Very Extended Ionized Gas Discovered around NGC 1068 with the Circumgalactic H$\alpha$ Spectrograph

TL;DR

This study used a new spectrograph to map faint ionized gas around NGC 1068, discovering a previously unknown large ionized gas ribbon that extends beyond known structures, revealing complex ionization processes in the galaxy's halo.

Contribution

First ultra-low surface brightness Hα mapping around NGC 1068 with the new Circumgalactic Hα Spectrograph, discovering a novel ionized gas ribbon beyond existing halo features.

Findings

Discovered a new ionized gas Ribbon 20 kpc from NGC 1068.

The Ribbon's Hα brightness ranges from 4 to 16 Rayleighs.

The Ribbon's morphology correlates with extended UV emission.

Abstract

We have performed wide-field, ultra-low surface brightness H$\alpha$ emission line mapping around NGC 1068 with the newly commissioned Circumgalactic H$\alpha$ Spectrograph (\chas). NGC 1068 is notable for its active galactic nucleus, which globally ionizes gas in the disk and halo. Line-emitting diffuse ionized gas is distributed throughout the galactic disk and large-scale ionized filaments are found well beyond the disk, aligned with the cone angle of the central jet. We report the discovery of a new Ribbon of ionized gas around NGC 1068 beyond even the known outer filamentary structure, located 20 kpc from the galaxy. The H$\alpha$ surface brightness of this Ribbon is on the order of the bright Telluric lines, ranging from $[4-16]$ R with fainter regions on the order of the sky background continuum. Unlike previous extended emission, the Ribbon is not as well aligned with the current axis of the central jet. It is not associated with any galactic structure or known tidal features in the halo of NGC 1068, though it may originate from a larger distribution of unmapped neutral atomic or molecular gas in the halo. The morphology of the Ribbon emission in H$\alpha$ is correlated with extended UV emission around NGC 1068. H$\alpha$ to UV flux ratios in the Ribbon are comparable to extended emission line ratios in the halos of NGC 5128, NGC 253, and M82. The H$\alpha$ excess in the Ribbon gas suggests ionization by slow-shocks or a mixture of in-situ star formation and photoionization and collisional ionization processes.