Euclid: Quick Data Release (Q1) -- Optical hotspots in powerful radio galaxies
Abstract
The hotspots of powerful radio galaxies are the sites of high-energy particle acceleration, but radio observations give us limited information on the electron energy distributions that they generate, which affects our overall understanding of their physics and energetics. Optical synchrotron counterparts to radio galaxy hotspots, which probe high electron energies, have been known for many years, but have only been studied in a small number of objects selected as being very bright in the radio. By combining Low-Frequency Array (LOFAR) observations with the Euclid Q1 data release in the Euclid Deep Field North, we identify 77 powerful radio galaxies with hotspots and show that a small but significant fraction of them, 7--15\%, may have detected infrared and optical counterparts to their hotspots. The hotspot spectral energy distributions in our clearest cases are consistent with particle acceleration up to (but not beyond) 1\,TeV in energy on the assumption of field strengths close to equipartition, whereas non-detected hotspots either do not reach such high electron energies or would have to have very different electron energy spectra or significantly sub-equipartition magnetic field strengths. Two new candidate optical hotspots are clearly detected matching high-resolution LOFAR data; one is remarkable as both the highest-redshift optical hotspot known to date (z ≈ 1.6) and also the first optical detection of the hotspot of the inner lobe of a restarting source. We discuss approaches to performing a large-scale study of hotspots with the full wide-area Euclid surveys, particularly when combined with the international baselines of LOFAR, enabling for the first time a statistical analysis of high-energy particle acceleration in hotspots.
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BibTeX
@misc{hardcastle2026euclid,
title = {{Euclid: Quick Data Release (Q1) -- Optical hotspots in powerful radio galaxies}},
author = {M.J Hardcastle and L Bisigello and M Bondi and M Magliocchetti and L.K Morabito and J Petley and F Ricci and H.J.A Rottgering and K Rubinur and D.J.B Smith and D Stern and B Altieri and S Andreon and N Auricchio and C Baccigalupi and M Baldi and A Balestra and S Bardelli and P Battaglia and A Biviano and E Branchini and M Brescia and S Camera and V Capobianco and C Carbone and J Carretero and S Casas and M Castellano and G Castignani and S Cavuoti and K.C Chambers and A Cimatti and C Colodro-Conde and G Congedo and C.J Conselice and L Conversi and Y Copin and A Costille and F Courbin and H.M Courtois and M Cropper and J.-G Cuby and A da Silva and H Degaudenzi and G de Lucia and C Dolding and H Dole and F Dubath and X Dupac and M Farina and R Farinelli and S Ferriol and P Fosalba and S Fotopoulou and M Frailis and E Franceschi and M Fumana and S Galeotta and K George and B Gillis},
howpublished = {HAL},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.21930},
eprint = {2609.21930},
archivePrefix = {arXiv},
url = {https://hal.science/hal-05781842v1},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Euclid: Quick Data Release (Q1) -- Optical hotspots in powerful radio galaxies
AU - M.J Hardcastle
AU - L Bisigello
AU - M Bondi
AU - M Magliocchetti
AU - L.K Morabito
AU - J Petley
AU - F Ricci
AU - H.J.A Rottgering
AU - K Rubinur
AU - D.J.B Smith
AU - D Stern
AU - B Altieri
AU - S Andreon
AU - N Auricchio
AU - C Baccigalupi
AU - M Baldi
AU - A Balestra
AU - S Bardelli
AU - P Battaglia
AU - A Biviano
AU - E Branchini
AU - M Brescia
AU - S Camera
AU - V Capobianco
AU - C Carbone
AU - J Carretero
AU - S Casas
AU - M Castellano
AU - G Castignani
AU - S Cavuoti
AU - K.C Chambers
AU - A Cimatti
AU - C Colodro-Conde
AU - G Congedo
AU - C.J Conselice
AU - L Conversi
AU - Y Copin
AU - A Costille
AU - F Courbin
AU - H.M Courtois
AU - M Cropper
AU - J.-G Cuby
AU - A da Silva
AU - H Degaudenzi
AU - G de Lucia
AU - C Dolding
AU - H Dole
AU - F Dubath
AU - X Dupac
AU - M Farina
AU - R Farinelli
AU - S Ferriol
AU - P Fosalba
AU - S Fotopoulou
AU - M Frailis
AU - E Franceschi
AU - M Fumana
AU - S Galeotta
AU - K George
AU - B Gillis
PY - 2026
DA - 2026/09/18
PB - HAL
DO - 10.48550/arxiv.2609.21930
UR - https://hal.science/hal-05781842v1
AB - The hotspots of powerful radio galaxies are the sites of high-energy particle acceleration, but radio observations give us limited information on the electron energy distributions that they generate, which affects our overall understanding of their physics and energetics. Optical synchrotron counterparts to radio galaxy hotspots, which probe high electron energies, have been known for many years, but have only been studied in a small number of objects selected as being very bright in the radio. By combining Low-Frequency Array (LOFAR) observations with the Euclid Q1 data release in the Euclid Deep Field North, we identify 77 powerful radio galaxies with hotspots and show that a small but significant fraction of them, 7--15\%, may have detected infrared and optical counterparts to their hotspots. The hotspot spectral energy distributions in our clearest cases are consistent with particle acceleration up to (but not beyond) 1\,TeV in energy on the assumption of field strengths close to equipartition, whereas non-detected hotspots either do not reach such high electron energies or would have to have very different electron energy spectra or significantly sub-equipartition magnetic field strengths. Two new candidate optical hotspots are clearly detected matching high-resolution LOFAR data; one is remarkable as both the highest-redshift optical hotspot known to date (z ≈ 1.6) and also the first optical detection of the hotspot of the inner lobe of a restarting source. We discuss approaches to performing a large-scale study of hotspots with the full wide-area Euclid surveys, particularly when combined with the international baselines of LOFAR, enabling for the first time a statistical analysis of high-energy particle acceleration in hotspots.
ER -