1924: Hubble classification scheme 1925: Hubble measures Cepheids (Period-Luminosity) in Andromeda case closed

Transcription

1 Galaxies

2 Curtis-Shapley debate on nature of spiral nebulae - distribution in the sky: zone of avoidance Curtis: extinction Shapley:? - apparent brightness of stars(?) observed in some nebulae Shapley: if novae, then nebulae too far and bright Curtis: Supernovae? - colors & spectra: not like MW? Shapley: spiral nebulae redder than MW Curtis:? - early measurements of rotation of nebulae: Shapley: too fast (>c) if nebulae as large as MW Curtis: flawed data 1924: Hubble classification scheme 1925: Hubble measures Cepheids (Period-Luminosity) in Andromeda case closed

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6 Galaxy Morphology Classification Scheme (Hubble 1924) Taxonomy: taxis = order,classification, nomos =law

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9 Galaxy Morphology Classification Scheme (Hubble)

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11 Galaxy Morphology Classification Scheme (Hubble)

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13 Galaxy Morphology Classification Scheme (Hubble) Taxonomy: taxis = order,classification, nomos =law

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16 Galaxy Morphology Classification Scheme (Hubble) Taxonomy: taxis = order,classification, nomos =law

17 Components of the Milky Way, by Mass: * Dark Matter Halo ~1.5-2x10 12 solar masses Stars ~2x10 11 solar masses Interstellar Gas ~6x10 10 solar masses Interstellar Dust ~5x10 7 solar masses Number of HI atoms in the ISM of the MW: (1/2)xMass of IS gas (solar units) x Mass of the Sun / mass of HI atom = (1/2)x6x10 10 solar x 2x10 33 gram /1.67x10-24 gram 4x10 67 HI atoms

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19 Curtis-Shapley debate on nature of spiral nebulae - distribution in the sky: zone of avoidance Curtis: extinction Shapley:? - apparent brightness of stars(?) observed in some nebulae Shapley: if novae, then nebulae too far and bright Curtis: Supernovae? - colors & spectra: not like MW? Shapley: spiral nebulae redder than MW Curtis:? - early measurements of rotation of nebulae: Shapley: too fast (>c) if nebulae as large as MW Curtis: flawed data 1924: Hubble classification scheme 1925: Hubble measures Cepheids in Andromeda case closed

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23 Galaxy Exotica

24 Galaxy encounters alter Morphology

25 The Antennae

26 The Mice NGC

27 The Cartwheel

28 Explanation: By chance, a collision of two galaxies has created a surprisingly recognizable shape on a cosmic scale - "The Cartwheel Galaxy". The Cartwheel is part of a group of galaxies about 500 million light years away in the constellation Sculptor (two smaller galaxies in the group are visible on the right). Its rim is an immense ring-like structure 100,000 light years in diamete r composed of newly formed, extremely bright, massive stars. When galaxies collide they pass through each other, their individual stars rarely coming into contact. However, the galaxies' gravitational fields are seriously distorted by the collision. In fact, the ring-like shape is the result of the gravitational disruption caused by a small intruder galaxy passing through a large one, compressing the interstellar gas and dust and causing a a star formation wave to move out from the impact point like a ripple across the surface of a pond. In this case the large galaxy may have originally been a spiral, not unlike our own Milky Way, transformed into the wheel shape by the collision. But... what happened to the small intruder galaxy? Text after NASA/apod

29 How do astronomers know that the "rim is an immense ring-like structure 100,000 light years in diameter composed of newly formed, extremely bright, massive stars"? What's the evidence?

30 Credit: J. Higdon

31 Toomre & Toomre 1972 Restricted 3-body problem

32 Galaxy Morphology Classification Scheme (Hubble) Taxonomy: taxis = order,classification, nomos =law

33 Morphological Classification

34 Integrated Galaxy Spectra MgI MgI Hα Spirals have Interstellar Gas Hα Ellipticals show absorption line spectra characteristic of older stellar population; spirals show emission lines, characteristic of star-formation regions.

35 Spiral and Irregular galaxies 21cm line emission of atomic Hydrogen they are rich in cold interstellar gas. Spirals also show strong emission of cold molecular gas. Their colors are bluer than those of Elliptical and S0 galaxies Spirals and Irregulars are actively forming stars Spirals have Interstellar Gas Ellipticals show absorption line spectra characteristic of older stellar population; spirals show emission lines, characteristic of star-formation regions.

36 Morphology-Density Relation Spirals/Irr The fraction of the population that is spiral decreases from the field to high density regions. S0 Low ρ Ellipticals High ρ [Dressler 1980]

37 Elliptical vs Spiral Galaxy Taxonomy Appearance and Color Kinematics Distribution in space Ellipticals Smooth falloff of light Not forming stars now Dominant motion: random orbits Prefer cluster cores Spirals Bulge+disk+arms Actively forming stars Dominant motion: circular orbits in disk Avoid cluster cores

38 See John Hibbard s Gallery of Rogues at astrores/ HIrogues

39 VLA maps

40 NGC 3628 Leo Triplet NGC 3627 NGC 3623

41 M96 Ring VLA map Arecibo map Schneider, Salpeter & Terzian 19

42 Optical galaxy HI

43 Go to My Documents/animations

44 Elliptical vs Spiral Galaxy Taxonomy Appearance and Color Kinematics Distribution in space Ellipticals Smooth falloff of light Not forming stars now Dominant motion: random orbits Prefer cluster cores Spirals Bulge+disk+arms Actively forming stars Dominant motion: circular orbits in disk Avoid cluster cores

45 [Van Albada, Bahcall, Begeman & Sancisi 1985]

46 The Local Group of Galaxies Includes ~35 galaxies MW & M31 are dominant Mostly dwarfs which cluster around giants

47 Substructure in the Local Group Giant spirals dsph (+dell) dirr dirr/dsph

48 M31, The Andromeda Galaxy Are the two apparently-smaller galaxies further away, or are they dwarf galaxies at the same distance?

49 Dwarf ellipticals: des The few des in the Local Group are all found around M31 M31 - Andromeda M32 N205

50 Dwarf irregulars: dis IC1613 barred Pegasus IC10 Phoenix NGC6822 SextansA NGC3109 SextansB

51 Dwarf irregulars: dis SextansA Typically HI rich (M HI ~ M ) Optical SF site(s) embedded in HI envelope Low metallicity (increases with increasing L); hence not much processing of material in stars Solid body rotation curves common; mostly dark matter! di s are found throughout the LG; some isolated Do stripping/tidal events turn dis into dsphs by removing their gas and causing star formation to cease?

52 The Magellanic Clouds LMC SMC The Magellanic Clouds are contained within a common HI envelope. The Magellanic Stream traces their interaction with the MW.

53 High Velocity Clouds?

54 The Magellanic Stream

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56 Dwarf spheroidals: dsphs Leo I Leo II Fornax Half of dwarfs in the Local Group are dsphs Draco Ursa Minor dsphs are the most common type of galaxy Faintest ones emit less optical light than globular clusters Some retain their gas

57 And How Are Galaxies Distributed in Space?

58 So you want to measure a velocity width? 1. Get good image of galaxy, measure PA, position slit 2. Pick spectral line, measure peak λ along slit 3. Center kinematically 4. Fold about kinematical center 5. Correct for disk inclination, using isophotal ellipticity Inner scale length Outer slope 6. You now have a rotation curve. Pick a parametric model and fit it. E.g.

59 Components of the Milky Way, by Mass: * Dark Matter Halo ~1.5-2x10 12 solar masses Stars ~2x10 11 Interstellar Gas ~6x10 10 solar masses solar masses Interstellar Dust ~5x10 7 solar masses Number of HI atoms in the ISM of the MW: (1/2)xMass of IS gas (solar units) x Mass of the Sun / mass of HI atom = (1/2)x6x10 10 solar x 2x10 33 gram /1.67x10-24 gram 4x10 67 HI atoms