H 0557−385: A Candidate Missing Link Between Radio-Quiet Seyferts and Young Radio Galaxies
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
Related
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BibTeX
@misc{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. Bottcher 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},
howpublished = {arXiv},
year = {2026},
month = nov,
doi = {10.48550/arxiv.2611.03818},
eprint = {2611.03818},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2611.03818},
}
RIS
TY - UNPB
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. Bottcher
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 - 2026/11/02
PB - arXiv
DO - 10.48550/arxiv.2611.03818
UR - https://arxiv.org/abs/2611.03818
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 (α_pl = −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 ≲1.8×10^3 yr, and a time-averaged jet power of ∼5×10^41 − 5×10^44 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
ER -