Our Galaxy The Milky Way

1. Our GalaxyThe Milky Way Chapter 19

2. Overview & History • Our Galaxy is a collection of stellar and interstellar matter – stars, gas, dust, neutron stars, black holes– held together by gravity. • Our view of the Galaxy….

3. History of Galactic (& Extragalactic) Astronomy • 1610 - Galileo discovered the Milky Way is comprised of many stars • 1755 - Immanuel Kant theorized that the galaxy has a planar structure, some nebulae might actually be entire other galaxies or island universes • 1774 -1781 - Messier catalog compiled including Andromeda galaxy as M31 • 1781-1802 - William and Caroline Herschel conducted first “all-sky survey” and cataloged 5000 nebulae, resolving some into their individual stars • Later (1845) William Parsons (Lord Rosse), using a 72-inch telescope, classified the nebulae into featureless ellipticals and whirlpool-like spiral nebulae • Much later (1888) Dreyer would add to their list to create the New General Catalog (NGC) and Index Catalog (IC)

5. 1785 - Herschel attempted to determine the shape and size of Galaxy Assumptions: • All stars have same intrinsic brightness • Star are arranged uniformly throughout the MW • He could see to the edge of the system • then dmax = 100.2(m_max-M+5) pc 1 parsec (pc) = 3x1013 km or 3.26 lightyears, m_max is apparent magnitude of faintest stars, M is the absolute magnitude assumed for all stars – absolute magnitude is the apparent magnitude a star would have at 10pc (e.g. the Sun has an apparent magnitude of -26.72 and an absolute magnitude of 4.8. Most stars in the night sky have apparent magnitudes between 1 and 6.) SUN Herschel could not account well for the effects of dust. More dust along the disk causes the distribution of stars to drop-off artificially – objects more than a few kpc from the Sun are obscured by dust.

6. Early 1900’s - Kapteyn used stellar parallax to estimate the true size of the Galaxy  Kapteyn Universe • 10kpc diameter and 2kpc thick with the Sun less than a kpc from the center (rather heliocentric) • Tried to estimate Rayleigh scattering due to ISM gas but determined it to be insignificant (because most obscuration is due to ISM dust absorption which has a smaller wavelength dependence) • Shapley (1919) observed that globular clusters are distributed asymmetrically in the sky and that if one assumes they are distributed about the center of the galaxy, this implies the Sun in not near the center of the Galaxy • Estimated distances to globular clusters using variable stars and P-M relationship • Concluded size to be 100kpc with Sun 15kpc from center • Still wrong…didn’t account for dust absorption which makes things look further away

7. Globular Clusters in our Galaxy Shapley realized that the globular clusters map out the true extent of our galaxy! Galactic Halo The hub of the galaxy is the Galactic Center - about 8 kpc from the Sun Actual size of the Galaxy and the Sun’s location not fully determined until 1950’s

8. In 1920, the National Academy of Science hosted the Great Debate concerning the nature of the Spiral Nebulae: were they island universes outside of the Milky Way? • Shapley had MW size too big and therefore argued “NO”, they are part of the Milky Way • Curtis (and many others at that time) believed the Kapteyn model of a much smaller MW and argued “YES”, they are separate galaxies. His notes about a variable star In 1922-1924 Edwin Hubble resolved the controversy using the superior 100-inch telescope at Mount Wilson. He observed Cepheid variables in Andromeda and, using the P-M relation, determined its distance to be 300kpc -- well outside of the MW (still off by a factor of 2 due to poor Cepheid calibrations) Note the date: 6 Oct 1923

9. Also in the early 1900’s, Lindblad was doing the first kinematic studies of the MW • Estimated mass in MW from all stars in Kapteyn’s model • Determined velocities of GCs to be as high as 250 km/s - much higher than escape velocity of Kapteyn model • Lindblad (1927) developed first detailed kinematic model of MW • Spherical component with random motions - HALO • Flattened component with rotational motion - DISK • Measured disk to rotate at 200 to 300 km/s near Sun Another spherical component exists in the center of the galaxy – BULGE Stars here also move on mostly random orbits

10. The three components of our galaxy, disk, halo and bulge, also differ in their mix of stellar populations • Population I: blue stars and open clusters accompanied by gas and dust primarily found in the disks of spiral galaxies like the Milky Way • Population II: red stars and globular clusters in spheroids and halo of Milky Way (also found primarily in elliptical galaxies) • Plotting stars on HR diagrams showed that the populations differed in age and metallicity: • Pop I young and metal rich • Pop II old and metal poor • Disk – mainly Pop I • Halo – mainly Pop II • Bulge – mix of Pop I and II Since most stars are smaller than the sun, the Milky Way actually contains far more than 23 billion stars – more like 200 billion

11. Differential Rotation • Everything in the Galaxy orbits around the Galactic center • Material closer to the center travels on faster orbits (takes less time to make one full orbit) • Similar to the way the planets orbit the Sun • Orbital periods at different distances from the Galactic center can tell us the distribution of mass in the Galaxy

12. M(R) = 0R (r) dV Motion at R depends only on M(R) That mass behaves as if centrally concentrated For an object with mass m at R, gravity must balance acceleration of circular motion GM(R)m/R2 = mv2/R M(R) = v(R)2R/G Measure v(R) and get M(R) Let ω(R) = v(R)/R, then M(R) = ω(R)2R3/G v(R) or ω(R) gives the rotation curve of the Galaxy M v R m

13. To determine galaxy rotation curve, define Galactic Coordinates b = galactic latitude in degrees above/below Galactic disk l = galactic longitude in degrees from GC Differential galactic rotation produces Doppler shifts in emission lines from gas in the Galactic disk

14. The Sun (and most stars) are on slightly perturbed orbits that resemble rosettes. This makes it difficult to measure relative motions of stars around the Sun. Need to establish a reference frame that is a perfect circular orbit about the Galactic Center. Local Standard of Rest - reference frame for measuring velocities in the Galaxy. Position of the Sun if its motion were completely governed by circular motion around the Galaxy. Cylindrical Coordinates for the Galactic plane

15. To determine the Suns motion wrt to LSR, we observe the average motions of all stars in the Sun’s vicinity and measure the following: Π - Πo = U (speed away from GC) = -10.4 km/s [7.5 +/-1 km/s] Z - Zo = W (speed towards NGP) = 7.3 km/s [6.8 (+/- 0.1) km/s] Θ - Θo = V (speed in direction of motion) = V = 14.8 km/s [13.5 (+/- 3) km/s] Parentheses values from Francis and Anderson (2009) The Sun is moving toward the Galactic center, faster than the LSR, and northward toward the NGP. Net motion is 19.5 km/s in the direction of constellation Hercules

16. Position and Velocity of the LSR in Galaxy Ro = 8 kpc Vo = 220 km/s = 225 kpc/Gyr Assuming a circular orbit, how long does it take the Sun to travel around the Galaxy? To = 2πRo/Vo = ___________ Myr How much mass is there interior to the Sun’s orbit? GMm/R2 = mv2/R M = Vo2Ro/G = __________ M helpful numbers: 3.1x1013 km/pc G=6.67x10-11 m3/kg/s2 M = 2x1030 kg

17. Determining the MW Rotation Curve Calculate Doppler shifts for material at P moving with velocity v at a distance d from the Sun. Radial velocity is: vr vr = Θ cos (90 – θ) – Θo cos (90 – l) Since cos(90 - x) = sin(x) To correlate with derivation in the textbook and figure 19.15, note θ + α = 90, v = Θ and vo = Θo vr = Θ sin θ – Θo sin l = Rω(R) sin θ – Ro ωo sin l Convert to l (since we can’t measure θ easily) First Oort equation: can now compute ω in terms of observables vr and l and known values ωo and Ro sin (180 – θ)/Ro =sin θ/Ro = sin l/R Putting this into the above equation gives

18. To map out vr throughout Galaxy, divide the Galaxy into quadrants based on value of galactic longitude. Quad I (l<90) - looking to material closest to GC, [ω - ω0] gets larger and vr increases. At point of closest approach (subcentral or tangent point) vr is at maximum for that line-of-site (los) and then continues to decrease to Sun’s orbit. Beyond Sun’s orbit, vr becomes negative and increases in absolute value. Quad II (180>l>90) - all los pass through orbits outside of the Sun’s. No maximum vr but increases with d. Quad III (270>l>180) - similar to Quad II but opposite signs. Quad IV (l>270) - similar to Quad I except reverse signs.

19. Getting the Rotation Curve of the MW • Determined from 21-cm line observations (radio wavelength) sensitive to atomic Hydrogen gas that permeates the Galaxy • Assume circular orbits and that there is at least some Hydrogen all along any given line-of-sight • Especially important to have measurable gas at subcentral/tangent point

20. Find maximum shift of 21-cm line along given line-of-sight (los) • Assign that Doppler shift to material at the tangent point (closest approach to GC) • Rmin = Ro sin l • ω(Rmin) = [vmax/(Ro sin l)] + ωo • By studying los longitude values from 0 to 90 degrees, Rmin will range from 0 to Ro • Limitations • No gas at subcentral point • Non-circular orbits • At Rmin = 0 and at Ro, difficult to measure curve (small Doppler velocity hard to determine)

21. Since there is no maximum Doppler shift for los away from GC, rotation curve beyond Ro is more difficult to determine • Need to measure the velocity and distance of material independently • Use Molecular Clouds : • get v from radial velocities of CO emission • get R (distance) from spectroscopic parallax of stars forming in clouds What does the MW rotation curve tell us?

22. GM(R<Ro) Radius V = Understanding rotation curves For planets in the Solar System, M is dominated by Msun, so M does not change much with R - Keplarian rotation curve Inside the Galaxy, M increases with radius, so velocity may stay constant or even increase with radius. Outside the Galaxy, as in the Solar System, M remains constant with increasing R (if most mass ends at visible edge). Then we would expect the rotation curve to slope downward in Keplarian-like motion.

23. Since the MW rotation curve shows no drop in velocities beyond the visible edge of the disk (around R=15 kpc), this indicates that some additional non-luminous material must be present  Dark Matter (matter too dim to be detected by current technology) Dark matter - not necessarily confined to the disk, likely to be distributed in the Galactic Halo What mass distribution do you need to get v(r) or Θ to be constant with radius?

24. What is the Dark Matter? Neutrinos: low mass particles that interact via gravity or weak nuclear force. Most neutrinos, produced from nuclear fusion, pass easily through the Sun. Common particles but low mass prevents them from contributing more than ~few percent of dark matter. WIMPs - Weakly Interacting Massive Particles – exotic subatomic particles predicted via supersymmetric extensions of the Standard Model. Predicted rest mass of 100 times that of proton. Being searched for in particle accelerator and large detector experiments. MACHOs - Massive Compact Halo Objects White Dwarf Stars, Red Dwarfs (0.2 Msun), Brown Dwarfs (<0.08 Msun), Neutron Stars, Black Holes. Recall that the more massive remnants result from relatively rare high, mass stellar progenitors.

25. The Search for Dark Matter (brown or white dwarfs): MACHOs • The faint foreground object (brown or white dwarf) bends the light of the background star because of its gravitational field • The light from the background star is focused or “lensed” by this effect and the star appears brighter. • MACHO results acount for only ~20% of dark matter

26. Slight Aside on Determining Distances • We get distances to nearby planets (e.g. radar measurements) • That sets the scale for the solar system (1 AU). • Given 1 AU plus stellar parallax, we find distances to nearby stars. • Use these nearby stars, with known distances, fluxes and luminosities, to calibrate Luminosity classes in HR diagram. • Then spectral class + Flux yields Luminosity + Distance for farther stars (Spectroscopic Parallax). • Cepheids (variable stars) use P-M relation to determined distances to nearby galaxies

27. Luminosity 4d2 Flux = The HR Diagram: Spectroscopic “Parallax” Example: 1) Determine Temperature from color or spectral type. 2) Determine Luminosity based on Main Sequence position. Main Sequence 3) Compare Luminosity with Flux (apparent brightness). 4) Use inverse square law to determine distance.

28. Distribution of Gas • To understand star formation on a galactic scale, we need to know how the interstellar gas is distributed • Radial distribution • Degree of confinement to the disk • Constant thickness HI = neutral hydrogen gas – observed via 21-cm spin-flip transition emission line HII= ionized hydrogen gas – referred to as HII regions and often observed via the hydrogen Balmer emission line Hα at 656 nm H2= molecular hydrogen (also neutral) – usually detected via the presence of other molecules such as CO (carbon monoxide)

29. Atomic gas - mass does not decrease very quickly (mass interior to Ro is 109 Msun and outside Ro is 2x109 Msun) • Molecular gas - falls off rapidly (109 Msun inside Ro and 5x108 Msun outside) • Feature in H2 called “molecular ring” • H2 distribution deduced from CO

30. How is ISM confined to the plane? Components more closely tied to the plane are younger HII regions and molecular clouds formed more recently than HI clouds and globular clusters Thus, molecular clouds may represent a more extreme Pop I than that represented by HI clouds.

31. Spiral Structure • Many external galaxies show spiral structure • Hard to “see” morphology of MW (since we are in it!) • Use other galaxy’s properties to determine the nature of the WM and conclude we are in a spiral galaxy

32. In other galaxies, HII regions and OB associations trace out spiral arms. • • Using spectroscopic parallax, we can place the nearby O and B stars at their proper distances. • • They appear to delineate spiral arms. • • Since O and B stars are young objects, spiral arms are associated with star formation. • Problem: Can’t see very far in the optical…

33. Large scale surveys of CO in the Galaxy identify many Giant Molecular Clouds. If spiral arms are associated with star formation, then they must also be traced out by the locations of GMCs. • GMC positions interior to Sun’s orbit in Galaxy have some distance ambiguity • Bits and pieces of arms • Less distance ambiguity outside of Solar orbit, and better evidence of arm-like morphology

34. The Nucleus of our Galaxy • Inner 500pc of Galaxy • Extinction makes optical studies impossible - use radio or IR • Observe ionized gas, line emission, dust, star clusters • Stellar density is 107 stars per pc3 (compared to 0.1 in the solar neighborhood) • If the Sun were near the GC • Nearest star would be 1000AU away • A million stars brighter than Sirius in the night sky • Total starlight more than 200 times brightness of the full Moon

35. Galactic CenterOptical vs Radio observations

36. Galactic Center: Radio Schematic 75pc 150pc

37. Star Formation in the Galactic Center • Molecular material in inner 200pc relatively hot and dense: 104 per cm3 and 70 K • High velocity dispersion (50 km/s) of molecules • Mass: 108 Msun • High density helps star formation but high temps don’t • SF rate ~ 1Msun/year

38. Radio emission shows bent arc of gas, filamentary structure • Also seen in IR • Thermal and synchrotron radiation • X-ray emission (produced when electrons from filaments collide with colder gas cloud) gives gas temperatures of T=107 to 108 K • Could result from past SN explosions

39. Supermassive Black Hole in the Galactic Center Radio image (10 pc across) shows feature known as SgrA West – center of this is SgrA* Radio image (80 pc across) shows feature SgrA and radio filaments Investigate IR stellar motions in region about 1pc across (~few ly) to estimate BH mass

40. Measure proper motions of stars around Galactic Center • Adaptive optics at Keck improved ground-based resolution to 0.5” in IR (stellar positions measured to 0.002”) • 90 stars identified and proper motions (largest at 1400 km/s!) centered about SgrA* to within 0.1” • Velocities consistent with Keplarian motion (all mass at center) • M = 2.6 +/- 0.2 x 106 Msun

41. Additional evidence - x-ray emission • Chandra X-ray image of Sgr A* showing nucleus and several thousand other X-ray sources. • During 2-week observation period, several X-ray flares occurred. • Rapidity of flares indicates they originate near the Schwarzchild radius of the BH. • Even during the flares, X-ray emission from the nucleus is relatively weak. Suggests that Sgr A* is a starved black hole, possibly because explosive events in the past have cleared much of the gas from around it. Curvature of the paths near SgrA* constrain the volume of the mass to ~ Schwarzchild radius (few x 106 km)  Supermassive Black Hole