Phys/Astro 689: Lecture 8Angular Momentum & the Cusp/Core Problem

Summary to DateWe first learned how to construct the Power Spectrum with CDM+baryons.

Found CDM agrees with the observed Power Spectrum on large scales; now trying to test small scales

Must follow galaxies in the non-linear regime to test small scales. Have learned about the tools to do this: DM-only and DM+baryonic simulations

Angular Momentum in Halos

Linear Tidal Torque Theory: matter acquires angular momentum due to forces from external matter (up to large distances).

Linear Tidal Torque Theory, originally worked out in White (1984).

Angular Momentum in Halos

Principal axes of tidal and inertia tensors not generally aligned for non-spherical volume, so net angular momentum results.

J acquisition stops at turn-around.

Angular Momentum in Halos

In TTT, so J ~ t

ZAVALA ET AL. (2008)

Angular Momentum in HalosIn TTT, DM and gas should initially have same J distribution

van

den

Bosc

h et

al. (

2002

)

The Problem

The angular momentum profile in real galaxies does not match predictions

van den Bosch et al. (2001)

The Problem The angular momentum profile in real galaxies does not match predictions (even when uncertainties are considered)

e.g., stellar M/L ratios, asymmetric drift

van den Bosch et al. (2001)

Does reionization help?

No

van den Bosch et al. (2003)

Big bulges are rare, and sometimes there is no bulge

Introduce bulgeless disk galaxies

A large bulge

A “bulgeless” disk

Big bulges are rare, and sometimes there is no bulge

DUTTON (2009)

Sersic profiles describe galaxy light distribution

Big bulges are rare, and sometimes there is no bulge

The first bulgeless disk galaxy simulation

Jonsson  (2006),  Jonsson  et  al.  (2010)

The Importance of Driving Outflows

Mvir ~ 1010 Msun “dwarf galaxy”

Edge on disk orientation

(arrows are velocity vectors)

Brook et al., (2011)

Outflows Remove Low Angular Momentum Gas

Add P(j) slide

HI +

All baryons ever in the galaxy

j/jtotvan den Bosch et al. (2001)Brook  et  al.  (2011)

Outflows Remove Low Angular Momentum Gas

Brook  et  al.  (2011)

Outflows Reduce the Inner Rotation Curve

see also: Governato et al., 2010, Nature, 463,  203, arXiv:0911.2237

No Feedback Feedback + Delayed Cooling

Teyssier et al. (2012)

High  threshold                                                            Low  threshold

See also: Saitoh et al. (2008), Ceverino & Klypin (2009)Robertson & Kravtsov (2008), Tasker & Bryan (2008)

This requires high resolution!

Outflows Reduce the Inner Rotation Curve

The effect of altering the SF density threshold

The effect of altering resolution

V circ%%=%

Governato et al., 2009, Nature, 463,  203, arXiv:0911.2237

!

Diffuse Star Formation

“Resolved” Star Formation

Radius (kpc) 0 1 2 3 4 5 6 7

18

Mag

/ars

ec2

Radius (kpc)

22

28 Mag

/ars

ec2

0 1 2 3 4

24!

26!

20!

22!

24!

“Observed” Surface Brightness Profile

The current state

Simulators can now make bulgeless disks

Realistic bulges up to a few 1011 in halo mass (Christensen et al. 2013)

Going to higher mass galaxies requires higher resolution. Realistic bulges in MW mass galaxies are yet to be achieved (but very close).

The Cusp/Core problem

All that low angular momentum material at the center of DM halos also leads to higher central densities than observed

It’s not just the normalization of the density, it’s also the distribution (slope of the density profile)

Best Test: Low Surface Brightness Galaxies

tend to be bulgless

have central surface brightnesses fainter than 23 mag/arcsec2

lie low on the mass-metallicity relation

dark matter dominated!

LSBs favor a constant density core

MOO

RE 19

94, N

ATUR

E

The Cusp/Core Problem

Parameterize density profile as !(r) ∝ r -"Simulations predict " ~ 1 (central cusp)

Observations show " ~ 0 (constant-density core)

But... your data sucks

Flores & Primack (1994)data: Carnignan & Freeman (1988),

Carnignan & Beulieu (1989)

van den Bosch et al. (2000)

Example degeneracyα  =  1.30 α  =  0.26 α  =  0.80

SOLID: BEST FIT MODEL, INCLUDING RESOLUTION EFFECTSGREEN: STARS

BLUE: DM HALODOTTED: HI

THIN RED: TOTAL

Simon et al. (2005)

Enter  the  Era  of  Better  Data

THINGS: The HI Nearby Galaxies Survey

resolution: 7”, 5km/s

Enter  the  Era  of  Better  Data

OH ET AL. (2011)

Theorists  counter  with  Non-­‐Circular  Motions

cold gas in a simulated dwarf galaxy

Valenzuela et al. (2006):

“true” rotation curve

“observed” HI rotation curve

Potential Core Creation Mechanisms: Dynamical Friction

(1) The effect of gravity causes light bodies in the parent halo to accelerate and gain momentum and kinetic energy. By conservation of energy and momentum, we may conclude that the heavier body will be slowed by an amount to compensate.

(2) Equivalently, the light bodies are attracted by gravity toward the larger body moving through the cloud, and therefore the density at that location increases (a gravitational wake). In the meantime, the object under consideration has moved forward. Therefore, the gravitational attraction of the wake pulls it backward and slows it down.

See also El-Zant (2001,2004), Tonini et al. (2006), Jardel & Sellwood (2009)

Potential Core Creation Mechanisms: Dynamical Friction

Perhaps the gas clumps are accelerated at the center of the galaxy rather than accreted (e.g., Mashchenko et al. 2006, 2008).

Potential Core Creation Mechanisms: Angular Momentum Arguments

DEL POPOLO (2009)

109 1014

J is transferred by resonance in bar pattern speed and orbits of DM in inner halo (see Weinberg & Katz, 2002)

But see Sellwood (2008)

HOLLEY-BOCKELMANN ET AL. (2005)

Potential Core Creation Mechanisms: Bars

What about outflows?

Galactic winds appear to be required to match the observed angular momentum distribution in galaxies.

Can they simultaneously solve the cusp/core problem?

Theorists accidentally made a DM core

•Bulgeless!•Exponential  stellar  disk,  Rd  ~  1  kpc•Gas  rich•Vc  <  60  km/sec•bursty  SFH•SFR  ~  0.01  Msun/yr

“typical”  Qield  dwarf

GOVERNATO ET AL. (2010)

Core creation due to rapid potential well fluctuations

How Are Cores Created?Bursty SF!

PONTZEN & GOVERNATO 2012

Outflows Flatten the DM Density Profile

Core Creation!

ρ ~ r-α

Galaxies in the THINGS survey have

average α~-0.3

Cores found by manyTeyssier et al. (2013), RAMSES (AMR) code

•  Navarro  et  al.,1996,  MNRAS,  283,  L73•  Read  &  Gilmore  2005,  MNRAS,  356,  107•  Mashchenko  et  al.,2006,  Nature,  442,  539•  Mashchenko  et  al.,  2008,  Science,  319,  174•  PaseLo  et  al.,  2010,  A&A,  514,  A47

•  Ogiya  &  Mori  2012,  arXiv:1206.5412•  de  Souza  et  al.,  2011,  MNRAS,  415,  2969•  Cloet-­‐Osselaer  et  al.,  2012,  MNRAS,  423,  735•  Maccio  et  al.,  2012,  ApJ,  744,  L9•  Teyssier  et  al.,  2013,  MNRAS,  429,  3068

Galaxies in the THINGS survey

have average α~-0.3

Lower  mass  galaxies  do  not  undergo  repeated  bursts  of  SF;  retain  

cusps

Core  Creation  varies  with  Mass!  because  SF  varies  with  mass

Governato et al., 2012, MNRAS, 422, 1231

Core creation requires enough E in stellar feedback (young stars, SNe) to unbind the cuspy DM

Penarrubia et al. (2012)

Core  Creation  varies  with  Mass!  because  there’s  not  enough  energy  at  low  masses

But Do Cores Exist? Stellar vs Gas Kinematics

Adams et al. (2011)VIRUS-P

NGC 2976

poster child for a core using gas,

stars are consistent with a cusp

And what happens at higher masses?

NEWMAN ET AL. (2013)

The current stateGeneral question about whether stars have enough energy to create cores

Do cores exist at high masses?

Ongoing observational tests; LSB dwarfs seem to have cores, higher masses are under debate

DI CINTIO ET AL. (2013)