GALAXIES - homepages.uc.eduvazct/stars/CH 24.pdf · • Elliptical galaxies have no spiral arms and...
Transcript of GALAXIES - homepages.uc.eduvazct/stars/CH 24.pdf · • Elliptical galaxies have no spiral arms and...
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GALAXIES
Chapter 24
• Morphology
– Hubble’s Galaxy Classification
• The Distribution of Galaxies in Space
– Extending the Distance Ladder
• Hubble’s Law
• Active Galactic Nuclei
• The Central Engine of an Active Galaxy
• Summary
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Hubble’s Galaxy Classification: Spirals
• We’re acquainted with Spiral Galaxies, the most
familiar one being the galaxy closest to us.
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Hubble’s Galaxy Classification: Spirals
• Spiral galaxies do not all look the same. Hubble
classified them by the size of the central bulge.
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Bode’s Galaxy
(M81)
Whirlpool Galaxy
(M51a)
Pinwheel Galaxy
(M101)
Hubble’s Galaxy Classification: Spirals
• All spiral galaxies are denoted by the letter “S”
• They are further classified by the letters “a”, “b”, “c”,
etc., according to the size of the bulge, so that:
– Type Sa has the largest central bulge,
– Type Sb has a smaller bulge, and
– Type Sc has the smallest bulge.
• The tightness of the spirals is strongly correlated
with the size of the bulge,
– Type Sa tends to have the most tightly bound spiral
arms, with types Sb and Sc progressively less tight.
• The correlation is not perfect. Andromeda is
classified as an Sb galaxy.
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Hubble’s Galaxy Classification: Spirals
• The components of spiral galaxies are the same as
in our own Galaxy: halo, bulge, disk, spiral arms.
– The bulge contains older stars on the outside and
younger stars inside,
– The halos contain older stars and globular clusters,
– The galactic disks are rich in gas and dust and
contains most of the star forming regions in the spiral
arms. Most of the Luminosity of these galaxies comes
from A – G type stars in the disk.
• It turns out that the larger the bulge the less gas and
dust contained by the disk: thus Sa galaxies have
the smallest amount of gas and dust in the disk and
Sc galaxies have the most© 2017 Pearson Education, Inc.
Hubble’s Galaxy Classification: Spirals
• Most spirals will not be seen face to face. However, we do
not need to see spiral arms to classify a galaxy as a spiral
galaxy. A disk, dust and star formation is good enough.
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The Spiral galaxy pair
NGC 4302 and NGC 4298.
The Sombrero Galaxy as
seen by Hubble.
Galaxy Classification: Barred Spirals
• Similar to the spiral galaxies are the barred spirals.
• They contain a pronounced “Bar” through the center.
• The spirals project from the ends of the bar.
• Barred spirals are designated the letters “SB” followed by the
letters “a”, “b”, “c”, etc. depending on the size of the bulge.
• In the case of SBc the spirals merge with the bar.
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Galaxy Classification: Ellipticals
• Elliptical galaxies have no spiral arms and no disk.
They come in many sizes,
– Giant Ellipticals contain trillions of stars
– Dwarf Ellipticals contain fewer than a million stars.
• Ellipticals contain very little, if any, cool gas and
dust, and show no evidence of ongoing star
formation. They contain mostly old stars.
• Many Ellipticals, however, have large clouds of very
hot gas (millions of K), extending far beyond the
visible boundaries of the galaxy.
• The presence of this hot gas is known by X-ray
observations.
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Galaxy Classification: Ellipticals
• The Giant Elliptical
NGC1316 in the Formax
Cluster.
• The Dwarf Elliptical
M110 is a satellite of
Andromeda
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Galaxy Classification: Ellipticals
• Some Giant Ellipticals are known to
contain disks of gas and dust in which
stars are forming.
• Dwarf Ellipticals are the most common
type of Elliptical galaxy.
• Stars in ellipticals have more or less
random motions.
• Ellipticals are assigned the letter “E”.
• They are classified according to their
shape, from
– E0 (almost spherical) to
– E7 (the most elongated).
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Galaxy Classification: Lenticulars
• Lenticular galaxies have a
thin disk and a flattened
bulge, but no gas and dust.
• They are known as
Lenticular because their
shape resembles a lens
• They are known as “S0”
galaxies or “SB0” galaxies,
if they have a bar.
• S0 and SB0 galaxies have a
disk and bulge, but no spiral
arms and no interstellar gas.
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Galaxy Classification: Irregulars
• The irregular galaxies have a wide variety of
shapes. They are classified as “Irr I” and “Irr II”.
• They typically contain between 100 million and 10
billion stars.
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Galaxy Classification: Irregulars
• The stars in irregular galaxies have random motions,
like their Elliptical counterparts.
• These galaxies have vigorous star formation.
• Irr I galaxies are by far the most common. They
have hardly any nucleus and are rich in gas and
dust, so they have active star formation.
• Irr I are bluish in appearance.
• Irr II galaxies redder than Irr I galaxies and rare.
They appear more filamentary.
• This type of galaxy is thought to result from normal
galaxies that have undergone strong interactions
with other galaxies.
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Galaxy Classification: Irregulars
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• Here are the Large and Small Magellanic Clouds,
which are two irregulars belonging to our local group.
Hubble’s Galaxy Classification
• A summary of galaxy properties by type
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Hubble’s “Tuning Fork”
• Hubble’s “tuning fork” is a convenient way to
remember the galaxy classifications, although it has
no deeper meaning.
• Eg. Spirals are not Ellipticals that grew arms!
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The Distance Ladder
• Cepheid variables allow measurement of galaxies to about 25 Mpc
away.
• However, most galaxies are farther away than 25 Mpc. New
distance measures are needed.
• There are two ways to do this:
– Tully-Fisher relation correlates a galaxy’s rotation speed
(which can be measured using the Doppler effect) to its mass,
and so to its Luminosity.
– Standard Candles: these are intrinsically bright, easily
recognizable objects whose Luminosity is known. RR Lyrae
variables and Cepheid variables are examples, but they are
generally not bright enough at very large distances. On the
other hand, Type I supernovae all have about the same
luminosity, as the process by which they happen doesn’t allow
for much variation. The same can be said for Planetary
Nebulae.© 2017 Pearson Education, Inc.
Measuring the Rotation Rate
• The rotation of a galaxy results in Doppler broadening of
its spectral lines.
• The faster the rotation the more the broadening.
• By measuring the broadening we can deduce the
rotation rate.
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The Tully Fisher Relation
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• The Tully-Fisher relation (TFR) is an empirical
relation between the rotational velocity and the
intrinsic luminosity of a galaxy. The relation is tighter
when the rotation rate is compared with the baryonic
mass. This latter form
is known as the
Baryonic Tully-Fisher
(BTR) relation.
By comparing the
intrinsic luminosity to
the apparent luminosity
we determine the distance.
The Distance Ladder
• With these additions, the cosmic distance ladder has
been extended to about 1 Gpc.
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Distribution of Galaxies: Local Group
• Here is the distribution of galaxies within about
1 Mpc of the Milky Way. We call it our local group.
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Distribution of Galaxies: Local Group
• There are three spirals in the Local Group—the Milky Way,
Andromeda, and M33 (the Triangulum Galaxy). There are
more than 54 galaxies in all most of them dwarfs.
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Distribution of Galaxies: Clusters
• A collection of galaxies between 2 and 10 Mpc across is called
a Cluster. Clusters are large, eg. the Virgo Cluster is a cluster
of about 2500 galaxies, whose center is roughly 16 Mpc away,
in the constellation of Virgo, with a diameter of about 3 Mpc.
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Distribution of Galaxies: Superclusters
• Clusters form Superclusters. The Laniakeia super-
cluster contains the local group and about 100,000 other
galaxies; it has a diameter of about 77 Mpc. [Video]
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Hubble’s Law
• Universal recession:
All galaxies (with a
couple of nearby
exceptions) seem to be
moving away from us,
with the redshift of their
motion correlated with
their distance.
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Hubble’s Law
• These plots show the
relation between
distance and
recessional velocity for
the five galaxies in the
previous figure, and
then for a larger sample.
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Hubble’s Law
• The relationship (slope of the line) is characterized by
Hubble’s constant H0:
recessional velocity = H0×distance
• The value of Hubble’s constant today is approximately
73.4 km/s/Mpc.
• Over the past year, there’s been some confusion about
its value because recent observations from ESA’s
Planck satellite indicates that its it more like 67.4
km/s/Mpc.
• Measuring distances using Hubble’s law works better
the farther away the object is; random motions are
overwhelmed by the recessional velocity.
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The Distance Ladder: Hubble’s Law
• This puts the final step on our distance ladder.
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Large Scale Structure of the Universe
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Large Scale Structure of the Universe
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Active Galactic Nuclei
• About 40 percent of galaxies don’t fit well into the “normal”
category — they are far too luminous and the radiation
does not look thermal. Such galaxies are called active
galaxies. They differ from normal galaxies in both the
luminosity and type of radiation they emit.
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Active Galaxies
• Most of a normal galaxy’s radiation is emitted in and
around the visible portion of the spectrum. This is
because most of the radiation is from stars.
• Radiation from stars obeys the black body curve.
• By contrast, the radiation from an Active Galaxy
does not obey the black body curve.
• On the contrary, it covers both longer and shorter
wavelengths.
• This is why the radiation from these galaxies is
called nonstellar radiation.
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Starburst Galaxies
• Many luminous galaxies are experiencing an outburst of
star formation, probably due to interactions with a
neighbor. These galaxies are called starburst galaxies.
• The radiation from a starburst galaxy will be spread
throughout the galaxy and distinctly stellar in nature.
• The active galaxies we will discuss now are those whose
activity is due to events occurring in and around the
galactic center. The centers are called Active Galactic
Nuclei or AGN for short:
– Seyfert Galaxies
– Radio Galaxies
– Quasars
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Active Galactic Nuclei
• We can tell when the radiation is primarily coming from
a small region by studying the variability of the light
emitted in different wavelengths.
• If the period of the variability, 𝑇, is small, then we
expect that the emitting region is also small. Strictly,
the size of a coherently emitting region cannot be
larger than the light travel time:
𝑆𝑖𝑧𝑒 = 𝑐 𝑇
• For example, if the variability is 1 day, then the size
will be 2.6 × 1010 km. This is only about 2000 times
the horizon size of the black hole at the center of our
galaxy.
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AGN: Seyfert Galaxies
• Seyfert galaxies
resemble normal
spiral galaxies, but
their cores are
thousands of times
more luminous.
• Seyferts have large
radio and infrared
luminosities but look
ordinary in visible.
• A Seyfert’s luminosity
can double or halve in
a fraction of a year.
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NGC 1566 lies about 13 Mpc away
AGN: Seyfert Galaxies
• The rapid variations in the luminosity of Seyfert
galaxies indicate that the core must be extremely
compact. The cores of Seyfert Galaxies have
luminosities roughly equal to the entire Milky Way,
or 5 × 1036 W
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AGN: Radio Galaxies
• Radio galaxies emit very strongly in the radio
portion of the spectrum.
• They may have enormous lobes, invisible to optical
telescopes, perpendicular to the plane of the galaxy.
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AGN: Radio Galaxies
• The lobes may extend
far out of the galaxy.
• They are made up of
multiple jets.
• A typical radio galaxy
has a luminosity that is
about 100 times the
Milky Way: 5 × 1038 W
• The galaxy Centaurus
A appears to be the
result of a collision.
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AGN: Radio Galaxies
• Radio galaxies may also be core dominated. That
is, most of the luminosity appears to be emitted from
near the center while no lobes appear.
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AGN: Radio Galaxies
• Core-dominated and radio-lobe galaxies are
probably the same phenomenon viewed from
different angles.
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AGN: Radio Galaxies
• Many active galaxies have jets,
and most show signs of
interactions with other galaxies.
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AGN: Blazars
• According to the theory of relativity, if a jet is
approaching the observer at speeds close to the
speed of light, the radiation emitted will be
concentrated in a cone (whose opening angle
depends on the speed) in the direction of the
motion.
• To such an observer, this jet will appear both very
intense (because the radiation is concentrated in a
cone) as well as greatly blue-shifted (by the Doppler
effect)
• If we observe them radiating gamma rays, they are
known as Blazars.
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AGN: Quasars
• Quasars:
Quasi-Stellar Radio
Sources are star-like in
appearance, but have
very unusual
spectral lines.
• For a long time the
spectral lines could not
be mapped to any
element.
• The first quasars to be
detected were labeled
3C 48 and 3C 273
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AGN: Quasars
• Eventually, it was realized that quasar spectra were
normal, but enormously red-shifted: 3C 48 was
redshifted about 37% and 3C 273 about 16%
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AGN: Quasars
• This means that 3C 48 is receding from us at about
1/3 the speed of light and 3C 273 at 1/7 the speed of
light!
• According to Hubble’s law this puts 3C 48 and 3C
273 very far away!
• Using Hubble’s constant of 70 km/s/Mpc we find
𝑑 =𝑣
𝐻Mpc
or
– 3C 48 is 1.3 Gpc away, and
– 3C 273 is 620 Mpc away!
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AGN: Quasars
• The farthest quasar observed
is 13.1 b.ly away.
• These distant quasars are very
old, dating back to an earlier
period of galactic evolution.
• They must be among the most
luminous objects in the galaxy
to be visible over such
enormous distances: 𝐿 = 5 ×1040 W.
• They are probably Radio
galaxies, in which a lobe is
viewed head on.
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AGN: The Central Engine
• Active galactic nuclei have some or all of the
following properties:
– High luminosity
– Non-stellar energy emission
– Variable energy output, indicating small nucleus
– Jets and other signs of explosive activity
– Broad emission lines, indicating rapid rotation
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AGN: The Central Engine
• This is the leading
theory for the energy
source in an active
galactic nucleus: a
black hole, surrounded
by an accretion disk.
The strong magnetic
field lines around the
black hole channel
particles into jets
perpendicular to the
plane of the accretion
disk.© 2017 Pearson Education, Inc.
AGN: The Central Engine
• In an active galaxy, the central black hole may be
billions of solar masses. Black holes have a small
horizon radius. A billion solar mass black hole has a
radius of only 3 × 109 km, which is only 20 AU.
• The black hole is probably spinning very fast.
• The accretion disk is whole clouds of interstellar gas
and dust;
• The rotational velocity of the accretion disk is very
high. We can say this because of the spectral line
broadening.
• Fast spinning, hot matter will generate powerful
magnetic fields.
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AGN: The Central Engine
• As the matter swirls around the central black hole, it
heats up by friction. The temperatures can rise to tens
of millions of degrees Kelvin.
• Accretion disks are efficient at releasing energy in the
form of radiation; they may radiate away as much as
10–20 percent of their mass energy before
disappearing into the black hole.
• Quick calculation: 20% of the mass energy of a 1 solar
mass star is 3.6 × 1046 Joules. To radiate at a rate of
about 100 times our galaxy an AGN would have to
consume about 1 solar mass every 2.3 years! At
10,000 times our galaxy, a quasar would have to
consume 1 solar mass every 8.3 days!
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AGN: The Central Engine
• The jets emerging from
an active galaxy can be
quite spectacular.
• They consist of material
(mostly charged)
blasted out from the
magnetic poles.
• They are a result of
matter being
accelerated by the
magnetic field produced
by the accretion disk.
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AGN: The Central Engine
• Measurements of the core of the galaxy M87
indicate that it is rotating very rapidly.
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AGN: The Central Engine
• One might expect the
radiation to be mostly
X-rays and gamma-rays,
but apparently it is often
“reprocessed” in the
dense clouds around the
black hole and reemitted
at longer wavelengths.
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AGN: The Central Engine
• Particles will emit synchrotron radiation as they
spiral along the magnetic field lines; this radiation is
decidedly non-thermal.
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Summary
• Hubble classification organizes galaxies according
to shape.
• Galaxy types include spiral, barred spiral, elliptical,
irregular.
• Objects of relatively uniform luminosities are called
“standard candles”; examples include RR Lyrae
stars, Cepheids and type I supernovae.
• The Milky Way lies within a small cluster of galaxies
called the Local Group.
• Other galaxy clusters may contain thousands of
galaxies.
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Summary
• Hubble’s law: Galaxies recede from us faster
the farther away they are.
• Active galaxies are far more luminous than normal
galaxies, and their radiation is nonstellar.
• Seyfert galaxies, radio galaxies, and quasars all
have very small cores; many emit high-speed jets.
• Active galaxies are thought to contain supermassive
black holes in their centers; infalling matter converts
to energy, powering the galaxy.
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