PE#4: It contains some useful diagrams and formula which we ll use today

Transcription

1 Sep 6, 2017 Overview of the MW PE#4: It contains some useful diagrams and formula which we ll use today HW#2 is due next Wed and is now posted. Don t wait for the last minute to start it. Includes a short in-class presentation (see instructions) Reading: Chapter 2 through 2.4 (about the Milky Way) If you have not had an astronomy class before, read some intro textbook/material on the Interstellar Medium. Next Monday s class will meet in SSB 511 (instead of here )

2 Homework #1 Everything in this class is graded on a curve. This homework was worth 25 points. (mean 22.9, sd 2) In general, just fine. When you write answers to non-quantitative questions Be sure to use your own language and explain what you are saying. Stuff copied from web sources generally doesn t tell the whole story. Include your citations; I should be able to find the details/facts you quote. If you haven t done it, you should turn it in asap; you need to understand the material covered. I have written comments on your work. If you have questions, please come see me.

3 HW for next week HW has three parts: don t leave for last minute Last part is 3 minute presentation on:

4 The Milky Way as a Galaxy Advantages: nearby Lots of detailed data We can observe stars of all masses (esp. low mass) Supermassive (4 x 10 6 M ʘ ) black hole at center (SgrA*) Stellar streams: evidence for satellite disruption; disk warping Magellanic Stream: evidence for interaction with MCs Disk/bulge/halo separate (a) age; (b) kinematics; (c) metallicity. Disadvantages: Unremarkable as galaxies go SMBH is not active Benign environment Mature universe Optical wavelengths heavily extincted

5 The Milky Way viewed edge-on

6 The Milky Way components

7 2MASS view of the MW Bulge 20% of Galaxy s light from the bulge, R~1 kpc. Stars: few Gyr old, metal-rich unlike the metal-poor stars of the halo; inner halo is also more round and does not show rotation Bulge rotates in prograde sense, like the Sun, but slower: <Vc> ~ 100 km/s A slight asymmetry of the bulge and additional kinematic data show that the Milky Way has a central bar extending to R=2-3 kpc. It is a Sbc galaxy or SABbc( r)

8 The Milky Way Stars contribute ~80% of the visible mass 80% in disk 20% in bulge Gas contributes ~20% of the visible mass Atomic gas (HI) ~ 2/3 of gas mass ( H one ) Molecular gas (H 2 ) ~ 1/3 of gas mass (molecular hydrogen Hot, ionized gas ( HII or H+) ( H two ) Dust Between stars Mostly in spiral arms and molecular clouds The gas and dust reside in the interstellar medium

9 The Structure of the Milky Way Stars: Thin disk Thick disk (older, less massive) Bulge and/or bar Gas: Atomic (HI) In diffuse clouds More extended than stars Molecular (H 2 ) In dense clouds Follow stars, spiral arms Ionized (HII) Low mass, low density, large volume Dust: Mostly in spiral arms & molecular clouds

10 Milky Way Overview The Milky Way serves as a laboratory for probing galaxy structure and evolution on scales impossible in other galaxies Galactic Archeology: Huge datasets of photometry and spectroscopy allow exploration of different components of the MW: Sloan Digital Sky Survey (SDSS) and the Sloan Extension for Galactic Exploration and Understanding (SEGUE) and APO Galactic EvolutionExperiment (APOGEE) RAdial Velocity Experiment (RAVE) GAIA (ESA mission; launched 19Dec2013 in progress) Luminosities, ages, metallicities, velocity dispersions for different populations/structures

11 Classic picture of MW formation Eggen, Lynden-Bell and Sandage 1962: collapse of the Milky Way Basic picture: collapse of rotating, spherical, gaseous halo First stars form in halo => oldest stars, globular clusters, Pop II Use kinematics and metallicities (Z) to infer model Low Z, higher σ in halo => older population Higher Z, higher V rot in plane => younger population Modern picture: not a single episode of collapse; more structure

12 Galactic Coordinates Differential rotation in the disk

13 Cartesian Galactic Coordinates Common use: centered on Sun Could also be centered on Galactic Center: beware of use! X, Y, X X points towards the Galactic Center Y points in the direction of the Sun s orbital motion Z is perpendicular to the disk U,V,W are the velocities in the directions X,Y,Z

14 Kinematics of the Galaxy To describe the velocity field of stars, decompose into components in the (R, θ,z) coordinate system. A star then has velocity components: V = (-10,5,7) km/s V 0 = 220 km/s P 0 ~ 230 Myr The Sun s motion is not simply circular in the plane; it also is currently moving inward (U < 0), faster than it would on a pure circular orbit [V > V(R 0 )], and away from the plane (W > 0). This extra motion is called the Sun s peculiar velocity V (U,V,W) The Local Standard of Rest (LSR): A fictitious rest-frame (mean motion of the Solar neighborhood, i.e. perfectly circular orbit in the plane of disk at the location of the Sun) whose components are: U LSR 0, V LSR V 0 V(R 0 ), W LSR 0 The LSR changes over time, but the timescale for this change is of order the orbital period (~230 Myr).

15 The latest in spectroscopic surveys SEGUE (SDSS): northern hemisphere, selected angular coverage, deep exposures, low resolution spectra but large wavelength coverage, 230,000 stars APOGEE (SDSS): infrared spectroscopy of 100,000 red giant stars, from both hemispheres (NM and Chile) RAVE: southern hemisphere, wide area, intermediate depth, intermediate resolution, limited wavelength coverage GAIA: Currently undertaking sky-wide survey to do astrometry, photometry and spectra ( nm).. Data release in 2018.

16 Questions (see today s PE) What is the Local Standard of Rest? What is the Sun s peculiar velocity? How big is it? Suppose we observe a star in the Solar neighborhood to have a radial velocity of -100 km/s. What does its velocity imply about the star?

17 Kinematics and populations Asymmetric drift: random motions of stars relative to LSR reveal differences in populations. Stars moving in circular orbits in the plane will have small velocities relative to the Sun (u, v ~ 0 km/s) whereas the halo stars will have high velocities (u ~0, v ~ -220 km/s)

18 Kinematics of the MW thick disk old disk Vertical structure related to age of component Velocity dispersions of nearby F stars Thick disk is discrete component thick disk appears at age ~ 10 Gyr Freeman 1991; Edvardsson et al 1993; Quillen & Garnett 2000

19 Thin and thick disks Most spirals (including the MW) have a second thicker disk component, believed to be the early thin disk heated by an accretion event. In some galaxies, it is easily seen : The thin disk The thick disk NGC a disk galaxy with a bright thick disk (Tsikoudi 1980)

20 MW:thick and thick disks Using the MW spherical polar coordinate R,Φ,z, we can approximate the density n(r,z,s) of stars of spectral type S by a double exponential of form: h R = 2 to 4 kpc, both for the thin (h z ~ 0.3 kpc) and the thick disk (h z ~ 1.5 kpc) Beyond R=15 kpc, the disk density is rapidly declining. The brightness distributions of other galaxies show similar downturns.

21 Structure and metallicity

22 Connection between kinematics & geometry spherical system (Pop II objects) thick disk (1) Thick disk of high-metallicity globular clusters (left-hand panel) is made of objects on low-inclination, nearly-circular orbits <=> the system has some prograde rotation. (2) Spherical system (right panel) has completely disorganized motions, no rotation on average; some clusters have prograde, some retrograde motion, Orbits are highly inclined.

23 Galactic rotation

24 Galactic Coordinates l = 90 o (direction of rotation) l = 0 o Differential rotation in the disk Along any longitude, the observed radial velocity reaches a maximum at the tangent point, where R = R min.

25 Galactic Rotation Relations V obs = V(R) cosα - V(R ) cos(90º-l ) = V(R) cosα - V(R ) sin l At any longitude, V obs (d) depends on the rotation curve characteristics. Notice that we observe the max. V r from an object at the tangent point along any l.o.s. R min R ʘ = sin l

26 Galactic Rotation Relations Using galactic coords l,b: V obs = V(R) cos α - V(R ) cos(90º- l) = V(R) cos α - V(R ) sin l R R min R ʘ = sin l Notice that we observe the max. V obs from objects at the tangent point along any l.o.s. V obs = RR RR VV RR sin l VV RR sin l = R V(R) R V(R ) R sin l At any longitude, V obs (d) depends on the characteristics of the Galactic rotation curve V(R). Define the angular velocity Ω RR = VV(RR) RR so Ω = Ω RR = VV(RR ) RR

27 Galactic rotation In the solar neighborhood Oort s constants A and B

28 Oort s Constants Local shear rate Local angular velocity Local vorticity Local slope of RC

29 EBHIS data service:

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35 Use Doppler Shift to Map Radial Velocity In Various Directions Starting point: Map intensity of neutral hydrogen along Galactic plane for various Galactic longitudes. From this, it has been shown that the neutral hydrogen appears to be concentrated in the spiral arms of the Milky Way.

36 Galactic Rotation l = 90 o l = 0 o Along any longitude, the observed radial velocity reaches a maximum at the tangent point, where R = R min.

37 Galactic Coordinates

38 Galactic Coordinates Q3 Q2 Q4 Q1

39 Questions (see today s PE) Suppose we observe a star at l = 52, b = -0.1 with radial velocity of -100 km/s. What does its velocity imply about the star?

40 Galaxy Rotation Curves Solid Body V(R) R Keplerian Decline V(R) R -1/2

41 Galaxy Rotation Curves

42 Milky Way Rotation Curve 8 kpc

43 Mass distribution from a Rotation Curve Gravitational acceleration G M(R) R 2 = V 2 (R) R Orbital acceleration M(R) = R V 2 (R) G A flat rotation curve implies that the mass increases linearly with radius.

44 Kinematic Distances Applicable for objects in/close to the Galactic Plane Assume a model for Galactic rotation V(R) Observe radial velocity and longitude Derive distance from model Milky Way Rotation: At R ~8 kpc, V(R )~ 220 km/s Period ~ ~ 2 X 10 8 years

45 Kinematic distances velocity In general, there is no way to determine the distance to an HI cloud (unless it is associated with another object, e.g. HII region). So, interpreting the HI line velocities requires a model of the rotational velocity field V rot (R)

46 Spiral structure in MW

47 The Milky Way has numerous arms

48 MW HI Map: z vs. Radius Verschuur & Kellerman, P. 348 The MW HI layer is warped! Sombrero effect

49 Galactic Archeology Detailed study of the kinematics and metallicities of galaxies in modern surveys allow the identification of streams which in turn reveal a complicated history More on this later P. Hard ing

50 2MASS View of the Milky Way What wavelengths correspond to J,H and K?

51 Stellar populations in the MW