Solving small scale structure puzzles with. dissipative dark matter

Transcription

1 Solving small scale structure puzzles with. dissipative dark matter Robert Foot, COEPP, University of Melbourne Okinawa, March 2016

2 Dark matter: why we think it exists Dark matter issues on small scales and dark matter scaling relations What type of particle physics might be implicated

3 Early Universe Cosmology: Cosmic Microwave Background Dark matter is needed to explain CMB. Matter in the Universe was once a rather homogeneous and isotropic plasma, where tiny perturbations were present which were the seeds of galaxies etc. At early times, the plasma behaves as a strongly coupled fluid which evolved in time subject to gravity and pressure. The CMB anisotropy spectrum provides information about this epoch, and can be successfully modelled with just a few parameters.

4 Early Universe Cosmology: Cosmic Microwave Background Dark matter also needed to explain CMB. Matter in the Universe was once a rather homogeneous and isotropic plasma, where tiny perturbations were present which were the seeds of galaxies etc. At early times, the plasma behaves as a strongly coupled fluid which evolved in time subject to gravity and pressure. The CMB anisotropy spectrum provides information about this epoch, and can be successfully modelled with just a few parameters.

5 Early Universe Cosmology: Cosmic Microwave Background Fit to the CMB allows one to infer the energy content of the Universe:

6 Dark matter in clusters: Bullet cluster Bullet cluster NASA/Chandra Red X-rays trace the baryonic matter in hot gas Blue Inferred dark matter distribution (from gravitational lensing) Some evidence that DM does not self-interact very strongly! e.g. S. W. Randall et al, arxiv: (ApJ 679, 1173 (2008). However details depend on the way dark matter is distributed thought the cluster.

7 Dark matter is needed around galaxies. Measurements of rotation curves in galaxies by Rubin and others in the 1970 s found that spiral galaxies like the Milk Way have asymptotically flat rotation curves. Conclusion: Galaxies like Milky Way embedded in roughly spherical dark matter halo.

8 Dark matter issues on small scales & galactic scaling relations

9 Dark matter issues on small scales Collisionless cold dark matter model predicts cuspy profile: DDO 47 Gas rich dwarf Density profiles obtained from observations more compatible with cored profiles. Ref: Gentile et al, 2005

10 Dark matter issues on small scales Collisionless cold dark matter model predicts cuspy profile: Density profiles obtained from observations more compatible with cored profiles. Oh et al, Little Things, 2015

11 Other dark matter issues on small scales Simulations predict many more satellite galaxies than are observed around Milky Way and Andromeda. Furthermore, most of the satellites that are observed appear to orbit on a polar plane. Ibata et al, 2013 Pawlowski et al, 2012

12 Other dark matter issues on small scales Simulations predict many more satellite galaxies than are observed around Milky Way and Andromeda. Furthermore, most of the satellites that are observed appear to orbit on a polar plane. The issue appears to be a wider problem: Some evidence for a deficit in field galaxies too. Zvala et al, 2009

13 Galaxies have interesting scaling relations a) Tully Fisher relation Tully Fisher relation relates total galaxy luminosity with rotational velocity: Due to baryons Due to dark matter Meyer et al, 2008

14 Galaxies have interesting scaling relations b) Dark matter core radius (R C ) scales with stellar Disk scale length With dark matter halo modelled with cored profile: and with baryonic disk modelled with exponential: Core radius is observed to scale with disk scale length: Dark matter property Baryonic property Donato et al, 2004

15 What type of particle physics might be implicated?

16 The possibilities for dark matter can be described in various ways. First, is the cold, warm, hot classification: Hot, cold or warm dark matter? Hot DM refers to particles, such as neutrinos, that were moving at nearly the speed of light at redshift z ~ 10 6 (or time t ~ 1 year), when the temperature T ~ 3 x 10 2 ev and the cosmic horizon first encompassed Msun the amount of dark matter contained in the halo of a large galaxy like the Milky Way. Free streaming of these particles destroys galaxy-scale fluctuations. Cold DM is the opposite limit, where dark matter mass is GeV scale or greater, and particles were very slow moving at t=1 year (T ~ 3 x 10 2 ev). Warm DM is the intermediate regime, Mass around a kev.

17 Moment please, three possibilities. The possibilities for dark matter particles are: a) DM is cold and (astrophysically) collisionless. b) DM is warm. c) DM is cold but collisional. Possibility a) is simple, gives good explanation for CMB, large scale structure, and also Bullet cluster, but requires baryonic physics to explain the small scale issues and galaxy scaling relations. Possibilities b) and c) can both explain CMB but also help in resolving the small scale issues, and c) can also explain galaxy scaling relations. However c) could have more difficulty in accounting for Bullet cluster.

18 WIMP dark matter candidates are a promising example of (astrophysically) collisionless dark matter. This is all well and good if baryonic physics solves the small scale structure issues and galactic scaling relations. But what if small scale structure issues and galaxy scaling relations result from dark matter physics?

19 Collisional dark matter: A plethora of possibilities One can envisage the possibility that dark matter has some properties similar to ordinary matter, e.g. charged under an unbroken U(1) gauge interaction (dark electromagnetism). E.g. consider a hidden sector comprising of a dark electron and dark proton, coupling to a massless dark photon: where is the covariant derivative. The term: represents kinetic mixing interaction. Provides a mechanism for ordinary matter to interact with dark matter on galactic scales. Ref: Foot, Volkas, Vagnozzi, Clarke, and many other people.

20 Dissipative dark matter Mirror dark matter is a theoretically constrained possibility, where dark matter arises from a hidden sector exactly duplicating the SM: & Gravity SM Kinetic mixing SM / U(1) The two systems can evolve semi-independently, being only relatively weakly coupled together via gravity and kinetic mixing interaction. Foot, Lew, Volkas, PLB 1991 Foot 2014 for detailed bibliography.

21 Dissipative dark matter with kinetic mixing Kinetic mixing is a renormalizable interaction, described by a dimensionless parameter,. The physical effect is to induce tiny ordinary electric charges for mirror particles, Q = e. This means that they can couple to ordinary photons: Important for cosmology, supernova s, Galactic structure, if Important for direct detection experiments, such as DAMA, XENON, LUX etc if

22 Formation of structure

23 Dissipative dark matter and CMB and LSS In the early universe, have two systems: The ordinary particles with temperature T and the dark sector particles with temperature T D. Dissipative dark matter can have acoustic oscillations. However such things can only occur before dark recombination. If T D < T, then this time occurs very early potentially before matter-radiation equality. At early times, only small scales can be affected, as large scales have not yet entered into the horizon. In fact such dark matter reproduces collisionless dark matter (for CMB/LSS) in the limit where T D /T 0. However even if we start with T D /T = 0 as an initial condition, kinetic mixing interactions will generate a nonzero T D /T. For mirror dark matter have : P.Ciarcelluti, Foot 2009, Foot 2012

24 Small scale power suppression Dark acoustic oscillations and dark diffusion damping operate prior to dark atom recombination. The four curves are for x = 0, 0.3, 0.5, 0.7 Bottom line: Dissipative dark matter can explain CMB and LSS, but requires x < 0.2. Foot, 2013, Berezhiani et al, 2001, 2005, Ignatiev, Volkas, 2004.

25 Small scale power suppression Dark acoustic oscillations and dark diffusion damping operate prior to dark atom recombination. Bottom line: Dissipative dark matter can explain CMB and LSS, but requires x < 0.2.

26 Small scale power suppression Easily calculate halo mass function using extended Press-Schechter formalism. Need to relate M halo to M baryons to connect with observations. Used simplest choice: S.Vagnozzi, Foot, 2016

27 Small scale power suppression Easily calculate halo mass function using extended Press-Schechter formalism. Smooth with Gaussian to obtain an estimate of baryonic mass function Need to relate M halo to M baryons to connect with observations. Used simplest choice: S.Vagnozzi, Foot, 2016

28 Top-down formation of satellite galaxies? Very small galaxies strongly suppressed. Can over solve missing satellite problem. Suggests different origin for satellites. Could be formed top-down out of nonlinear dissipative collapse of larger density perturbations. Satellites broke of from dark disk. Can provide simple explanation of the planar structure of satellites!

29 Formation of the Milky Way DM halo Consider a MW scale density perturbation, (t;r). Such a perturbation grows linearly until it reaches a critical overdensity, (t;r) ~ c, and collapses. Nonlinear collapse phase complicated, but envisage dark disk formation due to dissipative processes such as dark bremsstrahlung. Heating from dark SN can delay ordinary star formation, and possibly provide the mechanism for re-ionization of ordinary matter. Ordinary matter is primordial at this early time (no metal component). Hence it can only be heated to the ionization temperature ~ 10 ev. Star formation eventually occurs, and ordinary SN heat the dark disk. DM not in compact objects thereby expands into a roughly spherical plasma halo. dark electron dark dark photon If ordinary SN can convert ~ ½ core-collapse energy into light dark sector particles. The dark photons can provide a huge heat source: ~10 43 erg/s (for MW galaxy).

30 Formation of the Milky Way DM halo Consider a MW scale density perturbation, (t;r). Such a perturbation grows linearly until it reaches a critical overdensity, (t;r) ~ c, and collapses. Nonlinear collapse phase complicated, but envisage dark disk formation due to dissipative processes such as dark bremsstrahlung. Heating from dark SN can delay ordinary star formation, and possibly provide the mechanism for re-ionization of ordinary matter. Ordinary matter is primordial at this early time (no metal component). Hence it can only be heated to the ionization temperature ~ 10 ev. Star formation eventually occurs, and ordinary SN heat the dark disk. DM not in compact objects thereby expands into a roughly spherical plasma halo. Snaith et al, 2014 Dip can arise as dark halo expands past its equilibrium configuration

31 Galaxy dynamics with dissipative dark matter

32 Halo dynamics Euler equations Imagine that dark matter halo around disk galaxies take the form of a roughly spherical plasma composed of dark electrons, dark protons, dark photons. Their physical properties, density, bulk velocity v and pressure P, are described by Euler s equations of fluid dynamics: Halo heating supplied by supernova generated D via kinetic mixing mechanism. Cooling due (assumed to be mainly) bremsstrahlung.

33 Disk galaxies today If the system evolves to a steady-state configuration then the equations reduce to two relatively simple equations (with spherical symmetry assumed): Hydrostatic equilibrium Energy balance equation: Heating=cooling That is, two equations for two unknowns, the DM (r), T(r) distributions.

34 Disk galaxies today If dark bremsstrahlung dominates the cooling, then with If kinetic mixing induced processes in ordinary supernovae produce dark photons, then Matching heating and cooling rates then implies:

35 Disk galaxies today In the optically thin limit, the dark photon flux from supernovae in the disk is given (in spherical co-ordinates) by: Dark photon luminosity per unit (disk) area Dark matter mass density from the assumed dynamics is then: Supernovae rate per unit (disk) area

36 Properties of the steady state solution for generic galaxies with Gives R C r D (from geometry). for r << r D : 11

37 In fact, in this r << r D limit: is the central surface brightness. F. Lelli et al,mnras (July 21, 2013) Vol. 433 L30-L34

38 Specific galaxies NGC 2366 Little things, Oh et al, 2015 Herrmann et al, 2013

39 Specific galaxies DDO 50 : a non steady-state system This steady-state formula fails for some dwarfs, e.g. DDO 50. But DDO 50 in starburst phase. Explains why steady-state formula fails. Also, halo dynamics would be a main driver of oscillations in SFR, as halo contracts and expands due to time dependent heating, which in turn leads to feedback on SFR. DDO 50 K. McQuinn et al, 2010

40 . Dissipative dark matter leads to new avenues for direct detection If dark matter is collisional, other approaches for direct detection are possible. Dark matter can accumulate in the Earth and shield the detector from the halo dark matter wind. As the Earth spins on its axis the amount of shielding varies, leading to diurnal modulation. Can have very large such signal for southern hemisphere detector! J.Clarke. Foot 2016, S. Vagnozzi, Foot,2015.

41 Lot of dark matter detection laboratories but is something missing?

42 Stawell Underground Physics Laboratory (SUPL)

43 Conclusions Small scale puzzles and galactic scaling relations pose a serious challenge for (astrophysically) collisionless cold dark matter. Indeed, collisionless cold dark matter would require baryonic physics to explain these things, which seems to be asking a lot. However, there is no strong reason to believe that dark matter is astrophysically collisionless. Thus it seems a priori possible that the small scale puzzles and galactic scaling relations might instead result from nontrivial dark matter physics. I have argued that dissipative dark matter is a suitable candidate for this nontrivial dark matter physics. If dark matter is (astrophysically) collisional, then astrophysical probes (and also direct detection experiments) should be able to tell us something more about its nature.