[Paper Review] Distance and extinction to the Milky Way spiral arms along the Galactic centre line of sight

F. Nogueras-Lara, R. Schödel+1 more|arXiv (Cornell University)|Jun 8, 2021

Stellar, planetary, and galactic studiesPhysics and Astronomy101 references30 citations

TL;DR

This study determines the distances and extinction levels to four spiral arms along the line of sight toward the Galactic Centre using high-resolution JHKs photometry from the GALACTICNUCLEUS survey. By fitting synthetic colour-magnitude models to the data via χ² minimisation, it finds spiral arms at 1.6 ± 0.2, 2.6 ± 0.2, 3.9 ± 0.3, and 4.5 ± 0.2 kpc with corresponding extinctions of 0.35 ± 0.08, 0.77 ± 0.08, 1.68 ± 0.08, and 2.30 ± 0.08 mag in the H band, respectively, and confirms a consistent near-infrared extinction law across all features.

ABSTRACT

The position of the Sun inside the Milky Way's disc hampers the study of the spiral arm structure. We aim to analyse the spiral arms along the line-of-sight towards the Galactic centre (GC) to determine their distance, extinction, and stellar population. 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 $\chi^2$ 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 built extinction-corrected Ks luminosity functions (KLFs) to study the stellar populations present in the second and third spiral arm features. We determined the mean distances to the spiral arms: $d1=1.6\pm0.2$, $d2=2.6\pm0.2$, $d3=3.9\pm0.3$, and $d4=4.5\pm0.2$ kpc, and the mean extinctions: $A_{H1}=0.35\pm0.08$, $A_{H2}=0.77\pm0.08$, $A_{H3}=1.68\pm0.08$, and $A_{H4}=2.30\pm0.08$ mag. We analysed the extinction curve in the near infrared for the stars in the spiral arms and found mean values of $A_J/A_{H}=1.89\pm0.11$ and $A_H/A_{K_s}=1.86\pm0.11$, 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 obtained that they are homogeneous and might correspond to independent extinction layers. Finally, analysing the KLFs from the second and the third spiral arms, we found that they have similar stellar populations. We obtained two main episodes of star formation: $>6$ Gyr ($\sim60-70\%$ of the stellar mass), and $1.5-4$ Gyr ($\sim20-30\%$ of the stellar mass), compatible with previous work. We also detected recent star formation at a lower level ($\sim10\%$) for the third spiral arm.

Motivation & Objective

• . The Sun's position within the Galactic disc hinders detailed study of spiral arm structure due to high dust and gas extinction.
• Existing surveys lack sufficient depth and angular resolution to resolve fine structures in the highly obscured inner Galactic plane.
• There is a need for precise distance and extinction measurements to spiral arms along the line of sight to the Galactic Centre to understand the Milky Way's disc structure and stellar population.
• The study aims to characterise the distance, extinction, and stellar populations of spiral arms in the innermost Galactic disc using high-angular-resolution near-infrared photometry.
• It seeks to validate the consistency of the extinction law across different spiral arm features and assess their star formation histories.

Proposed method

• . Uses high-angular-resolution (0.2′′) JHKs photometry from the GALACTICNUCLEUS survey, covering the innermost Galactic bulge and nuclear stellar disc.
• Applies χ² minimisation to fit synthetic colour-magnitude models to observed CMDs (H vs. J−H) to derive distances and extinctions to spiral arm features.
• Constructs extinction maps in AKs using features in the Ks vs. H−Ks CMD, assessing homogeneity and spatial variations along the line of sight.
• Computes extinction ratios AJ/AH and AH/AKs for stars in each spiral arm to test the constancy of the extinction law across distances and extinction levels.
• Performs cross-validation by cross-matching GALACTICNUCLEUS sources with Gaia DR2 to estimate distances for the two nearest spiral arms.
• Analyzes Ks-band luminosity functions (KLFs) of the second and third spiral arms, fitting them with a linear combination of Parsec theoretical models to derive star formation histories (SFHs).

Experimental results

Research questions

• RQ1 . What are the distances and extinctions to the spiral arms along the line of sight toward the Galactic Centre?
• RQ2 . Is the near-infrared extinction law consistent across different spiral arms, independent of distance or absolute extinction?
• RQ3 . Are the extinction maps for each spiral arm homogeneous, suggesting distinct, independent extinction layers?
• RQ4 . What is the star formation history of the second and third spiral arms, and how do their stellar populations compare?
• RQ5 . How do the derived distances and extinctions compare with independent measurements from Gaia DR2?

Key findings

• . The mean distances to the four spiral arms are 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.
• . The mean extinctions in the H band are 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.
• . The extinction ratios AJ/AH = 1.89 ± 0.11 and AH/AKs = 1.86 ± 0.11 are consistent across all spiral arms, indicating a constant extinction law in the near-infrared.
• . Extinction maps for each spiral arm are homogeneous and likely represent independent extinction layers, with no significant spatial variations along the line of sight.
• . The second and third spiral arms host similar stellar populations, with two dominant star formation episodes: >6 Gyr ago (60–70% of stellar mass) and 1.5–4 Gyr ago (20–30% of stellar mass).
• . A minor recent star formation episode (≈10% of stellar mass) is detected for the third spiral arm at ages <1 Gyr.