Signatures of bulge triaxiality from kinematics in Baade's window

Hong Sheng Zhao, David N. Spergel, R. Michael Rich

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Abstract

We study a sample of 62 Baade's Window, (l,b)=(1,-4)°, K giants that have published proper motions, radial velocity, and metallicity. Using R0=8 kpc, we construct the velocity ellipsoids, namely the 3×3 velocity dispersion tensors, for the metal rich stars ([Fe/H]≥0) and metal poor stars ([Fe/H]≤-0.2). After diagonalizing the tensor, we find a vertex deviation characteristic of a nonaxisymmetric system. Eigenvalues for the two velocity ellipsoids (σ1, σ2, σ3) are (126, 89, 65)±13 km/s for the metal rich sample and (154, 77, 83)±25 km/s for the metal poor sample with their long axes pointing to two nearly perpendicular directions (lv,bv)=(-65±9°,+14±9°) and (lv,bv)=(25±14°,-11±14°), respectively. The vertex deviations of the velocity ellipsoids cannot be consistently explained by any oblate model. We are able to reject the hypothesis that the metal poor and metal rich populations are drawn from the same distribution at better than the 97% confidence level. We populate orbits in a realistic bar potential with a Gaussian velocity distribution, allowing us to simulate and interpret observations. We conclude that the data are consistent with a triaxial bulge pointing towards (l,b) with l<0° and b=0° as suggested by earlier work on gas dynamics and the observed light distribution. We also predict that low latitude (|6|≤4°) bulge fields should show the vertex deviation more strongly and would therefore be the best locations for future proper motion studies. In the classification scheme of Athanassoula et al. [A&A 127, 349 (1983)] the metal rich stars appear to occupy the B-family orbits which rotate in the prograde sense in the rest frame and have boxy shapes that are aligned with and supporting the bar. The metal poor stars in the sample lag behind the metal rich bulge and appear to occupy R-family orbits which rotate in the retrograde sense in the rest frame. They have nearly round loop shapes and are aligned perpendicularly to the bar, hence limit the triaxiality of the bar potential. The correlations between the metallicity and the orbit families can develop if the bulge forms dissipatively on a sufficiently long time scale. However, it is difficult to explain such correlations if most stars in the inner Galaxy form during the violent relaxation phase.

Original languageEnglish (US)
Pages (from-to)2154-2163
Number of pages10
JournalAstronomical Journal
Volume108
Issue number6
DOIs
StatePublished - Dec 1994

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science

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