Liang Cao, Shude Mao, David Nataf, Nicholas J. Rattenbury, Andrew Gould
We present a study of the luminosity density distribution of the Galactic bar using number counts of red clump giants (RCGs) from the OGLE-III survey. The data were recently released by \citet{Nat12} for 9019 fields towards the bulge and have $2.94\times 10^6$ RC stars over a viewing area of $90.25 \,\textrm{deg}^2$. The data include the number counts, mean distance modulus ($\mu$), dispersion in $\mu$ and full error matrix. We fit the data with several tri-axial parametric models, after excluding the bad sightlines, $2.65\times 10^6$ RC stars are remaining with the sky coverage of $81.47 \,\textrm{deg}^2$. We use the Markov Chain Monte Carlo (MCMC) method to explore the parameter space and find that the best-fit model is close to exponential ($E_2$ model), with the distance to the GC is 8.17 kpc, the ratio of semi-major and semi-minor bar axis scale lengths in the Galactic plane $x_{0},y_{0}$, and vertical bar scale length $z_0$, is $x_0:y_0:z_0 \approx 1.00:0.41:0.38$ (close to being prolate). The bar major axis is $\sim$ 0.68 kpc and has an angle of between $29^\circ$ and $32^\circ$, slightly larger than the value obtained from a similar study based on OGLE-II data. The number of estimated RC stars is $2.51 \times 10^6$, which is systematically lower than the observed value. We subtract the smooth parametric model from the observed counts and find that the residuals are consistent with the presence of an X-shaped structure in the Galactic centre, the excess to the estimated mass content is $\sim 5.3%$. The results can be used as a key ingredient to construct new density models of the Milky Way and will have implications on the predictions of the optical depth to gravitational microlensing and the patterns of hydrodynamical gas flow in the Milky Way.
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http://arxiv.org/abs/1303.6430
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