A catalogue of Type 2 active galactic nuclei in the 6dF Galaxy Survey

TL;DR

This paper presents a new catalogue of 5000 Type 2 active galactic nuclei identified from the 6dF Galaxy Survey, expanding southern sky AGN data with detailed spectral analysis and classification.

Contribution

It introduces a comprehensive catalogue of Type 2 AGN from the 6dF survey, utilizing a novel spectral fitting and visual inspection process for accurate identification.

Findings

Identified 10,492 narrow emission line galaxies in 6dFGS.

Classified 5000 as Type 2 AGN using BPT diagram.

Median redshift of AGN is approximately 0.032.

Abstract

Active galactic nuclei (AGNs) are the compact, energetic central regions of galaxies, powered by supermassive black holes that accrete surrounding gas and dust. Their optical spectra can be identified by strong emission-line signatures (broad and/or narrow lines). Those showing only narrow lines are classified as 'Type 2' AGN. Extensive surveys like SDSS cover AGN in the Northern Sky, but the equivalent coverage in the Southern Sky remains limited. We address this by presenting a new catalogue of Type 2 AGN from the 6dF Galaxy Survey (6dFGS), which has 136,304 spectra covering roughly 17{,}000~\mathrm{deg}^2, mainly of low-redshift galaxies. We use a median absolute deviation cut on the continuum-fitted spectrum to select emission line galaxies. AGN were identified by fitting the 6dFGS spectra with a modified Python QSO fitting tool (PyQSOFit). All selected spectra were visually inspected and corrected for fitting errors where necessary. 10,492 narrow emission line galaxies were identified in 6dFGS, including 5000 Type 2 AGN classified from Baldwin--Phillips--Terlevich (BPT) diagram. They have a median redshift of z$ \sim 0.032$ and a median $[\mathrm{O\,III}]$ luminosity of $\Log(L_{[\mathrm{O\,III}]}/ergs s^{-1}) \sim 40.04$.