Structure of the outer Galactic disc with Gaia DR2

Abstract

Context. The structure of outer disc of our Galaxy is still not well described, and many features need to be better understood. The second Gaia data release (DR2) provides data in unprecedented quality that can be analysed to shed some light on the outermost parts of the Milky Way. Aims. We calculate the stellar density using star counts obtained from Gaia DR2 up to a Galactocentric distance R=20 kpc with a deconvolution technique for the parallax errors. Then we analyse the density in order to study the structure of the outer Galactic disc, mainly the warp. Methods. In order to carry out the deconvolution, we used the Lucy inversion technique for recovering the corrected star counts. We also used the Gaia luminosity function of stars with MG < 10 to extract the stellar density from the star counts. Results. The stellar density maps can be fitted by an exponential disc in the radial direction hr = 2.07 ± 0.07 kpc, with a weak dependence on the azimuth, extended up to 20 kpc without any cut-off. The flare and warp are clearly visible. The best fit of a symmetrical S-shaped warp gives zw ≈ z + (37 ± 4.2(stat.) − 0.91(syst.))pc · (R/R ) 2.42±0.76(stat.)+0.129(syst.) sin(φ + 9.3 • ± 7.37 • (stat.) + 4.48 • (syst.)) for the whole population. When we analyse the northern and southern warps separately, we obtain an asymmetry of an ∼ 25% larger amplitude in the north. This result may be influenced by extinction because the Gaia G band is quite prone to extinction biases. However, we tested the accuracy of the extinction map we used, which shows that the extinction is determined very well in the outer disc. Nevertheless, we recall that we do not know the full extinction error, and neither do we know the systematic error of the map, which may influence the final result. The analysis was also carried out for very luminous stars alone (MG < −2), which on average represents a younger population. We obtain similar scale-length values, while the maximum amplitude of the warp is 20 − 30% larger than with the whole population. The north-south asymmetry is maintained.

Mapping the Galactic Metallicity Gradient with Open Clusters: The State-of-the-Art and Future Challenges

“…Our Galaxy, like many large spiral galaxies, is warped and flared in its outskirts. The Galactic warp and flare are traced by various populations: gas such as atomic HI e.g., [126][127][128], ionized hydrogen e.g., [129] and molecular clouds e.g., [130][131][132], and stellar populations e.g., [133][134][135][136], including Cepheids e.g., [137][138][139] and open clusters e.g., [63,140]. Amôres et al [141] investigated the dependence with the age of the structural parameters of the outer disc, including warp, flare, and disc truncation, finding strong evidence that the thin disc scale length, as well as the warp and flare shapes, changes with time.…”

A Case against a Significant Detection of Precession in the Galactic Warp

“…P20 derived the warp precession under an assumption that the amplitude of the warp corresponding to specific stellar populations (Classical Cepheids, Pulsars) is representative of the whole population. This is not consistent with data, as the stellar population traced by the Gaia survey is significantly older on average (∼5-6 Gyr) and it presents a much lower amplitude of the warp than the young population one (Chrobáková et al 2020;Wang et al 2020). For simplicity, we will refer to this population as an "old population" and to Cepheids as a "young population."…”

“…The model describes the warp as a series of tilted rings, where the amount of tilt is the Galactocentric distance raised to a power of w , C w is the warp amplitude, and φ w is the Galactocentric angle defining the warp's line of nodes. C w , w , and φ w are free parameters of the model, which were fitted in Chrobáková et al (2020). The 17 pc term compensates for the elevation of the Sun above the plane (Karim & Mamajek 2017).…”

“…The values of model parameters given by Chrobáková et al (2020) are C w = 1.17 • 10 −8 pc ± 1.34 • 10 −9 pc(stat.) +0pc −2.9•10 −10 pc (syst.)…”

GASKAP Pilot Survey Science II: ASKAP Zoom Observations of Galactic 21-cm Absorption

“…The molecular clouds, type II supernovae, and synchrotron emissivity all drop off with R following an exponential distribution with scale length similar to that of most stellar populations, typically 2.0 to 3.9 kpc (Miville-Deschênes, Murray and Lee 2017;Bland-Hawthorn and Gerhard 2016;Chrobáková et al 2020). But the WNM has a much larger radial extent.…”