HST unveils a compact mildly relativistic broad-line region in the candidate true type 2 NGC 3147
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Bianchi, Stefano; Antonucci, Robert; Capetti, Alessandro; Chiaberge, Marco; Laor, Ari; Bassani, Loredana; Carrera Troyano, Francisco Jesús
Fecha
2019-09Derechos
© 2019 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society
Publicado en
Monthly Notices of the Royal Astronomical Society: Letters, 2019, 488(1), L1-L5
Editorial
Oxford University Press
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Palabras clave
Galaxies: active
Galaxies: individual: NGC3147
Galaxies: Seyfert
Resumen/Abstract
NGC 3147 has been considered the best case of a true type 2 AGN: an unobscured AGN,
based on the unabsorbed compact X-ray continuum, which lacks a broad-line region (BLR).
However, the very low luminosity of NGC 3147 implies a compact BLR, which produces very
broad lines, hard to detect against the dominant background host galaxy. Narrow (0.1 arcsec × 0.1 arcsec) slit HST spectroscopy allowed us to exclude most of the host galaxy light, and
revealed an H α line with an extremely broad base (FWZI∼27 000 km s−1). The line profile
shows a steep cut-off blue wing and an extended red wing, which match the signature of a
mildly relativistic thin accretion disc line profile. It is indeed well fit with a nearly face on thin
disc, at i ∼ 23º , with an inner radius at 77 ± 15 rg, which matches the prediction of 62+¹⁸
−₁₄ rg from the RBLR–L1/2 relation. This result questions the very existence of true type 2 AGN. Moreover, the detection of a thin disc, which extends below 100 rg in an L/LEdd ∼ 10−⁴ system, contradicts the current view of the accretion flow configuration at extremely low accretion rates.
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