Lecture 13 AST4320

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AST4320 - Cosmology and extragalactic astronomy

Lecture 13

Structure of Dark Matter Halos & The Missing Satellites Problem.

1 AST4320 - Cosmology and extragalactic astronomy

Structure of Dark Matter Halos & The Missing Satellites Problem. Outline

The `Too-Big-to-Fail Problem’ the contemporary version of the missing satellite problem.

Dark matter: why cold, why pressureless?

Dark matter candidates / prospects for detection / possible detections?

2 Missing Satellite Problem (MSP) (see review by Weinberg et al. 2013, arXiv:1306.0913)

12 Left: simulated dark matter distribution in dark matter halo with M=10 Msun. Circles denote 9 most massive substructures or `satellites’. Right: Spatial distribution of observed Milky Way `satellites’. 3 Missing Satellite Problem (MSP) A quantitative comparison of # satellites at r < 400 kpc.

Klypin et al. 1999 simulated

Observed Number

Discrepancy apparent at vcirc < 40 km/s.

10 At vcirc<40 km/s (or M < 10 Msun) feedback mechanisms become efﬁcient.

4 Feedback Mechanism I: Photoionization Feedback. Observations indicate that gas in most of the volume outside of galaxies is photoionized.

More appropriate would be the `ﬁlter mass’ (takes into account time evolution of T and density during collapse of a cloud).

Gnedin 2000

Jeans Mass

Filter Mass Not possible (or more difﬁcult) for baryons to collapse into Dark 9 matter halos with M < 10 Msun.

This corresponds to

5 Feedback Mechanism II: Stellar Feedback.

Photoionization feedback provides natural explanation for Missing Satellite Problem, but... though plausible, no direct evidence exists for photoionization feedback in action

in contrast, there exists ample evidence for so-called `stellar’ feedback, e.g.

M82, nearby dwarf galaxy with a recent star burst. Red indicates H-alpha emission from outﬂowing gas. Outﬂow driven by supernova explosions, and/or radiation pressure.

6 Feedback Mechanism II: Stellar Feedback.

Stellar feedback is increasingly efﬁcient in removing gas from galaxies towards lower mass dark matter halos (derive on board).

Assume that we convert some fraction f* of baryonic matter into stars. The higher f*, the more stars explode as supernovae.

If supernovae drive out gas from galaxies, then this limits how much gas is available to star formation. In other words, supernova-feedback can put a limit on how large f* can be.

We derive (on the board + lecture notes 12) that this maximum f* scales with Vcirc as

In other words, supernova feedback suppresses star formation preferentially in low mass halos.

7 Missing Satellite Problem in Models that Include Feedback Munoz et al. 2009

Predicted number of satellites with no feedback

Predicted number of satellites with feedback*

* Note: feedback tuned to reproduce observations. 8 Missing Satellite Problem: Observational Advancements

2005: Discovery of new Milky Way companion Willman 1 (dwarf galaxy).

9 Missing Satellite Problem: Observational Advancements 2005: Discovery of new Milky Way companion Willman 1. This object is classiﬁed as an `ultra-faint dwarf’ galaxy (MV=-3.0; 400.000 times fainter than faintest, most distant galaxies we talked about earlier!)

Difﬁcult to ﬁnd these structures. Circles highlight an overdensity of faint, blue stars.

10 Missing Satellite Problem: Observational Advancements 2005: Discovery of new Milky Way companion Willman 1. This object is classiﬁed as an `ultra-faint dwarf’ galaxy (MV=-3.0; 400.000 times fainter than faintest, most distant galaxies we talked about earlier!)

Difﬁcult to ﬁnd these structures. Circles highlight an overdensity of faint, blue stars.

To date, another 15 ultra-faint dwarfs have been found!

11 Missing Satellite Problem in Summary

The Missing Satellite Problem refers to the apparent discrepancy between the predicted number of dark matter satellites and observed number of dwarf galaxies around the Milky Way

There is no shortage of baryonic processes which suppress the efﬁciency at which stars can form in low mass satellites of Milky Way.

New ultra-faint dwarf galaxies have only recently been discovered. Stellar kinematics suggests that there exists a large spread in relation between luminosity + dark matter halo mass.

Reasonable to (for now) regard the relation between low mass dark matter halos & `ultra-faint dwarfs’ as puzzle of galaxy formation physics (feedback) instead of a contradiction of the standard cold-dark matter paradigm.

A bigger - more pressing - problem is related to the most luminous satellites.

12 `Too Big To Fail’ Problem. Boylan-Kolchin et al. 2011/2012

Boylan-Kolchin took 6 hydrodynamical simulations designed to simulate `Milky-Way dark matter halos’ with a variety of mass & force resolution. Compared the properties of the most massive simulated satellites with the most luminous satellites. 13 `Too Big To Fail’ Problem. Boylan-Kolchin et al. 2011/2012

Satellite Luminosity function

observed

simulated

Models that best reproduce observed satellite luminosity function - and hence best `solves’ the missing satellite problems, predicts that all satellites have signiﬁcantly larger rotational velocities!

14 `Too Big To Fail’ Problem. Boylan-Kolchin et al. 2011/2012

Satellite Luminosity function

observed

simulated

Because rotational velocity provides a measure of enclosed mass, predicted satellites are too massive (`too big’).

15 `Too Big To Fail’ Problem. Boylan-Kolchin et al. 2011/2012

Because rotational velocity provides a measure of enclosed mass, predicted satellites are too massive (`too big’).

If the most luminous observed satellites around the Milky Way indeed reside in dark matter halos with vcirc < 25 km/s, then why do all 10 (!) more massive dark halos not have observed low-luminosity counter parts?

Why is star formation(relatively) efﬁcient SF in vcirc < 25 km/s, but much less so in the more massive objects (which presumably are more `immune’ to feedback effects, i.e. `too big to fail’).

`One possible explanation is that the matter concentration is less concentrated than what current dissipationless simulations predict’ (see next slides.)

16 From previous lecture: Observational Constraints on Dark Matter Halo Profiles `Rotation curves’ of gas rich dwarf galaxies. I showed this to illustrate the `cusp- core’ problem. (from Moore 1994)

Simulated proﬁles have cusps (density ~ r-1), while observed rotation curves favor `cored’ proﬁles (density is constant).

vROT

Radius (kpc) From previous lecture: Observational Constraints on Dark Matter Halo Profiles

`Rotation curves’ of gas rich (from Moore 1994) dwarf galaxies.

Cusp-core problem: simulated density proﬁles 1. are steeper. 2. have higher central densities 3. steeper rotation curves.

vROT Appears related to the discrepancy in predicted and observed circular velocities?

At ﬁxed r, can get signiﬁcant differences!

Radius (kpc) `Too Big To Fail’ Problem. Boylan-Kolchin et al. 2011/2012

There is a possible connection with the cusp-core problem, which also states that simulated dark matter proﬁles have higher central densities than what has been inferred from gas (and stellar) kinematics in dark-matter dominated galaxies (dwarf galaxies, and low-surface brightness galaxies)

19 The Small-Scales Crisis in Cold Dark Matter Cosmologies

Summary (Incl. Lecture 11.) When comparing the structure of dark matter halos as obtained from cosmological simulations to structure inferred from gas & stellar kinematics, some problems surfaced: Problem 1 ‘cusp-core problem’: Simulated density proﬁles are `cuspy’, inferred proﬁles are `cored’.

Some proposed solutions: 1. Cuspy proﬁles originally predicted in dark-matter only simulations. Baryonic physics can transform cusps into cores (e.g. via supernova feedback, but this requires a minimum stellar content). 2. Modify dark matter properties (next). 3. Modify gravity.

20 The Small-Scales Crisis in Cold Dark Matter Cosmologies

Summary (Incl. Lecture 11.) When comparing the structure of dark matter halos as obtained from cosmological simulations to structure inferred from gas & stellar kinematics, some problems surfaced:

Problem 2 ‘missing satellite problem’: The simulated and observed satellite distributions around the Milky Way are inconsistent, in the sense that simulations predict many more satellites.

Solution: 1. Feedback. Can bring down the predicted number to the observed number - which has increased in recent years (about 15 ultra-faint dwarfs have been discovered in the past 10 years)

Caveat: This problem has `transformed’ into the `too big to fail’ problem (discussed next). We are not missing satellites, rather simulations predict them to be too massive.

21 The Small-Scales Crisis in Cold Dark Matter Cosmologies

Summary (Incl. Lecture 11.) When comparing the structure of dark matter halos as obtained from cosmological simulations to structure inferred from gas & stellar kinematics, some problems surfaced:

Problem 2 (absorbed `missing satellite problem’) ‘too-big-to-fail- problem’: Predicted masses of Milky Way satellites are signiﬁcantly higher than observationally inferred values.

Solution: The problem appears connected to the cusp-core problems. So solutions range from baryonic physics, to modifying dark matter or gravity.

Interestingly: The `severity’ of the problem depends on mass of Milky-Way halo, which is still somewhat uncertain (see video).

22 The Small-Scales Crisis in Cold Dark Matter Cosmologies

Summary (Incl. Lecture 11.) When comparing the structure of dark matter halos as obtained from cosmological simulations to structure inferred from gas & stellar kinematics, some problems surfaced:

Problem 2 (absorbed `missing satellite problem’) ‘too-big-to-fail- problem’: Predicted masses of Milky Way satellites are signiﬁcantly higher than observationally inferred values.

Solution: The problem appears connected to the cusp-core problems. So solutions range from baryonic physics, to modifying dark matter or gravity.

Problem is highly relevant in current research:

Last week:

23 The Small-Scales Crisis in Cold Dark Matter Cosmologies

Summary (Incl. Lecture 11.) When comparing the structure of dark matter halos as obtained from cosmological simulations to structure inferred from gas & stellar kinematics, some problems surfaced:

Problem 2 (absorbed `missing satellite problem’) ‘too-big-to-fail- problem’: Predicted masses of Milky Way satellites are signiﬁcantly higher than observationally inferred values.

24 The Small-Scales Crisis in Cold Dark Matter Cosmologies

Summary (Incl. Lecture 11.) When comparing the structure of dark matter halos as obtained from cosmological simulations to structure inferred from gas & stellar kinematics, some problems surfaced:

Problem 2 (absorbed `missing satellite problem’) ‘too-big-to-fail- problem’: Predicted masses of Milky Way satellites are signiﬁcantly higher than observationally inferred values.

Solution: The problem appears connected to the cusp-core problems. So solutions range from baryonic physics, to modifying dark matter or gravity.

Problem is highly relevant in current research:

Last week:

25 The Small-Scales Crisis in Cold Dark Matter Cosmologies Finding ultrafaint dwarfs is hard.

26 The Nature of the Dark Matter

What is the dark matter, and why is it `cold’?

Cosmic microwave background and observed large-scale structure in the Universe (i.e. clustering of galaxies) provide constraints on content of Universe:

Ordinary matter (baryons, leptons, photons) make up ~ 4% of Universal energy density. `Dark energy’ accounts for ~73%. Dark matter accounts for the remaining ~23%.

`Just as the chocolate frosting glues the sprinkles together on the cupcake, dark matter binds baryons together to form galaxies, galaxy groups, and galaxy clusters.’ A. Peter, 2013, arXiv:1201.3942

27 The Nature of the Dark Matter

What is the dark matter, and why is it `cold’?

Dark matter is not: • baryonic: evidence from cosmic microwave background, large scale structure, and also from Big-Bang Nucleosynthesis (maybe more on this in later lecture)

• composed of `light’, mX < keV, particles. These particles would be `relativistic’ when T of the Universe was ~ 1 keV. This would suppress growth of structure on `small’ scales at levels that are at odds with Lyman alpha forest (next lecture) constraints.

This is illustrated on the next slide.

28 Constraints on (Warm) Dark Matter

Observational constraints mass power spectrum

`primordial’ P(k)

`Meszaros’ suppression

rH matter-radiation equality T~ eV

`rH‘ relativisitic WDM m ~ keV

Lyman alpha forest indicates that mDM > keV 29 Constraints on (Warm) Dark Matter

Viel et al. 2013

`Transmission’ Power Spectrum

Lyman alpha forest measures velocity off-sets. k is therefore shown in units of 1/velocity instead of 1/radius 30 Constraints on other Properties of Dark Matter

Constraints on electro-magnetic charge. If dark matter particles had a small electric or magnetic dipole moment, it would couple to the baryon-photon ﬂuid prior to recombination, and would alter small-scale ﬂuctuations in Cosmic-Microwave Background (see Sigurdson et al. 2004)

Constraints on self-interaction. `Self-interaction’ refers to interactions among (different species of) dark matter particles, mediated by e.g. `dark gauge bosons’. Could alter predicted structure of dark matter halos.

This can be thought of as to what extent dark matter is truly collision-less. Cluster lensing & X-ray data on cluster puts a limit on the collision cross-section of

(for get some feeling for these numbers, proton r ~ 1e-13 cm, m=1.6e-24 g).

31 Constraints on other Properties of Dark Matter

Recent example of self-interacting dark matter as a solution to the `cusp-core’ problem (with velocity dependent collision cross-section).

32 Constraints on other Properties of Dark Matter Density proﬁles in cosmological simulations that have self-interacting Dark Matter (SIDM).

Slope of density proﬁle ﬂattens from Cusp to Core.

Vogelsberger et al. 2012

Example of self-interacting dark matter as a solution to the `cusp-core’ problem (with velocity dependent collision cross-section).33 Constraints on other Properties of Dark Matter Density proﬁles in cosmological simulations that have self-interacting Dark Matter (SIDM).

Vogelsberger et al. 2012

SIDM reduces tension between kinematics in observed and simulated satellites. 34 Summary Empirical Constraints Dark Matter

Mass of dark matter particle > keV from Lyman alpha forest observations. Dark matter is at least colder than warm.

Collisionless nature of dark matter particle constrained by cluster lensing + X-ray data. Cross-section for `hard-sphere’ elastic scattering though recently some models of self-interacting DM have been put forward that bypass cluster constraints while addressing core-cusp + too big to fail problems in dwarf galaxies

Cosmic-Microwave Background limits the charge of the dark matter particle (see Sigurdson et al. 2004)

35 Some Dark Matter Candidates I: WIMPs

WIMP: Weakly Interacting Massive Particles. Popular because:

`Electro-weak’ energy scale at ~200 GeV, above which weak and electromagnetic interaction merges into the `electroweak’ interaction. It is thought that new particles* should exist around this mass-scale.

This new particle annihilates into quarks + antiquarks in the early Universe, until density and temperature drops sufﬁciently that annihilation becomes increasingly rare. The comoving number density nX `freezes’ out.

The`predicted’ mass density in this relic density of particles - for the standard assumptions for the mass and annihilation coupling strength - comes out at

The fact that particle physics considerations alone, can give the correct order of magnitude for WIMP mass density is referred to as WIMP Miracle.

* what these particles are depends on the new physics that is introduced at the electroweak scale. Examples of WIMPS are supersymmetric neutralino, Kaluza-Klein photon,... 36 Some Dark Matter Candidates II: Other New Particles Other candidates include: • Axions: hypothetical particle introduced to resolve the strong CP problem in QCD.

• Gravitinos: supersymmetric partner of graviton. Not as popular as WIMPs because hard to detect & tuning required to get matter density correct.

• Sterile Neutrinos: neutrinos that do not act electroweakly. Introduced to generate mass for `active’ neutrinos, explain neutrino experiment anomalies,...

• Hidden sector dark-matter: dark sector may be as rich as ordinary standard model, but not `communicate’ much at all. These sectors are referred to as `hidden’ sectors, which may contain `dark photons’.

Caution: I know little about this. There are many reviews on dark matter candidates out there (often with the obscure title `Dark Matter’). I followed Peter’s review that has many references in there.

37 Dark Matter Searches.

Searches for dark matter can be done in

• Colliders: given that dark matter is neutral and weakly interacting, they behave like giant neutrinos in colliders. Missing energy* in collisions may hint at existence of e.g. WIMPs. So far, no evidence for physics beyond standard model.

Moreover, even if hints for a WIMP are found, it is unclear whether it would be stable over cosmological times (let go longer than a ns).

* Missing energy makes it harder to detect lower mass dark matter particles, as it requires increasingly precisely accounting forHARD all energy. 38 Dark Matter Searches.

Searches for dark matter can be done via

• Direct detection: looking for the collision of a WIMP with an atomic nucleus in the LAB. Experiments include DAMA/LIBRA, CRESST, CoGeNT, XENON100, CDMS-II, COUPP.

39 Intermezzo: Nuclear Recoil

WIMP-Nucleus Interaction: WIMPs have ﬁnite cross-section for interacting with standard model particles. Momentum conservation during scattering of WIMPs by atomic nuclei gives rise to `recoil’ of nucleus with E ~few to tens of keV.

Nuclear recoil can be manifest through scintillation, collisional ionization

Why e.g. Xenon?: 1. transparent to own scintillation ﬂux (no subsequent absorption). 2. liquid xenon is so dense, neutrons cannot enter target (important, as neutron induced recoils indistinguishable from those by WIMPs).

40 Intermezzo: Nuclear Recoil

Nuclear recoil can be manifest through scintillation, collisional ionization

41 Intermezzo: Nuclear Recoil

Nuclear recoil can be manifest through scintillation, collisional ionization

42 Dark Matter Searches.

Searches for dark matter can be done via

• Direct detection: looking for the collision of a WIMP with an atomic nucleus in the LAB. Experiments include DAMA/LIBRA, CRESST, CoGeNT, XENON100, CDMS-II, COUPP.

A claimed detection by the DAMA Experiment. DAMA looked for annular modulations in their nuclear recoils. As the earth moves around the sun at ~30 km/s, and the sun moves through the dark matter halo at ~ 220 km/s, we expect the dark matter ﬂux to undergo an annual modulation.

43 Dark Matter Searches.

Searches for dark matter can be done via

• Direct detection: looking for the collision of a WIMP with an atomic nucleus in the LAB. Experiments include DAMA/LIBRA, CRESST, CoGeNT, XENON100, CDMS-II, COUPP.

44 Dark Matter Searches.

Searches for dark matter can be done via

• Direct detection: looking for the collision of a WIMP with an atomic nucleus in the LAB. Experiments include DAMA/LIBRA, CRESST, CoGeNT, XENON100, CDMS-II, COUPP.

Detection corresponds to WIMP Mass of ~ 10 GeV, and sigma ~ 1e-40 cm^2. Recall: proton r ~ 1e-13 cm.

45 Dark Matter Searches.

Searches for dark matter can be done via

• Direct detection: looking for the collision of a WIMP with an atomic nucleus in the LAB. Experiments include DAMA/LIBRA, CRESST, CoGeNT, XENON100, CDMS-II, COUPP.

Summary of constraints on WIMPS by different experiments.

46 Dark Matter Searches.

Searches for dark matter can be done via

• Direct detection: looking for the collision of a WIMP with an atomic nucleus in the LAB. Experiments include DAMA/LIBRA, CRESST, CoGeNT, XENON100, CDMS-II, COUPP.

Claimed detection by the DAMA Experiment inconsistent with upper limits by other experiments!

47 Dark Matter Searches.

Searches for dark matter can be done via

• Looking for Dark Matter Annhilation: WIMP annihilation in dark matter dense objects, since annihilation rate increase as (density)2. Good places include:

★ galaxy clusters Bonus: `WIMP miracle’ provides us with ★ milky way dwarf galaxies annihilation rate, no free parameters! ★ milky way halo ★ center of sun

• WIMPs annihilate into variety of standard model particles incl. neutrinos, and gamma-ray photons. WIMP searches have been performed with

★ gamma-ray telescopes such as Fermi & H.E.S.S. ★ neutrino telescopes (sun)

48 Dark Matter Searches.

Searches for dark matter can be done via

• Looking for Dark Matter Annhilation: H.E.S.S result from this week!

No detection..... 49 Dark Matter Detections?

Recent (Feb 2014) X-ray observations of nearby galaxy clusters.

Unidentiﬁed line at E=3.5 keV, associated with decaying dark matter? If so, we should see it in the Milky Way (?) Which we do not. Very hot topic. Ask Signe!

50 Dark Matter Detections?

Fermi Gamma Ray observations of the full sky.

A diffuse gamma-ray glow - centered on Milky Way center - has been observed. This so-called `Fermi-haze’ has been speculated to be a dark matter signal. 51 `Macro Dark Matter’ (journal club, yesterday)

Massive `non-particle’ dark matter particles (see arxiv:1410.2236).

Motivation: reaction rate between baryons and dark matter particles is ~

Moreover, }

Dark matter is `dark’ because it barely interacts with ordinary matter, i.e. GammaXb is low

Traditionally, a low is associated with a low

Alternatively, a low is associated with a high MX

High can be macroscopically high 1e-12-1e34 gr! `Macro dark matter’

52 `Macro Dark Matter’ (journal club, yesterday)

Weird non-particle dark matter particles (see arxiv:1410.2236):

• nuclearites • strangelets • strange baryon Q-balls • baryonic colour superconductors • compact composite objects `Macro dark matter’ (Macros) • compact ultradense objects • primordial black holes • cheese • ...

Pretty amazing world of possibilities.

53 `Macro Dark Matter’ Current observational constraints on macro-dark matter: white regions mark allowed parameter space.

54 `Macro Dark Matter’ Current observational constraints on macro-dark matter: white regions mark allowed parameter space.

Macros with mass of chocolate bar up to Earth mass allowed.

55 Summary Knowledge on Dark Matter

Mass of dark matter particle > keV from Lyman alpha forest observations. Dark matter is at least colder than warm.

Theoretically, the leading popular candidate is the WIMP. Universal mass density in WIMPs ~ dark matter density (`WIMP’ miracle).

Observational constraints on WIMPs, are improving.

However, stage is wide-open. Many other candidates including axions, hidden sector dark matter, macro dark matter,...

56 Outlook Next lecture: turn to the `bright’ side of extragalactic astrophysics.

Focus on Lyman alpha forest: • provide constraints on mass power spectrum on smallest scales (and hence dark matter properties) • provides insights into distribution & properties of gas in range of densities from linear regime to highly overdense gas in outskirts of galaxies.