2010A&A...515A..49R

We studied the stellar population in the central 6.6'x6.6'region of the ultra-deep (1Msec) Chandra Galactic field - the ``Chandra bulge field'' (CBF) approximately 1.5 degrees away from the Galactic Center - using the Hubble Space Telescope ACS/WFC blue (F435W) and red (F625W) images. We mainly focus on the behavior of red clump giants - a distinct stellar population, which is known to have an essentially constant intrinsic luminosity and color. By studying the variation in the position of the red clump giants on a spatially resolved color-magnitude diagram, we confirm the anomalous total-to-selective extinction ratio, as reported in previous work for other Galactic bulge fields. We show that the interstellar extinction in this area is <A<sub>F625W</sub>≥4 on average, but varies significantly between ∼3-5 on angular scales as small as 1 arcminute. Using the distribution of red clump giants in an extinction-corrected color-magnitude diagram, we constrain the shape of a stellar-mass distribution model in the direction of this ultra-deep Chandra field, which will be used in a future analysis of the population of X-ray sources. We also show that the adopted model for the stellar density distribution predicts an infrared surface brightness in the direction of the ``Chandra bulge field'' in good agreement (i.e. within ∼15%) with the actual measurements derived from the Spitzer/IRAC observations.