Galactic Gas Dynamics James Binney University of Oxford Saas Fee, - - PowerPoint PPT Presentation

Galactic Gas Dynamics

James Binney

University of Oxford

Saas Fee, January 2019

Basics

◮ Surveys of 21 cm H hyperfine line gave 1st global view of

MW (Oort, Kerr & Westerhout 1958)

◮ Followed in 1970s by similar surveys at small ℓ in 2.6 mm

line of CO

◮ Surveys yield (ℓ, v) plots:

◮ not easy to interpret ◮ but contain much diagnostic information

◮ Major task now with Gaia data:

◮ to bring together gas & star distributions & kinematics

Circular hypothesis

◮ Traditionally assume circular motion

◮ Then v(ℓ) = [Ω(R) − Ω(R0)]R0 sin ℓ ◮ ‘Terminal v’ the peak at R = R0 sin ℓ

→ Vc(R0 sin ℓ)

◮ At |ℓ| < 90◦ same v at 2 locations of same R ◮ → ambiguity in ‘kinematic distance’

Terminal velocities

Even now Φ(R) usually constrained within circular hypothesis

HI plot

◮ Observed (ℓ, v) planes show strings and ridges ◮ Spiral arms?

Spirals → ridges

CO plot

◮ CO (ℓ, v) plane shows

◮ Molecular ring ◮ Nuclear parallelogram ◮ Both CO and HI planes show material at ‘forbidden

velocities’

Evidence for a bar

◮ Binney+ (1991) argued parallelogram is a diagnostic of a

rotating bar

◮ hypothesised that gas spirals in along closed x1 orbits

◮ until it reaches cusped orbit ◮ The shock then transfers it to (closed) x2 orbits

Hydro simulations

◮ Sormani+ (2015a) tested hypothesis with hydro simulations ◮ Resolution and cs prove important

Hydro simulations

Gas reaches cusped orbit only at high cs and resolution

New interpretation of CO parallelogram

Bar-driven spirals (Sormani+ 2015b)

Bar drives spirals in gas

Bar-driven spirals (Sormani+ 2015b)

◮ Spirals reflect twisted streamlines

Dynamical model (Sormani+ 2015b)

◮ Can be modelled as driven, damped epicycles (Wada 1994) ◮ Major exes of closed orbits swing through 90◦ at LR

¨ x + κ2x = f(t) → ¨ x + λ ˙ x + κ2x = f(t)

Strong & weak bars

• If bar weak, epicycles

can be around circular

• rbit
• In strong bar must be

libration around eccentric closed orbit

Fitting observations (Sormani+ 2015c)

◮ Assume Φ(R, φ) = Φ(R) + Φ2(R) cos(2φ)

Models for Ωp = 40

Models for Ωp = 60

Constraints on Φ

◮ Extent of emission at vforbidden increases as Ωp drops

◮ Suggests Ωp = 30 − 40 km s−1/ kpc

◮ 3 kpc arm needs long bar rq > 1.5 kpc & strong quadrupole

◮ Argues against high Ωp

◮ Molecular ring also favours Ωp ∼ 40 ◮ Envelope at ℓ > 0 and permitted v favours Ωp = 50 − 60

but at ℓ < 0 envelope favours Ωp = 40

◮ Bumps on envelope: sensitive to quadrupole ◮ Too many parameters for a thorough model search

Asymmetry of CMZ

CO NH3

◮ 3 4 of molecular emission from ℓ > 0 ◮ Long-standing puzzle

Wiggle instability

◮ At high cs & resolution flow becomes unsteady ◮ Unsteadiness is code-independent except suppressed by

highly diffusive flux limiter (thick shocks)

◮ Vorticity arises in shocks (high shear) & moves into flow ◮ Vortices amplified in opposite shock ◮ Phenomenon discovered by Kim+ (2012) ◮ Shocks with periodic bdy conds are unstable (Sormani+

2017)

Sormani+ 2018

◮ High resolution 3d simulations of gas flow in fixed bar

(AREPO)

◮ Complex chemical network included ◮ Switch-on of bar → gas ∼on x1 & x2 orbits

Unsteady flow with phase changes (Sormani+ 2018)

HI much smoother than CO

Sormani+ 2018

CO in a handful of stringy clouds → strong asymmetry

Phenomenology of the CMZ

◮ Intense SF in R ∼ 200 pc x2 disc ◮ 90 cm radio-continuum map ◮ ‘hour-glass’ plume of soft X-ray emission (Bland-Hawthorn

& Cohen 2003)

CMZ

◮ Fermi bubble ◮ Stellar cpt discovered in APOGEE data (Sch¨

• nrich+ 2015)