Interstellar extinction and the distribution of stellar populations in the direction of the ultra-deep Chandra Galactic field

¹ Excellence Cluster Universe, Technische Universität München, Boltzmannstr. 2, 85748 Garching, Germany e-mail: mikej@mpa-garching.mpg.de
² Space Research Institute, Russian Academy of Sciences, Profsoyuznaya 84/32, 117997 Moscow, Russia
³ Visiting Fellow, Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
⁴ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA

Received: 22 October 2009
Accepted: 3 March 2010

Abstract

We studied the stellar population in the central 6.6´ × 6.6´ 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_F625W = 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.