,
20 tweets,
6 min read
Read on Twitter
New paper on the arxiv today with T. Haines, @elenadonghia , @cfl2126 & Lars Hernquist: we showed how the disequilibrium of a vertically perturbed disk affects our determination of the disk surface density with Jeans modelling arxiv.org/abs/1903.00607
Taking the 1st order moments of the collisionless Boltzmann eq leads to the Jeans eqs. Using the Jeans equation for vertical velocities of stars accross the Galactic disk in combination with the Poisson eq allows to determine the disk surface density as a function of height: nice
This method has been used countless times to determine the dynamical surface density of the Milky Way disk, and the associated density of dark matter in it. However, this is of course valid only at equilibrium
But back in 2012, Larry Widrow et al. discovered with SEGUE North-South asymmetries in the vertical density and velocity of stars accross the Galactic plane: "Galactoseismology" was born arxiv.org/abs/1203.6861
Soon later, various studies showed that the outer disk was actually a buckled mess, with disk stars actually ejected multiple kpc from the plane, see e.g. the recent work of @MariaBergemann et al arxiv.org/abs/1803.00563
The original kinematic finding of Widrow et al., which has been confirmed by various spectroscopic surveys such as RAVE, has recently taken yet a new turn with the publication of the Gaia DR2
Indeed, @AntojaTeresa et al. discovered that solar neighbourhood's stars vertical motions are correlated with their motions in the plane of the Galaxy, creating a groovy phase-space spiral! arxiv.org/abs/1804.10196
This has been attributed to various potential causes, such as a relic from the buckling phase of the bar formation arxiv.org/abs/1811.09205 , or to the impact of the Sagittarius dwarf galaxy on the Milky Way disk arxiv.org/abs/1808.00451
The recent pre-Gaia DR2 Sgr dwarf impact models of @cfl2126 demonstrated that they could simultaneously account for the morphology of the outer disk and amplitude of density and streaming motion fluctuations in the solar neighbourhood
We thus set out to analyze those simulations to check the validity of Jeans equations to evaluate the disk surface density in the presence of such non-equilibrium phenomena
We first showed that the Jeans method works perfectly fine at the beginning of the simulation, when the Sgr dwarf hasnt affected the Galactic disk just yet
But once the vertical perturbation is acting at full steam, things are different. First of all, because of the North-South asymmetries, it is best to perform the Jeans analysis separately in the North and South
In the most overdense regions of the disk, we noticed that the phase-space spiral is less well-defined than in the underdense regions. This is actually a nice prediction of the simulations, which could be checked with future Gaia data releases
Because of this reasonable phase-space distribution, we found out that the Jeans modelling *does* work reasonably well in such overdense regions
On this plot, the green (North) and red (South) lines, which are the result of our Jeans analysis (for the surface density in Msun/pc^2 as a function of z in kpc), fits the orange line (which is the true surface density in the simulation) reasonably well 
In the regions of intermediate (typical) density, we found out that the North and South Jeans analyses yield vastly different results. This is worrying, but an observer performing a Jeans analysis on the data would quickly notice the problem...
Things are however different in the most underdense regions. There, the Jeans modelling *does* give similar results in the North and South, but severely overestimates the disk surface density (and hence the dark matter density!)
The consequences of this are profound: it means that we cannot necessarily trust dark matter density estimates based on the equilbrium assumption. This is actually already an issue locally, in our neighbourhood, but is going to be much worse in the outer disk!
So what's next? Clearly, exciting times ahead, as we do need to develop non-equilibrium methods to determine the gravitational potential of the outer disk. Some preliminary approaches, but neglecting the disk self-gravity, have recently been proposed, e.g. arxiv.org/abs/1807.09819
Building on such methods to include the disk self-gravity in the equations is certainly a promising way forward. But equilibrium models, especially for the messy outer Milky Way disk, are dead! #End
