Warp signatures of the Galactic disk as seen in mid infrared from Midcourse Space Experiment

Abstract

The gross features in the distribution of stars as well as warm (T >~ 100 K) interstellar dust in the Galactic disk have been investigated using the recent mid infrared survey by Midcourse Space Experiment (MSX) at 8, 12, 14 & 21 micron bands. An attempt has been made to determine the location of the Galactic mid-plane at various longitudes, using two approaches : (i) fitting exponential functions to the latitude profiles and (ii) statistical indicators.The former method is successful for the inner Galaxy (-90 < l < 90), and quantifies characteristic angular scales along latitude, which have been translated to linear scale heights (z_h) and radial length scales (R_l) using geometric description of the Galactic disk. The distribution of warm dust in the Galactic disk is found to be characterised by R_l < 6 kpc and 60 < z_h <~ 100 pc, in agreement with other studies. The location of the Galactic mid-plane as a function of longitude, for stars as well as warm dust, has been searched for signatures of warp-like feature in their distribution, by fitting sinusoid with phase and amplitude as parameters. In every case, the warp signature has been detected. An identical analysis of the DIRBE/COBE data in all its ten bands covering the entire infrared spectrum (1.25-240 micron), also leads to detection of warp signatures with very similar phase as found from the MSX data. Our results have been compared with those from other studies.