Disk Galaxies of the Local Group: Dynamics from HST Proper Motions - - PowerPoint PPT Presentation

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)

First measured in 1912 (V. Slipher)

Vtan = ?

Vrad = 110 km/s

Proper Motion Challenge

• Wide range of Vtan possible from theories

(L argument, ΛCDM models, action modeling...): 0 < Vtan < 200 km/s = 55 μas/yr @ M31 distance ➥ 40 km/s = 11 μas/yr @ M31 distance

• Ground-based: NO
• VLBI in 2014?: H2O masers by Darling (2011)
• GAIA (Nov 2013) in 2018?: only bright M31 stars
• HST available now!

SPHEROID TIDALSTREAM

OUTERDISK

HST Proper Motions

• Observed in 2002-2004

with ACS/WFC

• Re-observed in 2010 with

ACS/WFC & WFC3/UVIS

Proper Motion Results

• Measurements taken

+ with different instruments + with different orientations + at different times + for different parts of sky are all consistent within error ➡ Systematics under control

• Final PM accuracy

= 12 μas/yr (∼ VLBI measurements for LG galaxies)

• Final result (combining Vtan from satellite kinematics):

Galacto Vtan = 17 km/s (68%: <34 km/s)

Weighted avg. of HST PMs Purely radial orbit (100% Solar motion)

Local Group Mass

Bayesian combination of constraints ➡ MLG ≈ MMW + MM31 = (3.2 ± 0.6) × 1012 M☉

Constraints

+ LG timing-argument (Full Vrad + Vtan) + Individual MMW & MM31 from satellites + M33 space motion + Bound M31-M33 pair

Future Orbital Evolution

• Orbital integration + N-body
• MW + M31 + M33
• First encounter @ ∼4 Gyr
• Merger @ ∼6 Gyr ➡ E galaxy

(3σ-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?

• Constraints:
• M33 bound to M31 (95% conf.)
• 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%]

1 2 3

What will happen to us?

What happens to the Sun?

• Sun will move outwards

[85%]

• May move to RGC > 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