A cross-match-free method for large-scale 3D extinction mapping with Gaia and 2MASS

3D extinction mapping procedures of the Milky Way face a fundamental trade-off: Gaia's precise astrometry and photometry enable high-resolution mapping, but only to limited distances due to visible-wavelength extinction; meanwhile infrared (IR) surveys such as 2MASS penetrate deeper into the Galactic plane but provide poor distance constraints, especially in the solar neighbourhood. Many studies have cross-matched these surveys to provide more wavelength coverage and, hence, better extinction estimates. This comes at a cost as the cross-matching limits depth to the visible survey's reach since highly extinguished stars detected in IR lack visible counterparts. We developed PyRedLine as an evolution of the REDLINE method, enabling simultaneous exploitation of multiple surveys without cross-matching. Our goal is to achieve both high spatial resolution at short distances through Gaia and extended the reach into the Galactic plane through 2MASS, with a target resolution of sim100 pc within several kpc. PyRedLine compares statistical distributions of observed stellar properties (colours, parallaxes, and absolute magnitudes) against predictions from the Besançon Galaxy Model reddened by parameterised extinction profiles. Using Bayesian inference with Markov chain Monte Carlo (MCMC) sampling, we can independently determine the extinction along each LOS by fitting the model predictions to Gaia DR3 and 2MASS observations simultaneously and then assembling these profiles into 3D maps. Our validation based on synthetic data confirmed a reliable structure detection out to sim8 kpc. We successfully mapped the Galactic plane from 240 łe l łe 303 and |b|łe 5 at 15' resolution. The resulting map reveals extinction to 13 kpc with up to 42 pc resolution. We recovered known structures, including the Vela molecular complex and Carina arm tangent, achieving a significantly finer resolution at short distances, compared to REDLINE, while maintaining a comparable depth. We detected distant features potentially associated with the Perseus arm that remain undetected in other surveys.