Disk Galaxies of the Local Group: Dynamics from HST Proper Motions S. Tony Sohn (STScI) + R. van der Marel, J. Anderson, G. Besla + others ApJ, 753, 7/8/9 [July 2012] I. HST measurement (Sohn et al.) II. Implied velocity + LG mass (vdMarel et al.) III. MW-M31-M33 future (vdMarel et al.) NASA Press Release + Live Conference (May 2012)
The Local Group (LG) V tan = ? V rad = 110 km/s First measured in 1912 (V. Slipher)
Proper Motion Challenge • Wide range of V tan possible from theories ( L argument, Λ CDM models, action modeling...): 0 < V tan < 200 km/s = 55 μ as/yr @ M31 distance ➥ 40 km/s = 11 μ as/yr @ M31 distance • Ground-based: NO • VLBI in 2014?: H 2 O masers by Darling (2011) • GAIA (Nov 2013) in 2018?: only bright M31 stars • HST available now!
HST Proper Motions OUTERDISK • Observed in 2002-2004 with ACS/WFC SPHEROID • Re-observed in 2010 with TIDALSTREAM ACS/WFC & WFC3/UVIS
Proper Motion Results • Measurements taken Purely radial orbit + with different instruments (100% Solar motion) + with different orientations + at different times + for different parts of sky Weighted avg. of HST PMs are all consistent within error ➡ Systematics under control • Final PM accuracy = 12 μ as/yr ( ∼ VLBI measurements for LG galaxies) • Final result (combining V tan from satellite kinematics): Galacto V tan = 17 km/s (68%: <34 km/s)
Local Group Mass Constraints + LG timing-argument (Full V rad + V tan ) + Individual M MW & M M31 from satellites + M33 space motion + Bound M31-M33 pair Bayesian combination of constraints ➡ M LG ≈ M MW + M M31 = (3.2 ± 0.6) × 10 12 M ☉
Future Orbital Evolution • Orbital integration + N-body • MW + M31 + M33 • First encounter @ ∼ 4 Gyr • Merger @ ∼ 6 Gyr ➡ E galaxy (3 σ -range: 5-7.5 Gyr) • M33 = slowly decaying orbit • All 3 galaxies orbit close to a single plane • Candidate Suns in red
What happens to M33? 2 • Constraints: - M33 bound to M31 (95% conf.) 1 - M33 probably interacted w/ M31 ➡ M31 will bring in M33 for sure • Possible future scenarios: 1. M31 hits first, then M33 [84%] 2. M33 may hit MW first [9%] 3. M33 may leave the LG! [7%] 3
What will happen to us?
What happens to the Sun? • Sun will move outwards [85%] • May move to R GC > 50 kpc [10 %] • May fly through M33 while dynamically bound to the MW-M31 remnant [20%]
What happens to the Earth? • Sun will still be a main-sequence star when M31 arrives in 4 Gyr, but... - Earth will be too hot for life as we know it • Sun will become a red giant in 6 Gyr, so... - Earth will likely be vaporized (out to Jupiter) • Chance of star-star interactions very small - Earth orbit likely to remain unperturbed
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