1	Galaxies & Quasars Lecture 12 Ay-1
2	Normal Galaxies - Hubble Classification 4 Types: Spirals flattened galactic disk with dust, gas, stars (spiral arms) + central bulge with dense nucleus + extended halo of faint old stars Barred Spirals elongated bar of stars, dust, gas crosses center; spiral arms “originate” from bar ends not bulge Ellipticals older spheroidal star system no spiral arms, no young stars, little gas, dust Irregulars irregular shape, lots of gas, young blue stars Elliptical galaxies are most massive – often seen at center of dense galaxy clusters
3	Spirals – classified by size of central bulge tightness of spiral pattern correlated with bulge size arms more knotty Sa ® Sc i/s gas (21 cm (H) & molecular line radiation) increases Sa ® Sc A & G stars in disk ® whitish color new O & B stars b(+ nebulae) in arms ® bluish color Barred spirals – SBa ® SBc (massive dark matter halo ® no bar?)
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5	Ellipticals – classified by shape E0 = most circular ® E7 = most elongated orientation could affect shape! giant ellipticals –few x mega parsec diameter, > 10¹² stars dwarf ellipticals – 1 kpc diameter, £ 10⁶ stars (our Galaxy d ~ 50 kpc) little i/s material, only old red, low mass stars (like halo)
6	Irregulars Irr I – misshapen spirals Irr II – filamentary, “explosive”
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9	Some Properties of Normal Galaxies S & SB Elliptical Irregular galaxies galaxies galaxies Mass (M_¤) 10⁹ – 4x10¹¹ 10⁵- 10¹³ 10⁸- 3x 10¹⁰ Luminosity (L_¤) 10⁸- 2x10¹⁰ 3x 10⁵ – 10¹¹ 10⁷ - 10⁹ Diameter (kpc) 5-250 1-200 1-10 % of observed 77% 20% 3% galaxies Hubble could be sequence in rotation properties ellipticals display little internal rotation – no disk Sa, SBa – sufficient rotation to form disk but bulge dominates
10	How far away are galaxies? 1755 Kant ® island universes 1845 Earl of Rosse®spiral M51 BUT Herschel ® planetary nebula in Draco 1920 Shapley-Curtis debate 1924 Hubble ® Cepheid in Andromeda galaxy Andromeda: distance = 750 kpc, diameter = 70 kpc
11	Measuring the distance to galaxies For pulsating variable stars (CEPHEIDS), observations of period ® luminosity Apparent brightness µ luminosity/(distance)² \ Distance measure - works for 1 kpc to 30 Mpc
12	Measuring the distance to galaxies Cepheids too faint to be standard candles in distant galaxies But all Type Ia supernovae reach same maximum brightness Cepheid + supernova in IC 4182 ® supernova as standard candle Supernovae enable distance measures from 1 Mpc to 1 Gpc
13	Nearby structure – the Local Group
14	Large-scale expansion of universe : Slipher (Lowell) & Curtis (Lick) noted redshifts of “spiral nebulae” Hubble, Humason (Mt Wilson) correlated distance of galaxies with red Doppler shift – the more distant the galaxy the greater the redshift ® the more distant a galaxy, the more rapidly it is moving away from us linear correlation between distance and recession speed if 2 x more distant, recedes twice as fast ¯ Hubble Flow
15	Hubble Law
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18	- and even larger scale structure group of galaxies held together by gravitational attraction = Galaxy Cluster Virgo cluster @18 Mpc, 2500 galaxies, 3Mpc wide Coma cluster @90 Mpc, 10,000? galaxies(80% elliptical) Virgo and Hercules clusters irregular (more mixed) CLUSTERS of CLUSTERS ® Superclusters Local Supercluster 40-50 Mpc wide, 10¹⁵ M_¤ Milky Way 20 Mpc from center of Local Supercluster
19	large scale structure in the universe
20	Active galaxies – Interacting/Colliding?
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22	Mergers tidal forces deform; stars hurled into space merging takes ~500 million yrs Milky Way & Andromeda?
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24	Dark matter large velocities for cluster galaxies ® mass of visible matter insufficient (x10) to bind clusters gravitationally Milky Way rotation curve ® mass > visible mass true for other galaxies Dark Matter dominates mass in outer regions
25	Gravitational Lensing ® Dark Matter
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