Two Red Clumps and the X-shaped Milky Way Bulge

From Two Micron All Sky Survey infrared photometry, we find two red clump (RC) populations coexisting in fields toward the Galactic bulge at latitudes |b|>55, ranging over ~13? in longitude and 20? in latitude. These RC peaks indicate two stellar populations separated by ~2.3 kpc; at (l, b) = (+1...

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Bibliographic Details
Published in:The Astrophysical journal 2010-12, Vol.724 (2), p.1491-1502
Main Authors: McWilliam, Andrew, Zoccali, Manuela
Format: Article
Language:English
Subjects:
Astronomy
ASTROPHYSICS, COSMOLOGY AND ASTRONOMY
Earth, ocean, space
Exact sciences and technology
GALAXIES
MAIN SEQUENCE STARS
MILKY WAY
MORPHOLOGY
PHOTOMETRY
SIMULATION
STARS
SUN
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Summary:From Two Micron All Sky Survey infrared photometry, we find two red clump (RC) populations coexisting in fields toward the Galactic bulge at latitudes |b|>55, ranging over ~13? in longitude and 20? in latitude. These RC peaks indicate two stellar populations separated by ~2.3 kpc; at (l, b) = (+1, -- 8) the two RCs are located at 6.5 and 8.8 ? 0.2 kpc. The double-peaked RC is inconsistent with a tilted bar morphology. Most of our fields show the two RCs at roughly constant distance with longitude, also inconsistent with a tilted bar; however, an underlying bar may be present. Stellar densities in the two RCs change dramatically with longitude: on the positive longitude side the foreground RC is dominant, while the background RC dominates negative longitudes. A line connecting the maxima of the foreground and background populations is tilted to the line of sight by ~20??4?, similar to claims for the tilt of a Galactic bar. The distance between the two RCs decreases toward the Galactic plane; seen edge-on the bulge is X-shaped, resembling some extragalactic bulges and the results of N-body simulations. The center of this X is consistent with the distance to the Galactic center, although better agreement would occur if the bulge is 2-3 Gyr younger than 47 Tuc. Our observations may be understood if the two RC populations emanate, nearly tangentially, from the Galactic bar ends, in a funnel shape. Alternatively, the X, or double funnel, may continue to the Galactic center. From the Sun, this would appear peanut/box shaped, but X-shaped when viewed tangentially.
ISSN:0004-637X
1538-4357
DOI:10.1088/0004-637X/724/2/1491