Extinction within 10 of the Galactic centre using 2MASS

Abstract

We extract J and Ks magnitudes from the 2MASS Point Source Catalog for approximately 6×106 stars with 8 < - Ks < - 13 in order to build an AK extinction map within 10◦ of the Galactic centre. The extinction was determined by fitting the upper giant branch of (Ks , J − Ks ) colour–magnitude diagrams to a dereddened upper giant branch mean locus built from previously studied bulge fields. The extinction values vary from AK =0.05 in the edges of the map up to AK =3.2 close to the Galactic centre. The 2MASS extinction map was compared to that recently derived from DENIS data. Both maps agree very well up to AK = 1.0. Above this limit, the comparison is affected by increased internal errors in both extinction determination methods. The 2MASS extinction values were also compared to those obtained from dust emission in the far-infrared using DIRBE/IRAS. Several systematic effects likely to bias this comparison were addressed, including the presence of dust on the background of the bulk of 2MASS stars used in the extinction determination. For the region with 3◦ <|b|<5◦, where the dust contribution on the far side of the Galaxy is ≈ 5 per cent, the two extinction determinations correlate well, but the dust emission AK values are systematically higher than those from 2MASS. A calibration correction factor of 76 per cent for the DIRBE/IRAS dust emission extinction is needed to eliminate this systematic effect. Similar comparisons were also carried out for the 1◦ < |b| < 3◦ and |b| < 0.5◦ strips, revealing an increasing complexity in the relation between the two extinction values. Discrepancies are explained in terms of the calibration factor, increasing background dust contribution, temperature effects influencing the dust emission extinction and limitations in the 2MASS extinction determination in very high extinction regions (|b| < 0.5◦). An asymmetry relative to the Galactic plane is observed in the dust maps, roughly in the sense that AK values are 60 per cent smaller in the south than in the north for 1◦ <|b|<5◦. This asymmetry is due to the presence of foreground dust clouds mostly in the northern region of the bulge.