The optically elusive, changing-look active nucleus in NGC 4156

TL;DR

This paper reports the discovery of a changing-look active galactic nucleus in NGC 4156, which transitioned from a type 2 to a nearly type 1, revealing new broad-line features and providing the first black hole mass estimate for this galaxy.

Contribution

It presents the first optical detection of broad-line emission in NGC 4156, confirming its changing-look nature and estimating its black hole mass from a single-epoch spectrum.

Findings

Detection of broad Hα and Hβ lines in 2019 data.

Confirmation of spectral variability between 2019 and 2022.

First black hole mass estimate for NGC 4156.

Abstract

We report on the changing-look nature of the active galactic nucleus (AGN) in the galaxy NGC 4156, as serendipitously discovered thanks to data acquired in 2019 at the Telescopio Nazionale Galileo (TNG) during a students' observing programme. Previous optical spectra had never shown any signatures of broad-line emission, and evidence of the AGN had come only from X-ray observations, being the optical narrow-line flux ratios unable to unambiguously denote this galaxy as a Seyfert. Our 2019 TNG data unexpectedly revealed the appearance of broad-line components in both the H$\alpha$ and H$\beta$ profiles, along with a rise of the continuum, thus implying a changing-look AGN transitioning from a type 2 (no broad-line emission) towards a (nearly) type 1. The broad-line emission has then been confirmed by our 2022 follow-up observations, whereas the rising continuum has no longer been detected, which hints at a further evolution backwards to a (nearly) type 2. The presence of broad-line components also allowed us to obtain the first single-epoch estimate of the black hole mass (log(MBH/Msun) $\sim$ 8.1) in this source. The observed spectral variability might be the result of a change in the accretion activity of NGC 4156, although variable absorption cannot be completely excluded.