The Bulge Radial Velocity Assay

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The Project


Overview

BRAVA is a large-scale radial velocity survey of the Galactic bulge. The aim of this survey is to advance the scientific knowledge of how our Galactic bulge and Milky Way formed. This can be done by using the radial velocities from the BRAVA stars surveyed to test and constrain dynamical models of the bulge, and to quantify the importance, if any, of cold stellar streams in the bulge and its vicinity.

M giants in the bulge were chosed to be dynamical probes for BRAVA. M giants are relatively bright (with luminosities 300 or more times that of the Sun's), they are known to be ubiquitous throughout the Galactic bulge, and they are cool enough to form titanium oxide (TiO) molecules in their atmospheres. Hence, they are excellent probes for a radial velocity survey.

The M giants were chosed from the Two Micron All Sky Survey catalog and used as targets for the Cerro Tololo Inter-American Observatory 4 m Hydra multiobject spectrograph.


Why the bulge is worth a survey
  • The bulge is thought of as a spheroid. "Spheroids" consist of 50-70% of the stellar mass in local Universe (Fukugitaet al. 1998). It is important to understand the properties of spheroids and their formation history.
  • Lyman-break (z> 3) galaxies metal-rich star formation and evidence for metal-rich winds. Chemical evolution.
  • Epoch, population of bulge formation? EROs? BzK, LBG? Other?
  • It is known that the Bulge has a unique formation history and stellar population. (link to extragalactic spheroids, high Mg2 index (Wortheyet al. 1993) metal poor stars?
  • Bulges host supermassive black holes, and our Bulge is no exception. However, how these supermassive black holes formed there is still unknown. Perhaps this is related to bulge formation? (Gebhardt et al., Ferrarese et al., Ghez et al.).
  • Metal rich halos are widespread: M31 (Durrell 1994, Rich 1996) other galaxies (Mouhcineet al. 2005).
  • There have already been ~15 hot Jupiter transit planets discovered using HST imaging of Bulge field (Sahuet al. 2006). The stars and other celestial objects located in the Galactic Bulge have proven time and time again to be very interesting in a wide range of applications.


Strategy
  • Use M giants as dynamical tracers and from the stellar dynamics, learn about our bulge. M giants are brighter than clump giants and can be observed in high extinction fields. M giants also trace the 2 micron light of the bulge.
  • Select M giants from the 2MASS survey. Hence they have excellent, uniform astrometry and photometry; ease of developing links to to spectra for a public database.
  • Take advantage of the fact that a clear red giant branch can easily seen in the 2MASS data; this helps distinguish clear bulge membership.
  • Use Cross correlation from 7000 -9000 Angstroms (include Ca IR triplet) to very accurately determine the stars radial velocity.
  • Obtain chemical abundances from either future IR studies or from modeling of optical spectra.
  • To observe these M giants, use 3x10 min exposures with the Hydra fiber spectrograph on the Blanco 4m telescope at the Cerro Tololo Inter-American Observatory (in Chile); about ~100 stars/field at R~4000 can thus be obtain.
  • Radial velocities have been determined for 9,000 stars to date.


Main Conclusions
  • BRAVA is a radial velocity survey of Galactic bulge M giants
  • Survey to date has covered strips at b=-4, -6 and -8, and the Southern minor axis
  • Bulge rotation curve and radial velocity dispersion profile measured
  • Departure from "solid body" rotation at b=-4
  • Cylindrical rotation at -8
  • Data agree remarkably well (independently) with both Fux and Shen N-body bars
  • No detection of cold streams
  • Coadded datasets at b=-4, -6, -8 are Gaussian with no evidence of dynamically independent sub populations
  • Dynamics unaffected by color, luminosity cuts
  • Please see the Publications section for more details


Implications
  • The M giant population shows kinematic uniformity on large scales.
  • The agreement with two independent N-body bar models is striking.
  • The dynamical processes responsible for creating a bar should accelerate stars without respect to abundance; the existence of an abundance gradient cannot be explained by N-body bar models.
  • There is no transition in population from "bar" to "bulge/spheroid" at b = -8, that might help explain the gradient.
  • Growing evidence (chemical, photometric) for early, rapid, bulgeformation; how does this fit in with BRAVA?





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: 05/04/2013