1. Dark Matter in Dwarf Galaxies
High-Resolution Measurements of the Density Profiles of Dwarf Galaxies
Josh Simon
UC Berkeley
Collaborators:
Leo Blitz
Alberto Bolatto
Adam Leroy

2. The Central Density Problem
cusp
core
Parameterize density profile as r(r) µ r -a
Observations show a ~ 0 (constant-density core)
Simulations predict 1  a  1.5 (central cusp)

3. Improvements Over Previous Work
2-D velocity fields
observations in Ha, 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!

4. Targets
NGC 2976
NGC 4605
NGC 5963
NGC 5949
NGC 6689
NGC 4625

5. Targets
NGC 2976
NGC 4605
NGC 5963
NGC 5949
NGC 6689
NGC 4625

6. See Simon et al. (2003) for more details
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 Ha and CO
BVRIJHK photometry to better model stellar disk
See Simon et al. (2003) for more details

7. NGC 2976 Velocity Field vobs = vsys + vrot cos q + vrad sin q
Fit a tilted ring model:
vobs = vsys + vrot cos q + vrad sin q Ha CO

8. NGC 2976 Rotation Curve Rotation velocity
Derived from combined CO and Ha velocity field

9. NGC 2976 Rotation Curve Significant radial motions in inner 30” (blue)
Rotation velocity
Radial velocity
Systemic velocity

10. NGC 2976 Rotation Curve
Power law provides a good fit to rotation curve out to 100” (1.7 kpc) (red)
Power law fit

11. Maximum Disk Fit Even with no disk, dark halo density profile is
r(r) = 1.2 r ± 0.09 M/pc3

12. Maximum Disk Fit Even with no disk, dark halo density profile is
r(r) = 1.2 r ± 0.09 M/pc3 HI H2

13. Maximum Disk Fit stars Even with no disk, dark halo density profile is
r(r) = 1.2 r ± 0.09 M/pc3
Maximal disk M*/LK =
0.19 M/L,K
stars

14. Maximum Disk Fit dark halo Even with no disk, dark
halo density profile is
r(r) = 1.2 r ± 0.09 M/pc3
Maximal disk M*/LK =
0.19 M/L,K
dark halo

15. Maximum Disk Fit No cusp! Even with no disk, dark
halo density profile is
r(r) = 1.2 r ± 0.09 M/pc3
Maximal disk M*/LK =
0.19 M/L,K
After subtracting stellar
disk, dark halo structure is
r(r) = 0.1 r ± 0.12M/pc3
No cusp!

16. What About the Systematics?
Beam-smearing
beam < 100 pc; > 1100 independent data points
Errors in geometric parameters
center position, PA, inclination, systemic velocity
Extinction
vHa = vCO
Asymmetric drift
After accounting for systematics, total uncertainty on density profile slope is ~ 0.1

17. Targets
NGC 5963
NGC 2976
NGC 4605
NGC 5949
NGC 6689
NGC 4625

18. NGC 5963: The NFW Galaxy Larger and more distant galaxy (D = 13 Mpc)
Compact inner spiral surrounded by very LSB disk

19. NFW profile also a good fit!
NGC 5963 Rotation Curve
Best fit: a = 1.28 power law
NFW profile also a good fit!
V200 ~ 90 km s-1, R200 ~ 130 kpc, rs = 7 kpc

20. Simon et al. (2004) for more details
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

21. See Simon et al. (2004) for more details
Galaxy #4: NGC 5949
More distant (14 Mpc), otherwise looks just like NGC 2976
NGC 5949
NGC 2976
See Simon et al. (2004) for more details

22. 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

23. Is There a Universal Density Profile?
NGC 2976
NGC 6689
NGC 5949
NGC 4605
NGC 5963
Five galaxies: a
0.01
0.80
0.88
1.28
No evidence for a universal density profile
large scatter compared to simulations
mean slope shallower than simulations
Also different from previous observations, though
e.g., a = 0.2 ± 0.2 (de Blok, Bosma, & McGaugh 2003)

24. 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 . . .

25. 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%

26. 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?

27. 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 . . .

28. Conclusions
1) Galaxy-to-galaxy scatter in density profile slope (Da = 0.46) is much larger than in simulations
2) Mean slope (a = 0.77) is shallower than predicted
3) Disagreement between observations and simulations is real, and systematics are only partially responsible