Taking the Measure of the Milky Way

1. The slides in this collection are all related and should be useful in preparing a presentation on SIM PlanetQuest. Note, however, that there is some redundancy in the collection to allow users to choose slides best suited to their needs.

2. Taking the Measure of the Milky Way Cover page from S. Majewski Key Project proposal • SIM will probe the structure of our Galaxy: • Fundamental measurements of: • Total mass of the Galaxy • Distribution of mass in the Galaxy • Rotation of the Galactic disk • How? • By observing samples of stars throughput the Galaxy • By sampling different star populations

3. Calibrating the Cosmic Distance Scale • Current parallax measurements reach only to about 100 pc. • Distances are derived from a staggered set of rungs, each relying on the previous step. • SIM will calibrate the rungs of the distance ladder throughout the Galaxy (10% to 25,000 pc)

4. Dynamics of Galaxy Groups within 5 Mpc You are here Simulation • Simulated 3-D motions projected onto a plane • ‘Smeared’ tracks show the simulated motions of galaxies • Circles show current positions SIM will test this model • SIM will measure current 2-D velocities across the sky Simulated ‘time-lapse’ photo of 30 galaxies closest to our Milky Way (1-billion year exposure)

5. NGC 6822

6. Holmberg II

10. Milky Way Studies Key issues: • Mass potential of MW from 3-D motions of tidal tails • Total mass from distant halo objects • Legacy orbit determinations for MW satellites • Legacy orbit determinations for all Galactic globulars • Surface density of disk (non-local Oort limit) • Dynamics of the bulge and the Galactic bar Science Drivers • Tidal Tails • Globular Clusters, Satellites, Open Clusters • Galactic Disk • Central Galaxy • Fundamental Parameters

11. Mass Potential: Halo Objects • Proper motions orbit shapes gravitational potential • Get radial dependence, q, triaxiality • With only RVs, have to make assumptions about,e.g., radial vs. isotropic orbits. • Orbit of Leo I (at 270 kpc) -- currently4X MW mass difference if bound vs. unbound. • Total MW mass comes from most distant (>90 kpc) objects

12. Mass Potential: Tidal Tails • RVs of tidal tails alone cannot yet distinguish between various potentials with flat rotation curveat large RGC (e.g., NFW vs. log) • Current problem of leading arm Sgr RVs vs. model • Any evidence of dynamical friction = independent satellite mass estimate (see poster 142.02) • Any evidence of deviations encounters w/lumps • Need SIM for 10-15 mas/yr accuracy to V=18-20

13. Star Clusters • Measure Proper Motion of every Galactic globular clusterand ~100 open clusters – a legacy for the astronomical community • With SIM PMs, we can search for correlations between orbits and streams. • Are all halo GCs found in streams? • Implications for Galaxy formation • Cluster dynamics/evolution tied to orbits • Bulge/disk globulars useful probe of central potential (least well known part of potential) • Need SIM for 10 mas/yr accuracy to V=18-20

14. Dwarf Satellites • Measure Proper Motion of every Galactic satellite galaxy • Orbits crucial to understanding formation and evolution of these most common and least understood types of galaxy • Crucial for understanding discrepancies between CDM theory and observations • Need SIM for 10 mas/yr accuracy to V=20

15. Galactic Disk • Determination of Galactic Rotation Curve • Difficult to do with Radial Velocities alone (have to assume circularity and have distances well known). • Influence of bar not yet established. • Can place MW on Tully-Fisher relation. • Need SIM for 10-15 mas/yr proper motion to V=16.

16. Galactic Disk/Bulge • Determine relative amounts of dark and luminous matter inside the solar circle • At large radii we have some knowledge from external galaxies (satellites, weak lensing) • At small radii (<~5 kpc) Dark Matter fraction is uncertain (5%? or 50%?) (maximum disk problem) • Theoretical predictions (strong Dark Matter cusps) vs. large Dark Matter cores observed in external galaxies. • Measuring Oort limit at other radii and bulge motions have the potential to answer these questions • Need SIM for 10-15 mas/yr motions to V=16

17. Galactic Center • Measure distance to Galactic Center • Distance to Sgr A* from orbit of S2 (Eisenhauer et al. 2003) • Accurate to 5% • 1-3% by 2010 • VLT and Keck • 3% error in Ro, QLSR 5% error in Mass

18. Galactic Center • SIM + HST Measurements to better than 1% • Roand QLSR are of fundamental importance; should not rely on SgrA* distances only. • Chance to confirm that the central black hole is indeed located at the Galactic Center for the MW. • SIM can sample a different and larger population of stars (bulge and bar) 100-500pc from Galactic Center. Probe potential over poorly studied distance range. • Test of parallax vs. dynamical center (distance at which bulge stars change direction of motion) • Need SIM for 6 mas parallaxes to V=16

19. Conclusions Taking Measure of the Milky Way Key Project will provide unique insight into the following key issues: • Mass potential of Galaxy from 3-D motions of tidal tails • Total mass from distant halo objects • Legacy orbit determinations for all MW satellite galaxies • Legacy orbits for all Galactic globulars/numerous open clusters • Surface density of disk (non-local Oort limit) • Dynamics of the bulge and the Galactic bar We will also independently: • Refine the distance measurement to the Galactic Center. • Refine measurement of the Galactic rotation curve.

20. Galactic Dynamics & Dark Halo of Our Galaxy • Study the ‘classical’ problems of size, mass distribution, and dynamics of the Galaxy, using stellar velocities • Example: Debris tail orbits (Sagittarius dwarf spheroidal galaxy orbits Milky Way) • Gravitational forces pull out ‘tidal tails’ of stars • The orbits of these tails trace the past history of the dwarf • They also trace the mass distribution of the Milky Way • Distances to 5% at 10 kpc, for stars with V < 20 • Proper motions to 0.1 km/s at 10 kpc • Combine with ground-based radial velocities ‘Tidal tail’ simulation: Dwarf galaxy in orbit around the Milky Way

21. Discovery of multiple tidal tail targets (Sgr, Pal 5, Monoceros/Argus, TriAnd)

22. Much new info on Sgr system • Initial 2MASS work reveals >360o arms • RV constraints • With only 4-D constraints, problems/contradictions remain: • Modeling of Sgr and Galactic halo Leading Arm RVs “wrong”; Prolate vs. Oblate ambiguity

23. Much new info on Sgr system (cont.) • New discovery -- two more wraps of Sgr debris. • Evidence that Sgr orbit evolved from large circular to smallerelliptical orbit about 2 Gyr ago due to collision with LMC. • Proper motions of faint stars needed to understand this complicated interaction independent confirmation of LMC m and halo potential

24. Discovery that halo is almost completely dominated by tidal streams • Obtaining halo properties from Jean’s equation applied to “random” halo stars more difficult.

25. Quasar Astrophysics Using Astrometry • Quasars are the most powerful objects in the universe • Many quasars emit twin jets of relativistic plasma • Optical observations average the entire region • Accretion disk, hot corona, jets • Jets have been studied by VLBI (radio) at ~100 µas scales • SIM will measure: • position shifts due to variability • color-dependent relative positions of the emission • These measurements will open up a research area only studied with VLBI SIM can determine if the visible light from quasars originates in hot gas around an accretion disk or from a relativistic plasma jet SIM can detect the orbital motions of two merging black holes in the centers of massive galaxies

26. Quasar Astrophysics Using Astrometry • Quasars are the most powerful objects in the universe - • Many quasars emit twin jets of relativistic plasma • Ground-based optical observations average the entire region • Accretion disk, hot corona, jets • SIM will measure: • position shifts due to variability (relative brightening or motion) • color-dependent relative positions of the emission • SIM will enable studies much closer to the central supermassive black hole than VLBI or ground based optical interferometry can reach

27. Understanding the Fundamental AGN Power Source • Accretion onto massive black holes fuels the energetic AGN phenomena-- but how does it work? Not just intrinsically interesting, but AGN directly affect the development of other structures in the universe (galaxies, clusters, 10-Mpc bubbles) • AGN and starbursts reionized the universe, forming bubbles around massive galaxies by z ~6, as shown in series of papers beginning with observations of Sloan high-z quasars 200,000 ly structure generated by jets in M87

28. The Most Basic Physics The “Core” What are the sizes and geometric relations between the components of the “core” region: jets, accretion disk, hot corona ? Which components dominate the optical emission? The structures we measure with SIM are either optical synchrotron emitters from particle populations distinct from the radio-synchrotron emitters, or thermal emitters from the hot corona or accretion disk.

29. Estimating the Emission Region Size Since we wrote our SIM proposal in 2000, Seyfert nuclei of NGC 4151 and NGC 1068 were detected by Keck Interferometer and VLTI, respectively, (Swain et al. 2003, ESO 2003). • Accretion disks are small • ~ 160 µas (at 15 Mpc - M87) • ~ 2 µas (at 3C345 - z = 0.6) • Hot corona region is also small • ~ 70 Rs corresponds to ~ 1 µas at z = 0.6 • Jet shocks are small- a shock in the jet of M87 outshone the nucleus! 4-wk variability size scale corresponds to 30 as (Perlman et al. 2003 ApJ Letters) • These sizes are well beyond the reach of ground based optical interferometers (e.g., Keck Interferometer and VLTI) - to study their motions,we need SIM.

30. Astrometric Signature of a Black Hole Binary Binary black holes may be ubiquitous in galactic centers, rather than rare, because mergers may stall out. Orbital motion of binary black holes is detectable with SIM, using astrometric reflex motion of their photocenter, just like we detect planets around stars.

31. AGN Science With SIM- More Compelling than Ever • AGN dust tori in nearby Seyferts are now being studied by the Keck Interferometer and VLTI, but only SIM can measure motions at the light-week physical scale • Discovery of the bulge mass- central black hole mass relationship triggered an avalanche of papers on accretion rates, quasar lifetimes, and the detection of fossil black holes. SIM can observe structures in low and high power AGN, addressing physics of a variety of central black hole masses and accretion rates. • Binary black holes may be ubiquitous rather than rare because mergers may stall out before the black holes coalesce (cf. NSF “Grand Challenge”). SIM can measure orbital motion directly by tracking motion of the photocenter.

32. Sample Permatile for relative astrometry • Select tile centers for objects of interest • Monitor optical brightness of candidates for next n years • Formulate SIM observing strategy

33. Optical Brightness Monitoring Quasars in the range of V~ 15-17, but a few are brighter, such as 3C273 Historically, the most compact quasars are the most variable, so we need to ensure the ones we observe with SIM are consistently the brightest. We have begun a monitoring program on the Palomar 60” telescope. Discussions with European (and other) monitoring groups have begun (international conf. in April 2005) Strong, non-coincidental agreement on source lists with GLAST mission (launch 2007) - brightest, most compact blazars are the ones they are most likely to detect