Astro 101 Fall 2013 Lecture 10 Galaxies (part I) T. Howard

1. Astro 101 Fall 2013 Lecture 10 Galaxies (part I) T. Howard

2. The Galaxy – part I -- Overview

3. New distance unit: the parsec (pc). Using Earth-orbit parallax, if a star has a parallactic angle of 1", it is 1 pc away. If the angle is 0.5", the distance is 2 pc. 1 Parallactic angle (arcsec) Distance (pc) = Closest star to Sun is ProximaCentauri. Parallactic angle is 0.7”, so distance is 1.3 pc. 1 pc = 3.3 light years = 3.1 x 1018cm = 206,000 AU 1 kiloparsec (kpc) = 1000 pc 1 Megaparsec (Mpc) = 10 6 pc

4. The Milky Way Galaxy This is NOT the Milky Way galaxy! It’s a similar one: NGC 4414.

6. All-sky projection 1.2, 1.6, 2.2 microns Atlas Image [or Atlas Image mosaic] courtesy of 2MASS/UMass/IPAC-Caltech/NASA/NSF.

7. The Milky Way Galaxy

8. Core of Galaxy (NASA – Spitzer) (false color)

9. How was Milky Way size and our location determined? Herschel (late 18th century): first map of Milky Way. Sun near center. Herschel’s Milky Way drawing . Sun 3 kpc

10. Shapley (1917) found that Sun was not at center of Milky Way Shapley used distances to Globular Clusters to determine that Sun was 16 kpc from center of Milky Way. Modern value 8 kpc.

11. Take a Giant Step Outside the Milky Way Artist's Conception Example (not to scale)

12. Artist's Conception from above ("face-on") see disk, with spiral and bar structure, and bulge (halo too dim) . Sun from the side ("edge-on")

13. The Three Main Structural Components of the Milky Way 1. Disk - 30 kpcdiameter - contains young and old stars, gas, dust. Has spiral structure - vertical thickness roughly 100 pc - 2 kpc (depending on component. Most gas and dust in thinner layer, most stars in thicker layer)

14. 2. Halo - at least 30 kpc across - contains globular clusters, old stars, little gas and dust, much "dark matter" - roughly spherical 3. Bulge - About 4 kpc across - old stars, some gas, dust - central black hole of 3 x 106 solar masses - spherical

16. young stars Gas, dust (ISM) Type: SBc ~ 1011 stars (mass ~ 1012Msun) Age ~ 13.2 GYr 8.3 kpc

17. Morphology of the Milky Way Disk: - 30 kpcdiameter - contains young and old stars, gas, dust. Has spiral structure - vertical thickness roughly 100 pc - 2 kpc (depending on component. Most gas and dust in thinner layer, most stars in thicker layer) Halo: - at least 30 kpc across - globular clusters, old stars, little gas and dust, "dark matter” - roughly spherical Bulge: - About 4 kpc across - old stars, some gas, dust - central black hole of 3 x 106 solar masses - spherical

18. Galaxies (believed to be) structurally similar M31 NGC 891 NGC 891 Close up [STSCI, HST]

19. Spiral Structure of Disk Spiral arms best traced by: Young stars and clusters Emission Nebulae Atomic gas Molecular Clouds (old stars to a lesser extent) Disk not empty between arms, just less material there.

20. from above ("face-on") see disk and bulge (halo too dim) Perseus arm Orion spur Sun Cygnus arm Sagittarius arm Carina arm from the side ("edge-on")

22. Stellar Orbits Halo: stars and globular clusters swarm around center of Milky Way. Very elliptical orbits with random orientations. They also cross the disk. Bulge: similar to halo. Disk: rotates.

23. Globular Clusters - few x 10 5 or 10 6 stars - size about 50 pc - very tightly packed, roughly spherical shape - billions of years old Clusters are crucial for stellar evolution studies because: 1) All stars in a cluster formed at about same time (so all have same age) 2) All stars are at about the same distance 3) All stars have same chemical composition

24. How Does the Disk Rotate? Sun moves at 225 km/sec around center. An orbit takes 240 million years. Stars closer to center take less time to orbit. Stars further from center take longer. => rotation not rigid like a phonograph record. Rather, "differential rotation". Over most of disk, rotation velocity is roughly constant. The "rotation curve" of the Milky Way

26. So if spiral arms always contain same stars, the spiral should end up like this: Real structure of Milky Way (and other spiral galaxies) is more loosely wrapped.

27. Problem: How do spiral arms survive? Given differential rotation, arms should be stretched and smeared out after a few revolutions (Sun has made 20 already): The Winding Dilemma

28. Proposed solution: Arms are not material moving together, but mark peak of a compressional wave circling the disk: A Spiral Density Wave Traffic-jam analogy:

29. Now replace cars by stars. The traffic jams are due to the stars' collective gravity. The higher gravity of the jams keeps stars in them for longer. Calculations and computer simulations show this situation can be maintained for a long time. Traffic jam on a loop caused by merging Not shown – whole pattern rotates

30. Gas clouds pushed together in arms too => high density of clouds => high concentration of dust => dust lanes. Also, squeezing of molecular clouds initiates collapse within them => star formation. Bright young massive stars live and die in spiral arms. Emission nebulae mostly in spiral arms. So arms always contain same types of objects. Individualobjects come and go.

31. observed curve Milky Way Rotation Curve Curve if Milky Way ended where radiating matter pretty much runs out. 90% of Matter in Milky Way is Dark Matter Gives off no detectable radiation. Evidence is from rotation curve: 10 Solar System Rotation Curve: when almost all mass at center, velocity decreases with radius ("Keplerian") Rotation Velocity (AU/yr) 5 1 30 1 10 20 R (AU)

32. Rotation curves of many normal galaxies are similar -- allows galaxy mass to be estimated

33. Not enough radiating matter at large R to explain rotation curve => "dark" matter! Dark matter must be about 90% of the mass! Composition unknown. Probably mostly exotic particles that hardly interact with ordinary matter at all (except gravity). Small fraction may be brown dwarfs, dead white dwarfs. Most likely it's a dark halo surrounding the Milky Way. Mass of Milky Way 6 x 1011 solar masses within 40 kpc of center.

34. Galactic Rotation curve – velocity vs. galactocentric radius

35. Rotation curve of M31 The M31 major axis mean optical radial velocities and the rotation curve, r <120 arcmin, superposed on the M31 image from the Palomar Sky Survey. Velocities from radio observations are indicated by triangles, 90< r <150 arcmin. Rotation velocities remain flat well beyond the optical galaxy, implying that the M31 cumulative mass rises linearly with radius. (Image by Rubin and Janice Dunlap.) Source: Physics Today, Dec. 2006

36. Several alternative explanations • Matter is “missing” (i.e., can’t be seen – nonluminous) • Probably view most prevalent is that of Cold Dark Matter (CDM) • What can’t it be? • Dust – no. Why?? • Gas – no. Why?? • What can it be? • Normal (i.e., baryonic) matter • MACHOs – MAssive Compact Halo Objects • White dwarfs, Red/Brown dwarfs • Neutron stars, BHs • Non-baryonic matter • WIMPs – Weakly Interacting Massive Particles • Weak interactions w/baryonic matter, no significant interactions w/light

37. Ruling Explanations In or Out • White & red dwarfs – HST can’t find enough of them • BHs – large – would probably leave signatures, disruption of Globulars in the halo  not seen • BHs – small – can’t be excluded, but how did they get there? • Compact gas clouds in halo  why not seen? Why don’t they form stars? • Cosmology theory predicts baryonic matter should be ~ 4% of the critical density of universe; luminous matter seems to be only about 0.8% • But when hot matter (i.e., intracluster gas) added  probably close • Although, error bars on this are large • Note, “CDM” vs. “HDM” (cold vs. hot) means: • Non-relativistic vs. relativistic

38. More on Dark Matter • Dark matter thought to be necessary in cosmology theory to explain clumpy distributions of matter w/ enough mass  galaxies form • Could have been either CDM or HDM • Usual predictions of CDM candidates require modifications or extensions to Standard Model of particles • Non-baryonic candidates • Neutrinos (neutrino oscillation  implies neutrino mass) • WIMPs – hypothetical • Possibly “heavy” neutrinos or something equally exotic • Axions

39. MOND – MOdified Newtonian Dynamics • Introduced c. 1983, M. Milgrom (Weizmann Inst., Israel) • ad hoc phenomenological fit to explain two things: • Galactic “flat” rotation curves, and • Tully-Fisher relation for spirals: M = (const.)*Va, a ~ 4 • Modifies Newtonian force for very small accelerations • Successfully predicts several astrophysical observations • Doesn’t require “missing mass” •  but some attempts made to derive it as an effect of CDM •  not completely successful • Not tied to any physical theory, i.e., a “fit to observations” • Some attempts to modify Newtonian gravity, inertia, or GR • Not complete • Not entirely popular but not ruled out; has a few vocal adherents

40. Massive Black Holes -- leading theory to explain the emission and properties of many active galaxies (AGNs) -- Central BH may be Billions of solar masses -- Accretion disk made of large clouds of gas And dust -- X-rays and gamma rays from BH are converted to longer wavelengths by clouds

42. Milky Way -- Evidence for Central BH • Dynamical – orbits of stars near • Galactic center, esp. S2, indicate • Central mass of ~ 4 million Msun • Only a BH dense enough to have • this much mass in such a small • volume of space • Corresponds to location of • radio source Sgr A • Smaller compact source Sgr A* • at center, diameter 0.3 AU • Observations also in x-rays • and gamma rays

43. Milky Way – Central Black Hole Central area (enlarged)  Center of Galaxy and Constellation Sagittarius (Sgr)

44. Other views of the Galactic Center Near-infrared X-rays (Chandra) infrared

45. “flare” as BH accretes gas(period ~ 48 hr) Hot gas (10 million K) BH area

46. Galaxies

47. Early drawings of nebulae by Herschel (1811). Stars, gas or both? Distances? First “spiral nebula” found in 1845 by the Earl of Rosse. Speculated it was beyond our Galaxy. 1920 - "Great Debate" between Shapley and Curtis on whether spiral nebulae were galaxies beyond our own. Settled in 1924 when Hubble observed individual stars in spiral nebulae.

48. The Variety of Galaxy Morphologies

49. More on bars… Milky Way schematic showing bar Another barred galaxy A bar is a pattern too, like a spiral.

50. bulge less prominent, arms more loosely wrapped Irr increasing apparent flatness disk and large bulge, but no spiral Galaxy Classification Hubble’s 1924 "tuning fork diagram" Spirals Ellipticals Irregulars barred unbarred E0 - E7 Irr I Irr II SBa-SBc Sa-Sc "misshapen truly spirals" irregular