# HST unveils a compact mildly relativistic Broad Line Region in the candidate true type 2 NGC 3147

## TL;DR

HST spectroscopy of NGC 3147 reveals a compact, mildly relativistic broad line region and a thin accretion disk at very low luminosity, challenging the true type 2 AGN classification and current accretion models.

## Contribution

This study provides direct evidence of a relativistic broad line region and a thin accretion disk in a low-luminosity AGN, questioning the existence of true type 2 AGN.

## Key findings

- Detection of an extremely broad Hα line with FWZI ~27,000 km/s.
- Identification of a nearly face-on thin disk at ~77 r_g.
- Contradiction of current models by finding a thin disk at L/L_Edd ~ 10^{-4}.

## 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"x0.1") slit HST spectroscopy allowed us to exclude most of the host galaxy light, and revealed an H$\alpha$ line with an extremely broad base (FWZI$\sim27\,000$ km s$^{-1}$). The line profile shows a steep cutoff blue wing and an extended red wing, which match the signature of a mildly relativistic thin accretion disk line profile. It is indeed well fit with a nearly face on thin disk, at $i\sim23^\circ$, with an inner radius at $77\pm15$ r$_g$, which matches the prediction of $62^{+18}_{-14}$ r$_g$ from the $R_{\rm BLR} \sim L^{1/2}$ relation. This result questions the very existence of true type 2 AGN. Moreover, the detection of a thin disk, which extends below 100 r$_g$ in an $L/L_{\rm Edd}\sim10^{-4}$ system, contradicts the current view of the accretion flow configuration at extremely low accretion rates.