Some properties of dust outside the galactic disk

Abstract

The joint use of accurate near- and mid-infrared photometry from the 2MASS and WISE catalogues has allowed the variations of the extinction law and the dust grain size distribution in high Galactic latitudes (|b| > 50°) at distances up to 3 kpc from the Galactic midplane to be analyzed. The modified method of extrapolation of the extinction law applied to clump giants has turned out to be efficient for separating the spatial variations of the sample composition, metallicity, reddening, and properties of the medium. The detected spatial variations of the coefficientsE(H − W1)/E(H − Ks), E(H − W2)/E(H − Ks), and E(H − W3)/E(H − Ks) are similar for all high latitudes and depend only on the distance from the Galactic midplane. The ratio of short-wavelength extinction to long-wavelength one everywhere outside the Galactic disk has been found to be smaller than that in the disk and, accordingly, the mean dust grain size is larger, while the grain size distribution in the range 0.5–11 µm is shifted toward coarse dust. Specifically, the mean grain size initially increases sharply with distance from the Galactic midplane, then decreases gradually, approaching a value typical of the disk at |Z| ≈ 2.4 kpc, and, further out, stabilizes or may increase again. The coefficients under consideration change with coordinate Z with a period of about 1312 ± 40 pc, coinciding every 656 ± 20 pc to the south and the north and showing a significant anticorrelation between their values in the southern and northern hemispheres at intermediate Z. Thus, there exists a unified large-scale periodic structure of the interstellar medium at high latitudes within at least 5 kpc. The same periodic variations have also been found for the extinction coefficient R _V within 600 pc of the Galactic midplane through the reduction of different photometric data for stars of different classes.