A100H Exploring the Universe: Discovering Galaxies. Martin D. Weinberg UMass Astronomy

Transcription

1 A100H Exploring the Universe: Discovering Galaxies Martin D. Weinberg UMass Astronomy April 05, 2016 Read: Chap 19 04/05/16 slide 1

2 Exam #2 Returned by next class meeting Discussion of commonly missed questions Still waiting for grades on make-up exams... sorry about that! Read: Chap 19 04/05/16 slide 2

3 Exam #2 Returned by next class meeting Discussion of commonly missed questions Still waiting for grades on make-up exams... sorry about that! Today: Discovering galaxies Clues to galaxy formation How we discovered the Milky Way Galaxies in the Universe Read: Chap 19 04/05/16 slide 2

4 Exam #2 Returned by next class meeting Discussion of commonly missed questions Still waiting for grades on make-up exams... sorry about that! Today: Discovering galaxies Clues to galaxy formation How we discovered the Milky Way Galaxies in the Universe Your questions? Read: Chap 19 04/05/16 slide 2

5 Observing the gas (review from last class) We observe star-gas-star cycle operating in Milky Ways disk using many different wavelengths of light Read: Chap 19 04/05/16 slide 3

6 Observing the gas (review from last class) We observe star-gas-star cycle operating in Milky Ways disk using many different wavelengths of light Read: Chap 19 04/05/16 slide 3

7 Where will the gas be in 1 trillion years? (A) Blown out of galaxy (B) Still recycling just like now (C) Locked into white dwarfs and low-mass stars Read: Chap 19 04/05/16 slide 4

8 Where will the gas be in 1 trillion years? (A) Blown out of galaxy (B) Still recycling just like now (C) Locked into white dwarfs and low-mass stars Read: Chap 19 04/05/16 slide 4

9 Where do stars form in the Galaxy? Ionization nebulae are found around short-lived high-mass stars, signifying active star formation Read: Chap 19 04/05/16 slide 5

10 Where do stars form in the Galaxy? Ionization nebulae are found around short-lived high-mass stars, signifying active star formation Read: Chap 19 04/05/16 slide 5

11 Where do stars form in the Galaxy? Reflection nebulae scatter the light from stars Why do reflection nebulae look bluer than the nearby stars? Read: Chap 19 04/05/16 slide 6

12 Where do stars form in the Galaxy? Reflection nebulae scatter the light from stars Why do reflection nebulae look bluer than the nearby stars? For the same reason that the sky is blue... Read: Chap 19 04/05/16 slide 6

13 Where do stars form in the Galaxy? How many kinds of nebulae in this image? Read: Chap 19 04/05/16 slide 7

14 Where do stars form in the Galaxy? Halo: No ionization nebulae, no blue stars no star formation Disk: Ionization nebulae, blue stars star formation Read: Chap 19 04/05/16 slide 8

15 Where do stars form in the Galaxy? Halo: No ionization nebulae, no blue stars no star formation Disk: Ionization nebulae, blue stars star formation Read: Chap 19 04/05/16 slide 8

16 Where do stars form in the Galaxy? Halo: No ionization nebulae, no blue stars no star formation Disk: Ionization nebulae, blue stars star formation Read: Chap 19 04/05/16 slide 8

17 Where do stars form in the Galaxy? Most of star formation in disk happens in spiral arms Read: Chap 19 04/05/16 slide 9

18 Where do stars form in the Galaxy? Most of star formation in disk happens in spiral arms Ionization nebulae Blue stars Gas clouds and dust Read: Chap 19 04/05/16 slide 9

19 Where do stars form in the Galaxy? are waves of star formation! Read: Chap 19 04/05/16 slide 9

20 Wave pattern occur in the disk of the Milky Way and other galaxies Earliest thought: material waves Read: Chap 19 04/05/16 slide 10

21 Wave pattern occur in the disk of the Milky Way and other galaxies Earliest thought: material waves Read: Chap 19 04/05/16 slide 10

22 Density waves Not material waves: density waves Example: waves in traffic flow Cars bunch together and spread out as a density wave passes through traffic Slow vehicle, line painting Read: Chap 19 04/05/16 slide 11

23 Solution to the winding problem The orbits of stars are not quite circles but ellipses Where orbits bunch, gravity is enhanced Extra gravity correlates the orbits and prevents the wave from wrapping quickly Read: Chap 19 04/05/16 slide 12

24 Young stars in the arms are waves of star formation 1. Gas clouds get squeezed as they move into spiral arms 2. Squeezing of clouds triggers star formation 3. Young stars flow out of spiral arms Read: Chap 19 04/05/16 slide 13

25 Young stars in the arms The spiral density waves cause a density enhancement which triggers star formation In a galaxy like the Milky Way there will be a progression of star formation across the spiral arm. Gas falls into arm, compresses Forms molecular clouds Forms stars Asssociations and clusters trail arm Read: Chap 19 04/05/16 slide 14

26 Clues to Galaxy formation Halo Stars: % heavy elements (O, Fe,... ), only old stars Disk Stars: 2% heavy elements, stars of all ages Read: Chap 19 04/05/16 slide 15

27 Clues to Galaxy formation Halo Stars: % heavy elements (O, Fe,... ), only old stars Disk Stars: 2% heavy elements, stars of all ages Read: Chap 19 04/05/16 slide 15

28 Clues to Galaxy formation Halo Stars: % heavy elements (O, Fe,... ), only old stars Disk Stars: 2% heavy elements, stars of all ages Read: Chap 19 04/05/16 slide 15

29 Stellar Populations Astronomers (since mid 20th century) divide stars into two Populations: Population I: Disk and Open Cluster stars Population II: Spheroid and Globular Cluster stars Distinguished by: Location, Age, and Chemical Composition Read: Chap 19 04/05/16 slide 16

30 Population I Stellar Populations Location: Disk and Open Clusters Age: Mix of young and old stars Composition: Metal rich (roughly solar composition) Environment: Often gas rich, especially for the young stars 70% Hydrogen 28% Helium 2% metals Read: Chap 19 04/05/16 slide 17

31 Population II Stellar Populations Location: Spheroid and Globular Clusters Ages: Oldest stars, > 10 Gyr Composition: Metal Poor (0.1-1% solar) Environment: gas poor, no star formation 75% Hydrogen 24.99% Helium 0.01% metals Read: Chap 19 04/05/16 slide 18

32 of Galaxy Formation Read: Chap 19 04/05/16 slide 19

33 of Galaxy Formation Our galaxy probably formed from a giant gas cloud Read: Chap 19 04/05/16 slide 20

34 of Galaxy Formation Halo stars formed first as gravity caused cloud to contract Read: Chap 19 04/05/16 slide 21

35 of Galaxy Formation Remaining gas settled into spinning disk Read: Chap 19 04/05/16 slide 22

36 of Galaxy Formation Stars continuously form in disk as galaxy grows older Read: Chap 19 04/05/16 slide 23

37 of Galaxy Formation Stars continuously form in disk as galaxy grows older VERY oversimplified Read: Chap 19 04/05/16 slide 23

38 of Galaxy Formation Detailed studies: Halo stars formed in clumps that later merged Read: Chap 19 04/05/16 slide 24

39 Galaxy Formation (summary) What clues to our galaxys history do halo stars hold? Halo stars are all old, with a smaller proportion of heavy elements than disk stars, indicating that the halo formed first How did our galaxy form? Our galaxy formed from a huge cloud of gas, with the halo stars forming first and the disk stars forming later, after the gas settled into a spinning disk Read: Chap 19 04/05/16 slide 25

40 of the Milky Way Spectral types to estimate luminosity L Herschel ( 1800) and Kapteyn ( 1920) counted stars to infer the shape of the Galaxy Brightness-distance relation, b = L/(4πR 2 ), to map the locations of the stars His answer: we are near the center of an elliptical distribution of stars! Read: Chap 19 04/05/16 slide 26

41 of the Milky Way Spectral types to estimate luminosity L Herschel ( 1800) and Kapteyn ( 1920) counted stars to infer the shape of the Galaxy Brightness-distance relation, b = L/(4πR 2 ), to map the locations of the stars His answer: we are near the center of an elliptical distribution of stars! What happened? Read: Chap 19 04/05/16 slide 26

42 of the Milky Way Spectral types to estimate luminosity L Herschel ( 1800) and Kapteyn ( 1920) counted stars to infer the shape of the Galaxy Brightness-distance relation, b = L/(4πR 2 ), to map the locations of the stars His answer: we are near the center of an elliptical distribution of stars! What happened? Interstellar extinction! Read: Chap 19 04/05/16 slide 26

43 How did astronomers figure this out? 1920s: Harlow Shapley measured distance to globular clusters using RR Lyrae variables Deduced the correct size and shape of the Milky Way Read: Chap 19 04/05/16 slide 27

44 How did astronomers figure this out? 1920s: Harlow Shapley measured distance to globular clusters using RR Lyrae variables Deduced the correct size and shape of the Milky Way Read: Chap 19 04/05/16 slide 27

45 How did astronomers figure this out? 1920s: Harlow Shapley measured distance to globular clusters using RR Lyrae variables Deduced the correct size and shape of the Milky Way Read: Chap 19 04/05/16 slide 27

46 How did astronomers figure this out? 1920s: Harlow Shapley measured distance to globular clusters using RR Lyrae variables Deduced the correct size and shape of the Milky Way Read: Chap 19 04/05/16 slide 27

47 General problem with studying Milky Way Like being inside a forest, and seeing only the trees Because our view is obscured, it looks as if there are about as many stars in all directions Need to use radio waves or other forms of radiation to see through interstellar matter What we now know: The Milky Way is a spiral galaxy containing about 100 billion stars Read: Chap 19 04/05/16 slide 28

48 Radio emission from neutral hydrogen The n = 1 level (ground state) of H is split into 2 levels separated by a very small energy This splitting is due intrinsic spin of electron and proton Behave like small magnets When the North poles are aligned, the energy is higher and vice versa Read: Chap 19 04/05/16 slide 29

49 Radio emission from neutral hydrogen The n = 1 level (ground state) of H is split into 2 levels separated by a very small energy This splitting is due intrinsic spin of electron and proton Behave like small magnets When the North poles are aligned, the energy is higher and vice versa Read: Chap 19 04/05/16 slide 29

50 Radio emission from neutral hydrogen HI emission is rare (per atom) but hydrogen is plentiful in the Galaxy Read: Chap 19 04/05/16 slide 30

51 Rotation of the Disk Measure using the Doppler Effect Read: Chap 19 04/05/16 slide 31

52 Rotation of the Disk Measure using the Doppler Effect Stars: Doppler shifts of stellar absorption lines Ionized Gas: emission lines from HII regions Atomic Hydrogen (HI) Gas: Cold H clouds emit a radio emission line at a wavelength of 21-cm Can trace nearly the entire disk beyond where the stars have begun to thin out Read: Chap 19 04/05/16 slide 31

53 the Milky Way Distribution of gas in velocity along the line of sight distance map Read: Chap 19 04/05/16 slide 32

54 the Milky Way NGC 1232 (similar to the Milky Way, face on... ) Read: Chap 19 04/05/16 slide 33

55 the Milky Way NGC 4565 (similar to the Milky Way, edge on... ) Read: Chap 19 04/05/16 slide 34

56 Artist s view of the Milky Way from above Milky Way: four-armed barred spiral Read: Chap 19 04/05/16 slide 35

57 Read: Chap 19 04/05/16 slide 36

58 Hubble Deep Field Our deepest images of the universe show a great variety of galaxies, some of them billions of light-years away Read: Chap 19 04/05/16 slide 37

59 Galaxies and Cosmology A galaxys age, its distance, and the age of the universe are all closely related The study of galaxies is thus intimately connected with cosmology the study of the structure and evolution of the universe Read: Chap 19 04/05/16 slide 38

60 Three Types of Galaxies Hubble Ultra Deep Field Read: Chap 19 04/05/16 slide 39

61 Three Types of Galaxies Hubble Ultra Deep Field Read: Chap 19 04/05/16 slide 39

62 Three Types of Galaxies Hubble Ultra Deep Field Read: Chap 19 04/05/16 slide 39

63 Three Types of Galaxies Hubble Ultra Deep Field Read: Chap 19 04/05/16 slide 39

64 Three Types of Galaxies Hubble Ultra Deep Field Read: Chap 19 04/05/16 slide 39

65 Three Types of Galaxies Read: Chap 19 04/05/16 slide 40

66 Three Types of Galaxies Disk Component: stars of all ages, many gas clouds Spheroidal Component: bulge & halo, old stars, few gas clouds Read: Chap 19 04/05/16 slide 41

67 Three Types of Galaxies Disk Component: stars of all ages, many gas clouds Spheroidal Component: bulge & halo, old stars, few gas clouds Read: Chap 19 04/05/16 slide 42

68 Three Types of Galaxies Disk Component: stars of all ages, many gas clouds Spheroidal Component: bulge & halo, old stars, few gas clouds Read: Chap 19 04/05/16 slide 42

69 Three Types of Galaxies Disk Component: stars of all ages, many gas clouds Spheroidal Component: bulge & halo, old stars, few gas clouds Read: Chap 19 04/05/16 slide 42

70 Why does ongoing star formation lead to a blue-white appearance? (A) There aren t any red or yellow stars (B) Short-lived blue stars outshine others (C) Gas in the disk scatters blue light Read: Chap 19 04/05/16 slide 43

71 Why does ongoing star formation lead to a blue-white appearance? (A) There aren t any red or yellow stars (B) Short-lived blue stars outshine others (C) Gas in the disk scatters blue light Read: Chap 19 04/05/16 slide 43

72 Other Types Galaxies Barred Spiral Galaxy: Has a bar of stars across the bulge Read: Chap 19 04/05/16 slide 44

73 Other Types of Galaxies Lenticular Galaxy: Has a disk like a spiral galaxy but much less dusty gas (intermediate between spiral and elliptical) Read: Chap 19 04/05/16 slide 45

74 Elliptical Galaxies Elliptical Galaxy: All spheroidal component, virtually no disk component Read: Chap 19 04/05/16 slide 46

75 Elliptical Galaxies Elliptical Galaxy: All spheroidal component, virtually no disk component Red-yellow color indicates older star population Read: Chap 19 04/05/16 slide 46

76 Irregular Galaxies Large Magellanic Cloud Read: Chap 19 04/05/16 slide 47

77 Irregular Galaxies Large Magellanic Cloud Blue-white color indicates ongoing star formation Read: Chap 19 04/05/16 slide 48

78 Hubble s Galaxy Types Spheroid dominates Disk Dominates Read: Chap 19 04/05/16 slide 49