Galaxies & Quasars

	Lecture 12 Ay-1

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

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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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)

Irregulars

	Irr I – misshapen spirals
	Irr II – filamentary, “explosive”

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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

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

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

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

Nearby structure – the Local Group

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

Hubble Law

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- 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

large scale structure in the universe

Active galaxies – Interacting/Colliding?

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Mergers

	tidal forces deform; stars hurled into space
	merging takes ~500 million yrs
	Milky Way & Andromeda?

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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

Gravitational Lensing ® Dark Matter

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