Integral Field Spectroscopy of the circumnuclear region of the Radio Galaxy Pictor A

TL;DR

This study uses integral field spectroscopy to analyze the circumnuclear region of Pictor A, revealing a low-metallicity, elongated structure likely formed from a past merger, with complex kinematics and AGN photoionization.

Contribution

First detailed integral field spectroscopic analysis of Pictor A's nuclear region, identifying a low-metallicity, merger-related structure with unique kinematic features.

Findings

ELF is photoionized by the AGN.

ELF's kinematics suggest a non-rotational origin.

Low [NII]/Hα ratio indicates low metallicity.

Abstract

We present optical integral field spectroscopy of the inner $2.5 \times 3.4$ kpc$^2$ of the broad-line radio galaxy Pictor A, at a spatial resolution of $\approx 400$ pc. Line emission is observed over the whole field-of-view, being strongest at the nucleus and in an elongated linear feature (ELF) crossing the nucleus from the south-west to the north-east along PA $\sim 70^\circ$. Although the broad double-peaked H$\alpha$ line and the [OI]6300/H$\alpha$ and [SII]6717+31/H$\alpha$ ratios are typical of AGNs, the [NII]6584/H$\alpha$ ratio (0.15 - 0.25) is unusually low. We suggest that this is due to the unusually low metallicity of the gas. Centroid velocity maps show mostly blueshifts to the south and redshifts to the north of the nucleus, but the velocity field is not well fitted by a rotation model. Velocity dispersions are low (< 100 km s$^{-1}$) along the ELF, ruling out a jet-cloud interaction as the origin of this structure. The ELF shows both blueshifts and redshifts in channel maps, suggesting that it is close to the plane of the sky. The ELF is evidently photoionized by the AGN, but its kinematics and inferred low metallicity suggest that this structure may have originated in a past merger event with another galaxy. We suggest that the gas acquired in this interaction may be feeding the ELF.