Two more disk galaxies with global gas counterrotation

TL;DR

This study discovers extended counterrotating gas disks in two early-type disk galaxies, suggesting they are in early stages of minor mergers contributing to lenticular galaxy formation.

Contribution

First detection of full-size counterrotating gaseous disks in NGC 2551 and NGC 5631 using combined spectral observations, revealing different orientations and early merger stages.

Findings

Full-size counterrotating gaseous disks identified

One disk coplanar, the other inclined to stellar disk

Evidence of early minor-merger stages in galaxy evolution

Abstract

We report a discovery of extended counterrotating gaseous disks in early-type disk galaxies NGC 2551 and NGC 5631. To find them, we have undertaken complex spectral observations including integral-field spectroscopy for the central parts of the galaxies and long-slit deep spectroscopy to probe the external parts. The line-of-sight velocity fields have been constructed and compared to the photometric structure of the galaxies. As a result, we have revealed full-size counterrotating gaseous disks, the one coplanar to the stellar disk in NGC 2551 and the other inclined to the main stellar disk in NGC 5631. We suggest that we observe the early stages of minor-merger events which may be two different stages of the process of lenticular galaxy formation in rather sparse environments.