Some Properties of Dust Outside the Galactic Disk (1606.08609v1)

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 coefficients E(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 microns 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 $|Z|\approx2.4$ kpc, and, further out, stabilizes or may increase again. The coefficients under consideration change with coordinate $Z$ with a period of about 1312\pm40 pc, coinciding every 656\pm20 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 Rv within 600 pc of the Galactic midplane through the reduction of different photometric data for stars of different classes.

• The paper analyzes interstellar dust properties outside the Galactic disk using 2MASS/WISE data, revealing that mean dust grain sizes are larger than within the disk.
• Extinction coefficients change periodically with Galactic coordinate Z, showing an anticorrelation between northern and southern hemispheres over a 1312 ± 40 pc period.
• The ratio of short- to long-wavelength extinction decreases with distance from the Galactic plane, suggesting coarse dust is more prevalent at high latitudes.

Analysis of Dust Properties in High Galactic Latitudes

The paper presents a comprehensive paper on the characteristics of interstellar dust grains located outside the Galactic disk, utilizing photometric data from the 2MASS and WISE catalogues. The joint use of near- and mid-infrared photometry enables an insightful analysis and extrapolation of the extinction law and dust grain size distribution at high Galactic latitudes.

Methodology and Data Utilization

For this analysis, the paper leveraged extensive IR photometry data from the 2MASS and WISE surveys, collectively covering over 900 million stars. The research focuses on stars at high latitudes, ensuring high photometric accuracy, and applies a modified method of extinction law extrapolation, particularly targeting clump giants. Clump giants are preferred due to their evolved nature and stable spectral energy distribution, assisting in accurate distance estimation even with extinction values neglected for high latitudes.

Results and Interpretation

Key results highlight the variations in the extinction coefficients—E(H−W1)/E(H−Ks), E(H−W2)/E(H−Ks), and E(H−W3)/E(H−Ks)—with significant findings noted:

• Size Distribution and Extinction Variations: Outside the Galactic disk, the mean dust grain size is larger than within the disk. A notable trend reveals that the ratio of short- to long-wavelength extinction decreases with distance from the Galactic plane, stabilizing or potentially increasing beyond 2.4 kpc.
• Periodic Structure: The coefficients change periodically with the Galactic coordinate Z, with a significant anticorrelation observed between the northern and southern hemispheres. This oscillation, recurring every 656 pc within a broader 1312 ± 40 pc period, suggests a large-scale periodic structure of the interstellar medium extending at least 5 kpc.

Implications and Future Directions

The research provides critical insights into the Galactic diffusion of dust, indicating that coarse dust is more prevalent at high latitudes compared to the Galactic plane. The periodic pattern observed could be indicative of a density wave mechanism akin to that responsible for the spiral arm structure of the Galactic disk.

This paper opens up future research opportunities to explore the global structure of the interstellar medium, investigate mechanisms of dust sorting and distribution, and improve the mapping accuracy of the Galactic dust environment. Further integration of data from forthcoming IR astronomical surveys may refine these observations and reduce uncertainties in the cyclic variations of dust properties.