Kinematic classification of non-interacting spiral galaxies

TL;DR

This paper introduces a kinematic classification method for non-interacting spiral galaxies using HI rotation curves, employing hierarchical clustering to identify five classes and exploring their correlations with galaxy properties.

Contribution

The study develops a new classification scheme based on rotation curve parameters and hierarchical clustering, linking kinematic classes to galaxy morphology and other properties.

Findings

Galaxies with higher rotational velocities tend to be earlier types (Sb/Sbc).

Increasing maximum rotational velocity correlates with brightness, size, and star formation rate.

Lower velocities are associated with higher HI to dynamical mass ratios.

Abstract

Using neutral hydrogen (HI) rotation curves of 79 galaxies, culled from the literature, as well as measured from HI data, we present a method for classifying disk galaxies by their kinematics. In order to investigate fundamental kinematic properties we concentrate on non-interacting spiral galaxies. We employ a simple parameterized form for the rotation curve in order to derive the three parameters: the maximum rotational velocity, the turnover radius and a measure of the slope of the rotation curve beyond the turnover radius. Our approach uses the statistical Hierarchical Clustering method to guide our division of the resultant 3D distribution of galaxies into five classes. Comparing the kinematic classes in this preliminary classification scheme to a number of galaxy properties we find that our class containing galaxies with the largest rotational velocities has a mean morphological type of Sb/Sbc while the other classes tend to later types. Other trends also generally agree with those described by previous researchers. In particular we confirm correlations between increasing maximum rotational velocity and the following observed properties: increasing brightness in B-band, increasing size of the optical disk (D_25) and increasing star formation rate (as derived using radio continuum data). Our analysis also suggests that lower velocities are associated with a higher ratio of the HI mass over the dynamical mass. Additionally, three galaxies exhibit a drop in rotational velocity amplitude of >~ 20% after the turnover radius. However recent investigations suggest that they have interacted with minor companions which is a common cause for declining rotation curves. (Figures 12, 14, 16 and 17 are interactive in the electronic pdf version of this paper.)