The kinematic signature of the Galactic warp inGaiaDR1

Abstract

Context. The mechanism responsible for the warp of our Galaxy, as well as its dynamical nature, continues to remain unknown. With the advent of high precision astrometry, new horizons have been opened for detecting the kinematics associated with the warp and for constraining possible warp formation scenarios for the Milky Way. Aims. The aim of this contribution is to establish whether the first Gaia data release (DR1) shows significant evidence of the kinematic signature expected from a long-lived Galactic warp in the kinematics of distant OB stars. As the first paper in a series, we present our approach for analyzing the proper motions and apply it to the subsample of Hipparcos stars. Methods. We select a sample of 989 distant spectroscopically-identified OB stars from the new reduction of Hipparcos, of which 758 are also in the first Gaia data release (DR1), covering distances from 0.5 to 3 kpc from the Sun. We develop a model of the spatial distribution and kinematics of the OB stars from which we produce the probability distribution functions of the proper motions, with and without the systematic motions expected from a long-lived warp. A likelihood analysis is used to compare the expectations of the models with the observed proper motions from both Hipparcos and Gaia DR1. Results. We find that the proper motions of the nearby OB stars are consistent with the signature of a kinematic warp, while those of the more distant stars (parallax <1 mas) are not. Conclusions. The kinematics of our sample of young OB stars suggests that systematic vertical motions in the disk cannot be explained by a simple model of a stable long-lived warp. The warp of the Milky Way may either be a transient feature, or additional phenomena are acting on the gaseous component of the Milky Way, causing systematic vertical motions that are masking the expected warp signal. A larger and deeper sample of stars with Gaia astrometry will be needed to constrain the dynamical nature of the Galactic warp.

Evidence of a dynamically evolving Galactic warp

“…For each star we use Gaia DR2 galactic coordinates l and b, the parallax , and the component of the proper motion perpendicular to the galactic plane µ b , derived from Gaia's astrometry (see Methods). We only consider µ b as the Galactic warp primarily manifests itself kinematically as systematic motions perpendicular to the Galactic plane 18 . We also construct the covariance matrix Σ ,µ b * , which includes the astrometric uncertainties on and µ b , together with their correlation ρ ,µ b , neglecting the uncertainties in the coordinates (l, b).…”

The Galactic warp revealed by Gaia DR2 kinematics

“…We adopt a simple density model for the Galactic disc ρ(l, b, r), consisting of an exponential disc in Galactocentric radius R and vertical height z, with a radial scale length L R = 2.6 kpc (Bland-Hawthorn & Gerhard 2016) and vertical scale height h z = 150 pc (larger than the known scale height for OB stars, Poggio et al 2017). We assume for the Sun R =8.34 kpc (Reid et al 2014) and z = 25 pc (Bland-Hawthorn & Gerhard 2016).…”

GaiaData Release 2

“…Several works have discussed the possibility that the Galactic warp may be generated by the interaction with the Magellanic Clouds (Burke 1957;Weinberg & Blitz 2006) or Sagittarius (Bailin 2003), while other scenarios suggest that a warped structure in a galaxy disc may be generated by a dark matter halo distribution that is off-centred or tilted with respect to the baryonic one (Bailin & Steinmetz 2003), by bending instabilities (Revaz & Pfenniger 2004) in the disc, or by misaligned infall of material (Ostriker & Binney 1989;Quinn & Binney 1992). These scenarios predict either long-lived, transient, or repeatedly excited structures, and it is clear that to understand the origin of the Galactic warp, we need to understand its dynamical nature, since, for example, a long-lived warp would leave a specific signature in the kinematics of stars in the outer disc (Abedi et al 2014;Poggio et al 2017).…”