Galactic spiral structure revealed by Gaia Edr3

E. Poggio, et al. – incl., T. Cantat-Gaudin, P. Ramos, E. Zari, R. Andrae, M. Fouesneau

Abstract: Using the astrometry and integrated photometry from the Gaia Early Data Release 3 (EDR3), we map the density variations in the distribution of young Upper Main Sequence (UMS) stars, open clusters and classical Cepheids in the Galactic disk within several kiloparsecs of the Sun. Maps of relative over/under-dense regions for UMS stars in the Galactic disk are derived using both bivariate kernel density estimators and wavelet transformations. The resulting overdensity maps exhibit large-scale arches, that extend in a clumpy but coherent way over the entire sampled volume, indicating the location of the spiral arms segments in the vicinity of the Sun. Peaks in the UMS overdensity are well-matched by the distribution of young and intrinsically bright open clusters. By applying a wavelet transformation to a sample of classical Cepheids, we find that their overdensities possibly extend the spiral arm segments on a larger scale (≃10 kpc from the Sun). While the resulting map based on the UMS sample is generally consistent with previous models of the Sagittarius-Carina spiral arm, the geometry of the arms in the III quadrant (galactic longitudes 180∘<l<270∘) differs significantly from many previous models. In particular we find that our maps favour a larger pitch angle for the Perseus arm, and that the Local Arm extends into the III quadrant at least 4 kpc past the Sun’s position, giving it a total length of at least 8 kpc.

Figure 1: Panel A: Face-on view of the UMS P18 dataset in the Galactic disk. The position of the Sun is shown by the white cross in (X,Y)=(0,0). The Galactic center is to the right, in (X,Y)=(R⊙,0), and the Galaxy is rotating clockwise. Panel B: Same as Panel A, but showing the measured overdensity using the UMS P18 dataset, based on a local density scale length 0.3 kpc. Only points with Σ(x,y)> 0.003 are plotted, in order to remove regions where the statistics is too low. The corresponding plot for the UMS Z21 sample can be found in Figure 6. Panel C: Same as Panel A, but showing the wavelet transformation at the scale 3 (size∼0.4 kpc). A different version of Panel B and C using a larger scale length can be found in Figure 7 (see Appendix). The maps shown in panel B and C are publicly available at https://github.com/epoggio/Spiral_arms_EDR3.git.

Figure 2: Top left panel: Same as Figure 1B, but with some geometric references superimposed. Dotted lines from left to right show the points with constant Galactocentric radius R=12,10,8,6 kpc, respectively. Dashed lines show the five selected slices (A,B,C,D,E), which are separated by 10 deg in Galactic azimuth ϕ and of 2 deg in width. Panels A to E: The profile of the measured overdensity as a function of R for the UMS P18 sample (black dots) and the Z21 sample (white squares). Error bars show bootstrap uncertainties, calculated as explained in Appendix B.3. For computational reasons, black dots/white squares and relative errorbars were calculated for each slice and then connected using splines, as shown by the dotted (dashed) lines, to give a visual impression of how the overdensity varies at different radii. Each peak of the measured overdensity has been identified with a spiral arm in the Milky Way, as indicated by the vertical grey lines and the corresponding labels. The identification of each peak is not due to an assumed specific model, but is simply based on the geometric appearance of the top left panel (see text).

Figure 3: Same as Figure 1B, but compared to the distribution of the young and instrinsically bright open clusters sample (see Section 2.2), shown by the black dots. The size of the dots is proportional to the number of cluster members brighter than absolute magnitude MG¿ 0 (see text). Solid lines show the spiral arm model of Taylor & Cordes (1993), based on HII regions.