2025A&A...704A.118M

Aims. We present REDLINE, a new technique for mapping three-dimensional interstellar extinction across the Galactic plane, with the aim of revealing the distribution of interstellar dust at distances where traditional mapping approaches face limitations, particularly beyond 4 kpc from the Sun.
Methods. Our approach relies solely on near-infrared stellar observations to achieve a higher sensitivity through high-extinction regions. Building on our previous mapping techniques, REDLINE employs Markov chain Monte Carlo (MCMC) methods to constrain the line-of-sight dust distribution. A further methodological improvement is the incorporation of an enhanced Besançon Galaxy model (BGM), which provides more accurate stellar population predictions. We applied this refined framework to survey data covering the entire Galactic plane at |b| ≤ 1^°, resulting in a map with 10^' angular resolution but on a 5^' grid and with a 100 pc resolution in distance.
Results. REDLINE successfully maps the structure of the Galactic disc, particularly in the inner Galaxy. While showing general agreement with existing maps of the large-scale dust distribution, our method provides superior sensitivity at greater distances compared to star-by-star analyses that excel at closer ranges. We identified several key spiral arm features and provide new constraints on dust distribution in previously poorly mapped regions. A significant finding from the REDLINE map indicates that dust in the Galactic disc is distributed in a highly non-uniform manner, revealing a fragmented structure with clumps of matter aligning along expected spiral patterns, rather than continuous spiral arms, and highlighting regions of reduced density that deviate from idealised logarithmic models. The seamless and extended view provided by REDLINE demonstrates its value for advancing understanding of the Milky Way's interstellar medium and spiral structure.