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{∼ }27 000 km s<SUP>-1</SUP>). 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<SUP>°</SUP>, with an inner radius at 77 ± 15 r<SUB>g</SUB>, which matches the prediction of 62^{+18}_{-14}r<SUB>g</SUB> from the R<SUB>BLR</SUB>-L<SUP>1/2</SUP> relation. This result questions the very existence of true type 2 AGN. Moreover, the detection of a thin disc, which extends below 100 r<SUB>g</SUB> in an L/L<SUB>Edd</SUB> ∼ 10<SUP>-4</SUP> system, contradicts the current view of the accretion flow configuration at extremely low accretion rates.