A Tale of Two Dust Disks in Our Milky

Abstract

Cosmic dust plays a vital role in stellar and galactic formation and evolution, but its three-dimensional structure in the Milky Way has remained unclear due to insufficient precise reddening and distance measurements. Although early studies typically adopted a single-disk model, we detect two distinct components at Galactocentric distances of 5-14 kpc, enabled by photometric, spectroscopic, and astrometric measurements of over 5 million stars. The thin dust disk's scale height increases radially from 60 to 200 pc, while the thick disk grows from 300 to 800 pc. For the first time, we find the thin and thick dust disk correlates spatially with molecular and atomic hydrogen disk, respectively. The thin, thick, and combined disks have scale lengths of 9.6+1.2-1.1 kpc, 4.2+0.4-0.3 kpc, and 6.6+0.3-0.3 kpc, respectively. The gas-to-dust ratio shows an exponential radial gradient, increasing from around 60 at 5 kpc to around 470 at 14 kpc. These findings provide new insights into dust morphology in the Galaxy and raise fundamental questions that require further investigation.

Figures (9)

• Figure 1: Spatial distribution of selected LAMOST and APOGEE stars in Galactic coordinates. Blue points represent LAMOST stars, and yellow points represent APOGEE stars.
• Figure 2: Comparison of star-pair reddening estimates with SFD reddening values for selected individual stars. The points in the left and middle panels are divided into bins, with blue open circles indicating the median values. The blue dashed lines representing the lines of equality. The right panel shows the histogram distributions and Gaussian fits of $E(B-V)_{SFD-LAMOST}$ (blue lines) and $E(B-V)_{SFD-APOGEE}$ (orange lines) for the selected stars.
• Figure 3: Extinction-distance plots for two sightline examples. The left panel shows a low-latitude sightline at $(l,b)=(153.3^\circ,6.6^\circ)$, while the right panel shows a medium-latitude sightline at $(l,b)=(265.5^\circ,-52.0^\circ)$. Gray dots represent the $E(B-V)$ values of individual stars within 5 kpc. The blue diamonds indicate the median $E(B-V)$ for each distance bin.
• Figure 4: Distribution of dust gradients in the $R-Z$ plane. (a) Distribution of all dust reddening gradients in the $R-Z$ coordinate system. The color represents the value of reddening gradients, with gray indicating negative values. The sun icon represents the position of the Sun. (b) Same as (a), but pixelated. Gray bins indicate regions lacking sufficient data. (c) Single-disk model fit to (b). (d) Fitting residuals of the single-disk model. (e) Two-disk model fit to (b). (f) Fitting residuals of the two-disk model.
• Figure 5: Distribution of errors in pixelated reddening gradients in the $R-Z$ coordinate system.The color represents the estimated errors in reddening gradients, while gray bins indicate regions lacking sufficient data.