Bulge-Disk Decompositions and Structural Bimodality of Ursa Major Cluster Spiral Galaxies

TL;DR

This study performs detailed bulge-disk decompositions of spiral galaxies in the Ursa Major cluster, confirming a bimodal distribution in their structural parameters and linking it to dynamical properties.

Contribution

It introduces an improved 1D bulge-disk decomposition method that accounts for complex features and confirms the bimodality in galaxy structures with statistical significance.

Findings

Disk central surface brightnesses are bimodal with 80% confidence.

Evidence of a luminosity function minimum at M_K' ~ -22.

Structural bimodality correlates with dynamical bimodality from HI line widths.

Abstract

We present bulge and disk (B/D) decompositions of existing K'-band surface brightness profiles for 65 Ursa Major cluster spiral galaxies. This improves upon the disk-only fits of Tully et al. (1996). The 1996 disk fits were used by Tully & Verheijen (1997) for their discovery of the bimodality of structural parameters in the UMa cluster galaxies. It is shown that our new 1D B/D decompositions yield disk structural parameters that differ only slightly from the basic fits of Tully et al. and evidence for structural bimodality of UMa galaxies is maintained. Our B/D software for the decomposition of 1D surface brightness profiles of galaxies uses a non-linear minimization scheme to recover the best fitting Sersic bulge and exponential disk while accounting for the possible presence of a compact nucleus and spiral arms and for the effects of seeing and disk truncations. In agreement with Tully & Verheijen, we find that the distribution of near-infrared disk central surface brightnesses is bimodal with an F-test confidence of 80%. There is also strong evidence for a local minimum in the luminosity function at M_K' ~ -22. A connection between the brightness bimodality and a dynamical bimodality, based on new HI line widths, is identified. The B/D parameters are presented in an Appendix.