The Age and Structure of the Galactic Bulge from Mira Variables

TL;DR

This study analyzes Mira variables in the Galactic bulge to reveal age-dependent structural differences, indicating a bar structure for younger stars and a spheroidal distribution for older stars, refining the Galactic center distance estimate.

Contribution

It provides the first comprehensive analysis combining multiple surveys to show age-related structural variations in the Galactic bulge using Mira variables.

Findings

Longer period Miras trace a bar structure inclined to the line of sight.

Shorter period Miras are distributed spheroidally in the outer bulge.

Estimated distance to the Galactic center is 8.9 kpc.

Abstract

We report periods and JHKL observations for 648 oxygen-rich Mira variables found in two outer bulge fields at b=-7 degrees and l=+/-8 degrees and combine these with data on 8057 inner bulge Miras from the OGLE, Macho and 2MASS surveys, which are concentrated closer to the Galactic centre. Distance moduli are estimated for all these stars. Evidence is given showing that the bulge structure is a function of age. The longer period Miras (log P > 2.6, age about 5 Gyr and younger) show clear evidence of a bar structure inclined to the line of sight in both the inner and outer regions. The distribution of the shorter period (metal-rich globular cluster age) Miras, appears spheroidal in the outer bulge. In the inner region these old stars are also distributed differently from the younger ones and possibly suggest a more complex structure. These data suggest a distance to the Galactic centre, R0, of 8.9 kpc with an estimated uncertainty of 0.4 kpc. The possible effect of helium enrichment on our conclusions is discussed.