Extinction map of the Galactic centre: OGLE-II Galactic bulge fields

Takahiro Sumi

doi:10.1111/j.1365-2966.2004.07482.x

Abstract

We present the reddening [E(V−I)] and extinction maps in the V band (AV) and the I band (AI) for 48 Optical Gravitational Lensing Experiment II (OGLE-II) Galactic bulge (GB) fields, covering a range of −11° < l < 11°, with the total area close to 11 deg2. These measurements are based on two-band photometry of red clump giant (RCG) stars in OGLE-II VI maps of the GB. We confirm the anomalous value of the ratio of total to selective extinction RVI≡AV/E(V−I) = 1.9–2.1, depending on the line of sight, as measured recently by Udalski. By using the average value of RVI= 1.964 with the standard deviation sdev= 0.085, we measured E(V−I), AV and AI, and we obtained extinction and reddening maps with a high spatial resolution of 26.7–106.8 arcsec, depending on the stellar density of each field. We have assumed that average, reddening corrected colours of RCGs are the same in every field. The maps cover the ranges 0.42 < E(V−I) < 3.5, 0.83 < AV < 6.9 and 0.41 < AI < 3.4 mag, respectively. The zero-points of these maps are calibrated by using V−K colours of 20 RR Lyrae ab variables (RRab) in Baade's window. The apparent reddening corrected I-band magnitudes of the RCGs change by +0.4 mag while the Galactic coordinate l varies from +5° to −5°, indicating that these stars are in the Galactic bar. The reddening corrected colour of RRab and RCGs in the GB are consistent with colours of local stars, while in the past these colours were claimed to be different.

Highlights

A study of stellar populations and stellar dynamics in the Galactic Bulge (GB) is important for understanding how bulges formed, what are their populations, gravitational potential and structure.Several gravitational microlensing survey groups have found hundreds of events towards the Galactic center and disc (EROS: Derue 1999; OGLE: Udalski et al 2000; Wozniak et al 2001; MACHO: Alcock et al 2000; MOA: Bond et al 2001; Sumi et al 2003a), and thousands are expected in the upcoming years by MOA 1, OGLE-III 2 and other collaborations
To measure the ratio of total to selective extinction, i.e. RI ≡ AI /E(V − I), we make use of the position of Red Clump Giant (RCG) in the (I,V − I) Color Magnitude Diagram (CMD), as it was done by Udalski (2003), but contrary to Wozniak & Stanek (1996) and Stanek (1996), who used the (V,V − I) CMD
In a given bin we measure the average colour and magnitude of RCGs within the circle with a radius of 0.4 mag in the CMD centered at the colour V − I RC on the reddening line

Summary

INTRODUCTION

A study of stellar populations and stellar dynamics in the Galactic Bulge (GB) is important for understanding how bulges formed, what are their populations, gravitational potential and structure. They used RCGdominated parts of the CMDs for determining the offsets, the clump being seen at fainter magnitudes and redder colours in subfields with higher extinction They applied this method to the OGLE-I data and found the ratio of total to selective extinction RV I ≡ AV /E(V − I) = 2.44. Paczynski et al (1999) and Sumi et al 2003a applied this method to OGLE-II (14′.2 × 14′.2 with resolution of 20′′ × 20′′) and MOA (16 deg with resolution of 3.45′ × 3.45′) data respectively They first made a reddening map for their fields because determining the reddening E(V − I) (horizontal shift in the CMD) is easier than AV and AI (vertical shift in the CMD). Details of the instrumentation setup can be found in Udalski, Kubiak & Szymanski (1997)

THE RATIO OF TOTAL TO SELECTIVE EXTINCTION

Relative Reddening

Zero point

DISCUSSION AND CONCLUSION