Ultra light axion dark matterandsmall scale problems
Kohei Hayashi (JSPS fellow)
ICRR, The University of Tokyo

1

Contents
1. Introduction
1.1 Cold dark matter theory and dwarf spheroidal galaxies
1.2 Small scale problems in ΛCDM models
2. Ultralight axion dark matter as a solution to small scaleproblems
3. Non-sphericity of ultralight axion dark matter halos in
the Galactic dwarf satellites
4. Revisit core-cusp problem
5. Summary!2

Introduction!3

Cold dark matter theory
anddwarf spheroidal galaxies!4

Dark Universe!5Λ Cold Dark Matter model
ESA and the Planck Collaboration

Large scale (> 1Mpc)
Remarkable success!
Small scale (<1 Mpc)
What’s going on?

!6?

The structures on small scales (<1Mpc):
Milky Way and its neighbors!7

The structures on small scales (<1Mpc):
Milky Way and its neighbors

Galactic Archaeology / Near Field Cosmology:
Unveiling the nature of dark matter and its role in galaxy formation!8

Dwarf Spheroidal Galaxy (dSph): basic properties~50 dwarf satellites
are discovered around MW!9

Dwarf Spheroidal Galaxy (dSph): basic properties
size: ~0.3kpc
Mstar: 106Msun

size: ~0.25kpc
Mstar: 104MsundSph

Bootes I©NAOJ

SculptordSph©ESO~50 dwarf satellites
are discovered around MW✤ no gas, no current SF✤ smallest and oldest galaxy

Fossil records of galaxy formation!10

dSphs: dark-matter dominated system
Dynamical mass-to-light ratioM(DM+star) /Mstar

McConnachie (2012)

M(DM+star)
Mstar= 101 ∼ 103

視線速度分布DSphs are ideal sites forstudying the nature of DM!
!11V-band absolute magnitude [mag]Small scale problemsin ΛCDM models!12Small-scale challenges to ΛCDM paradigm
Definition of “small scales”

Mvirial < 1011M⊙, k > 3Mpc−1, r < 1Mpc

⟹ rvir < 150kpc, Vvirial < 50km/s

i.e., galaxy and dwarf-galaxy scales

Bullock &Boylan-Kolchin (2017)✦ Missing satellite problem (Moore+99, Klypin+99)- Overabundance of dark subhalos✦ Core-cusp problem (de Blok 2002, Gilmore+ 07)- Cuspy central density in CDM halos vs. cores in observed galaxies✦ Too-big-to-fail problem (Boylan-Kolchin+ 11)- Most massive subhalos are more concentrated than observed luminous satellites+ the other problems (Pawlowski+ 12, KH & Chiba 12)(satellite planes, shapes of dark halo, etc…)!13

Small-scale challenges to ΛCDM paradigm
Definition of “small scales”

Mvirial < 1011M⊙, k > 3Mpc−1, r < 1Mpc

⟹ rvir < 150kpc, Vvirial < 50km/s

i.e., galaxy and dwarf-galaxy scales

Bullock &Boylan-Kolchin (2017)✦ Missing satellite problem (Moore+99, Klypin+99)- Overabundance of dark subhalos✦ Core-cusp problem (de Blok 2002, Gilmore+ 07)- Cuspy central density in CDM halos vs. cores in observed galaxies✦ Too-big-to-fail problem (Boylan-Kolchin+ 11)- Most massive subhalos are more concentrated than observed luminous satellites+ the other problems (Pawlowski+ 12, KH & Chiba 12)(satellite planes, shapes of dark halo, etc…)!14

Solution to core-cusp problem✦ Core-cusp problem- Cuspy central density in CDM halos vs. cores in observed galaxies

Dark matter density[M⊙ /pc3]

Gilmore+ (2007)NFW cusp

Possible solutions:- Baryonic feedback

Stellar feedbacks such as SNe cantransform central cusp into coreddark matter profiles.
- Alternative dark matter models
The other dark matter models
motivated by particle physics(SIDM, SIMP, Axion..) can create acored density profiles without
relying on any baryon effects.

Caveats:- Incomplete observed data
- Uncertainties on dynamicalRadius[kpc]analysis!15

Solution to core-cusp problem- Baryonic feedback
Governato+ 12
Silk & Mamon 12• Supernova feedback is dominatedin less-massive galaxies.
• Numerical simulations havepredicted that this feedback
process can transform central cusp
into cored dark matter profiles.
!16

Solution to core-cusp problem- Baryonic feedback
Recent high resolution simulations argue that the impact of baryonic feedback
is negligible on classical and ultra faint dwarf galaxy mass scales.

Bullock & Boylan-Kolchin (2017)!17

Solution to core-cusp problem- Alternative DM models (WDM, SIDM, SIMP, Ultra light axion,…)• Suppress the matter
power spectrum onsmall scales.
• Dark matter cannot beconcentrated on smaller
spatial scales.
• Create a cored dark
matter density profiles
without relying on any
baryon physics.

Bozek+ 15

Ultra light Axion Dark Matteras a solution to small scale problems!19

Ultralight axion dark matter (ULADM)• The lightest particle among
dark matter candidates(mψ~10-22 eV)• Create a core (~kpc) comesfrom quantum pressure
hrcore ∼ λdB ≡mψ v

Schive et al. (2014)

dSph

Sculptor• central soliton core + outer
NFW DM profile
Soliton-core dark matter density profileρcρsoliton(r) =[1 + 0.091(r/rc)2]8mψ

rc −4−3ρc = 1.9 × 10 ( −23[Mpc]⊙)()
10eVpc12−2!20

Ultralight axion dark matter (ULADM)- Current constraints on particle mass of ULADM
KH & Obata 19����� ��������(���� �� ��� ����)MW Substructures(Ursa Minor & Fornax)
MW Substructures(Sextans)�������������

Excluded byUV Luminosity function-2410������� �� �����������������������)�� ��(���������--2310

�����-α ������-2210!21-2110m [eV]

-20 ψ10

Constraining particle mass of ULADM
Chen et al. (2017)

Soliton-core dark matter density profileρcρsoliton(r) =[1 + 0.091(r/rc)2]8mψ

rc −4−3ρc = 1.9 × 10 ( −23[Mpc]⊙)()
10eVpc−2

l.o.s velocity dispersion [km/s]

12

Dynamical models for starsex) Spherical Jeans equation(Theory)
σl.o.sFIT(obsrved)
σl.o.s∞22
ν(r)v2Rr2
σl.o.s(R) =1−βdra 2)(I(R) ∫R
rr2 − R2• Current constraint from Spherical mass models−22mψ = 1.79+0.35
×10eV (2σ)
−0.33

!22

Radius [pc]

Major systematic uncertainty: Spherical Symmetry
1. Observed dSphs are
NOT spherical shapeq’=b/a=0.68

q’=0.44

Sculptor

Ursa Minor

2. DM models predict
NON-spherical DM halo

credit: Aquarius project

σl.o.s[km/s]

3. 1D spatial information

Radius [pc]!23Major systematic uncertainty: Spherical Symmetry
1. Observed dSphs are
NOT spherical shapeq’=b/a=0.68

q’=0.44

Sculptor

Ursa Minor

2. DM models predict
NON-spherical DM halo

credit: Aquarius project

σl.o.s[km/s]

3. 1D spatial information

Radius [pc]Non-spherical mass model!24

Non-sphericity ofultralight axion dark matter halos
in the Galactic dwarf satellites
Hayashi and Obata (2019), arXiv: 190203054!25

Non-spherical dynamical mass models
Non-spherical dark matter density profile

Non-spherical stellar profileρcρsoliton(r) =[1 + 0.091(r/rc)2]8mψ

r*2 −5/23L
ρ*(r*) =1 + 2][34πrprp2z
r*2 = R 2 + 2q

rc −4−3ρc = 1.9 × 10 ( −23[Mpc]⊙)()
10eVpc12−22z
r2 = R2 + 2Q

DM haloaxial ratio

Axisymmetric Jeans equations

2 2(vz , vϕ)(theory)
σl.o.svR2 is unknown parameter asβz = 1 − vz2 /vR2

FIT

Unobservable(obsrved)
σl.o.s!26

stellaraxial ratio

Constraints on ULADM via non-spherical analysis
Hayashi & Obata (2019), 1902.03054

����� ��������(���� �� ��� ����)MW Substructures(Ursa Minor & Fornax)
MW Substructures(Sextans)�������������

Excluded byUV Luminosity function-2410������� �� �����������������������)�� ��(���������--2310

�����-α ������-2210!27-2110m [eV]

-20 ψ

10

Constraints on ULADM via non-spherical analysis
Hayashi & Obata (2019), 1902.03054

����� ��������(���� �� ��� ����)��� ������MW Substructures(Ursa Minor & Fornax)

MW Substructures(Sextans)�������������

Excluded by
UV Luminosity function-2410������� �� �����������������������)�� ��(���������--2310

�����-α ������
-2210!28-2110m [eV]

-20 ψ

10

Constraints on ULADM via non-spherical analysis+0.07
Q = 0.21−0.04

Hayashi & Obata (2019), 1902.03054

Draco dSphlog(rc) = 2.89+0.07−0.07log(mψ10−23 eV) = 0.58+0.13−0.14−log(1 − βz) = − 0.18+0.09−0.07

inclination = 77.13+8.38−10.91!29

Constraints on ULADM via non-spherical analysis
Hayashi & Obata (2019), 1902.03054
Stellar & DM halo axial ratio of Draco• Draco has strongly elongatedq = (b/a)star = 0.69
Q = (b/a)DM = 0.21
STARdark halo, Q~0.2.

• Draco’s ULADM halo ismuch more flattened thanN-body predictions andstellar distributions.

DM• Further understanding ofbaryonic and DM physics
should be needed.

Schive et al. (2014)Q ∼ 1.0!30

Summary• ΛCDM theory faces the serious challenges on

q’=b/a=0.68

dwarf galaxy scales.

• Ultralight axion dark matter is one of the dark

matter candidates, because it can resolve small
scale problems.

Sculptor• The MW dSphs are ideal sites for studying the����� ��������(���� �� ��� ����)nature of dark matter because these are DMdominated systems.

• To obtain realistic limits on DM models, weconstruct new dynamical modeling with taking
into account non-sphericity.

• Our mass models place less stringentconstraints on ULADM mass but require
unphysically elongated ULADM halos.

• We revisit core-cusp problem and find that thediversity of inner slopes of DM profiles in theclassical dwarfs.
!31��� ������

MW Substructures(Ursa Minor & Fornax)

MW Substructures(Sextans)�������������

Excluded by
UV Luminosity function
10-24(���������-���������������� �� ��������������)

�� ��

10-23

10-22

�����-α ������

10-21mψ [eV]10-20