Red Versus Blue Disks in Triaxial Dark Matter Halos (wrt the Milky - PowerPoint PPT Presentation

Red Versus Blue Disks in Triaxial Dark Matter Halos (wrt the Milky Way) Victor P. Debattista Debattista et al. 2008 Valluri et al. 2010 Valluri et al. 2012 Valluri et at. 2013 Debattista et al. 2013 Debattista et al. 2015 Earp et al. 2017

Dark matter halos are triaxial Bardeen+ 86, Barnes & Efstathiou 87, Frenk+ 88, Dubinski & Carlberg 91, Jing & Suto 02, Bailin & Steinmetz 05, Allgood+ 06, Butsky+16 12% R Vir 5% R Vir Butsky+ 16

Wang+ 08 <N R ( θ )> is the number of pairs with angle of the central galaxy randomised Conclusion: red centrals are aligned with their halos (as traced by satellites), blue galaxies are more randomly orientated. (Profoundly important to understanding if we want to use weak lensing to measuring the equation of state of dark energy, e.g. Kirk+ 12.)

What about the Milky Way? Law & Majewski 10 Based on the location and velocities of stars in the Sagittarius stream, MW halo is triaxial with c/a = 0.72 and b/a = 0.99 (pot). Nearly oblate with intermediate axis perpendicular to plane of disk Also Martinez-Delgado+04; Johnston+05; Helmi 04; Deg & Widrow 13

Briggs 90 R c ϕ c Transform the ORIENTATION of a vector into a 2D cylindrical polar plot, with tilt angle represented as radius angle in (x,y) plane from x-axis represented as cylindrical angle ϕ The angles ( θ i , ϕ i ) specifying the orientation of the annulus are plotted in cylindrical coordinates as ϕ c = ϕ i and R c = θ i / θ max

DM only Controlled experiments A rigid disk placed inside a triaxial halo with its spin axis along the halo’s intermediate axis. The disk mass is grown from 0 to MW mass; in the process the shape of the halo changes. The disk is then given equilibrium kinematics and released. Debattista+ 15 No gas

Controlled experiments A series dark matter + adiabatic gas mergers producing a triaxial halo with gas angular momentum along the intermediate axis. At that point gas cooling and star formation are switched on. Disk is not able to form or persist in an intermediate axis orientation. What modelling step fails? Assumption that the MW disk in one of the global symmetry planes of the halo is broken

No gas Discs in short axis vs long axis orientations Tilt angle In the absence of gas, the only stable disk orientation is for the spin axis along the High mass disc + 2% satellite minor axis of the halo Debattista+ 15

Mass evo. Stellar L evo. 8 Gyr evo. Torque from DM In the presence of cooling gas, the spin of the cooling gas determines the orientation of the disk. Simulations find that the angular momentum axis of the corona evolves with time (e.g. Ro š kar+ 10)

Ro š kar+ 10 The evolution of gas angular momentum is driven by hydrodynamic forces, not by the torque from the triaxial halo The inner corona and cool disk gas are misaligned throughout

Sample A: MW mass galaxies Subsample B: MW mass and no mergers or large interactions Drawn from a cosmological zoom simulation of a volume 25 × 25 × 25 Mpc 3 The tilting rate of most of the MW- like galaxies in this cosmological volume are > 0.28 ° Gyr -1 = Gaia detection limit (Perryman+ 14) Earp+ 17

Sample A: MW mass galaxies Subsample B: MW mass and no mergers or large interactions Tilting does not appear to correlate strongly with distance to largest similar mass galaxy or to tilting rate of the coronal angular momentum Earp+ 17

NIHAO simulations 20 Secular growth MW mass ∆ θ ⇤ / ∆ t [ � Gyr � 1 ] 15 10 5 0 � 10 . 8 � 10 . 6 � 10 . 4 � 10 . 2 � 10 . 0 � 9 . 8 log 10 sSFR [yr � 1 ] Hints that the tilting rate is larger as the sSFR Earp+ in progress increases but secondary effects too

Conclusions • A disc perpendicular to the intermediate axis of a triaxial halo (as proposed for the MW by Law & Majewski 2010) is NEVER stable. • Without gas cooling, the most natural orientation for a disc is perpendicular to the short axis. Satellites help discs reach this lowest- energy orientation. • When gas cools onto discs, the angular momentum delivered by the gas is a stronger driver than the halo torques on the orientation of the disc. This delivery of gas is the driving mechanism for the difference between the intrinsic alignments of red versus blue galaxies. Direct tidal torques do not dominate the orientation of disks. • The MW is accreting gas which has angular momentum misaligned with the stellar disk. It should therefore be tilting. • In cosmological simulations, isolated MW-like disks tilt at a mean rate of 6.3 deg/Gyr. The Gaia detection limit is 0.28 deg/Gyr The tilting rate is correlated with misalignment between disk and coronal spins.