We investigate the dependence of the extended X-ray emission from the halos of optically luminous early-type galaxies on the small-scale (the nearest neighbor distance) and large-scale (the average density inside the 20 nearest galaxies) environments. We cross-match the 3rd Data Release of the Second XMMNewton Serendipitous Source Catalog (2XMMi-DR3) to a volume-limited sample of the Sloan Digital Sky Survey (SDSS) Data Release 7 with Mr < −19.5 and 0.020 < z < 0.085, and find 20 early-type galaxies that have extended X-ray detections. The X-ray luminosity of the galaxies is found to have a tighter correlation with the optical and near infrared luminosities when the galaxy is situated in the low large-scale density region than in the high large-scale density region. Furthermore, the X-ray to optical (r-band) luminosity ratio, LX/Lr, shows a clear correlation with the distance to the nearest neighbor and with large-scale density environment only where the galaxies in pair interact hydrodynamically with seperations of rp < rvir. These findings indicate that the galaxies in the high local density region have other mechanisms that are responsible for their halo X-ray luminosities than the current presence of a close encounter, or alternatively, in the high local density region the cooling time of the heated gas halo is longer than the typical time between the subsequent encounters.

ABSTRACT
1. INTRODUCTION
2. SAMPLE SELECTION AND DATA ANAL-YSIS
    2.1 The Match between SDSS Optical and XMM-Newton X-Ray Surveys
    2.2 Environmental Parameters
    2.3 Analysis of X-ray Data
3. ENVIRONMENTAL DEPENDENCE OF X-RAY LUMINOSITIES
    3.1 LX-Lr and LX-LK Relations
    3.2 Environmental Effect on LX/Lr
4. SUMMARY
REFERENCES

• EUNBIN KIM(School of Space Research, Kyung Hee University)
• YUN-YOUNG CHOI(Department of Astronomy & Space Science, Kyung Hee University)
• SUNGSOO S. KIM(School of Space Research, Kyung Hee University, Department of Astronomy & Space Science, Kyung Hee University)