The Kinematic Properties of the Extended Disks of Spiral Galaxies: A Sample of Edge-On Galaxies

TL;DR

This study investigates the outer regions of 17 edge-on spiral galaxies using deep spectroscopy, revealing extended thin disks, consistent disk-like kinematics, and some anomalies, thus enhancing understanding of galaxy disk structures beyond the optical radius.

Contribution

It provides new observational evidence of extended thin disks and detailed kinematic properties in the outer regions of edge-on spiral galaxies using deep spectroscopic data.

Findings

H-alpha emission detected beyond R_25 in many galaxies

Outer disk kinematics are generally disk-like with flat rotation curves

Strong limits on counter-rotating gas out to 1.5 R_25

Abstract

We present a kinematic study of the outer regions (R_25<R<2 R_25) of 17 edge-on disk galaxies. Using deep long-slit spectroscopy (flux sensitivity a few 10^-19 erg s^-1 cm^-2 arcsec^-2), we search for H-alpha emission, which must be emitted at these flux levels by any accumulation of hydrogen due to the presence of the extragalactic UV background and any other, local source of UV flux. We present results from the individual galaxy spectra and a stacked composite. We detect H-alpha in many cases well beyond R_25 and sometimes as far as 2 R_25. The combination of sensitivity, spatial resolution, and kinematic resolution of this technique thus provides a powerful complement to 21-cm observations. Kinematics in the outer disk are generally disk-like (flat rotation curves, small velocity dispersions) at all radii, and there is no evidence for a change in the velocity dispersion with radius. We place strong limits, few percent, on the existence of counter-rotating gas out to 1.5 R_25. These results suggest that thin disks extend well beyond R_25; however, we also find a few puzzling anomalies. In ESO 323-G033 we find two emission regions that have velocities close to the systemic velocity rather than the expected rotation velocity. These low relative velocities are unlikely to be simply due to projection effects and so suggest that these regions are not on disk-plane, circular orbits. In MCG-01-31-002 we find emission from gas with a large velocity dispersion that is co-rotating with the inner disk.