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

The gross features in the distribution of stars as well as warm ($T \ga 100$ 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 and 21 μm 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 ($-90^{\circ} < l \le 90^{\circ}$), and quantifies characteristic angular scales, γ, along latitude. These γs have been translated to linear scale heights (zh) and radial length scales (Rl) using geometric description of the Galactic disk. The distribution of warm dust in the Galactic disk is found to be characterised by $R_{\rm l} < 6$ kpc and $60 \la z_{\rm h} \la 100$ pc, consistent with other studies. The location of the Galactic mid-plane as a function of longitude (in all 4 MSX bands), 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. Carrying out an identical analysis of the DIRBE/COBE data (with lower angular resolution) in all its ten bands covering the entire infrared spectrum (1.25–240 μm), 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.