Dark Matter in Dwarf Galaxies

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

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

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

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

6. 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 • Fit a tilted ring model: • vobs = vsys + vrotcos q + vradsin 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 -0.27 ± 0.09 M/pc3

12. Maximum Disk Fit • Even with no disk, dark • halo density profile is • r(r) = 1.2 r -0.27 ± 0.09 M/pc3 HI H2

13. Even with no disk, dark • halo density profile is • r(r) = 1.2 r -0.27 ± 0.09 M/pc3 • Maximal disk M*/LK = • 0.19 M/L,K Maximum Disk Fit stars

14. Even with no disk, dark • halo density profile is • r(r) = 1.2 r -0.27 ± 0.09 M/pc3 • Maximal disk M*/LK = • 0.19 M/L,K Maximum Disk Fit dark halo

15. Even with no disk, dark • halo density profile is • r(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(r) = 0.1 r -0.01 ± 0.12M/pc3 • No cusp! Maximum Disk Fit

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

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

19. 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. 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. 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. Galaxy #5: NGC 6689 • ~11 Mpc away, slightly more highly inclined and more massive See Simon et al. (2004) for more details

23. Five galaxies: a NGC 2976 0.01 NGC 6689 0.80 NGC 5949 0.88 NGC 4605 0.88 NGC 5963 1.28 Is There a Universal Density Profile? • 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. 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 . . . Distinguishing Cores From Cusps NGC 5949 NGC 6689

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