The large scale dust lanes of the Galactic bar

Context. Trails of dust inside galactic bars are easily observable in external galaxies. However, information on the dust lanes of the Milky Way bar is harder to obtain due to our position within the Galactic disc. Aims. By comparing the distribution of dust and gas in the central regions of the Galaxy, we aim to obtain new insights into the properties of the offset dust lanes leading the bar's major axis in the Milky Way. Methods. On the one hand, the molecular emission of the dust lanes is extracted from the observed CO l - b - V distribution according to the interpretation of a dynamical model. On the other hand, a three dimensional extinction map of the Galactic central region constructed from near-infrared observations is used as a tracer of the dust itself and clearly reveals dust lanes in its face-on projection. Comparison of the position of both independent detections of the dust lanes is performed in the (l, b) plane. Results. These two completely independent methods are used to provide a coherent picture of the dust lanes in the Milky Way bar. In both the gas and dust distributions, the dust lanes are found to be out of the Galactic plane, appearing at negative latitudes for l > 0 degrees and at positive latitudes for l < 0 degrees. However, even though there is substantial overlap between the two components, they are offset from one another with the dust appearing to lie closer to the b = 0 degrees plane. Conclusions. Two scenarios are proposed to explain the observed offset. The first involves grain destruction by the bar shock and reformation downstream. Due to the decrease in velocity caused by the shock, this occurs at lower z. The second assumes that the gas and dust remain on a common tilted plane, but that the molecular gas decouples from the Milky Way's magnetic field, itself strong enough to resist the shear of the bar's shock. The diffuse gas and dust remain coupled to the field and are carried further downstream. This second scenario has recently been suggested in order to explain observations of the barred galaxy NGC 1097.