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					Galaxies
First spiral nebula found in 1845 by the Earl of Rosse.
Speculated it was beyond our Galaxy.




1920 - "Great Debate" between Shapley and Curtis on whether spiral
nebulae were galaxies beyond our own. Settled in 1924 when Edwin
Hubble observed individual stars in spiral nebulae.
The Variety of Galaxy Morphologies
More on bars…

Milky Way schematic
showing bar




Another barred galaxy



A bar is a pattern too,
like a spiral.

Bar simulation
                            Galaxy Classification
     Spirals                       Ellipticals               Irregulars

 barred unbarred                    E0 - E7              Irr I        Irr II
SBa-SBc Sa-Sc                                        "misshapen       truly
                                                       spirals"     irregular
First classified by Hubble in 1924 => "tuning fork diagram"




                                                    bulge less prominent,
                                                    arms more loosely wrapped
                                                                                Irr

   increasing apparent flatness
                                  disk and large
                                  bulge, but no spiral
                                                 bulge less prominent,
                                                 arms more loosely wrapped
                                                                             Irr

increasing apparent flatness
                               disk and large
                               bulge, but no spiral
Still used today. We talk of a galaxy's "Hubble type"

Milky Way is an SBbc, between SBb and SBc.
What the current structure says about a galaxy’s evolution is
Still active research area.

Ignores some notable features, e.g. viewing angle for ellipticals,
number of spiral arms for spirals.
                     Sa vs. Sc galaxies




Messier 81 – Sa galaxy               Messier 101 – Sc galaxy
                               Irr I vs. Irr II

   Irr I (“misshapen spirals”)                    Irr II (truly irregular)




            bar



             poor beginnings
             of spiral arms

    Large Magellanic Cloud                        Small Magellanic Cloud


These are both companion galaxies of the Milky Way.
                              Ellipticals
Similar to halos of spirals, but generally larger, with many more
stars. Stellar orbits are like halo star orbits in spirals.
Stars in ellipticals also very old, like halo stars.
                                                       An elliptical


      Orbits in a spiral
 A further distinction for ellipticals and irregulars:


          Giant              vs.               Dwarf

      1010 - 1013 stars                  106 - 108 stars
      10's of kpc across                 few kpc across




Dwarf Elliptical NGC 205

             Spiral M31

    Dwarf Elliptical M32
In giant galaxies, the average elliptical has more stars than the
average spiral, which has more than the average irregular.


What kind of giant galaxy is most common?

Spirals - about 77%
Ellipticals -   20%
Irregulars -     3%

But dwarfs are much more common than giants.
 "Star formation history" also related to Hubble type:


                                 Ellipticals formed all their stars early on,
amount of star
                                 no gas left. Stars are old, red, dim.
formation


                     1                                          14 (now)
                             time (billions of years)



                                 Spirals still have star formation, and gas.
amount of star                   Luminous, massive, short-lived stars make
formation                        spirals bluer than ellipticals


                     1       time (billions of years)          14 (now)

    Irregulars have a variety of star formation histories.
                         Distances to Galaxies
For "nearby" (out to 20 Mpc or so) galaxies, use a very bright class of
variable star called a "Cepheid".

luminosity                                                         average
                                                                   luminosity



                                 time (days or weeks)




                                                              Cepheid star in
                                                              galaxy M100
                                                              with Hubble.
                                                              Brightness
                                                              varies over a
                                                              few weeks.
From Cepheids in Milky Way star clusters (with known
distances), it was found that period (days to weeks) is
related to average luminosity.

              (average)




So measure period of Cepheid in nearby galaxy, this gives
star's average luminosity. Measure average apparent
brightness. Now can determine distance to star and galaxy.

Has been used to find distances to galaxies up to 25 Mpc.
         Get used to these huge distances!


                    Milky Way



30 kpc


           Milky Way to Andromeda
                   700 kpc




          Milky Way to Virgo Cluster
                   17 Mpc
                         Structures of Galaxies

Groups

A few to a few dozen galaxies
bound together by their
combined gravity.

No regular structure to them.




                                The Milky Way is part of the Local Group of
                                about 30 galaxies, including Andromeda.
    Clusters
        Larger structures typically containing thousands of galaxies.




Center of Virgo Cluster of about 2500 galaxies.   Center of the Hercules Cluster


    Galaxies orbit in groups or clusters just like stars in a stellar cluster.

    Most galaxies are in groups or clusters.
Superclusters
  Recognizable structures containing clusters and groups.
  10,000's of galaxies.




   The Local Supercluster consists of the Virgo Cluster, the Local
   Group and several other groups.
To search for structure on larger scales, need a new method of finding
distances beyond about 25 Mpc.

One is the Tully-Fisher Relation. Please read how this works.

The second is:

                         Hubble's Law

In 1912, Slipher used spectra of "spiral nebulae" to find
essentially all of them are receding from us, that is, show
“redshifted” spectral lines.
Spectra of galaxies
in clusters of
increasing distance

  Calcium absorption
  lines
   In 1920's, Hubble used Cepheids to find distances to galaxies. Showed
   that redshift or recessional velocity is proportional to distance:



                V = H0 x D              (Hubble's Law)
  velocity (km / sec)            Distance (Mpc)

              Hubble's Constant (km / sec / Mpc)


Or graphically. . .                                Current estimate:

                                                   H0 = 60 -75 km/sec/Mpc

                                                   If H0 = 70 km/sec/Mpc, a
                                                   galaxy at 1 Mpc moves
                                                   away from us at 70 km/sec,
                                                   etc.
  So get spectrum of a galaxy, measure its redshift, convert it to a
  velocity, and determine distance.




Results from a
mid 1980's
survey.

                                                            Assumes H0 = 65
                                                            km/sec/Mpc. Note
                                                            how scale of
                                                            structure depends on
                                                            this.



 Hubble's Law now used to unveil Large Scale Structure of the
 universe. Result: empty voids surrounded by shells or filaments,
 each containing many galaxies and clusters. Like a froth.
Result from “2dF” survey out to 600 Mpc
The local
universe300 Mpc
                        Galaxy Interactions and Mergers
 Galaxies sometimes come near each other, especially in groups and clusters.

Large tidal force can
draw stars and gas
out of them => tidal
tails in spirals.

Galaxy shapes can
become badly
distorted.
Galaxies may merge.




Some ellipticals may be mergers of two or more spirals.

Since ellipticals have old stars, most mergers must have occurred long ago.
Interactions and mergers are simulated by computers.




                          Simulations of interacting and merging galaxies
Interactions and mergers also lead to "starbursts": unusually
high rates of star formation at centers of merging pairs
                       How do Galaxies Form?
Old idea: a single large collapsing cloud of gas, like Solar Nebula.

New idea: observations indicate that "sub-galactic" fragments of size
several hundred parsecs were the first things to form. Hundreds might
merge to form a galaxy.




             600 kpc
                                                Few hundred pc
Deep Hubble image. Small fragments contain several billion stars each.
May merge to form one large galaxy. Looking back 10 billion years.
                    Schematic of galaxy formation




Subsequent mergers of large galaxies also important for galaxy
evolution. Large galaxies continue to swallow small ones today.
       The Milky Way is still accreting dwarf galaxies




Artist’s impression of
tidally stripped stream of stars
from Sag. dwarf. Predicted in
simulations. Later found
observationally.
Tidally stripped stars from a small galaxy orbiting NGC 5907
In some starbursts, supernova rate so high that the exploded gas combines
to form outflow from disk.
Sometimes a galaxy may pass right through another one, creating a
ring galaxy.

                  Hubble image of The “Cartwheel” galaxy
   Another Ring Galaxy: AM 0644-741




(as seen at Satellite Coffee on Montgomery)
 "Star formation history" also related to Hubble type:


                                 Ellipticals formed all their stars early on,
amount of star
                                 no gas left.
formation


                     1                                          14 (now)
                             time (billions of years)



                                 Spirals still have star formation, and
amount of star                   gas. More so Sc's than Sa's.
formation
                                                                      Sc
                                                                      Sb
                                                                      Sa
                     1       time (billions of years)          14 (now)

    Irregulars have irregular star formation histories.
VLA observations show a bridge of atomic gas
connecting Cartwheel and a more distant galaxy.
                          How do Galaxies Form?
    Old idea: a single large collapsing cloud of gas, like Solar Nebula.

    New idea: observations indicate that "sub-galactic" fragments of size
    several hundred parsecs were the first things to form. Hundreds might
    merge to form a galaxy.

Image from HST of possible
growth of large galaxy by
merger of small (1-5 kpc)
fragments. “Spiderweb”
galaxy.

This is 11 billion years ago,
when universe was 3 billion
years old.
Another group

				
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