Mapping the Asymmetric Thick Disk I. A Search for Triaxiality

TL;DR

This study investigates the asymmetry in faint blue star distribution in the inner Galaxy to determine if it indicates a triaxial Thick Disk, concluding it does not based on extended survey data.

Contribution

The paper provides new wide field photometry data extending the survey to higher Galactic longitudes, testing the triaxiality hypothesis of the Thick Disk.

Findings

No excess of faint blue stars at l > 55 degrees.

Asymmetry is not due to a triaxial Thick Disk.

Extended survey confirms previous asymmetry observations.

Abstract

A significant asymmetry in the distribution of faint blue stars in the inner Galaxy, Quadrant 1 (l = 20 to 45 degrees) compared to Quadrant 4 was first reported by Larsen & Humphreys (1996). Parker et al (2003, 2004) greatly expanded the survey to determine its spatial extent and shape and the kinematics of the affected stars. This excess in the star counts was subsequently confirmed by Juric et al. (2008) using SDSS data. Possible explanations for the asymmetry include a merger remnant, a triaxial Thick Disk, and a possible interaction with the bar in the Disk. In this paper we describe our program of wide field photometry to map the asymmetry to fainter magnitudes and therefore larger distances. To search for the signature of triaxiality, we extended our survey to higher Galactic longitudes. We find no evidence for an excess of faint blue stars at l > 55 degrees including the faintest magnitude interval. The asymmetry and star count excess in Quadrant 1 is thus not due to a triaxial Thick Disk.