Transcript Chapter 15, Galaxies - Institute for Astronomy

Chapter 15, Galaxies
Galaxies come in different size and shape. In the previous chapter, we talked about how
galaxies provide an environment for the stars to be born and die, and enrich the heavy
element content of the galaxy. In this chapter, we will talk about
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Galaxy Classification
Location of Galaxies in the Universe
Galaxy Evolution
Quasars and AGN (Active Galactic Nuclei)…
Spiral Galaxies
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NGC 1300 – barred spiral
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NGC 4594 – Sombrero
Galaxy. Large bulge, small
NGC (New General Catalog)
4594…
Elliptical and Irregular
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Large Magellanic Cloud, irregular galaxy
M87 – Elliptical Galaxy in Virgo
Galaxy Classification
Galaxies come in different size and shape…
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Spiral Galaxies
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Barred Spiral :
Spiral galaxy with a bar.
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Lenticular galaxy:
Spiral galaxy without spiral arms.
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Formed by gas clouds with large initial angular momentum.
Elliptical Galaxies
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Similar to the bulge of the spiral galaxies.
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Formed by gas clouds with small initial angular momentum.
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Formed by high density clouds—more efficient cooling, faster star formation,
exhausting the gas supply before the galaxy has time to collapse into the disk.
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Formed by collision and merging of galaxies.
Irregular Galaxies
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Usually more distant galaxies.
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Starburst galaxies
– Galaxies with high star forming rate. Merging galaxies?
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Quasars, Active Galactic Nuclei
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Distant objects. Extremely luminous—could be 1,000 times the luminosity
of the Milky Way.
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Supermassive Black holes?
Galaxy Facts
Grouping…
• Spiral galaxies are usually found in loosely
associated small group (tens) of galaxies
• Ellipticals are commonly found in large cluster
of galaxies containing hundreds or thousands of
galaxies extending over tens of millions of
light-years.
Size…
• Most of the large galaxies are spiral galaxies…
• While some of the largest galaxies are giant
elliptical galaxies, the most common type of
galaxy in the universe is small elliptical galaxy.
• Very small ellipticals (or dwarf spheroidals, less
than a billion stars) are often found near large
spirals – our Milky Way galaxy has 10 or more
nearby…
Where are the Galaxies?
Where are the galaxies located? Are they located within the Milky Way, or are
they much further away from us than the stars?
• Before the 1920s, there were no reliable methods of measuring the
distance to the galaxies. Many people believed that the galaxies were
located within the Milky Way…
How do we measure the distance of objects far away in the universe, much
farther than the distance that can be measured by stellar parallax?
• Measurement of distance farther than the reach of stellar parallax
rely on our ability to find objects with known luminosity…
• In 1924, Edwin Hubble determined the distance to the Andromeda
galaxy using Cepheid variables, thus proving that the galaxies are
located far beyond the stars in the Milky Way galaxy.
Measuring Cosmological Distance
The principle method of measuring astronomical distance is the
distance-luminosity relation
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However, the apparent brightness B is the only thing we can measure
accurately in most of the cases.
L 4D 2 B
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 If we know the distance D, we can determine the luminosity L.
 If we know the luminosity L, then we can determine the distance D.
Need to find Standard Candles — astronomical objects with known
luminosity.
 Main sequence stars.
 Cepheid variables.
 White dwarf supernovae.
 Galaxies (using Tully-Fisher Law).
The Cosmological Distance Ladder
Methods of Measuring Distance and their useful range…
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Radar ranging – D < 10-4 light-years
Parallax – D < 103 light-years
Standard Candles
 Main sequence stars – D < 105 light-years
 Cepheid variables – D < 107 light-years
 White dwarf supernovae – D < 1010 light-years
Hubble’s Law – 1010 ly and beyond…
Calibrating the Cosmic Tape
Measure
We rely heavily on the standard candles for the measurement of the cosmological
distance. How do we make sure that these standard candles are truly standard?
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Use independent measurements to check the luminosity of the standard candle.
For example, we can use parallax measurements of the distance to main
sequence stars to check measurements of distance using main-sequence
fitting. If we do this for a few of them, then we can verify the assumption
that the main sequence stars are good standard candle. However, this method
works only for stars that are relatively close by.
The standard candles we had verified in close range can now be used (by
extrapolation) to measure the distance to more remote objects. This new
distance measure then allows us to calibrate the next standard candle.
For example we used the distance measured by observation of Cepheid
variables to check the assumption of the constancy of the white dwarf
supernovae.
Keep going…
Main Sequence Fitting
Main sequence stars with the same color should
have the same luminosity. So, if we compare the
(pseudo) H-R diagram of a star cluster with
unknown distance (using their apparent
brightness instead of luminosity) to that of a
group of main sequence stars with known
distance, then we can determine the distance to
this new cluster.
For example…
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Hyades (in constellation Taurus) is a open
cluster about 150 light years away. It’s
distance is close enough to be measured by
stellar parallax.
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Comparing the H-R Diagrams of Hyades
with that of Pleiades, we can determine that
the distance of Pleiades should be 2.75
times farther than Hyades…or, 150*2.75 =
410 light years.
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New parallax measurement of Hyades by
Hipparcos (ESO Space Interferometry
mission) yielded a distance of 438 light
years
The two parallel lines shows that the apparent
brightness of all the different type of main sequence
stars are decreased by a factor of 7.5
Cepheid Variables
Cepheid variable stars are population I (metal-rich) yellow giant stars with periodic
luminosity variation….
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Their periods range from a few days to over 100 days,
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Their luminosities range from 1000 to 30,000 L⊙,
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The high luminosity makes it possible to identify them from a large distance…
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Their luminosity and period are strongly correlated. Therefore, we can determine their
luminosity by simply measuring their periods!
The luminosity of Cepheid
variabls are strongly
correlated to their
periodicity…
Cepheid Variables in M100
The period-luminosity relation of Cepheid variables were discovered Henrrietta Leavitt in
1912. Edwin Hubble identified Cepheid variables in Andromeda galaxy (about 2.5 million
light-years away) in 1924, and used the luminosity-distance relation to demonstrate that
galaxies are much farther than the stars….
• There are well over 1,000 Cepheid variables known today…for example, the Polaries!
• In 1994, Hubble Space Telescope observed a Cepheid variable in the face-on spiral
galaxy M100 in Virgo Cluster located at a distance of 56 million light-years…this is the
most distant distant Cepheid observed so far….
http://hubblesite.org/newscenter/archive/releases/1994/49/
Tully-Fisher Relation
Although this is not discussed in our text
book, the luminosity of the spiral galaxies
are related to their rotational speed,. This
was discovered by B. Tully (of UH/IfA)
and J.R. Fisher in 1977. Therefore, the
luminosity of the spiral galaxies can be
determined simply by measuring their
rotational speed…
 Spiral galaxies are good standard
candles also!
 The slope of the luminosity-rotation
rate curve is different for different
type of spiral galaxies…
White Dwarf Supernova
Every time the hydrogen shell is ignited, the mass
of the white dwarf may increase (or decrease, we
don’t know for sure yet).
• The mass of the white dwarf may gradually
increase,
• At about 1 M⊙, the gravitation force
overcomes the electron degenerate pressure,
and the white dwarf collapses, increasing
temperature and density until it reaches
carbon fusion temperature.
• The carbon inside the white dwarfs are
simultaneously ignited. It explodes to form a
White dwarf supernova. (Type I).
• Nothing is left behind from a white dwarf
supernova explosion (In contrast to a
massive-star supernova, which would leave a
neutron star or black hole behind). All the
materials are dispersed into space.
White Dwarf Supernova is a
very important standard
candle for measuring
cosmological distance…
White Dwarf and Massive Star
Supernovae
Because the mass of white dwarfs when they explode as supernovae is always
around 1.0 M⊙, its luminosity is very consistent, and can be used as a standard
candle for the measurement of distance to distant galaxies (Chapter 15).
The amount of energy produced by white dwarf supernovae and massive star
supernovae are about the same. But the properties of the light emitted from these
two types of supernovae are intrinsically different, allowing us to distinguish them
from a distance.
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Massive star supernovae spectrum is
rich with hydrogen lines (because
they have a large outer layer of
hydrogen).
White dwarf supernovae spectra do
not contain hydrogen line (because
white dwarfs are mostly carbon, with
only a thin shell of hydrogen).
The light curve is different.
Supernovae from Distant
Galaxies
These snapshots, taken by NASA's Hubble Space Telescope, reveal
five supernovae, or exploding stars, and their host galaxies.
– The arrows in the top row of images point to the supernovae.
The bottom row shows the host galaxies before or after the stars
exploded. The supernovae exploded between 3.5 and 10 billion
years ago.
Distance and Redshift
In addition to distance, Hubble also measured the redshift of the galaxies…and when
combined with distances derived from observation of Cepheid variables and the brightest
stars in galaxies, Hubble found that, the more distant a galaxy, the greater its redshift is,
and hence the faster it is moving away from us…
→
the universe is expanding!
Hubble’s Law
From the redshift and distance measurements,
we can express the recession speed V of a
galaxy located at a distant d away from us by
V = d  H0
The value of the Hubble’s Constant is
H0 = 20~24 [km/sec] / million light-year
Once the value of H0 is determined, we can
use measured recession speed to infer the
distance of galaxies using the formula
d = V / H0