Slide Deck - Constraining Self-Interacting Dark Matter: Insights from Equal Mass Mergers of Galaxy Clusters

Merging galaxy clusters have been touted as one of the best probes for constraining self-interacting dark matter, but few simulations exist to back up this claim. We simulate equal mass mergers of 10^15 M⊙ halos, like the El Gordo and Sausage clusters, with cosmologically-motivated halo and merger parameters, and with velocity-independent dark-matter self-interactions. Although the standard lore for merging clusters is that self-interactions lead to large separations between the galaxy and dark-matter distributions, we find that maximal galaxy-dark matter offsets of ≲ 20 kpc form for a self-interaction cross section of σ_SI/m_χ = 1 cm2/g. This is an order of magnitude smaller than those measured in observed equal mass and near equal mass mergers, and is likely to be even smaller for lower-mass systems. While competitive cross-section constraints are thus unlikely to emerge from offsets, we find other signatures of self-interactions which are more promising. Intriguingly, we find that after dark matter halos coalesce, the collisionless galaxies (and especially the Brightest Cluster Galaxy [BGC]) oscillate around the center of the merger remnant on stable orbits of 100 kpc for σ_SI/m_χ = 1 cm^2/g for at least several Gyr, well after the clusters have relaxed. If BCG miscentering in relaxed clusters remains a robust prediction of SIDM under the addition of gas physics, substructure, merger mass ratios (e.g., 10:1 like the Bullet Cluster), and complex cosmological merger histories, the observed BCG offsets may constrain σ_SI/m_χ≲ 0.1 cm^2/g---the tightest constraint yet.

Transcript

1. S. Y. Kim A. H. G. Peter D. Wittman galaxy clusters equal mass mergers constraining self-interacting dark matter: insights from of
2. How good is this assumption? F568%3' CDM a success! Γ = σ mχ ρv CDM typically assumed to be collisionless, i.e. = 0.
3. F568%3' CDM a success! density radius SIDM! (“cored”) CDM! (“cuspy”) How good is this assumption? CDM typically assumed to be collisionless, i.e. = 0. Γ = σ mχ ρv If 0, collisional or “self-interacting.” Γ = σ mχ ρv
4. The Sausage Cluster Jee+ 2015
5. a galaxy cluster contains dark matter
6. collisionless! galaxies a galaxy cluster contains dark matter
7. highly! collisional gas collisionless! galaxies dark matter a galaxy cluster contains
8. a galaxy cluster merger
9. a galaxy cluster merger
10. a galaxy cluster merger
11. a galaxy cluster merger
12. a galaxy cluster merger
14. more collisional
15. more collisional Γ = σ mχ ρv 0 cm2/g
16. more collisional Γ = σ mχ ρv 0 ~104 cm2/g
17. more collisional Γ = σ mχ ρv 0 ~104 cm2/g ?
18. how do we get from to ? Γ = σ mχ ρv
19. DM galaxies 0 100 -2000 -1000 0 1000 2000 distance from barycenter (kpc) galaxy-DM offset (kpc) to : the simulations 1st pericenter passage coalescence DM galaxies BCGs CDM 0 cm2/g 1015 M! equal mass mergers
20. DM galaxies 0 100 -2000 -1000 0 1000 2000 distance from barycenter (kpc) galaxy-DM offset (kpc) to : the simulations 1st pericenter passage coalescence DM galaxies BCGs CDM 0 cm2/g no offsets
21. to : the simulations DM galaxies 0 100 -2000 -1000 0 1000 2000 distance from barycenter (kpc) galaxy-DM offset (kpc) 1st pericenter passage coalescence DM galaxies 0 100 -2000 -1000 0 1000 2000 distance from barycenter (kpc) alaxy-DM offset (kpc) DM galaxies 100 -2000 -1000 0 1000 2000 distance from barycenter (kpc) offset (kpc) DM galaxies 100 -2000 -1000 0 1000 2000 distance from barycenter (kpc) ) DM galaxies -2000 -1000 0 1000 2000 distance from barycenter (kpc) DM galaxies BCGs SIDM 3 cm2/g
22. to : the simulations DM galaxies 0 100 -2000 -1000 0 1000 2000 distance from barycenter (kpc) galaxy-DM offset (kpc) 1st pericenter passage coalescence DM galaxies 0 100 -2000 -1000 0 1000 2000 distance from barycenter (kpc) alaxy-DM offset (kpc) DM galaxies 100 -2000 -1000 0 1000 2000 distance from barycenter (kpc) offset (kpc) DM galaxies 100 -2000 -1000 0 1000 2000 distance from barycenter (kpc) ) DM galaxies -2000 -1000 0 1000 2000 distance from barycenter (kpc) DM galaxies BCGs SIDM 3 cm2/g 40 kpc offset! typically largest just after pericenter
23. to : Γ = σ mχ ρv full simulation results
24. to : Γ = σ mχ ρv full simulation results fiducial SIDM 1 cm2/g
25. to : Γ = σ mχ ρv full simulation results SIDM 1 cm2/g
26. to : Γ = σ mχ ρv full simulation results SIDM 1 cm2/g
27. to : Γ = σ mχ ρv full simulation results SIDM 1 cm2/g
28. to : Γ = σ mχ ρv full simulation results SIDM 1 cm2/g
29. to : Γ = σ mχ ρv full simulation results SIDM 1 cm2/g
30. to : Γ = σ mχ ρv full simulation results 20 kpc SIDM 1 cm2/g
31. to : Γ = σ mχ ρv full simulation results CDM 0 cm2/g SIDM 1 cm2/g 20 kpc
32. to : Γ = σ mχ ρv full simulation results SIDM 3 cm2/g CDM 0 cm2/g SIDM 1 cm2/g 20 kpc 50 kpc
33. to : Γ = σ mχ ρv full simulation results SIDM 10 cm2/g SIDM 3 cm2/g CDM 0 cm2/g SIDM 1 cm2/g 20 kpc 50 kpc
34. to : Γ = σ mχ ρv full simulation results SIDM 10 cm2/g SIDM 3 cm2/g CDM 0 cm2/g SIDM 1 cm2/g 20 kpc 50 kpc
35. to : Γ = σ mχ ρv full simulation results SIDM 10 cm2/g SIDM 3 cm2/g CDM 0 cm2/g SIDM 1 cm2/g 20 kpc 50 kpc cosmological
36. to : El Gordo 100, 400 (± 140?) kpc Sausage Cluster 160 ±130 kpc 220 ± 240 kpc the observations
37. equal mass mergers: a summary
38. equal mass mergers: a summary expected offsets are 20-50 kpc!
39. equal mass mergers: a summary ( ) smaller than obs. uncertainties + too small to explain observed offsets expected offsets are 20-50 kpc!
40. equal mass mergers: a summary ( ) smaller than obs. uncertainties + too small to explain observed offsets expected offsets are 20-50 kpc! however…
41. alternative constraints? DM galaxies 0 100 -2000 -1000 0 1000 2000 distance from barycenter (kpc) galaxy-DM offset (kpc) 1st pericenter passage coalescence DM galaxies 0 100 -2000 -1000 0 1000 2000 distance from barycenter (kpc) alaxy-DM offset (kpc) DM galaxies 100 -2000 -1000 0 1000 2000 distance from barycenter (kpc) offset (kpc) DM galaxies 100 -2000 -1000 0 1000 2000 distance from barycenter (kpc) ) DM galaxies -2000 -1000 0 1000 2000 distance from barycenter (kpc) DM galaxies BCGs SIDM 3 cm2/g to : Γ = σ mχ ρv
42. DM galaxies 0 100 -2000 -1000 0 1000 2000 distance from barycenter (kpc) galaxy-DM offset (kpc) BCGs 1st pericenter passage coalescence alternative constraints? DM galaxies BCGs SIDM 3 cm2/g to : Γ = σ mχ ρv
43. DM galaxies 0 100 -2000 -1000 0 1000 2000 distance from barycenter (kpc) galaxy-DM offset (kpc) BCGs 1st pericenter passage coalescence galaxies & BCGs oscillate! alternative constraints? DM galaxies BCGs DM galaxies BCGs SIDM 3 cm2/g to : Γ = σ mχ ρv
44. DM galaxies 0 100 -2000 -1000 0 1000 2000 distance from barycenter (kpc) galaxy-DM offset (kpc) BCGs 1st pericenter passage coalescence galaxies & BCGs oscillate! alternative constraints? DM galaxies BCGs DM galaxies BCGs SIDM 3 cm2/g should observe ! miscentered BCGs in relaxed clusters to : Γ = σ mχ ρv
45. alternative constraints? 1 cm2/g: 100 kpc 3 cm2/g: 200 kpc 10 cm2/g: 300 kpc to : Γ = σ mχ ρv
46. to : alternative constraints? Γ = σ mχ ρv 1 cm2/g: 100 kpc 3 cm2/g: 200 kpc 10 cm2/g: 300 kpc scales with! cross section!
47. to : alternative constraints? Γ = σ mχ ρv 1 cm2/g: 100 kpc observed: 10s of kpc σ/m 0.1 cm2/g 3 cm2/g: 200 kpc 10 cm2/g: 300 kpc scales with! cross section!
48. equal mass mergers: a summary alternative methods may! provide better SIDM constraints ( BCG miscentering could give 0.1 cm2/g ) ( ) smaller than obs. uncertainties + too small to explain observed offsets expected offsets are 20-50 kpc!
49. DM galaxies 0 100 -2000 -1000 0 1000 2000 distance from barycenter (kpc) galaxy-DM offset (kpc) 1st pericenter passage coalescence DM galaxies BCGs CDM 0 cm2/g alternative constraints? to : Γ = σ mχ ρv
50. alternative constraints? DM galaxies 0 100 -2000 -1000 0 1000 2000 distance from barycenter (kpc) galaxy-DM offset (kpc) 1st pericenter passage coalescence DM galaxies 0 100 -2000 -1000 0 1000 2000 distance from barycenter (kpc) alaxy-DM offset (kpc) DM galaxies 100 -2000 -1000 0 1000 2000 distance from barycenter (kpc) offset (kpc) DM galaxies 100 -2000 -1000 0 1000 2000 distance from barycenter (kpc) ) DM galaxies -2000 -1000 0 1000 2000 distance from barycenter (kpc) DM galaxies BCGs SIDM 3 cm2/g to : Γ = σ mχ ρv
51. to : Γ = σ mχ ρv alternative constraints?
52. equal mass mergers: a summary alternative methods may! provide better SIDM constraints ( ) smaller than obs. uncertainties + too small to explain observed offsets expected offsets are 20-50 kpc! BCG miscentering could give 0.1 cm2/g (plus halo-halo separation, merger fraction)
53. towards a general model trends? non-1015 M! mergers? unequal mass mergers?
54. v particles from opposite halos interact
55. v capture exchange escape three outcomes both bound one bound none bound particles from opposite halos interact
56. v capture exchange escape three outcomes both bound one bound none bound particles from opposite halos interact given velocity distribution + escape velocity, ! can compute likelihood of each outcome
57. v1 particles from! opposite halos given velocity distribution + escape velocity, ! can compute likelihood of each outcome v2 self-interaction (isotropic scattering) v’1 v’2
58. v1 particles from! opposite halos given velocity distribution + escape velocity, ! can compute likelihood of each outcome v2 self-interaction (isotropic scattering) v’1 v’2 Maxwell-Boltzmann distribution Phalo1(v) = PMB(v, σv) Phalo2(v) = PMB(v − vcol, σv) with (for cored profiles) σv = vesc/4
59. v1 particles from! opposite halos given velocity distribution + escape velocity, ! can compute likelihood of each outcome v2 self-interaction (isotropic scattering) v’1 v’2 escape velocity vesc ∼ GM R ∼ M1/3 Maxwell-Boltzmann distribution Phalo1(v) = PMB(v, σv) Phalo2(v) = PMB(v − vcol, σv) with (for cored profiles) σv = vesc/4
60. outcome probabilities vesc ~ 3200 km/s 1015 Msun mergers
61. collision velocity (~4200 +- 200) km/s outcome probabilities vesc ~ 3200 km/s 1015 Msun mergers
62. ~11% chance of escape 1 cm2/g: 10% lost 3 cm2/g: 23% lost 10 cm2/g: 48% lost vesc ~ 3200 km/s collision velocity (~4200 +- 200) km/s 1015 Msun mergers outcome probabilities Nlost N = 1 − exp − σSI mχ ΣP(esc)
63. v capture exchange escape three outcomes both bound one bound none bound particles from opposite halos interact momentum loss (“drag”) mass loss (tails)
64. 1015 Msun mergers velocity exchange
65. 1015 Msun mergers velocity exchange ~30% change
66. outcome probabilities, more generally our inputs scale as vcol ∼ M1/3 vesc ∼ M1/3 σv ∼ vesc ∼ M1/3
67. outcome probabilities, more generally our inputs scale as vcol ∼ M1/3 vesc ∼ M1/3 σv ∼ vesc ∼ M1/3 outcome probabilities same across all masses! less ejected in smaller M
68. outcome probabilities, more generally unequal mass mergers?
69. outcome probabilities, more generally unequal mass mergers? let . inputs now scale as: q = M1/M2 σv = σ2 v,1 + σ2 v,2 = σv,1 1 + q−2/3 vcol ∼ M1(1 + q−1)
70. outcome probabilities, more generally unequal mass mergers? let . inputs now scale as: q = M1/M2 σv = σ2 v,1 + σ2 v,2 = σv,1 1 + q−2/3 vcol ∼ M1(1 + q−1) for a 10:1 merger, , are 75% smaller; P(esc) = 0.56! vcol ∼ M1(1 + q−1) σv = σ2 v,1 + σ2 v,2 = σv,1 1 + q−2/3 36%, 66%, 93% of lower-mass cluster lost for 1, 3, 10 cm2/g
71. outcome probabilities, more generally unequal mass mergers? much more likely to be ejected and form tails! let . inputs now scale as: q = M1/M2 σv = σ2 v,1 + σ2 v,2 = σv,1 1 + q−2/3 vcol ∼ M1(1 + q−1) for a 10:1 merger, , are 75% smaller; P(esc) = 0.56! vcol ∼ M1(1 + q−1) σv = σ2 v,1 + σ2 v,2 = σv,1 1 + q−2/3 36%, 66%, 93% of lower-mass cluster lost for 1, 3, 10 cm2/g
72. SIDM mergers summary alternative methods may! provide better SIDM constraints too small to explain observed offsets expected offsets are 20-50 kpc BCG miscentering could give 0.1 cm2/g but tails more likely in unequal mass mergers underlying processes scale with mass