NORMAL GALAXIES

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NORMAL GALAXIES

• COLLECTIONS OF STARS, GAS (and DARK MATTER)
  HUGE VARIETY OF TYPES

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What are the three major types of galaxies?
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Hubble Ultra Deep Field
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Hubble Ultra Deep Field
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Hubble Ultra Deep Field
Spiral Galaxy
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Hubble Ultra Deep Field
Spiral Galaxy
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Hubble Ultra Deep Field
Elliptical Galaxy
Elliptical Galaxy
Spiral Galaxy
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Hubble Ultra Deep Field
Elliptical Galaxy
Elliptical Galaxy
Spiral Galaxy
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Hubble Ultra Deep Field
Elliptical Galaxy
Elliptical Galaxy
Irregular Galaxies
Spiral Galaxy
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Basic Galactic Facts

• SIZES 1 kpc -- 100 kpc across 109 M?
  -- 1013 M? dwarf galaxies down to 107 M?
• BASIC STRUCTURES SPIRAL -- Milky Way most
  overall
• ELLIPTICAL -- most dwarfs and giants
• IRREGULARS -- often satellites'
• LOCATIONS Isolated (Field) Groups
  (Local group includes 50) Clusters (100s to
  1000s of galaxies) Superclusters (clusters of
  clusters!)
• Voids (galaxies not there)

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ELLIPTICAL GALAXIES

• Ellipticity E0 -- E7 (round to flatest)
  Projected on the sky
• (1 - b/a) x 10
• Can be more elliptical than they are seen to be
  (Projection Effect)
• Often really tri-axial
• Some (e.g. Centaurus A) include dust disk -- big
  elliptical swallowed a spiral!

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Elliptical Galaxy Shapes

• M49 is close to a circle E2
• M84 is an average E3
• M110 is a dwarf E (Andromeda satellite) E5

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SPIRAL GALAXIES

• Classify by Hubble Type S0 Sa--Sc SBa--SBc
  (tuning fork diagram)
• S0 disk seen, but no spiral arms
• Sa prominent nucleus, tightly wound arms
• Sb significant nucleus, moderate arms
• Sc small nucleus, patchy loose arms
• SBa, SBb, SBc central bar from which arms emerge
• Milky Way is a SBb (weak bar though, and between
  SBb and SBc)

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Regular Spiral Types
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M101-pinwheel galaxy 51 HST images
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Barred Spiral Types
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S0 (lenticular) Galaxies
S0 and SB0 have disk and bulge but no visible
spiral arms
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Irregular Galaxies Magellanic Clouds
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Irregular Galaxies Interactions and
Starbursts
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Key Info on Galaxy Types

• Colors E's red, S's various, Irr's, blue
• Populations E Pop II Irr Pop I
• S disk, Pops III halo, Pop II
• Sizes E's 1-100 kpc, S's 3-50 kpc
  Irr's 1-15 kpc
• of Stars Ellipicals 107 (dwarfs) to
  1013 (cD central dominant) Spirals 1010 - 1012
  Irregulars 107 - 1011

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Gas Content and Star Formation

• Gas Content E up to 30 of ordinary
  matter (baryonic) mass, but very hot (T gt 107
  K) S typically 5-15 of baryonic mass, in
  many ISM phases (10 K lt T lt 106 K) Irr
  typically 20-50 of baryonic mass, in many ISM
  phases
• Star Formation E very little, if any,
  currently (so RED) S moderate amount in disk
  (some BLUE with many YELLOW and RED) Irr
  often lots currently (so BLUE)

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Thought Question

• Why does ongoing star formation lead to a
  blue-white appearance?
• A. There arent any red or yellow stars
• B. Short-lived blue stars outshine others
• C. Gas in the disk scatters blue light

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Thought Question

• Why does ongoing star formation lead to a
  blue-white appearance?
• A. There arent any red or yellow stars
• B. Short-lived blue stars outshine others
• C. Gas in the disk scatters blue light

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What Determines Galactic Shapes?

• The quasi-spherical shapes of Ellipticals as
  well as halo and bulge stars in spirals arise
  from their stars' original RANDOM VELOCITIES.
• The orbits of stars in spiral galaxies come from
  stars mainly forming in a flattened disk,
  supported by ROTATION.
• The odd shapes of Irregulars are not well
  understood but many probably arise from tidal
  distortion by bigger galaxies.

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Formation of a Spiral Galaxy
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Summary The Hubble Sequence
Like stars on the Main Sequence galaxies are born
into a type in the Hubble Sequence and they DO
NOT usually move along the Hubble Sequence.
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Mergers and Cannabalism

• The biggest E and S galaxies have almost
  certainly MERGED WITH comparable sized galaxies,
  or CANNABALIZED several smaller galaxies over
  billions of years.
• Typically SS ? E, SE ? E, EE ? E So as the
  universe ages the fractions of S's goes down,
  E's up.
• BUT sometimes mergers induce more star formation
  and spiral disks

Cen A new dust disk from S swallowed by big E
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Where are they found?

• Most Es (except dwarfs) near CENTERS OF CLUSTERS.
  
• Most S's in the FIELD or toward EDGES OF
  CLUSTERS.
• Irr's locations are less well known probably
  like Spirals.

Coma Cluster about 100 Mpc away
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DISTRIBUTION OF GALAXIES

• Most galaxies are in clusters
• Most clusters are part of superclusters.
• Our Local Group has about 50 members. MW LMC,
  SMC, Draco, Fornax, Sculptor, Leo etc is one
  sub-group Andromeda (M31) M32, M33, NGC 147
  and more is another sub-group
• Total extent about 1 Mpc (M31 is 700 kpc
  from MW)

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The Local Group
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Some Properties of CLUSTERS

• The nearest CLUSTER is the Virgo Cluster, about
  15 Mpc away
• Clusters vary in size and richness, from 100 up
  to over 5000 galaxies.
• Within clusters, E's and S0's dominate the
  central parts (90 or so) but S's and SB's
  dominate the outskirts of clusters.

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Cluster Merger Movie

• Many clusters grow through mergers of smaller
  clusters
• Some clusters are still growing today
• Collisions can heat gas in clusters (intracluster
  medium) to 108K, giving off X-rays

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THE COSMIC DISTANCE LADDER

• Review of VARIABLE STARS
• Giants and supergiants will PULSATE in the
  INSTABILITY STRIP above the MS for A and F
  stars, where variations in He opacity drive
  increases and decreases in R and T.
• Some variable stars calibrate distances All RR
  LYRAE stars are nearly the same luminosity,
  some 70 times the Sun's. Periods between 2 and
  24 hours.
• CEPHEID VARIABLES have luminosites proportional
  to their periods from 200 L? (for 1 day) to
  10000 L? (for 50 days)
• Both are "STANDARD CANDLES" that allow DISTANCE
  DETERMINATIONS to NEARBY CLUSTERS and many,
  relatively nearby GALAXIES.

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Next Step Tully-Fisher Relation

• There is a very strong correlation between
  rotational speeds and luminosities for Sc
  galaxies. Why?
• Roughly rotation speed mass luminosity
• Measure brightness and estimate luminosity, then
  distance.
• The 21 cm H I line is broader in the faster
  rotating galaxies IR magnitudes give better
  estimates of total brightness
• This Tully-Fisher relation is good out to 200 Mpc!

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Cosmic Distance Ladder, IllustratedType 1a SNe
take us out beyond 1 Gpc
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White-dwarf supernovae can also be used as
standard candles
Type Ia SNe as Standard Candle
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Apparent brightness of white-dwarf supernova
tells us the distance to its galaxy (up to 10
billion light-years)
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HUBBLE's LAW

• Back in 1920's Edwin Hubble found that nearly all
  galaxies showed REDSHIFTS!
• Even more interesting, the fainter the galaxy,
  therefore, probably the more distant the galaxy,
  the greater the redshift.
• When distances (r) were calibrated using Cepheid
  variables, Hubble found
• v H0 r where v c (??/?) is the
  expansion velocity and z ??/? is the
  redshift. (So v cz)

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Galaxy Spectra and Hubbles Law

• Discover Hubble's Law

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Using Hubbles Law

• If one knows enough galaxy distances from
  independent measurements and has z's for all of
  those galaxies then one gets a value for
• H0 average of all (v/r) measurements.
• This yields 50 lt H0 lt 100 km/s/Mpc and most
  likely, H0 72 km/s/Mpc (with an error of 3
  km/s/Mpc)
• An example say a line of 5000 Å is seen at 5500
  Å
• z ??/? (5500 Å -5000 Å)/5000 Å 0.10 So v
  cz 0.10 x 3.00 x 105 km/s 3.00 x 104 km/s
  If H0 75 km/s/Mpc, then
• r v/ H0 (30,000 km/s)/(75 km/s/Mpc) 400 Mpc

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Cause of Hubble's Law
Distances between faraway galaxies change while
light travels Astronomers think in terms of
lookback time rather than distance
distance?
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Copernican Principle, Expanded

• VERY IMPORTANT POINT The expansion of the
  Universe shown by the Hubble Law should be
  independent of location in the Universe. EVERYONE
  WOULD SEE AN EQUIVALENT EXPANSION AWAY FROM THEM.
  
• In other words, we do not believe we are at a
  special place in the universe.