Origin of the high v_los feature in the Galactic bar

TL;DR

This study uses simulations to explain the high line-of-sight velocity features in the Milky Way's bar, attributing them to young, dynamically cool stars on specific orbits formed near the bar.

Contribution

It demonstrates that high v_los features can be reproduced by a simulated galaxy with a growing bar, matching APOGEE observations without parameter adjustments.

Findings

High v_los features are associated with young, cool stars on specific orbits.

Simulated v_los distributions match APOGEE data without scaling.

High v_los features diminish at higher latitudes, explaining observational non-detections.

Abstract

We analyse a controlled N-body + smoothed particle hydrodynamics simulation of a growing disc galaxy within a non-growing, live dark halo. The disc is continuously fed with gas and star particles on near-circular orbits and develops a bar comparable in size to the one of the Milky Way (MW). We extract line of sight velocity v_los distributions from the model and compare it to data recently obtained from the APOGEE survey which show distinct high velocity features around v_los ~ 200 km/s. With an APOGEE like selection function, but without any scaling nor adjustment, we find v_los distributions very similar to those in APOGEE. The stars that make up the high v_los features at positive longitudes l are preferentially young bar stars (age <~ 2-3 Gyr) which move away from us along the rear side of the bar. At negative l, we find the corresponding low v_los feature from stars moving towards us. At l>10 degrees the highest v_los stars are a mixture of bar and background disc stars which complicates the interpretation of observations. The main peak in v_los is dominated by fore- and background stars. At a given time, ~40-50 per cent of high v_los stars occupy x_1 like orbits, but a significant fraction are on more complex orbits. The observed feature is likely due to a population of dynamically cool, young stars formed from gas just outside the bar and subsequently captured by the growing bar. The high v_los features disappear at high latitudes |b|>~2 degrees which explains the non-detection of such features in other surveys.