HST unveils a compact mildly relativistic broad-line region in the candidate true type 2 NGC 3147

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${\sim }27\, 000$ 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 ∼23°, 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.