Distance and extinction to the Milky Way spiral arms along the Galactic centre line of sight

Abstract

Context. The position of the Sun inside the disc of the Milky Way significantly hampers the study of the spiral arm structure given the high amount of dust and gas along the line of sight, and the overall structure of this disc has therefore not yet been fully characterised. Aims. We aim to analyse the spiral arms in the line of sight towards the Galactic centre (GC) in order to determine their distance, extinction, and stellar population. Methods. We use the GALACTICNUCLEUS survey, a JHKs high-angular-resolution photometric catalogue (0.2″) for the innermost regions of the Galaxy. We fitted simple synthetic colour-magnitude models to our data via χ2 minimisation. We computed the distance and extinction to the detected spiral arms. We also analysed the extinction curve and the relative extinction between the detected features. Finally, we studied extinction-corrected Ks luminosity functions (KLFs) to study the stellar populations present in the second and third spiral arm features. Results. We determined the mean distances to the spiral arms: d1 = 1.6 ± 0.2 kpc, d2 = 2.6 ± 0.2 kpc, d3 = 3.9 ± 0.3 kpc, and d4 = 4.5 ± 0.2 kpc, and the mean extinctions: AH1 = 0.35 ± 0.08 mag, AH2 = 0.77 ± 0.08 mag, AH3 = 1.68 ± 0.08 mag, and AH4 = 2.30 ± 0.08 mag. We analysed the extinction curve in the near-infrared for the stars in the spiral arms and find mean values of AJ/AH = 1.89 ± 0.11 and AH/AKs = 1.86 ± 0.11, in agreement with the results obtained for the GC. This implies that the shape of the extinction curve does not depend on distance or absolute extinction. We also built extinction maps for each spiral arm and find them to be homogeneous and that they might correspond to independent extinction layers. Finally, analysing the KLFs from the second and third spiral arms, we find that they have similar stellar populations. We obtain two main episodes of star formation: > 6 Gyr (∼60 − 70% of the stellar mass), and 1.5 − 4 Gyr (∼20 − 30% of the stellar mass), compatible with previous work. We also detect recent star formation at a lower level (∼10%) for the third spiral arm.

On the stochastic nature of Galactic cosmic-ray sources

“…We notice that in the Spiral model the Sun is correctly placed in the inter-arm region between Sagittarius-Carina and Perseus arms. In this model, the distance to the closest spiral arm, in the direction of the Galactic center, is about 1.5 kpc, well compatible with recent determinations, as e.g., [70].…”

Self-consistent modelling of the Milky Way’s nuclear stellar disc

“…Here we consider the P17 model with Ω p = 37.5 km s −1 kpc −1 which was found to be a good match to the VIRAC proper motions in Clarke et al (2019) and, with gas dynamical modelling, to the observed distribution of interstellar gas in the (𝑙, 𝑣 los )-diagram (Sormani et al 2015;Li et al 2016Li et al , 2021. We neglect spiral arms in the Galactic Disc, which contain too few stars to have a significant impact on the normalised kinematic histograms used in our fitting procedure (Nogueras-Lara et al 2021b).…”

Jeans modelling of the Milky Way’s nuclear stellar disc

“…Here we consider the P17 model with Ω p = 37.5 km s −1 kpc −1 which is a good match to the VIRAC proper motions (Clarke et al 2019) and, with gas dynamical modelling, to the observed distribution of interstellar gas in the (𝑙, 𝑣 los )-diagram (Sormani et al 2015;Li et al 2016Li et al , 2021. We neglect spiral arms in the Galactic Disc, which contain too few stars to have a significant impact on the normalised kinematic histograms used in our fitting procedure (Nogueras-Lara et al 2021b).…”