Galaxies: Their Structure and Evolution

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Galaxies Their Structure and Evolution
Slides which are important are marked by

• Physics 360/Geology 360
• Astronomy
• J. Swez

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Hubble Deep Field
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Island Universes or Nebula?

• Galaxies appear as either nearby gaseous nebula
  or else island universes with stars like our
  galaxy
• Finding their distances was required to tell the
  difference
• Edwin Hubble found the distance to the Andromeda
  galaxy with a Cepheid Variable
• Nearest Galaxy Large Magellanic Cloud, 150,000 ly
  from the Sun

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When did the study of galaxies begin?
Charles Messier discovered galaxies as early as
the 28th century. He began the Messier Catalog
(e.g. M31) Sir William Hershel (19th century)
began the classification system known today as
NGC (New General Catalog). Sir Hershel was also
known for his advances in photography. Edwin
Hubble (1899-1853) classified galaxies in the
20th century in terms of Spiral, Elliptical and
Irregular. The key to galactic classification
was the measurement of distances. The key to the
measurement of distances was Cephid Varibles
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How we found distances for remote stars. Remember
this only works up to a 100 parsecs (or 250
parsecs from apace). Certainly not galactic
distances!
Review Slide. This you should know from Chapter
12.
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Variable Star Light Curves
Remember! Cephids are classed as Type I and Type
II depending on the shape of their period curve.
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Pulsating Star HR Diagram
Review Slide! You should already know about HR
diagrams.
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Cepheid Period-Luminosity Relationship

• Cepheid variables are luminous variable stars
• The period of their variation is regular
• The period corresponds to luminosity (Period
  Luminosity Law)
• Cepheids are good distance indicators (standard
  candles)
• More Slowly they PulsateMore Luminous
• RR Lyrae 40 L, Cephids up to 106 L (an L is a
  solar mass unit)

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Distance From Cepheids
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Hubble Tuning Fork Diagram
From Thomas Arny (text) page 485
An organizational tool, not necessarily tied to
structure or evolution of galaxies.
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The Hubble Tuning Fork Further Explanation

• This page was copied from Nick Strobel's
  Astronomy Notes. Go to his site at
  www.astronomynotes.com for the updated and
  corrected version.

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Spiral Galaxies
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Barred Spiral Galaxies
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The giant elliptical galaxy M87, also called
Virgo A, is one of the most remarkable objects in
the sky. It is perhaps the dominant galaxy in the
closest big cluster to us, the famous Virgo
Cluster of galaxies M87's diameter of apparently
about 7' corresponds to a linear extension of
120,000 light years, more than the diameter of
our Milky Way's disk. However, as M87 is of type
E1 or E0, it fills a much larger volume, and thus
contains much more stars (and mass) than our
galaxy, certainly several trillion (1012) solar
masses (J.C. Brandt and R.G. Roosen have
estimated 2.7 trillion). This galaxy is also of
extreme luminosity, with an absolute magnitude of
about -22.
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• This is a 2048x2048 CCD image of the Coma
  Cluster. This cluster has a recessional velocity
  of about 7000 km/s and is the densest cluster in
  our local region of the Universe. In contrast to
  the Hercules cluster, Coma has almost no spiral
  galaxies in its central regions. The cluster is
  strongly virialized and has a hot intracluster
  medium which generates strong X-ray emission.
  This image shows the central few 100 kpc of the
  Coma cluster. At a slightly larger radius ,
  images reveal that Coma is still rather devoid of
  spiral galaxies. It is generally believed that
  the cold hydrogen gas in the disks of spiral
  galaxies is swept out of them as they orbit
  through the intracluster medium of Coma.

An example of clusters of galaxies
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Elliptical Galaxies
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• Malin 1 is an excellent example of a very Low
  Surface Brightness Spiral Galaxy. This galaxy was
  only recently discovered and can barely be seen
  in this 30 minute exposure using a 100-inch
  telescope in Chile. The galaxy disk covers about
  half of the image Frame yet is barely visible.
  There are lots of these out there.

http//zebu.uoregon.edu/images/malin1.gif
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M77 Spiral Galaxy M77 (NGC 1068), type Sb, in
Cetus Discovered 1780 by Pierre Méchain.
This magnificient galaxy is one of the biggest
galaxies in Messier's catalog, its bright part
measuring about 120,000 light years, but its
faint extensions (which are well visible e.g. in
the DSSM image) going perhaps out to nearly
170,000 light years. Its appearance is that of a
magnificient spiral with broad structured arms,
which in the inner region show a quite young
stellar population, but more away from the
center, are dominated by a smooth yellowish older
stellar population.
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SO Galaxy
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Irregular Galaxies
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• M101 is a High Surface Brightness Galaxy It has
  lots of active star formation and many H II
  regions. This exposure was through a blue filter
  with an exposure time of 30 seconds with a
  52-inch telescope using 4.851 re-imaging optics
  in front of the CCD

http//zebu.uoregon.edu/images/m101big.gif
Can you classify M101 according to the Hubble
Classification?
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• This is a 2048x2048 CCD image of the Hercules
  cluster. This cluster has a recessional velocity
  of 11,000 km/s. It is noteworthy in that it
  contains a number of spiral galaxies, many of
  which are intereacting. This is a sure sign that
  the cluster still has substructure and is not
  fully virialized yet.

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Andromeda Galaxy M31 a color picture
Spiral Galaxy M31 (NGC 224), type Sb, Andromeda
Galaxy among the brightest of the Messier
Galaxies
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Elliptical Galaxy M32 (NGC 221), type E2, in
Andromeda A Satellite of the Andromeda Galaxy,
M31
M32 is the small yet bright companion of the
Great Andromeda Galaxy, M31, and as such a member
of the Local Group of galaxies. It can be easily
found when observing the Andromeda Galaxy, as it
is situated 22 arc minutes exactly south of M31's
central region, overlaid over the outskirts of
the spiral arms.
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How would you classify this galaxy (lower left of
picture)?

• M31N205 from McDonald Observatory 10/27/95

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http//www.seds.org/messier/galaxy.html
M51 (NGC 5194), type Sc, in Canes Venatici The
distance to this galaxy is thought to be 37 Mly
(million light years) but a recent (2001) STScI
Press Release gave 31 million light years.
According to our present understanding, the
pronounced spiral structure is a result of M51's
current encounter with its neighbor, NGC 5195
(the fainter one in Messier's description).
Discovered in 1773 by Charles Messier as the
famous Whirlpool Galaxy. Its companion, NGC 5195,
was discovered in 1781 by his friend, Pierre
Méchain
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Galaxy Types
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Galactic Evolution

• The age of the oldest stars in each galaxy is
  about the same
• All galaxies are about the same age
• Galaxies are distinguished by the amount of
  star-forming material
• Different galaxy types correspond to
• different star formation rates, or
• large scale changes in gas/dust content

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Galaxy Collision - core
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Galaxy Collision
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Galaxy Collisions
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Ring Galaxies
A result of galactic collisions. The central gas
is disrupted.
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Galactic Collisions / Mergers/Cannibals

• Galaxies may collide
• Collisions occur between gas, not stars
• Collisions - rapid star formation / gas loss
• This produces ring galaxies and ellipticals
• A few mergers - giant galaxy
• Galactic cannibals and giant ellipticals

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Galaxy Clusters

• Poor Clusters
• 10-100 galaxies
• Spirals, Irregulars and lastly dwarf ellipticals
• Local Group - our galaxy cluster
• Rich Clusters
• Thousand galaxies
• Ellipticals, SO, with spirals at edges only
• Giant Ellipticals at the center

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Local Group

• Our poor cluster of galaxies
• Dominated by Milky Way and Andromeda
• Probable additional large galaxy near Milky Way

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Rich Galaxy Clusters and Local Supergroup

• Rich clusters have thousands of galaxies
• Hercules cluster

• Galaxy clusters organize into groups
• Local Supergroup

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Large Scale Structure
Large scales show a bubble and void structure
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Active Galaxies / Radio Galaxies/Quasars

• Read about them in the news!
• Active Galaxies
• Galaxies that have bright, varying cores.
  Abnormally lare radiation from a tiny region in
  the galactic core
• Radio Galaxies
• Galaxies with massive radio emissions from jets
• Quasars
• Distant active/radio galaxies
• Seyfert Galaxies a spiral galaxy with abnormally
  luminous core from a region less than a ly year
  across
• Jets in Quasars sometimes move with superluminal
  motion
• Most distant Quasar about 10 billion ly

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Radio Galaxies
Some galaxies show massive, radio emissions These
are tied to massive jets in the core
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Galactic Accretion Disk
Core!
Jet from core of M87
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Accretion Disk and Jet Formation
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The Hubble Law
In 1914 Vesto Slipher (lived 1870--1963)
announced his results from the spectra of over 40
spiral galaxies (at his time people thought the
spiral nebulae'' were inside the Milky Way). He
found that over 90 of the spectra showed
redshifts which meant that they were moving away
from us. Edwin Hubble and Milton Humason found
distances to the spiral nebulae. When Hubble
plotted the redshift vs. the distance of the
galaxies, he found a surprising relation more
distant galaxies are moving faster away from us.
Hubble and Humason announced their result in
1931 the recession speed H distance, where H
is a number now called the Hubble constant. This
relation is called the Hubble Law and the Hubble
constant is the slope of the line.
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The Redshift and the Hubble Law
In 1920s astronomers discovered galaxies are
moving away from one another The spectrum of a
galaxy is the spectrum of all its component stars
added together If a galaxy is moving toward or
away from us its spectral lines will be
Doppler-Shifted-e.g. motion away from us
lengthens the wavelength Nearly all galaxies are
moving away Hubble discovered in 1920 that V HD
where H is the Hubble Constant 65 km/s per
Mpc Thus by measuring the red shift we can find
the galactic distance (See Problem 16.2)
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Left Red Shift for a galaxy nearer Right Red
Shift for a more distant Galaxy
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The Hubble Law as applied to the recession of
galaxies
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Gravitational Lenses

• How do they Work?
• Einsteinmatter bends light, see Fig. 16.26
• Why do we think they Exist
• Multiple quasar images but idential spectral
  history
• Existence of Black Holes and Dark Matter verified?

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Why is there believed Dark Matter to Exist
Figure from Arny, an Introduction to Astronomy p
495

• The line with the dots is the theoretical curve
  for a galaxy consisting only of stars
• The observed line is with the xs
• Only the material between the stars orbit and
  the galaxys center contributes to the gravity
  force
• Something must keep the outer stars from flying
  out
• Answer Dark Matter