CAHA/PPAK Integral-field Spectroscopic Observations of M81 -- I. Circumnuclear ionized gas

TL;DR

This study uses optical integral-field spectroscopy to analyze the ionized gas in M81's central region, revealing a nuclear spiral, ionized gas mass, possible outflows, and diverse ionization mechanisms.

Contribution

It provides the first detailed integral-field spectroscopic analysis of M81's circumnuclear ionized gas, highlighting complex ionization sources and gas dynamics.

Findings

Ionized gas concentrated within a 200 pc core and nuclear spiral.

Estimated ionized gas mass of approximately 2×10^5 solar masses.

Evidence of bi-conical outflow signatures near the nucleus.

Abstract

Galactic circumnuclear environments of nearby galaxies provide unique opportunities for our understanding of the co-evolution between super-massive black holes and their host galaxies. Here we present a detailed study of ionized gas in the central kiloparsec region of M81, which hosts the closest prototype low-luminosity active galactic nucleus, based on optical integral-field spectroscopic observations taken with the CAHA 3.5m telescope. It is found that much of the circumnuclear ionized gas is concentraed within a bright core of $\sim$200 pc in extent and a surrounding spiral-like structure known as the nuclear spiral. The total mass of the ionized gas is estimated to be $\sim2\times10^5\rm~M_\odot$, which corresponds to a few percent of the cold gas mass in this region, as traced by co-spatial dust extinction features. Plausible signature of a bi-conical outflow along the disk plane is suggested by a pair of blueshifted/redshifted low-velocity features, symmetrically located at $\sim$ 120 -- 250 pc from the nucleus. The spatially-resolved line ratios of [N\,{\sc ii}]/H$\alpha$ and [O\,{\sc iii}]/H$\beta$ demonstrate that much of the circumnuclear region can be classified as LINER (low-ionization nuclear emission-line region). However, substantial spatial variations in the line intensities and line ratios strongly suggest that different ionization/excitation mechanisms, rather than just a central dominant source of photoionization, are simultaneously at work to produce the observed line signatures.