HSTPROMO The HST Proper Motion Collaboration Dark Matter in the Local Group Roeland van der Marel (STScI) Stellar Dynamics • Many reasons to understand the dynamics of stars, clusters, and galaxies in the nearby Universe • Formation: The dynamics contains an imprint of initial conditions • Evolution: The dynamics reflects subsequent (secular) evolution • Structure: dynamics and structure are connected • Mass: Tied to the dynamics through gravity è critical for studies of dark matter 2 Line-of-Sight (LOS) Velocities • Almost all observational knowledge of stellar dynamics derives from LOS velocities • Requires spectroscopy – Time consuming – Limited numbers of objects – Brightness limits • Yields only 1 component of motion – Limited insight from 1D information 2 – 3D velocities needed for mass modeling: M = σ 3D Rgrav / G • Many assumptions/unknowns in LOS velocity modeling 3 Proper Motions (PMs) • PMs provide much added information, either by themselves (2D) or combined with LOS data (3D) • Characteristic velocity accuracy necessary – 1 km/s at 7 kpc (globular cluster dynamics) – 10 km/s at 70 kpc (Milky Way halo/satellite dynamics) – 100 km/s at 700 kpc (Local Group dynamics) – 0.03c at 70 Mpc (AGN jet dynamics) • Corresponding PM accuracy – 30 μas / yr (~ speed of human hair growth at Moon distance) • Observations – Ground-based: NO – VLBI: LIMITED (some water masers) – GAIA: FUTURE (but not crowded or faint) – HST: YES (0.006 HST ACS/WFC pixels in 10 yr) 4 Proper Motions with Hubble • Key Characteristics: – High spatial resolution – Low sky background – Exquisite telescope stability – Long time baselines – Large Archive – Many background galaxies in any moderately deep image • Advantages: – Depth: Astrometry of faint sources – Multiplexing: Many sources per field (N = 102 –106, Δ~ 1/√N) 5 HSTPROMO: The Hubble Space Telescope Proper Motion Collaboration (http://www.stsci.edu/~marel/hstpromo.html) • Set of many HST investigations aimed at improving our dynamical understanding of stars, clusters, and galaxies in the nearby Universe through PMs. • New observations and detailed theory components • Status/Achievements – 10+ years of work – 28 projects awarded in highly competitive peer-review – $3M in NASA research funding – 23 refereed papers, many others in preparation – 7 press releases – 2 Astronomy Picture of the Day 6 HSTPROMO Team • Lead Coordinators – Science Applications : Roeland van der Marel – Astrometric Methods : Jay Anderson • Key Members + Project PIs: – Andrea Bellini, Gurtina Besla, Paolo Binachini, Mike Boylan- Kolchin, Julio Chaname, Alis Deason, Tuan Do, Raja Guhathakurta, Nitya Kallivayalil, Davide Massari, Eileen Meyer, Imants Platais, Elena Sabbi, Sanmo Tony Sohn, Mario Soto, Michele Trenti, Laura Watkins • Associates, Advisors, Other/Former Members – many other PIs/CoIs/co-authors on individual HST projects 7 Illustration: Globular Cluster NGC 6681 (M70) and Sagittarius dSph Compact galaxies (distant) Sgr dSph (28 kpc) Disk/bulge (foreground) M70 (9 kpc) [Massari et al. 2013, Bellini et al. 2013] 8 Sampling of Key Results wrt Milky Way and Local Group Dark Matter • M31 (Sohn et al. 2012, vdM et al. 2012a,b) • Leo I (Sohn et al. 2013, Boylan-Kolchin et al. 2013) • Large and Small Magellanic Clouds (LMC/SMC) (Kallivayalil et al. 2006a,b and 2013; Besla et al. 2007, 2010; vdM & Kallivayalil 2013) • Milky Way Halo stars (Deason et al. 2013) 9 Milky Way M31 Halo MW SMC satellites Magellanic Stream Leo I LMC M33 M31 satellites [Grebel 2000] 10 11 M31 Galactocentric Velocity • Agreement HST vs. satellites (vdM & Guhathakurta 2008) • Final weighted average – vW = -125 ± 31 km/s – vN = -74 ± 28 km/s (12 μas/yr accuracy) • Most of heliocentric velocity Radial is reflex solar motion in MW Orbit – V¤ = 239 ± 10 km/s (McMillan 2011) • Galactocentric [Sohn et al. 2012, vdMarel et al. 2012] – Vtan = 17 km/s (< 34 km/s @68%) • Consistent with MW-M31 Direct Collision Course! 12 Local Group Mass • Individual virial masses of the MW and M31 – estimated from many techniques 12 – MMW ≈ MM31 ≈ (1.5 ± 0.4) × 10 M¤ • Timing argument mass from M31+MW orbit 12 – MMW + MM31 = (4.9 ± 1.6) x 10 M¤ – Cosmic variance uncertainty (Li & White 2008) dominates over observational errors – Timing argument helps little to constrain LG mass • Bayesian combination of constraints 12 – MMW + MM31 = (3.2 ± 0.6) x 10 M¤ 13 14 Leo I dSph • Unusually distant and rapidly moving galaxy – Galactocentric: r = 261 ± 13 kpc v(radial) = 168 ± 3 km/s • Mass estimates from equilibrium modeling of Milky Way satellite [Sohn et al. 2013] system (e.g., Watkins et al. 2010) – significantly affected by whether Leo I assumed bound or not 15 Leo I HST HST Results average • Proper Motion – Galactocentric v(tangential) = Radial 101 ± 34 km/s Orbit • Orbit integrations [Sohn et al. 2013] – Leo I entered MW virial radius for first time 2.3 ± 0.2 Gyr ago – Leo I had pericenter at 91 ± 36 kpc at 1.0 ± 0.1 Gyr ago – No indication that other galaxies have influenced Leo I orbit 16 Leo I compared toΛCDM subhalos • Highest resolution numerical simulations of Galaxy-size dark matter halos (Aquarius) • 99.9% of subhalos in the simulations are bound to their host: likely true for Leo I • Assuming that Leo I is the least bound Milky Way satellite, MMW = 12 [Boylan-Kolchin et al. 2013] (1.6 ± 0.3) x 10 M¤ 17 PM and Orbit of Magellanic Clouds • Traditional view – Clouds have orbited Milky Way many times – Logarithmic Milky Way halo implies ~2 Gyr period • HST PM measurements LMC Field 1 of 22; 3 epochs – Reflex motion of QSO wrt 1x1 pixel box LMC/SMC stars over 7 years – Clouds move faster than traditionally believed è wider, longer-period orbit [Kallivayalil et al. 2013] 18 Dark Mass and First Infall • Orbit integrations in CDM-motivated Milky Way halo imply >4 Gyr period, with Clouds possibly on first infall • Orbit depends on Milky Way and LMC masses – a) Multiple past pericenters, low LMC mass due to tides – b) First infall, high LMC mass consistent with halo occupation models • Relative LMC-SMC PM + Expectation LMC-SMC bound – First infall models preferred Big • Consistent with Bang Present – Blue gas-rich nature of Clouds [Besla et al. 2007; Kallivayalil et al. 2013] – Cosmological simulations 19 New Magellanic Stream Models • Traditional models with Stream formed by tidal or ram pressure interaction with Milky Way do not work in a first-passage scenario • Possible to form Stream formed from gas stripped in LMC-SMC interaction before entering Local Group [Besla et al. 2010] 20 LMC Internal Kinematics • First ever measurement of PM rotation field of any galaxy (1 revolution in ~ 250 Myr) • Yields new insights LMC geometry, structure, distance [van der Marel & Kallivayalil 2013] 21 LMC PM Rotation Curve • Consistent with LOS rotation curve [van der Marel & Kallivayalil 2013] • Depends on stellar population 22 PMs of Distant Milky Way Halo Stars • Kinematics of metal-poor Milky Way halo known from – LOS velocities out to ~50-100 kpc (e.g., BHB stars) – Ground-based/Hipparcos PMs in solar neighborhood (<10 kpc) • Lack of PMs at large distances is problem for understanding – halo formation mechanisms – MW mass (anisotropy degeneracy) • Multi-epoch HST PM+CMD data allows identification of MSTO halo stars at 10-100 kpc [Deason et al. 2013] 23 A Shell in the Milky Way Halo? • 13 halo stars indentified at 24 ± 6 kpc in 3 M31 fields – Direction of Andromeda-Triangulum overdensity – Distance similar to known break in halo density profile • Velocity anisotropy more tangential (β= 0.0 ± 0.2) than in solar neighborhood (β= 0.6 ± 0.1) • We are extending this work from 3 to 150 fields, to get a PM sample for ~1000 halo stars • Possible indication for shell in halo from past accretion event? 24 25 Other HSTPROMO Projects • Internal dynamics of ~25 Globular Clusters – Central black holes, equipartition, multiple populations, … • Runaway O/B Stars – 30 Doradus in LMC, Galactic Center clusters • Stellar Streams [Monday talk by Sohn] – Sagittarius, Orphan • dSph Galaxy internal PM dynamics – Draco, Sculptor • Local Group infall of distant dSph galaxies – Leo T, Tucana, Cetus, … • Relativistic Flows in AGN jets – M87, 3C273, …. 26 Conclusions • HST fantastic tool for Proper Motions and Local Group / Milky Way Dynamics – First/Best PM measurements of M31, Leo I, LMC, SMC, distant halo stars – Many other exciting programs ongoing • Buildup of Local Group through galaxy/satellite infall continues to present day • Dark Matter in Local Group 12 – Milky Way : MMW > 10 M¤ [95% confidence] 12 – Local Group : MLG = (3.2 ± 0.6) x 10 M¤ • Movies: will run while you ask me questions 27 .