The Galactic Bulge Structure Detected by “2MASS Variables”

Shinjirou KOUZUMA and Hitoshi YAMAOKA

Department of Physics, Faculty of Sciences, Kyushu University

4-2-1 Ropponmatsu, Chuo-ku, Fukuoka 810-8560

1. Introduction

Late-type stars are main constituents of the central region of our Galaxy. The structure of the Galaxy has been often discussed using late-type stars as the tracers: e.g. star counts of IRAS sources (Nakada et al. 1991), red clump stars in the bulge (e.g. Stanek et al. 1994).

Here, we report our discovery of large-amplitude variables (most of them should be late-type stars) by comparing overlapping regions in the 2MASS public images in the Galactic center and discuss about the structure of the Galaxy.

2. Data Analysis

The 2MASS (Two Micron All Sky Survey) is the project which observed whole sky in the near-infrared wavelengths (J: 1.25μm, H: 1.65μm, Ks: 2.17μm). Usually, in order to observe without spatial gaps, it is necessary to overlap the neighboring images. In other words, a part of region in the images is observed multiple times in different epochs. For picking up variables using overlapping regions, we had to perform several steps: identification of the overlapping regions, relative photometry, comparing of relative magnitudes, confirming the variability. It is difficult to perform these steps by person-hands because of the vast amount of the images (we used about 70,000 images in this study), so we invented semi-automated system for doing that.

3. Result and Discussion

Investigated region is the central region of the Galaxy (). As a result of investigation, we could have detected 134 variables. In Fig. 1, the number of detection in seems to be larger than in . We noticed this asymmetry and investigated detailed spatial distribution by plotting the number density for every Galactic longitude.

Fig. 2 shows the number density distribution. The decrease from to can be explained by the Galactic bar structure. The reason is that the number of detection in should be larger than in because the edge of the bar in is closer than in . Moreover, if structure reflects uniform bar, extraordinary decreases at and are strange. These are interpreted as either of following: statistical error, difference of extinction by dust, existence of inner structures.However, previous studies suggested the existence of inner structure in (e.g. Alard 2001). Therefore, if structure is real one, it is highly probable that structure is also real inner structure newly detected. To confirm this structure, it is necessary to improve the accuracy of statistics.

References

Alard, C. 2001, A&A, 379, L44

Nakada, Y., Deguchi, S., Hashimoto, O., Izumiura, H., Onaka, T., Sekiguchi, K., & Yamamura, I. 1991, Nat, 353, 140

Stanek, K. Z., Mateo, M., Udalski, A., Szymański, M., Kałużny, J., & Kubiak, M. 1994, ApJ, 429, L73