Modelling the Galactic Interstellar Extinction Distribution in Three Dimensions

Aims. The Two Micron All Sky Survey, along with the Stellar Population Synthesis Model of the Galaxy, developed in Besancon, is used to calculate the extinction distribution along different lines of sight. By combining many lines of sight, the large scale distribution of interstellar material can be deduced. Methods. The Galaxy model is used to provide the intrinsic colour of stars and their probable distances, so that the near infrared colour excess, and hence the extinction, may be calculated and its distance evaluated. Such a technique is dependent on the model used, however we are able to show that moderate changes in the model parameters result in insignificant changes in the predicted extinction. Results. This technique has now been applied to over 64 000 lines of sight, each separated by 15´, in the inner Galaxy ($|l|\le$ 100°, $|b|\le$ 10°). We have projected our three dimensional results onto a two dimensional plane in order to compare them with existing two dimensional extinction maps and CO surveys. We find that although differences exist due to the different methods used or the medium traced, the same large scale structures are visible in each of the different maps. Using our extinction map, we have derived the main characteristics of the large scale structure of the dust distribution. The scale height of the interstellar matter is found to be $125^{+17}_{-7}$ pc. The dust distribution is found to be asymmetrically warped, in agreement with CO and HI observations of the ISM. However, the slope of the dust warp and the galactocentric distance where it starts are found to be smaller than the values measured for the external HI disc: for positive longitudes the angle is $\theta=$ $89\degr$, it starts at 8.7 kpc from the Galactic center and grows with a slope of 0.14, while at negative longitudes, the angle of the maximum is at $\theta=$ $272\degr$, the starting radius 7.8 kpc and the slope 0.11. Finally, the presence of dust is detected in the Galactic bulge. It forms an elongated structure approximately 5.2 kpc long and lies at an angle of 30  ± $5\degr$ with respect to the Sun-Galactic centre direction. This may be interpreted as a dust lane along the Galactic bar. This resulting extinction map will be useful for studies of the inner Galaxy and its stellar populations.

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