Dark Matter in Dwarf Galaxies

Josh SimonUC Berkeley

Collaborators:

Leo BlitzAlberto BolattoAdam Leroy

High-Resolution Measurements of the Density Profiles of Dwarf Galaxies

The Central Density Problem

• Parameterize density profile as (r) r -

• Observations show ~ 0 (constant-density core)• Simulations predict 1 1.5 (central cusp)

cusp core

Improvements Over Previous Work

• 2-D velocity fields• observations in H, CO, and HI • can detect noncircular motions

• Nearby targets = high spatial resolution (~100 pc) • Multicolor optical/near-IR imaging

• better stellar disk model

• Concentrate on the simplest galaxies• low mass, no bulges, no bars

• Test for systematics!

TargetsNGC 4605 NGC 5963

NGC 5949 NGC 6689 NGC 4625

NGC 2976

NGC 6689NGC 5949

TargetsNGC 4605 NGC 5963

NGC 4625

NGC 2976

NGC 2976

• Sc dwarf galaxy in the M 81 group (D = 3.5 Mpc)

• Gas-rich, no bulge, no bar, no spiral arms• High-quality data:

• 2-D velocity fields in H and CO

• BVRIJHK photometry to better model stellar disk

See Simon et al. (2003) for more details

NGC 2976 Velocity Field

H CO

• Fit a tilted ring model:vobs = vsys + vrot cos + vrad sin

NGC 2976 Rotation Curve

• Rotation velocity

• Derived from combined CO and H velocity field

NGC 2976 Rotation Curve

• Rotation velocity

• Radial velocity

• Systemic velocity

• Significant radial motions in inner 30” (blue)

NGC 2976 Rotation Curve

• Power law provides a good fit to rotation curve out to 100” (1.7 kpc) (red)

• Power law fit

Maximum Disk Fit

• Even with no disk, dark halo density profile is(r) = 1.2 r -0.27 ± 0.09 M/pc3

Maximum Disk Fit

HI

H2

• Even with no disk, dark halo density profile is(r) = 1.2 r -0.27 ± 0.09 M/pc3

Maximum Disk Fit

stars

• Even with no disk, dark halo density profile is(r) = 1.2 r -0.27 ± 0.09 M/pc3

• Maximal disk M*/LK = 0.19 M/L,K

Maximum Disk Fit

dark halo

• Even with no disk, dark halo density profile is(r) = 1.2 r -0.27 ± 0.09 M/pc3

• Maximal disk M*/LK = 0.19 M/L,K

Maximum Disk Fit

• Even with no disk, dark halo density profile is(r) = 1.2 r -0.27 ± 0.09 M/pc3

• Maximal disk M*/LK = 0.19 M/L,K

• After subtracting stellar disk, dark halo structure is (r) = 0.1 r -0.01 ± 0.12M/pc3

• No cusp!

• Beam-smearing• beam < 100 pc; > 1100 independent data points

• Errors in geometric parameters• center position, PA, inclination, systemic velocity

• Extinction• vH = vCO

• Asymmetric drift

• After accounting for systematics, total uncertainty on density profile slope is ~ 0.1

What About the Systematics?

NGC 6689 NGC 4625

NGC 2976

TargetsNGC 4605

NGC 5963

NGC 5949

NGC 5963: The NFW Galaxy

• Larger and more distant galaxy (D = 13 Mpc)

• Compact inner spiral surrounded by very LSB disk

NGC 5963 Rotation Curve

NFW profile also a good fit!

V200 ~ 90 km s-1, R200 ~ 130 kpc, rs = 7 kpc

Best fit: = 1.28 power law

Galaxy #3: NGC 4605

• Nearby (4.3 Mpc), LMC-mass, CO-rich pure disk galaxy

See Bolatto et al. (2002) and Simon et al. (2004) for more details

Galaxy #4: NGC 5949

• More distant (14 Mpc), otherwise looks just like NGC 2976

NGC 2976

NGC 5949

See Simon et al. (2004) for more details

Galaxy #5: NGC 6689

• ~11 Mpc away, slightly more highly inclined and more massive

See Simon et al. (2004) for more details

• No evidence for a universal density profile• large scatter compared to

simulations• mean slope shallower

than simulations

Is There a Universal Density Profile?

NGC 2976

NGC 6689

NGC 5949

NGC 4605

NGC 5963

Five galaxies:

• Also different from previous observations, though e.g., = 0.2 ± 0.2 (de Blok, Bosma, & McGaugh 2003)

Puzzles

1) Radial motions - what’s causing them?• Bar, triaxial dark matter halo, intrinsically elliptical disk

• Not only present in our sample - most 2D velocity fields show evidence for them

• Could have been missed in other galaxies due to long-slit observations . . .

Are Galaxy Halos Triaxial?

• Triaxial DM halos cause noncircular motions in disks

• 4/5 galaxies show measurable orbital ellipticity

• Lower limits on the potential ellipticity range from 0.5% to 3%

Puzzles

1) Radial motions - what’s causing them?• Bar, triaxial dark matter halo, intrinsically elliptical

disk

• Not only present in our sample - most 2D velocity fields show evidence for them

• Could have been missed in other galaxies due to longslit observations . . .

2) How can a rotation curve be fit by both a pseudo-isothermal profile and a cuspy power law?

Distinguishing Cores From Cusps

• Only exquisite data can distinguish cores from cusps in these galaxies• Even then, the galaxies have to be very well behaved• If you look for cores, you will find them. Same for cusps. Phrasing the

debate as cores vs. cusps may not be the most useful approach . . .

NGC 6689NGC 5949

1) Galaxy-to-galaxy scatter in density profile slope ( = 0.46) is much larger than in simulations

Conclusions

2) Mean slope ( = 0.77) is shallower than predicted

3) Disagreement between observations and simulations is real, and systematics are only partially responsible