Simulating a slow bar in the low surface brightness galaxy UGC 628

TL;DR

This paper models the low surface brightness galaxy UGC 628 with an N-body simulation, reproducing its slow bar and mass distribution, and challenges the assumption that such galaxies are dark matter dominated everywhere.

Contribution

It provides a detailed dynamical model of UGC 628 showing how a slow bar can form in a submaximal disc with realistic initial conditions.

Findings

UGC 628's inner region is baryon dominated post-bar formation.

The galaxy's dark matter fraction aligns with high surface brightness galaxies.

The disc mass fraction at peak potential is not a reliable indicator of overall dynamical importance.

Abstract

We present a disc-halo N-body model of the low surface brightness galaxy UGC 628, one of the few systems that harbours a "slow" bar with a ratio of corotation radius to bar length of $\mathcal{R} \equiv R_c/a_b \sim 2$. We select our initial conditions using SDSS DR10 photometry, a physically motivated radially variable mass-to-light ratio profile, and rotation curve data from the literature. A global bar instability grows in our submaximal disc model, and the disc morphology and dynamics agree broadly with the photometry and kinematics of UGC 628 at times between peak bar strength and the onset of buckling. Prior to bar formation, the disc and halo contribute roughly equally to the potential in the galaxy's inner region, giving the disc enough self gravity for bar modes to grow. After bar formation there is significant mass redistribution, creating a baryon dominated inner and dark matter dominated outer disc. This implies that, unlike most other low surface brightness galaxies, UGC 628 is not dark matter dominated everywhere. Our model nonetheless implies that UGC 628 falls on same the relationship between dark matter fraction and rotation velocity found for high surface brightness galaxies, and lends credence to the argument that the disc mass fraction measured at the location where its contribution to the potential peaks is not a reliable indicator of its dynamical importance at all radii.