1 / 29

Pushing HST’s Astrometry capabilities to Its Limits: Proper Motion of M31

Pushing HST’s Astrometry capabilities to Its Limits: Proper Motion of M31. S. Tony Sohn (STScI) J. Anderson & R. P. van der Marel (STScI). 1. Image credit: Robert Gendler. Proper Motions of Local Group Galaxies.

nash-richards
Download Presentation

Pushing HST’s Astrometry capabilities to Its Limits: Proper Motion of M31

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Pushing HST’s Astrometry capabilities to Its Limits:Proper Motion of M31 • S. Tony Sohn (STScI) • J. Anderson & R. P. van der Marel (STScI) 1 Image credit: Robert Gendler

  2. Proper Motions ofLocal Group Galaxies • The Local Group (LG) = “An astrophysical laboratory for studying the dynamical interactions among galaxies.” • Many galaxies lack PMs, preventing complete dynamical studies.ex) Leo I - bound or unbound to the MW? ➠ Total Mass of MW • Proper motion of M31:✓ The total mass of Local Group.✓ M31-Milky Way orbit and the merger scenario (Vtan < 200 km/s).✓ M31-M33 orbit and past interaction between the two.✓ M31 internal kinematics - dynamical evolution.

  3. Proper Motions ofLocal Group Galaxies • Piatek et al. (2002, 2003, 2005, 2006, & 2007)- Milky Way dSph satellites: Fornax, Carina, Ursa Minor, and Sculptor- Used QSOs near center of fields as reference points • Kallivayalil et al. (2006a, 2006b) - LMC/SMC PM with QSOs • Brunthaler et al. (2005, 2006) - M33 & IC10 PMs with H2O masers • van der Marel & Guhathakurta (2008) - M31 VTAN estimated using statistical methods for M31 satellites and outer LG galaxies:<VW> = −78±41 km/s, <VN> = −38±34 km/s (0.01~0.02 mas/yr)

  4. OUTERDISK SPHEROID TIDALSTREAM M31 Data • 3 Fields in M31: SPHEROID / OUTERDISK / TIDALSTREAM • 1st epoch: ACS/WFC F606W + F814W (PI: Tom Brown; ID 9453,10265)➥ 120 / 53 / 53 orbits of integration➥ Construction of master frames, CMDs, star/galaxy identifications • 2nd epoch: ACS/WFC F606W 1 orbit + WFC3/UVIS F606W 2 orbits➥ ACS data observed with same pointing and orientation as EP1➥ WFC3 data for additional control of systematics (better scale and CTE)➥ Time baselines: 7 / 5 / 5 years

  5. SPHEROID

  6. Absolute Proper Motions withBackground Galaxies • General procedure of measuring the PMs of stars. • How to measure the positions of galaxies? • How to choose a reliable sample of galaxies? • What are the uncertainties of our approach?

  7. Median-combine SExtractor + visual inspection Stars Galaxies individual images individual images individual images master frame Templates 108 frames (1st epoch) Epoch 2 Epoch 2 Transformations Linear + Distortion

  8. Transformation 5x5 pixels Measuring Positions of Galaxies Brightest pixel near the center = “Handle” Individual frame Template (GSF) Master frame Trial offsets

  9. −10.2 −9.7 −8.4 −9.1 −8.7 σX = 0.12 σY = 0.40 σX = 0.42 σY = 0.06 σX = 0.07 σY = 0.19 σX = 0.03 σY = 0.06 σX = 0.13 σY = 0.06 −11.1 σX = 0.01 σY = 0.01 −10.3 σX = 0.03 σY = 0.02 −9.4 σX = 0.05 σY = 0.06 −8.2 σX = 0.07 σY = 0.07 −7.4 σX = 0.11 σY = 0.11

  10. How Well Does GSF Work in Real Data?

  11. 146 264 406 250 78 Selection of Background Galaxies 1087 background galaxies used for PM calculations

  12. M31 Spheroid Field

  13. SPHEROID FIELD OUTERDISK FIELD vdM&G (2008) vdM&G (2008) σ/√N = 0.032 mas/yr σ/√N = 0.016 mas/yr = 0.002 pix • Systematics ➯ The PSFs have changed significantly between the first and second epoch data. • ACS post processing + WFC3 data will provide solutions.

  14. Average PSF Residuals Epoch 1 Epoch 2

  15. GSF-Fitting PM Derivations: Further Scientific Applications • “Proper Motion of Leo I: Constraining the Milky Way Mass”(ID 12270; PI Sohn) - 25 km/s level precision at 260 kpc➥ Constrain orbit of Leo I to check the bound status to MW • “Mass of the Local Group from PMs of Distant Dwarf Galaxies”(ID 12273; PI van der Marel) - Cetus, Leo A, Tucana, and SAGDIGAveraged 35 km/s level precision at the edge of LG (~1 Mpc)➥ Constrain orbits of galaxies near the turn-around radius of the LG • Potential for measuring transverse velocities of more distant galaxies beyond the Local Group.

  16. Tucana Dwarf Galaxy - ACS/WFC F814W

  17. Benefits of using bG galaxies as Reference Sources • Abundance of sources provides √N advantage in averaging. • Sample the whole detector, reducing systematic errors. • Ubiquity - absolute PMs can be derived in virtually any field. • Many exciting scientific applications for Local Group and beyond. - Work in progress...

  18. The End

  19. Proper Motion • Typical units: mas/yr or mas/century • To get accurate proper motions, measure positions with small errors, increase the time baseline, or both. • With photographic plates, usually deal with time baseline. • With HST, precise positions can be measured.

More Related