On the Origin of the Local Group in Modified Newtonian Dynamics
Ingo Thies
with Indranil Banik, Pavel Kroupa, Benoit Famaey
BonnGravity2019 Motivation
•Origin of the Disks of Satellites of the Milky Way and M31 still unclear. •ΛCDM does not easily provide suitable scenarios. •However, MOND allows an ancient close encounter between MW and M31 due to lower dynamical friction. •A close encounter may lead to inclined ejecta disks.
⇒ Try a past-encounter scenario in MOND!
1 Motivation
•Preliminary work by Bílek et al. (2018): Phantom of Ramses particle simulation. •Banik, O’Ryan & Zhao (2018): Restricted QUMOND N- body simulation parameter scan. •Tidal debris located mainly between 100 and 200 kpc from their host-galaxy centres. •Orbital pole of debris in agreement with observations.
Results to be verified in a fully self-consistent parameter scan.
2 Galaxies observed forming within tidal tails (Mirabel+ 1992)
MW satellite galaxies lie Satellites were formed within a thin plane from tidal debris. (Pawlowski & Kroupa 2013). Alternatives not very Analogous situation for M31 likely (Pawlowski+ 2014 (Ibata+ 2013) and references therein)
Should only contain baryons as DM can’t cool and form dense tidal tails (Wetzstein+ 2007)
MW and M31 satellite galaxies have high internal Internal dynamics can’t be velocity dispersions, explained by Newtonian requiring strong self-gravity gravity (Kroupa, 2015) (McGaugh & Wolf, 2010) 3 Newtonian gravity
4 Modified Newtonian Dynamics • Integrate MW-M31 relative trajectory back to near Big Bang (redshift 19) • Adjust mass so no peculiar velocity early on (galaxies on Hubble flow initially – no time for gravity to act)
Banik (2017) 5 MW-M31 trajectory
1
0.8
0.6
0.4 M too low 0.2 M just right
MW-M31 separation, Mpc separation, MW-M31 M too high 0 0 2 4 6 8 10 12 14 Time after Big Bang, Gyr Flyby inevitable under wide range of assumptions
Astronomy & Astrophysics 557, Letter 3 (Zhao+, 2013) 6 External field in QMOND
Recap: Basic field equation
~g = (⌫~g ) r
•Add Newtonian external field, •MONDify the sum.
~g = ~g + ~g
g = g ⌫(g )
7 Simulation Setup 0
•Software: Phantom of Ramses (PoR). •Custom patch based on “merger” patch by D. Chapon (2010). •Particle component set up from DICE templates, gas component as exponential disk with sech2 vertical profile.
8 Simulation Setup I
&RUN_PARAMS &POISSON_PARAMS cosmo=.false. epsilon=1e-4 pic=.true. a0_ms2=1.2e-10 mond=.true. g_ext_dir_L_degrees=276.0 Activate_g_ext=.true. g_ext_dir_b_degrees=30.0 poisson=.true. g_ext_ms2=3.6d-12 hydro=.true. H_0_kms_Mpc=67.3 sink=.true. Omega_Lambda_0=2.15 nrestart=33 / nremap=20 nsubcycle=1,1,2,2 ncontrol=1 nstepmax=2000000 /
9 Simulation Setup II
&BOUNDARY_PARAMS nboundary=6 bound_type= 1, 1, 1, 1, 1, 1 ibound_min=-1, 1, 0, 0, 0, 0 ibound_max=-1, 1, 0, 0, 0, 0 jbound_min= 0, 0, -1, 1, 0, 0 jbound_max= 0, 0, -1, 1, 0, 0 kbound_min= 0, 0, 0, 0, -1, 1 kbound_max= 0, 0, 0, 0, -1, 1 /
(see also Banik & Zhao, arXiv:1509.08457)
10 Simulation Setup III
Vcirc_dat_file1='../data/Andromeda_rc_500kK-80pct.txt' Vcirc_dat_file2='../data/Milky_Way_rc_500kK-60pct.txt' ic_part_file_gal1='../data/Andromeda_500kK-80pct.txt' ic_part_file_gal2='../data/Milky_Way_500kK-60pct.txt' gal_center1=8.14954179918350,-110.956651001079,-7.53653996516632 gal_center2=-19.0155975314282,258.898852335852,17.5852599187214 Vgal1=12.6480587546639,75.8944296788073,24.8859801483908 Vgal2=-29.5121370941988,-177.087002583795,-58.0672870129007 gal_axis1=-0.464011733729835,-0.761667097832811,-0.452279054390071 gal_axis2=0.,0.,-1.
(only relevant parts of the namelist file parameters are shown)
11 Simulation
•Starts about 1 Gyr before encounter (9 Gyr before present). •MW and M31 initially both edge-on. •Constant external field added. •No star formation – constant stellar component only. •1.1 million stellar particles + 50% gas fraction.
Movie 1: Stellar component Movie 2: Gas component
12 13 14 Results
•Encounter flings material out of both galaxies •Debris orbits inclined to galactic disks
But...
•Low-mass debris disks hard to see in video. •Gas portion quite diffuse due to resolution limits. •Comparison with observations requires quantitative analysis.
15 Analysis of Encounter
•Extraction of particle positions from snapshot near present. •Extraction of gas cell positons (RDRAMSES, Florent Renaud 2015). •2D histogram of ejecta orbital poles (stars+gas). •1D histogram of ejecta radial distribution. •Regions close to the galactic disks omitted. •Compare density centers with observations.
16 Directions of the key vectors
At 2σ, all orbital pole directions are feasible (orthogonal to MW- (ApJ, 753:7) M31 line)
Tidal torques cause disk-satellite plane misalignment 17 Parameter Scan
•Encounter orbit constrained by observations (HST). •Orbital pole first free parameter along great circle. •Angular momentum second free parameter given by tangential velocity. •Iterative angular corrections for dissipative effects. •➜ Found best-fit orbital pole at lon=187.75° and lat=42.46°.
18 Effect of angle (horizontal) and angular momentum (vertical)
19 Best fit model Milky Way
20 Best fit model Andromeda
21 Best fit model Milky Way
22 Best fit model Andromeda
23 Directions of the key vectors Best fit orbital pole
At 2σ, all orbital pole directions are feasible (orthogonal to MW- (ApJ, 753:7) M31 line)
➜ Encounter trajectory pole agrees with HST observations 24 Summary
•Existence of satellite disks cannot be explained in ΛCDM. •MOND paradigm allows an ancient encounter between the MW and M31. •Ejecta (gas+stars) mostly between 50 and 200 kpc around both galaxies. •Orbital poles of ejecta about 8 Gyr after encounter in agreement with observations ⇒ Local Group can at least partly modelled in MOND. •Best-fit encounter orbital pole well within observational limits.
25 26 Best fit distance evolution (analytical model)
27 Local Group galaxies flying outwards at high speeds
Only Milky Way or Andromeda could fling them out, if they were fast-moving
In Newtonian gravity, they were always moving slowly Gravity is modified if given reasonable masses
Try very high mass: a close flyby leads to high speeds MNRAS: 459, 2237 (Banik & Zhao, 2016) Galaxies merge as dark matter halos overlap