Phys/Astro 689: Lecture 11 Tidal Debris
Goals (1) We ll explore whether we can trace the accretion events that should have formed the Milky Way. (2) We ll discuss the impact of tidal debris on direct detection experiments.
Ways to destroy a satellite Evaporation (internal evolution) Tidal Stripping Resonant Heating Disk Shocking
Evaporation What are the typical timescales for evaporation? Assume tail of Maxwellian velocity distribution is above vesc On board: find tevap > Hubble time.
Tidal Stripping As a satellite is accreted onto a parent galaxy, it feels the gravitational pull of the parent galaxy The force of gravity varies across the satellite (nearest to parent center feels stronger pull) DM or stars outside the tidal radius will be stripped
Tidal Stripping
Tidal Stripping TIDAL STRIPPING OF SAGITTARIUS (CREDIT: KATHRYN JOHNSTON)
Tidal Stripping The gravitational force holding stars to the satellite is determined by the satellite's mass. But stars are being stripped away as the satellite orbits, so its mass is dropping. Since its mass is dropping, its harder for the satellite to hold on to its stars. So its stars get stripped even faster. So its mass drops even faster. So it's even harder to hold on to its stars, so it loses them even more faster. Etc...
Resonant Heating If spin of the parent galaxy s disk is aligned with and of similar magnitude to the orbital angular speed of the satellite, get resonant heating For non-circular orbits of the satellite, response is temporary
Resonant Heating
Disk Shocking When a body passes through the disk, it feels a rapidly varying gravitational field mean density of a subhalo (M=10 7, r=1kpc) = 0.002 Msun/pc 3, while mean density of disk is 0.2 Msun/pc 3 outside the tidal radius, the disk dominates the gravitational field of the subhalo, adds energy
Why do streams form? Imagine a subhalo naturally evaporating, ejecting stars in all directions The maximum distance ejected is set by the maximum energy difference between the escaped star and the satellite On board: change in E of orbit affects the period of the orbit, but depends on ejection angle (stars ejected perpendicular to orbit experience least change)
Stream Survival In a spherical potential, the L of an orbit is conserved. Stream remains confined. In a triaxial potential, orbits are stable if the orbital plane coincides with a stable axis; otherwise, precession of the orbit occurs L = Iω is conserved
Stream Survival Let the density of the Galaxy be described by ρ(s), 0.5 < q < 1.0 (q = 1 is a spherical halo) consider flatted potential: precession rate is:
q=1 IBATA ET AL. 2001
q=0.9 IBATA ET AL. 2001
q=0.75 IBATA ET AL. 2001
q=0.5 IBATA ET AL. 2001
Stream Survival Point: The spatial coherence of the stream after a given time depends on the shape of the galactic potential Conversely: streams can be used to measure the shape of the Galactic potential Sagittarius: 0.9 < q < 1.0 Streams at different galactic radii can measure how shape changes with r.
Can we identify accreted substructure?
Can we identify accreted substructure? Number of streams as a function of galactic radius (in 2kpc side boxes) Sats with large sigma and small r will be easiest to detect HELMI ET AL. 2003
Can we identify accreted substructure? substructure becomes kinematically cold HELMI ET AL. 2003
Can we identify accreted substructure? Helmi et al. 1999 use Hipparcos data to identify possible stream within 40pc of Sun in action space.
Can we identify accreted substructure? Using radial velocities alone, and more distant (8-20kpc), SEGUE has identified velocity components in the inner halo SCHLAUFMAN ET AL. (2009)
Can we identify accreted substructure? Using radial velocities alone, and more distant (8-20kpc), SEGUE has identified velocity components in the inner halo, and follow up shows that they are chemically coherent SCHLAUFMAN ET AL. (2011)
Surveys Geneva-Copenhagen survey (Nordstrom et al. 2004) 6D phase space and metallicities for 13,240 stars V ~8.9 proper motion ~1.8 mas/yr out to ~40pc from Sun
Results based on Geneva-Copenhagen simulation predictions: after 8Gyr grey = w/in 5 kpc of sun, black = w/in 1.5kpc of sun HELMI ET AL. 2006
Results based on Geneva-Copenhagen N04 DATA WHAT A SMOOTH GALAXY WOULD LOOK LIKE
Results based on Geneva-Copenhagen HELMI ET AL. 2006
Surveys RaVE: the Radial Velocity Experiment 2003-2013 483,300 stars ~2km/s precision 9 < I < 13
Surveys GAIA: launch 20 Dec 2013 (38 days from now!) < 10 (down to 24) micro parallax < 10 micro /yr in proper motion to V ~15 ~0.2 milli /yr at V~20 1 billion Galaxy stars to V~20 radial velocities to a few km/s full 6D phase space
Surveys Gaia vs Hipparcos
Predictions for Gaia Helmi et al. 2000 evolve 33 satellites over 12 Gyr. After convolving by Gaia detection limits, find them in invariant space. Develop a clump finding algorithm, can recover ~50% of the original satellites
Implications for Direct Detection A Standard Halo Model is typically adopted to describe the velocity distribution function (VDF) of DM expected near the Sun Debris flows alter this expectation Baryons alter this expectation (dark disks)
Tidal Flows RAVE finds no streams (no coherent flows like Sgr) near the sun (8 kpc 3 ); Seabroke et al. 2008 But a contribution from more evolved debris is likely to still exist
The VDF of Halos MAO, STRIGARI ET AL. (2013) SHM: Maxwellian with peak at 220 km/s, tail to vesc ~ 550km/s (black line) Colors show results from DM-only simulations instead (at varying radii from halo center)
The VDF of Halos Kuhlen et al. (2012): relatively recent accretions of massive subhalos no longer exist spatially, but do exist in velocity space GALACTIC REFERENCE FRAME EARTH REFERENCE FRAME
The VDF of Halos Kuhlen et al. (2012): relatively recent accretions of massive subhalos no longer exist spatially, but do exist in velocity space GALACTIC REFERENCE FRAME EARTH REFERENCE FRAME
KUHLEN ET AL. 2012 Implications for Direct Detection More high E events are predicted Is VL2 representative?
The VDF of Halos The latest formulation of the best fit halo VDF MAO, STRIGARI, ET AL (2013)
Implications for Direct Detection MAO, STRIGARI, ET AL (2013) Deviations from SHM in recoil spectrum
Implications for Direct Detection MAO, STRIGARI, & WECHSLER (2013)
A Dark Disk? Baryonic simulations find that accreted subhalos are focused into the disk plane, enhancing the local DM density KUHLEN ET AL. 2013