NGC 3147: a prototypical low-luminosity active galactic nucleus with double-peaked optical and ultraviolet lines

TL;DR

NGC 3147, a low-luminosity AGN, exhibits double-peaked broad emission lines in optical and UV, variability in line profiles, and a spectral energy distribution lacking a thermal UV bump, consistent with low accretion activity.

Contribution

This study confirms the double-peaked emission lines in NGC 3147 and provides a detailed optical-to-X-ray spectral energy distribution, revealing characteristics typical of low-luminosity AGN.

Findings

Double-peaked broad emission lines confirmed in optical and UV.

Spectral energy distribution shows no thermal UV bump, with a flat alpha_ox.

Variability observed in emission line profiles and inner disc radius.

Abstract

A previous narrow-slit ($0.1$ arcsec) Hubble Space Telescope observation unveiled a broad relativistic H$\alpha$ profile in NGC3147, a low-luminosity ($\mathrm{L_{bol}}\sim10^{42}$ erg s$^{-1}$), low-Eddington ratio ($\mathrm{L_{bol}/L_{Edd}}\sim10^{-4}$) active galactic nucleus (AGN), formerly believed to be a candidate true type 2 AGN intrinsically lacking the broad-line region. The new observations presented here confirm the double-peaked profile of the H$\alpha$ line, which further shows variability both in flux and in the inner radius of the emitting disc with respect to the previous epoch. Similar disc line profiles are also found in prominent ultraviolet (UV) lines, in particular Ly$\alpha$ and C IV. The new data also allow us to build a simultaneous subarcsec optical-to-X-ray spectral energy distribution of NGC3147, which is characterized by the absence of a thermal UV bump, and an emission peak in the X-rays. The resulting very flat $\alpha_{ox}=-0.82$ is typical of low-luminosity AGN, and is in good agreement with the extrapolation to low luminosities of the well-known trend with luminosity observed in standard AGN. Indeed, we are possibly observing the accretion disc emission in NGC3147 in the optical, close to the expected peak. On the other hand, the steep -2 UV power law may be Comptonization of that cold disc by a warm corona, what is instead generally observed as a `soft excess' in more luminous AGN.