dark matter in dwarf galaxies josh simon uc berkeley collaborators: leo blitz alberto bolatto adam...
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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!
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 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
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 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
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