Chapter 17: Galaxies

    Galaxy Types
   Cosmic Distances
     Cosmic Ages
   Galaxy Evolution
             Islands of Stars
• Hubble Deep Field:
  Tiny slice of the sky
  imaged during 10 days
  with HST.
• Galaxies of many
  sizes, colors and
• The observable
  universe contains >
  8x1010 galaxies.
                Galaxy Types
• Spirals have flat disks and a bulge. The disks are
  filled with cool gas and dust out of which new
  stars form. They usually display spiral arms.
• Ellipticals are redder and rounder than spirals.
  They contain very little cool gas and dust. They
  can be rather large (~1012 stars).
• Irregulars do not appear neither disklike nor
  rounded. They can be rather small (~108 stars).
Spiral Galaxies
        • Similar structure to the
          Milky Way.
        • Bulge+halo=Spheroidal
        • Disk component contains
          ISM of gas and dust.
        • Spirals with large bulges
          tend to have less gas and
       Peculiar Spiral Galaxies
Barred spiral galaxies
  have a straight bar of
  stars cutting across the
Lenticular (lens-shaped)
  galaxies have disks but
  do not appear to have
  spiral arms.
Groups of Galaxies
         • ~75-85% of large galaxies
           are spiral or lenticular.
         • Spirals are often found in
           loose groups.
         • Our local group has two
           large spirals (Milky Way
           and M11).
         • Lenticulars are common in
           clusters of galaxies which
           can contain up to few
           thousands of galaxies.
             Elliptical Galaxies
• Ellipticals look like the
  bulge and halo of a spiral
  galaxy without a
  significant disk
• ISM in ellipticals is
  mainly hot, low-density,
  X-ray emitting gas. Little
  dust and cold gas.
• Large ellipticals make up
  ~50% of large galaxies in
  large cluster cores.
Dwarf Ellipticals
         • Dwarf elliptical galaxies
           are small elliptical
           galaxies which have less
           than 109 stars and are
           often found near larger
           spiral galaxies.
         • At least 10 dwarf elliptical
           galaxies belong to the
           Local Group.
Irregular Galaxies
         • Miscellaneous class for
           galaxies that are neither
           elliptical nor spiral.
         • Distant galaxies are more
           likely to be irregular than
           those nearby. Irregulars
           were more common when
           the universe was younger.
         • The Magellanic Clouds
           are two small irregular
           galaxies that orbit the
           Milky Way.
        Hubble’s Galaxy Classes
The galaxy classification
  system proposed by
  Edwin Hubble remains
  widely used. Letter E for
  elliptical with a number
  for the elongation. S for
  spirals. SB for barred
  spirals. SO for lenticulars.
  Lower case letters indicate
  the size of the relative
  sizes of bulge and disk.
          Cosmic Distances
• Radar measurements of solar system size.
• Parallax measurements of the distances to
  nearby stars.
• Standard candles are objects for which we
  are likely to know the true luminosity.
  Some astronomical objects make good
  standard candles, but never perfect.
         Main-Sequence Fitting
• Measure parallax to
  nearby star cluster
  (Hyades, Pleiades).
• Compare MS of
  distant cluster to that
  of a nearby one.
• Luminosity-distance
  formula (chapter 13):
           Cepheid Variables
• Pulsating variable bright stars that follow a
  simple period-luminosity relation.
• The longer the time period between peaks in
  brightness, the greater the stellar luminosity.
• Cepheids are primary standard candles to
  determine distances in the Milky Way and
  other galaxies.
              Hubble’s Law
• Edwin Hubble determined the distance to M11
  (1924) and other spiral galaxies using Cepheids.
• In 1929, he announced that the more distant a
  galaxy is, the greater its redshift and hence the
  faster it is moving away from us.
• Hubble’s law: v=H0xd where v is the recession
  velocity, d stands for distance and H-naught is
  Hubble’s constant expressed in units of km/s/Mpc.
Using Hubble’s Law to Measure
• We can use a galaxy’s recession velocity to
  determine its distance: d=v/H0
• However, galaxies may have peculiar motions that
  change their velocity, particularly in the Local
  Group and nearby galaxy clusters.
• The distances we find with Hubble’s law are only
  as accurate as our best knowledge of Hubble’s
   Measuring Hubble’s Constant
• One of the main missions
  of HST.
• Distant Cepheids can be
  measured with HST up to
  30 Mpc (108 l.y.) reaching
  the nearest galaxy clusters
  such as the one in Virgo.
  However, this is not
  enough to calibrate
  Hubble’s constant.
White Dwarf Supernovae
           • Cepheid distances are
             used to calibrate the
             distances to WD
           • HST has been used to
             determine the Cepheid
             distance to several
             historical WD supernovae.
           • As expected WD
             supernovae are good
             standard candles.
           Tully-Fisher Relation
• Both the luminosity and
  the rotation speed of a
  spiral galaxy depend on
  the mass, and hence they
  are connected with a
  simple relation.
• This relation allows to use
  large spiral galaxies as
  standard candles.
• As of 2002, H0=65 +/-10
The Distance Chain
         • Radar ranging: Solar-
         • Parallax: Solar-
         • MS fitting: Milky Way.
         • Cepheids: Galaxies up to
           30 Mpc.
         • WD supernovae and TF
           relation: Distant galaxies.
         • Hubble’s law: Universe.
           Universal Expansion
• The Universe is expanding
  as a whole.
• Gravity is working to slow
  the expansion rate and in
  the densest regions it wins
  and creates galaxies.
• Matter is evenly
  distributed on large scales
  (Cosmological Principle).
• No center, no edges.
Cosmic Ages
      • Hubble constant=1/(age of
        the universe).
      • It is called constant
        because it is the same at
        all locations in the
      • Age of Universe = 12-18
        billion years.
      • Cosmological redshift is
        the stretching out of
        photon wavelenghts due to
        the universal expansion.
      Family Album of Galaxies
• Redshifts are used to
  estimate ages of galaxies.
• Each picture shows each
  galaxy at a single stage in
  its life.
• Younger galaxies appear
  smaller because they are
  more distant. They tend to
  be irregular in shape.
       Cosmological Horizon
• It is a boundary in time, not in space. We
  cannot see back to a time before the
  universe began.
• Lookback time to the cosmological horizon
  is equal to the age of the universe.
            Galaxy Evolution
• Our telescopes cannot yet see the details of what
  happened back in the time when galaxies formed
  the first stars.
• Theoretical models of galaxy formation assume
  that H and He gas filled all of space almost
  uniformly when the universe was very young.
  Certain regions were slightly denser than others.
• Within about 109 years after the BB, gravity
  overcame expansion in the denser regions and
  formed protogalactic clouds.
       A Basic Model of Galaxy
• Stars of the spheroidal
  population formed first
  from blobby clouds with
  little rotation.
• Stars of the disk
  population formed later
  after the cloud had been
  flattened by conservation
  of angular momentum.
Why Elliptical Galaxies have no
                • Explanation A: If a
                  protogalactic cloud has
                  little angular momentum,
                  its gas might not form a
                  disk at all.
                • Explanation B: If a
                  protogalactic cloud is very
                  dense, gravity could
                  collapse clumps into stars
                  before they have time to
                  settle onto a disk.
                • Distant giant galaxies
                  have formed all their stars.
             Galaxy Collisions
• The average distances
  between galaxies is not
  very much larger than
  their sizes.
• Collisions are inevitable.
• The Antennae are a pair of
  colliding spiral galaxies.
• Distorted looking galaxies
  were more common in the
  early universe when
  galaxies were closer
Simulations of Galaxy Collisions
                • Computer models show
                  that a collision between
                  two spiral galaxies can
                  create an elliptical galaxy.
                • The central dominant
                  galaxies found in dense
                  clusters are usually giant
                  elliptical galaxies that
                  grew by consuming many
                  other smaller galaxies.
           Galactic Cannibalism
• The central dominant
  galaxy in the cluster Abell
  3827 has multiple clumps
  that probably once were
  the centers of individual
• Giant ellipticals can be
  over 10 times more
  massive than the Milky
Starburst Galaxies
         • The Milky Way produces
           an average of 1 star per
         • Starburst galaxies form
           about 100 stars per year. It
           must be a short-lived
         • Superbubbles erupt out
           from the disk in a galactic
         • Starbursts result from tidal
Active Galactic Nuclei and QSOs
                • AGNs are unusually
                  bright galactic centers, and
                  quasars (QSOs) are the
                  very brightest of them.
                • They shine up to 1,000
                  times the Milky Way.
                • QSOs are found primarily
                  at great distances. They
                  arouse when galaxies were
Seyfert Galaxies
        • AGNs less powerful than
          QSOs that make up about
          1% of the present-day
          galaxy population.
        • Our best radio wave
          images show that AGNs
          must be smaller than 1 pc.
        • Significant luminosity
          changes in ~hours imply a
          size similar to the Solar
        Radio Galaxies and Jets
• Radio galaxies are closely
  related to QSOs.
• Much of the radio
  emission comes from pairs
  of radio lobes produced by
  charged particles spiraling
  around magnetic fields at
  speeds near to c.
• Energy is transported by
  jets spurting from the
The QSO 3C345
       • Series of pictures
         show blobs of plasma
         moving outward from
         the QSO at close to c.
     Supermassive Black Holes
• The energy in AGNs
  comes from matter
  falling into a
  supermassive black
• Gravity converts the
  potential energy of the
  infalling matter into
  kinetic and thermal
Evidence for supermassive BHs
               • The relatively nearby
                 AGN M87 features a
                 bright nucleus and a jet.
               • HST spectra shows a
                 pattern of emission lines
                 characteristic of gas
                 orbiting around a BH with
                 a mass of ~3x109 solar
                 masses and a radius of up
                 to 18 parsecs.
              Loose Ends
• Where are the first stars?
• How did density enhancements first
  appeared in the universe?
• How giant black holes form?
• When do QSOs stop shining?
             The Big Picture
• Chain of distance measurements culminating with
  Hubble’s law.
• Universal expansion implies a finite age of 12-16
  billion years.
• Galaxies grown from protogalactic clouds of gas
  and collisions are important.
• Tremendous energy output of AGNs is probably
  powered by accretion onto a supermassive black
  hole, which still must remain in present-day

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