FORMATION OF DARK GALAXIES 1812.07000 (JCAP 03 (2019) 036) Daniel Egana-Ugrinovic Perimeter Institute Jae Hyeok-Chang Rouven Essig Stony Brook University Chris Kouvaris CP3-Origins This talk is carbon neutral. www.cooleffect.org
A pessimistic scenario: Dark matter does not have any detectable non-gravitational interactions with the SM Liu Bolin 2
If the dark sector interacts only gravitationally with us… …what can we learn from its particle nature?
A COMPLETELY DARK, DARK SECTOR Progress can be made based uniquely in astronomical and cosmological observations. High precision astronomical observatories (LSST, GAIA,LIGO, etc.) will test the behavior of DM on small scales. How do we turn this experimental program into a dark matter theory program? 4
SM FORMS STARS DUE TO DISSIPATION SM forms compact objects since: 1. It has self-interactions 2. The baryonic gas can dissipate energy (can cool ) Properties of these objects (how did they form, sizes, masses) gives information on particle interactions. Microscopic Properties 5
ASTRONOMICAL PROPERTIES FIXED BY LAGRANGIAN PARAMETERS Example: Chandrasekhar limit for White dwarf M C ∼ M 3 Pl m 2 p In principle, it is possible to obtain a map between astronomical properties and lagrangian parameters 6
CAN DM FORM GALAXIES AND COMPACT OBJECTS? Halo and star formation is a complex problem in the SM. Chemistry, multiple cooling rates… Example: the SM cooling rate functions Sutherland & Dopita, O, C, N ApJS, 88, 253 Bremsstrahlung, Ne, Fe, Si collisional excitation, Molecules ionization, (only H2 in primordial gas ) H, He recombination… All dependent on metallicity… coll. exc. brems only (atom-e coll) 7
OUTLINE A complete history of structure formation in a dissipative dark-sector 1. Present the simplest dark-sector model that has cooling and self- interactions 2. Discuss the initial, linear evolution of dark-sector perturbations starting from the primordial power spectrum. 3. Continue into the non-linear regime, and discuss galactic evolution and the formation of exotic compact objects , or “dark stars”. 8
Simplified Models for Dark-Sector Astronomy The Standard Model Our talk 9
A MODEL WITH ONLY TWO PARTICLES Dark-electron dark-photon model Ψ e D Ψ e D − 1 4 F µ ν F µ ν + m 2 i ¯ Ψ e D γ µ D µ Ψ e D − m e D ¯ γ D A µ A µ D , e + e − D , γ D m e D , m γ D , α D This model has only three parameters 10
COSMOLOGICAL ABUNDANCES In general, all species may have a significant cosmological abundance. symmetric asymmetric CDM Baryons γ D e D e D Dark sector is asymmetric: How to generate the 1. No annihilations within a compact object. asymmetry? 2. Avoids complications of bound states. Petraki, Pearce, Kusenko 1403.1077 11
SYMMETRIC PART DEPLETED BY ANNIHILATIONS The symmetric part is depleted by annihilations e − D γ D e + D symmetric asymmetric CDM Baryons e D e D γ D m e D � 1 / 2 T e D | e D dec Condition for � 1 / 2 � 10 − 2 α D ≥ 4 . 6 × 10 − 7 efficient depletion T SM | e D dec 1 MeV f of symmetric part 12
DARK SECTOR COLD FEW DARK PHOTONS Dark photons may lead to overclosure or large ∆ N e ff Dark photon matter density ρ γ D ∼ m γ D T 3 γ D asymmetric CDM Baryons e D γ D � 1 / 3 1 keV � 1 / 3 g ∗ S | γ D dec Overclosure bound T γ D | γ D dec ≤ 0 . 2 T SM | γ D dec 10 m γ D ∆ N e ff T γ D | BBN ≤ 0 . 5 T SM | BBN (see also DAO, Cyr-Racyne et.al.1310.3278) 13
MATTER BUDGET OF OUR MODEL Asymmetric part is a small component of matter: no bounds form bullet cluster/halo shapes f ≡ ρ e D 0 ≤ 10% Katz et. al. 1303.1521 ρ DM 0 asymmetric CDM Baryons 14
ONLY THREE PROCESSES MATTER Dark-electron Compton scattering Bremsstrahlung self-interactions 15
DARK ELECTRON THERMODYNAMICS Even within this simple model there are three thermodynamic regimes ` mfp vs. L MW ` mfp = 1 / ( n � ) , �� � �� � z eq = 3400 α 2 �� � σ C ∝ m 2 �� - � e D �� - � σ M ∝ α 2 m 2 e D �� - � m 4 γ D �� - � �� - � �� - � �� - � �� � �� � �� � �� - � �� � �� � 16
Objective Study the formation of dark-electron galaxies and their substructure We will concentrate on the formation and evolution of the dark electron galaxy within our Milky Way 17
STRUCTURE FORMATION HAS TWO MAIN STAGES 1. Linear growth of matter overdensities δ = δρ ρ < 1 2. Non-linear evolution of the resulting(dark) matter clumps 18
LINEAR GROWTH OF PERTURBATIONS Initial conditions: Harrison-Zeldovich power spectrum ≡ (2 π ) 3 P ( k ) = Ak ⌦ ↵ δ k δ ∗ k Linear evolution of perturbations ∂ 2 ⇥ c 2 s k 2 /a 2 − 4 π G ρ 0 ⇤ t δ k ( t ) + 2 H ∂ t δ k ( t ) + δ k = 0 s T e + 4 πα n e c s = Kouvaris et.al. 1507.00959 m 2 e m 2 m e γ Matter overdensities grow only on scales larger than the Jeans length 19
JEANS CRITERION DECIDES WHICH PERTURBATIONS GROW The Jeans criterion is ◆ 3 / 2 ✓ M > m J = π π 6 c 3 ρ e D , s ρ 0 ( z ) G δρ ρ ∝ a 20
ONLY IN PARTS OF PARAMETER SPACE A MW CAN BE FORMED �� � �� � �� - � �� - � �� - � �� - � �� - � �� - � α 2 SM σ C ∝ �� - � �� - � m 2 e D �� - � �� - � �� - � �� - � �� - � �� - � �� - � �� - � �� - � �� - � �� - � �� � �� � �� � �� � �� � �� � White: regions of parameter space where a Milky Way-sized perturbation may grow after equality 21
GALAXY GOES NON-LINEAR AT z ≈ 2 At some point, perturbations become non-linear δρ ρ ∼ 1 Gravitational pull overcomes Hubble expansion: perturbations “turn-around” ◆ 1 / 2 ✓ 1 t ff ∼ H − 1 t ff ≡ 16 π G ρ The galaxy’s turnaround redshift can be estimated by k 3 MW 2 π 2 P ( k MW , z ta ) = 1 z ta ≈ 2 → 22
Summary of linear perturbation growth Primordial regions of under- and over-densities � δρ ρ ∼ a δρ Density contrast grows as � � ρ � 0 δρ Nonlinearities and turnaround ρ ∼ 1 ◆ 1 / 2 ✓ 3 π t ff ∼ H − 1 t ff = 32 Gm e n e Nonlinear regime: self-gravitating gas decoupled from Hubble flow 23
Summary of linear perturbation growth Primordial regions of under- and over-densities � δρ ρ ∼ a δρ Density contrast grows as � � ρ � 0 δρ Nonlinearities and turnaround ρ ∼ 1 ◆ 1 / 2 ✓ 3 π t ff ∼ H − 1 t ff = 32 Gm e n e Nonlinear regime: self-gravitating gas decoupled from Hubble flow 24
Summary of linear perturbation growth Primordial regions of under- and over-densities � δρ ρ ∼ a δρ Density contrast grows as � � ρ � 0 δρ Nonlinearities and turnaround ρ ∼ 1 ◆ 1 / 2 ✓ 1 t ff ∼ H − 1 t ff ≡ 16 π G ρ Nonlinear regime: self-gravitating gas decoupled from Hubble flow 25
Summary of linear perturbation growth Primordial regions of under- and over-densities � δρ ρ ∼ a δρ Density contrast grows as � � ρ � 0 δρ Nonlinearities and turnaround ρ ∼ 1 ◆ 1 / 2 ✓ 1 t ff ∼ H − 1 t ff ≡ 16 π G ρ Nonlinear regime: self-gravitating gas decoupled from Hubble flow 26
STRUCTURE FORMATION HAS TWO MAIN STAGES 1. Linear growth of matter overdensities 2. Non-linear evolution of the resulting(dark) matter clumps δ = δρ ρ � 1 27
ASTRONOMY BEFORE BIG COMPUTERS ◆ 3 / 2 ✓ Jeans Mass: M > m J = π π 6 c 3 ρ e D , s ρ 0 ( z ) G max mass of gas that for collapse to happen pressure can support Fragmentation: Jeans mass decreases as mother halo collapses Low, Linden-Bell 1976 Halo fragmentation is the origin of stars Rees, Ostriker, 1977 Silk, 1977 28
JEANS MASS EVOLUTION IS FIXED BY ENERGY CONSERVATION First law of thermodynamics s Λ BS = 32 α 3 ( D ρ e D T e D T e D ) dE thermal = − PdV − Λ cooling dt e − m γ D /T eD √ π m 4 m e D e D d log T e D = 2 m e D P e D − 2 t collapse d log ρ e D 3 ρ e D T e D t cooling ◆ − 1 ✓ d log ρ e D t cooling ≡ 3 T e D 1 , t collapse ≡ . m Λ cooling dt Specifies a contour in the density-temperature plane as the galaxy collapses 29
THE HISTORY OF A GALAXY: SETUP �� �� �� �� �� �� �� �� �� �� We will now follow �� �� �� �� the evolution of a �� - � �� �� �� � �� � �� � �� � �� �� �� �� � 10 10 M � �� � dark electron halo �� � �� � (1% of our Milky Way) �� � �� � �� - � �� - � �� - � �� - � �� � �� � �� � �� � �� � �� �� 30
THE HISTORY OF A GALAXY: FREE-FALL �� �� �� �� Free-fall �� �� and heating �� �� �� �� �� �� d log T d log ρ = 2 �� �� �� - � �� �� �� � �� � �� � �� � �� �� �� �� � 3 �� � �� � Free-fall time �� � �� � �� � ◆ 1 / 2 ✓ 3 π �� - � t ff = �� - � 32 Gm e n e �� - � �� - � �� � �� � �� � �� � �� � �� �� 31
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