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Home , Leo (constellation), Leo I (dwarf galaxy)

HSTPROMO The HST Proper Motion Collaboration

Dark Matter in the Roeland van der Marel (STScI)

Stellar Dynamics

• Many reasons to understand the dynamics of , clusters, and in the nearby

• 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

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 ( 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 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+ 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 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 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)

• I (Sohn et al. 2013, Boylan-Kolchin et al. 2013)

• Large and Small (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 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 – 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 () • 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- 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, clusters • Stellar Streams [Monday talk by Sohn] – Sagittarius, Orphan • dSph Galaxy internal PM dynamics – , • Local Group infall of distant dSph galaxies – , , , … • 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