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-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.
- Publication:
-
Monthly Notices of the Royal Astronomical Society
- Pub Date:
- September 2019
- DOI:
- 10.1093/mnrasl/slz080
- arXiv:
- arXiv:1905.09627
- Bibcode:
- 2019MNRAS.488L...1B
- Keywords:
-
- galaxies: active;
- galaxies: individual: NGC3147;
- galaxies: Seyfert;
- Astrophysics - High Energy Astrophysical Phenomena;
- Astrophysics - Astrophysics of Galaxies
- E-Print:
- 6 pages, 3 figures, accepted for publication in MNRAS Letters