_{\mathrm{inj}}^{\mathrm{south}}=-0.6$, fitting a Tribble spectral ageing model results in a spectral age of ${\tau }_{\mathrm{spec}}=1.1\pm 0.1$ Myr. We conclude that our data on 4C 43.15 indicates that inverse Compton losses could become comparable to or exceed synchrotron losses at higher redshifts and that inverse Compton losses could be a viable explanation for the $\alpha -z$ relation.

Accepted to a special issue of A&A on sub-arcsecond imaging with LOFAR. 13 pages, 9 figures

References in corpus (20)

• WSClean: an implementation of a fast, generic wide-field imager for radio astronomy
• Revised equipartition & minimum energy formula for magnetic field strength estimates from radio synchrotron observations
• Distant Radio Galaxies and their Environments
• A search for distant radio galaxies from SUMSS and NVSS: III. radio spectral energy distributions and the z-alpha correlation
• Physical Properties of Very Powerful FRII Radio Galaxies
• Sub-arcsecond imaging with the International LOFAR Telescope I. Foundational calibration strategy and pipeline
• A multifrequency study of giant radio sources-II. Spectral ageing analysis of the lobes of selected sources
• LBCS: the LOFAR Long-Baseline Calibrator Survey
• RAiSE III: 3C radio AGN energetics and composition
• LOFAR imaging of Cygnus A -- Direct detection of a turnover in the hotspot radio spectra
• Subarcsecond international LOFAR radio images of Arp 220 at 150 MHz: A kpc-scale star forming disk surrounding nuclei with shocked outflows
• Compression of interferometric radio-astronomical data
• Giant Lyman-alpha nebulae around z>2 radio galaxies: evidence for infall
• The LOFAR long baseline snapshot calibrator survey
• The radio core structure of the luminous infrared galaxy NGC 4418
• Spectral ageing in the era of big data: integrated vs resolved models
• Investigating the spectral age problem with powerful radio galaxies
• High-resolution international LOFAR observations of 4C~43.15 -- Spectral ages and injection indices in a high-z radio galaxy
• LOFAR observations of 4C+19.44. On the discovery of low frequency spectral curvature in relativistic jet knots
• A VLA-GMRT Look at 11 Powerful FR II Quasars

Cited by in corpus (10)

• Measuring cavity powers of active galactic nuclei in clusters using a hybrid X-ray-radio method -- A new window on feedback opened by subarcsecond LOFAR-VLBI observations
• A machine learning based approach to gravitational lens identification with the International LOFAR Telescope
• Radio spectral properties of star-forming galaxies between 150-5000MHz in the ELAIS-N1 field
• High-resolution international LOFAR observations of 4C~43.15 -- Spectral ages and injection indices in a high-z radio galaxy
• A low frequency sub-arcsecond view of powerful radio galaxies in rich-cluster environments: 3C 34 and 3C 320
• VLBI Imaging of high-redshift galaxies and protoclusters at low radio frequencies with the International LOFAR Telescope
• Monster radio jet (>66 kpc) observed in quasar at z$\sim$5
• Core prominence as a signature of restarted jet activity in the LOFAR radio-galaxy population
• The first high-redshift cavity power measurements of cool-core galaxy clusters with the International LOFAR Telescope
• The LOFAR sub-arcsecond view of the high-redshift radio relic in PSZ2G091.83+26.11