The outer disc in shambles: Blind detection of Monoceros and the ACS with Gaia’s astrometric sample

Abstract

Context. The Gaia astrometric sample allows us to study the outermost Galactic disc, the halo, and their interface. It is precisely at the very edge of the disc where the effects of external perturbations are expected to be the most noticeable. Aims. Our goal is to detect the kinematic substructure present in the halo and at the edge of the Milky Way (MW) disc and provide observational constraints on their phase-space distribution. Methods. We download, one HEALpix at a time, the proper motion histogram of distant stars, to which we apply a wavelet transformation to reveal the significant overdensities. We then analyse the large coherent structures that appear in the sky. Results. We reveal a sharp yet complex anticentre dominated by Monoceros (MNC) and the Anticentre Stream (ACS) in the north – which we find have intensities comparable to the Magellanic Clouds and the Sagittarius stream – and by MNC South and TriAnd at negative latitudes. Our method allows us to perform a morphological analysis of MNC and the ACS, both of which span more than 100° in longitude, and to provide a high purity sample of giants with which we track MNC down to latitudes as low as ∼5°. Their colour-magnitude diagram is consistent with extended structures at a distance of ∼10−11 kpc that originated in the disc, with a very low ratio of RR Lyrae over M giants, and with kinematics compatible with the rotation curve at those distances or slightly slower. Conclusions. We present a precise characterisation of MNC and the ACS, two previously known structures that our method reveals naturally, allowing us to detect them without limiting ourselves to a particular stellar type and, for the first time, using only kinematics. Our results will allow future studies to model their chemo-dynamics and evolution, thus constraining some of the most influential processes that shaped the MW.

The Sagittarius stream in Gaia eDR3 and the origin of the bifurcations

“…The data was analysed in a similar fashion to Antoja et al (2020) and Ramos et al (2021) with a few, but important, improvements. Overall, the goal of our technique is to detect kinematic substructures -those related to the Sgr stream in particular-in the proper motion histograms throughout our whole sample by analysing in parallel each HEALpix bin in the sky.…”

Galactic spiral structure revealed byGaiaEDR3

“…There are many types of WT but, for the purpose of this work, we used the stationary wavelet transformation, also known as à trous algorithm, in its redundant form and using the B-spline mother wavelet. We computed the WT using the Multiresolution Analysis software (Starck et al 1998), which has already been applied successfully to other astrophysical problems (for more details on the WT transform see, e.g., Ramos et al 2018;Antoja et al 2020;Ramos et al 2021)…”

“…We have computed the WT using the Multiresolution Analysis (MRA) software (Starck et al 1998), which has already been applied successfully to other astrophysical problems (for more details on the WT transform see, e.g. Ramos et al 2018;Antoja et al 2020;Ramos et al 2021) The advantatge of using this implementation of the WT in front of others is that the number of pixels are preserved and that each scale has zero mean. As a consequence of the latter, positive coefficients are directly related to overdensities and negatives ones, to underdensities, which is why panels A and B of Fig.…”

Photo-astrometric distances, extinctions, and astrophysical parameters for Gaia DR2 stars brighter than G = 18

“…Cantat-Gaudin et al 2018;Castro-Ginard et al 2020), distant regions of the Milky Way (e.g. Ramos et al 2021;Gaia Col-laboration et al 2021a;Zari et al 2021) and the Local Group (e.g. Gaia Collaboration et al 2018b;Antoja et al 2020;Gaia Collaboration et al 2021d), the Galactic potential (e.g.…”