The First Distance Constraint on the Renegade High Velocity Cloud Complex WD

TL;DR

This study provides the first distance constraint on high-velocity cloud complex WD, showing it is closer than 4.4 kpc and likely ejected from the solar neighborhood, challenging previous assumptions about its galactic motion.

Contribution

It introduces the first direct distance measurement to complex WD and reveals its non-co-rotation with the Galactic disk, suggesting a local ejection origin.

Findings

Complex WD is closer than 4.4 kpc from the Sun.

Complex WD is not in galactic co-rotation.

Likely ejected from the solar neighborhood.

Abstract

We present medium-resolution, near-ultraviolet VLT/FLAMES observations of the star USNO-A0600-15865535. We adapt a standard method of stellar typing to our measurement of the shape of the Balmer epsilon absorption line to demonstrates that USNO-A0600-15865535 is a blue horizontal branch star, residing in the lower stellar halo at a distance of 4.4 kpc from the Sun. We measure the H & K lines of singly-ionized calcium and find two isolated velocity components, one originating in the disk, and one associated with high-velocity cloud complex WD. This detection demonstrated that complex WD is closer than ~4.4 kpc and is the first distance constraint on the +100 km/s Galactic complex of clouds. We find that Complex WD is not in corotation with the Galactic disk as has been assumed for decades. We examine a number of scenarios, and find that the most likely is that Complex WD was ejected from the solar neighborhood and is only a few kpc from the Sun.