H 0557–385: A Candidate Missing Link Between Radio-Quiet Seyferts and Young Radio Galaxies
8 affiliations
- 1School of Physics and Astronomy, University of Leicester , University Rd, Leicester LE1 7RH ,
- 2INAF/IASF Palermo , via Ugo La Malfa 153, I-90146 Palermo ,
- 3Montana State University Department of Physics, , P.O. Box 173840, Bozeman, MT 59717-3840 ,
- 4Centre for Space Research, North West University , Potchefstroom 2520 ,
- 5INAF–Istituto di Astrofisica e Planetologia Spaziali , Via Fosso del Cavaliere, 00133, Roma ,
- 6U.S. Naval Research Laboratory , 4555 Overlook Avenue Southwest, WA, DC 20375 ,
- 7Durham University Centre for Extragalactic Astronomy, Department of Physics, , South Road, Durham DH1 3LE ,
- 8Inter-University Centre for Astronomy and Astrophysics , Ganeshkhind, Pune 411007 ,
Abstract
The Seyfert 1 galaxy H 0557–385 (z = 0.034) has long been classified as radio-quiet on the basis of its radio-to-optical flux ratio, yet its radio properties are physically inconsistent with this designation. We present a broadband (0.076–258.8 GHz) radio spectral analysis combining new Australia Telescope Compact Array (ATCA) observations with archival Murchison Widefield Array (MWA), Australian Square Kilometre Array Pathfinder (ASKAP) and Atacama Large Millimeter/submillimeter Array (ALMA) data. The spectrum requires a two-component model: a compact, self-absorbed component with a stratified gigahertz-peaked (GPS)/high-frequency-peaked (HFP) spectral shape (turnover at 9.4+0.6−0.5 GHz), superposed on an extended, optically thin power-law lobe (αpl = −0.78) that dominates below ~1.6 GHz. The derived parameters — a source size of ~15 pc, an equipartition magnetic field of 2.8 mG, a synchrotron age of ≲ 1.8 × 103 yr, and a time-averaged jet power of ~5 × 1041–5 × 1044 erg s−1 — are characteristic of compact steep-spectrum and GPS young radio galaxies, not of canonical radio-quiet AGN. The two-component structure admits two interpretations that the present data cannot distinguish: the extended emission may be the growing lobe of a genuinely young, expanding radio galaxy, or relic plasma from an earlier episode of a transient or recurrent jet. Under either, H 0557–385 is better understood as an intermediate object: optically a Seyfert, but with the radio-physical properties of a newly ignited or recently re-triggered radio galaxy. It is therefore a candidate missing link between radio-quiet AGN and large-scale radio galaxies. Distinguishing these scenarios will require high-resolution imaging and multi-epoch monitoring.
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BibTeX
@article{chauhan20260557385,
title = {{H 0557–385: A Candidate Missing Link Between Radio-Quiet Seyferts and Young Radio Galaxies}},
author = {J Chauhan and T D Russell and A Lohfink and M Böttcher and F Panessa and R L C Starling and K Nyland and C Done and S Chattopadhyay and T Ghosh and R Misra and M Del Santo},
journal = {Monthly Notices of the Royal Astronomical Society},
year = {2026},
month = nov,
doi = {10.1093/mnras/stag2102},
url = {https://academic.oup.com/mnras/advance-article/doi/10.1093/mnras/stag2102/9012720},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - H 0557–385: A Candidate Missing Link Between Radio-Quiet Seyferts and Young Radio Galaxies
AU - J Chauhan
AU - T D Russell
AU - A Lohfink
AU - M Böttcher
AU - F Panessa
AU - R L C Starling
AU - K Nyland
AU - C Done
AU - S Chattopadhyay
AU - T Ghosh
AU - R Misra
AU - M Del Santo
PY - 2026
DA - 2026/11/04
JO - Monthly Notices of the Royal Astronomical Society
DO - 10.1093/mnras/stag2102
UR - https://academic.oup.com/mnras/advance-article/doi/10.1093/mnras/stag2102/9012720
AB - The Seyfert 1 galaxy H 0557–385 (z = 0.034) has long been classified as radio-quiet on the basis of its radio-to-optical flux ratio, yet its radio properties are physically inconsistent with this designation. We present a broadband (0.076–258.8 GHz) radio spectral analysis combining new Australia Telescope Compact Array (ATCA) observations with archival Murchison Widefield Array (MWA), Australian Square Kilometre Array Pathfinder (ASKAP) and Atacama Large Millimeter/submillimeter Array (ALMA) data. The spectrum requires a two-component model: a compact, self-absorbed component with a stratified gigahertz-peaked (GPS)/high-frequency-peaked (HFP) spectral shape (turnover at 9.4^+0.6_−0.5 GHz), superposed on an extended, optically thin power-law lobe (αpl = −0.78) that dominates below ~1.6 GHz. The derived parameters — a source size of ~15 pc, an equipartition magnetic field of 2.8 mG, a synchrotron age of ≲ 1.8 × 103 yr, and a time-averaged jet power of ~5 × 1041–5 × 1044 erg s−1 — are characteristic of compact steep-spectrum and GPS young radio galaxies, not of canonical radio-quiet AGN. The two-component structure admits two interpretations that the present data cannot distinguish: the extended emission may be the growing lobe of a genuinely young, expanding radio galaxy, or relic plasma from an earlier episode of a transient or recurrent jet. Under either, H 0557–385 is better understood as an intermediate object: optically a Seyfert, but with the radio-physical properties of a newly ignited or recently re-triggered radio galaxy. It is therefore a candidate missing link between radio-quiet AGN and large-scale radio galaxies. Distinguishing these scenarios will require high-resolution imaging and multi-epoch monitoring.
SN - 0035-8711
SN - 1365-2966
ER -