The warp of the Galactic stellar disk detected in IRAS source counts

Abstract

We investigate the tentative evidence for a warp in the Galactic stellar disk, similar to the well-known warp seen in the H I gas, using a sample of ~ 90,000 IRAS point sources as disk tracers. The IRAS sources were selected on the basis of infrared colors, and most of them are probably evolved, dust-shell stars. Analysis of their distribution near the Galactic plane shows a systematic variation of characteristic latitude with the longitude. The sources are found preferentially above the plane near l^(II) ~ 90°-100°, and below the plane near l^(II) ~ 240°-270°, with the amplitude of the deviation increasing at the fainter flux levels (i.e., for more distant sources). Simple sine-wave models provide good fits to the data, whereas the null hypothesis (no variation with the longitude) can be rejected with a high degree of significance. This behavior is at least qualitatively as expected, if the IRAS sources follow the H I warp. We conclude that the stellar disk of our Galaxy is warped at large radii in a way similar to the H I layer, and that the warp is an important characteristic of our Galaxy as a whole. Since most of the IRAS stars are relatively old (several Gyr), we propose that the warp is a long-lasting phenomenon, possibly caused by asymmetries of the mass distribution in the outer regions of the Galactic dark halo. The projected amplitude of the IRAS warp may be lower than expected if the stars and the gas trace each other at all radii. We tentatively explain this as a signature of the radial truncation of the Galactic stellar disk. We also find that the mean disk plane, as delineated by the IRAS sources, is ~ 0°1-0°2 below b^(II) = 0°; this may reflect the z-distance of the Sun above the disk plane.