The Bulge Radial Velocity Assay

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A key in interpreting observings of our galaxy is the concept of distinct stellar populations in the Milky Way. Each stellar population is characterized by a different spatial distribution, kinematic structure, metal content, and age range. The Milky Way's bulge has been seen as a distinct stellar population since Baade's surveys of RR Lyrae stars and the well-known concentration of red giants toward the Sagittarius region (Blanco 1965). Further, the Cosmic Background Explorer (COBE) satellite clearly showed that the Galaxy has an unambiguous, asymmetric, peanut-shaped bulge. This is shown on plot on the right, which shows the results from the Diffuse InfraRed Background Experiment (DIRBE) instrument mounted on COBE.

Right: Diffuse Infrared Background Experiment (DIRBE) observations of the bulge at 1.25, 2.2, 3.5 and 4.5 microns. Intensities are represented using color contours spaced logarithmically. Calibrated data at a fiexed solar elongation of 90 degrees, after removal of foreground signal from interplanetary dust, correction for extinction and subtraction of the empirical model for the disk. (Figure taken from Weiland 1994, ApJ, 415, 81).

The shape of the bulge is flattened and elliptical in appearance. There is a noticeable longitudinal asymmetry about l=0 in the bulge brightness contours. 

DIRBE observations provided a unique view of the Galctic bulge. After corrections for extincion and light from the stellar disk, the bulge shape is more accurately described as a box than as a peanut. Asymmetries in the bulge brightness contours are qualitatively consistent with the model predictions for a triaxial bar located at the center of the Galaxy, with its near end in the first quadrant. The evidence for a triaxial (barred) bulge is convincing. Not only is the COBE 2 micron light distribution modeled as a bar oriented toward positive Galactic longitude (Dwek et al. 1995; Binney et al. 1997), but star counts of red clump stars (Stanek et al. 1997; Babusiaux & Gilmore 2005) show a barlike structure. Microlensing events further show a central bar that is pointed roughly in the direction of the Sun.

As evidence of a bar in the Milky Way has grown, it has become clear that the Galactic bulge offers a potential laboratory to investigate the dynamics of the nearest bar in detail with radial velocities and eventually with proper motions as well as abundances. This recent evidence has prompted renewed interest in the dynamical modeling of the Galactic bar/bulge system in order to determine, in part, the dynamical formation and evolution of galaxies, e.g., the unsolved issue of whether a cold dark matter halo cusp is compatible with the microlensing and rotation of the Milky Way bar (Zhao et al. 1995; Klypin et al. 2002).

Simple schematic of the bulge/bar orientation with respect to the Sun/Galactic center line of sight. Exact values of the angle are uncertain but range from 15 to 35 degrees. On this diagram, positive Galactic latitude (b) points up out of the page.



The dynamical model for the bulge/bar has a number of important implications. Large samples of uniform radial velocity data are of great value in constraining not only the bar versus axisymmetric models but also the nature of the orbit families supporting the bar. Furthermore, the interpretation of the microlensing events in the bulge depends on the use of an accurate dynamical model (Han & Gould 2003). The recent discovery of planetary transit host stars in the bulge (Sahu et al. 2006) gives an additional incentive to improve our knowledge of the bulge/bar model, as it is debated whether stars on noncircular orbits are incapable of bearing planets, if their orbits stray too near the Galactic center. The relationship to the disk, thick disk, and halo populations is of great interest. Only with a realistic dynamical model can the issue of long-term stability be explored. There is also the issue o f whether the bar formed due to the buckling of the disk or is a yet more ancient population.

Right: The BRAVA idea.

BRAVA is a survey based on red giants, the stars which comprise the bulk of the 2.4 micron light of the bulge. M giants offer a dynamical probe that is populous, is luminous, traces the light, and is easily utilized for velocity measurements. M giants are also extremely well studied and have been identified, and their giant branch characterized, over the whole of the bulge, from 3 degrees to 12 degrees (420-1675 pc, adopting R_0=8 kpc). Given their significant contribution to the integrated light, these stars are excellent dynamical probes. The M giants, being luminous, are also usable as probes even in regions of high extinction and for exploring the fringes of the bulge. BRAVA obtained radial velocities of M giants using the HYDRA multiobject spectragraph mounted on the BLANCO 4m telescope.  Observation began in 2005 and continued through 2009, yielding more than 40 individual bulge fields and a hand full of disk fields.

The above plot shows the observed BRAVA fields, up to 2009, overplotted on the 2MASS infrared image of the Milky Way. Black circles represent fields observed by BRAVA. The size of the circles corresponds to the 40 minute field of view of the instrument. One degree corresponds to 140 pc for a distance of 8 kpc; the b=-4 degree strip lies roughly 550 pc south of the Galactic center. The Galactic plane is avoided due to heavy extinction.

The National Science Foundation provides support for BRAVA through grant AST-0709479.

Bulge and BLANCO telescope
Department of Physics and Astronomy
The Bulge Radial Velocity Assay, or BRAVA was conceived as a survey of the line-of-sight velocity distribution of red giants across the bulge, to be compared with self-consistent dynamical models.

Last updated on: 09/29/2010