jsimon santacruz

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Science-Technology

Published on August 29, 2007

Author: Mahugani

Source: authorstream.com

Dark Matter in Dwarf Galaxies:  Dark Matter in Dwarf Galaxies Josh Simon UC Berkeley Collaborators: Leo Blitz Alberto Bolatto Adam Leroy High-Resolution Measurements of the Density Profiles of Dwarf Galaxies The Central Density Problem:  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 Improvements Over Previous Work:  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! Targets:  Targets Targets:  Targets 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 Ha and CO BVRIJHK photometry to better model stellar disk See Simon et al. (2003) for more details NGC 2976 Velocity Field:  NGC 2976 Velocity Field Ha CO Fit a tilted ring model: vobs = vsys + vrot cos q + vrad sin q NGC 2976 Rotation Curve:  NGC 2976 Rotation Curve Rotation velocity Derived from combined CO and Ha velocity field NGC 2976 Rotation Curve:  NGC 2976 Rotation Curve Rotation velocity Radial velocity Systemic velocity Significant radial motions in inner 30' (blue) NGC 2976 Rotation Curve:  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:  Maximum Disk Fit Even with no disk, dark halo density profile is r(r) = 1.2 r -0.27 ± 0.09 M/pc3 Maximum Disk Fit:  Maximum Disk Fit HI H2 Even with no disk, dark halo density profile is r(r) = 1.2 r -0.27 ± 0.09 M/pc3 Maximum Disk Fit:  Maximum Disk Fit stars Maximum Disk Fit:  Maximum Disk Fit dark halo Maximum Disk Fit:  Maximum Disk Fit What About the Systematics?:  Beam-smearing beam andlt; 100 pc; andgt; 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 What About the Systematics? Targets:  Targets NGC 5963: The NFW Galaxy:  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:  NGC 5963 Rotation Curve NFW profile also a good fit! V200 ~ 90 km s-1, R200 ~ 130 kpc, rs = 7 kpc Best fit: a = 1.28 power law Galaxy #3: NGC 4605:  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:  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:  Galaxy #5: NGC 6689 ~11 Mpc away, slightly more highly inclined and more massive See Simon et al. (2004) for more details Is There a Universal Density Profile?:  No evidence for a universal density profile large scatter compared to simulations mean slope shallower than simulations Is There a Universal Density Profile? Also different from previous observations, though e.g., a = 0.2 ± 0.2 (de Blok, Bosma, andamp; McGaugh 2003) Puzzles:  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?:  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:  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:  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 6689 NGC 5949 Conclusions:  1) Galaxy-to-galaxy scatter in density profile slope (Da = 0.46) is much larger than in simulations Conclusions 2) Mean slope (a = 0.77) is shallower than predicted 3) Disagreement between observations and simulations is real, and systematics are only partially responsible

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