Galactic spiral structure revealed by Gaia EDR3
  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 $180^\circ < l < 270^\circ$) 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.
