http://messier.seds.org/Pics/M/MESSIERS
-BIG.jpg
During the years from 1758 to 1782 Charles
Messier, a French astronomer (1730 - 1817),
compiled a list of approximately 100 diffuse
objects that were difficult to distinguish from
comets through the telescopes of the day.
Discovering comets was the way to make a
name for yourself in astronomy in the 18th
century -- Messier's first aim was to catalog
the objects that were often mistaken for
comets.
What are nebulae??
• 1755 Kant proposed nebulae are “island
universes”
• Are Nebulae within or outside of our
galaxy??
• Nebula comes from a Greek word meaning
Cloud
April 1920, National Academy of
Sciences , Washington, DC
• Harlow Shapley (had determined size of
Milky Way Galaxy) pushed that nebulae were
relatively close by objects.
• Heber Curtis championed view that spiral
nebulae were rotating systems of stars like our
own galaxy.
• Debate was heated but eventually a draw as
neither side could provide convincing
evidence for either view
Edwin Hubble
• Oct 6, 1923 Historic Photo of M31(Andromeda
Galaxy)
• Discovered a Cepheid variable star (thought it was
a nova)
• Used formulas to determine the galaxy was 750
kpc (2.5 million ly) away and it was 70 kpc in
diameter (recall Milky Way is only 50 kpc in
diameter)
• Nebula was not “Near by”
• December 30, 1924 Hubble presents his
findings to American Astronomical Society
• Shapley-Curtis “Debate” effectively settled
• The Universe is a big place!!
Hubble Ultra-Deep Field
http://antwrp.gsfc.nasa.gov/apod/ap040309.html
Hubble Classification of Galaxies
•
•
•
•
•
4 Classes of Galaxies
S – Spirals
SB – Barred Spirals
E – Ellipticals
Irr – Irregulars
Spirals
• Arched lanes of stars
• Spiral Arms containing young hot blue stars and associated
H II regions
• Strong heavy elements present so lots of Population I stars
in spiral arms
• Central bulges low star formation rate dominated by
Population II stars
Spirals – Sub-divided
• Sa – Large central bulges
~4% of galactic mass in gas and dust
• Sb – Medium sized central bulges
~8% of galactic mass in gas and dust
• Sc – Small central bulges
~25% of galactic mass in gas and dust
Barred Spiral Galaxies
• Spirals originate at the end of a bar shaped region
which passes through the nucleus of the galaxy
rather than originating from the nucleus itself.
• Hubble again applied a, b, and c to rate the size
of the central bulge with a being largest and c
being smallest
• As with ordinary spirals this difference of a
through c may be due to amount of gas and dust
in the galaxy.
• Bars appear to form naturally in Spirals from
computer simulations
• Barred Spirals outnumber Spirals by about
two to one
• Milky Way may be Barred Spiral
• Bars appear to form naturally in Spirals from
computer simulations
• Barred Spirals outnumber Spirals by about
two to one
• Milky Way may be Barred Spiral
• A theory why all Spirals don’t have bars
proposes that if a galaxy is surrounded by a
large enough halo of Dark Matter bar will not
form.
• Milky Way may have such a halo?
Ellipticals
• No Spiral Arms
• Further sub-divided by numbers
– E0 is roundest
– E7 is flattest
– Category based on observation from Earth, may not be
completely accurate
• Mostly Population II stars, little to no new star formation
occurring in these galaxies
Hubble Tuning Fork Diagram
• Hubble originally proposed that it represented evolution of
galaxies
• Ellipticals would evolve into Spirals
• Ellipticals have little to no rotation
• Modern view is diagram represents grouping by rotation
• Ellipticals are galaxies with little to no rotation Spirals
“class c” have greatest rotations
Types of Irregular Galaxies
• Type I – Many OB associations and H II
regions
• Type II – asymmetrical shapes implying
they resulted from collisions
with other
galaxies or have
violent activity occur within
their nucleus
• Tidal forces from Milky Way believed to be
source of star formation in the two
Magellanic Clouds
How to Determine Distances to Galaxies??
• Standard Candles
– Luminous to shine at great distances
– Luminosity known to a great degree of precision
– Light curve should be identifiable, what kind of variable star
– Common enough to allow for many measurements and hence
many checks
• Use
Intensity α 1/d2
“Standard” Candles??
• Not all objects are “Standard”
• Type Ia Supernova’s – Not all have same peak luminosity.
• Relationship between rate of decrease in luminosity and
peak luminosity
• Tully-Fisher Relation
– Use 21 cm radiation
– Line is broadened due to one edge of galaxy
is rotating towards us, blue-shifted, and one
edge is rotating away from us, red-shifted.
– Rotational velocity provides Mass
– Mass provides number of stars
– Number of provides luminosity
*****
• Fundamental Plane (3 points define a
plane)
– Relate Size, Motion, and Brightness
– Measure Motion and Brightness, determine
Size
– Use apparent Size to Actual Size to get
distance
Tully-Fisher relation is a correlation for spiral
galaxies between their luminosity and how fast they
are rotating.
The idea is that the bigger the galaxy is, faster it is
rotating. That means that if you know the rotation
velocity of the spiral galaxy, you can tell by using
this Tully-Fisher relation its intrinsic brightness
(that is, how bright that galaxy really is).
By comparing the intrinsic brightness with the
apparent magnitude (what you actually observe -because the further the galaxy, the dimmer it
"appears"), you can calculate its distance.
http://www.noao.edu/staff/shoko/tf.html
For a cluster of galaxies where you can
observe tens of galaxies, you can measure the
distance to each galaxy via Tully-Fisher
relation.
Then take the average to calculate the
distance to that cluster. (Because these
clusters are so far away, you can assume that
the actual size of the cluster itself can be
ignored with respect to its distance from us).
http://www.noao.edu/staff/shoko/tf.html
• Here, Tully-Fisher relations of
two clusters at different
distances are shown. The one
shown in the lower part
represent a cluster called Abell
1367 which is much farther
than the Fornax cluster which
is shown in the upper part of
the diagram. The relative
difference between the
distances of two clusters is
estimated by measuring Delta
D as indicated.
http://www.noao.edu/staff/shoko/tf.html
Lasers vs. Masers

LASER – Light Amplification by Stimulated Emission of
Radiation

MASER – Microwave Amplification by Stimulated
Emission of Radiation
Spontaneous
Stimulated
http://www.worldoflasers.com/laserprinciples.htm
http://www.worldoflasers.com/laserprinciples.htm
http://www.worldoflasers.com/laserprinciples.htm
1920’s Hubble and Humanson
• Photographed spectra of many galaxies with the 100 inch
Mt Wilson telescope
• They found most spectra were red-shifted (ie moving
away from us)
• They found the amount of red-shifting was directly
proportional to how close they were to us, more distant
galaxies had greater red-shifts than nearby galaxies
• Redshift (z)
• Doppler Shift
• Hubble Law
  0 
z

0
0

v

0 c
v
z
c
( z  1)  1 v

2
( z  1)  1 c
v  H 0d
H0 is between 40 and 90 km/s/Mpc
2
Relativistic Vs Non-Relativistic Redshifts for speed
v
z
c
vnon rel  zc
( z  1) 2  1 v

2
( z  1)  1 c
vrel
• Consider z = 0.32
 ( z  1) 2  1 
c
 
2
 ( z  1)  1 
vnon rel  .32c
(.32  1)  1
0.7424

c
c  .27c
2
(.32  1)  1
2.7424
2
vrel
vnon rel  2.0c
• Consider z = 2.0
vrel
(2.0  1) 2  1
8

c  c  .8c
2
(2.0  1)  1
10
Uncertainty in H0
• Redshifts fairly reliable
• Distances are not
• Book uses 73 km s-1 Mpc-1
Grouping of Galaxies
• Groups of Galaxies are called Clusters
• Rich Clusters have many galaxies
• Poor Clusters have few galaxies and are sometimes
called groups
• Poor Clusters outnumber Rich Clusters
• Milky Way (our galaxy) is part of the Local Group ~30
galaxies (most are dwarf ellipticals)
Categorizing Clusters
• Regular – Spherical in shape obvious
concentration of galaxies at its center
• Irregular – Not Regular!
• Local Group is irregular
Shape of Clusters
• Shape of Cluster is determined by which type of galaxies
dominate its make-up
• Regular Clusters have mostly ellipticals and lenticular
galaxies
• Irregular Clusters have a more even mixture of galaxies
Groupings of Clusters
• Clusters group to form Superclusters
• Typically dozens of clusters spread over a a region of
space 30 Mpc across
1980’s mapping of clusters
• Use redshifts to get a 3-dimensional mapping of
galaxy locations.
• Discovery of spherical voids which are either
empty or contain Hydrogen clouds or dim
galaxies
• Voids are about 30 to 120 Mpc in diameter
• Galaxies are concentrated in “Sheets” on the
surfaces surrounding and between the Voids
• Think soapsuds in a sink with soapfilms
surrounding air bubbles
Great Walls of Galaxies
• In the 3-D map we find two large
structures the “Great Wall” in the northern
part of the map and the “Southern Wall” in
the southern part of the map.
• Great Wall extends ~150 Mpc along the
arc and 5 Mpc perpendicular to the
surface
• Southern Wall extends ~100 Mpc
• On scales above 100 Mpc, distribution of
galaxies appears to be fairly uniform
• 21 cm radiation show connections of Hydrogen gas
between galaxies indicating previous interactions
• As with the M 82, M 81 and NGC 3077,
Milky Way and Large Magellanic Cloud have similar
Hydrogen gas connections
Colliding Galaxies?
• Collisions produce hot intracluster gas at temperatures
between 107 and 108 k
• Collisions unlikely to result in Star on Star collisions, too
much empty space between
• Starburst galaxies due to compression of gas within the
galaxy causing star birth
The Antennae Galaxies
http://apod.nasa.gov/apod/ap971022.html
M64: The Sleeping Beauty Galaxy -Gas rotates opposite direction of the stars!!
http://antwrp.gsfc.nasa.gov/apod/ap040211.html
Warped Spiral Galaxy ESO 510-13 (150 Million ly)
http://antwrp.gsfc.nasa.gov/apod/ap030607.html
Warped Spiral Galaxy ESO510-13 (150 Million ly)
http://antwrp.gsfc.nasa.gov/apod/ap990512.html
NGC 1410/1409: Intergalactic Pipeline
http://antwrp.gsfc.nasa.gov/apod/ap010112.html
Cluster is 8 Billion ly away
http://antwrp.gsfc.nasa.gov/apod/ap990722.html
Sagittarius Dwarf to Collide with Milky Way
http://antwrp.gsfc.nasa.gov/apod/ap980216.html
Colliding Galaxies
• Galactic deformations due to tidal forces
• Stars may be ejected from galaxy
• Often the colliding galaxies repeatedly collide until they
merge and form one new galaxy – Galactic Cannibalism!!
*****
• Milky Way on track to collide with Andromeda (M 31) in ~
6 billion years
Third Method to form Spiral Arms?
• Close encounters between galaxies may provide another
way to form spiral arms in addition to Density Wave model
or Self-Propagating Star Formation
http://phys.org/news/2013-04-insightsspiral-galaxies-arms.html
Why do clusters stay together?
• Observed measurements of galactic speeds from Doppler
shift measurements indicate that galaxies are moving too
fast to remain in a cluster.
• The amount of mass required to keep them gravitationally
bound is not present in the visible mass
• Answer is of course Dark Matter
Where is the Dark Matter?
• 1970’s found hot x-ray emitting gas within clusters
• Measuring amount of x-ray radiation provides for about
10% of the needed missing mass
Where is Dark Matter?
• Shape of rotation curves for different galaxies tell us that
the edge of the galaxy is not where the visible part ends
as the rotation should drop back down like Kepler’s law
predicts
• Dark Matter must extend out past the visible edges of
galaxies
http://www.nytimes.com/2013/04/04/science/space/n
ew-clues-to-the-mystery-of-dark-matter.html?_r=2&
Where is Dark Matter?
• No evidence for Dark Matter in Intergalactic Space
• Dark Matter appears to be within and in the immediate
space surrounding galaxies
• The visible components appear to trace the locations of
Dark Matter
• Looking at distant objects (galaxies,
clusters, quasars, etc) is looking back in
time
• Galaxies were bluer in the past
• More vigorous star birth and lots of hot
short-lived O and B stars
• Rich clusters with redshifts of z=0.4
correspond to ~4 billion years ago had
30% spiral galaxies, while nearby rich
clusters (today) have ~ 5% spiral galaxies
Where did the Spirals Go?
• Galactic collisions and mergers
• Collisions often repeated would increase star formation,
possibly using up available gas. Spirals no longer seen
as they are not making new stars along spiral arms
• Tidal forces among galaxies could have disrupted spiral
arm patterns
• Events listed previously while they do
destroy Spirals, they do not create
Ellipticals or Lenticular Galaxies.
• Ellipticals are mostly composed of Pop II
stars that appear to have formed over 4
billion years ago in a burst of star
formation.
• Spirals seem to have a more steadier
approach to star formation
Theories of Galaxy Creation
• 1960’s
• One Huge Primordial Gas Cloud Condenses
• Type of Galaxy depends on Star Formation rate
– Slow rate  Disk shaped galaxy probably Spiral
– Fast rate  Elliptical shaped galaxy formed
• This would account for distribution of Pop II
stars in Halo and Pop I stars in main disk
• Also accounts for highly elliptical orbits of Halo
objects
Theories of Galaxy Creation
• 1977
• Galaxies formed from the merging of several
smaller clouds instead of one large isolated
cloud
• Third theory speculates a large number of fairly
small clouds merge to form a Galaxy
Open Questions on Galaxy Formation
• Where did subgalactic gas clouds come from?
• What happened in early Universe to cause
primordial Hydrogen and Helium to clump into
the size clouds they did? Why not bigger or
smaller?
• What about the 90% of the Universe’s mass
that is Dark Matter? What is it?
http://www.nasa.gov/press/2014/january/nasa-and-esa-space-telescopeshelp-solve-mystery-of-ultra-compact-burned-out/#.U0fwLFcXKSo
• The research, supported by several ground-based
telescopes, solves a 10-year-old mystery about the
growth of the most massive elliptical galaxies we see
today. It provides a clear picture of the formation of the
most massive galaxies in the universe, from their initial
burst of star formation through their development of dense
stellar cores to their ultimate reality as giant ellipticals.
• January 2014
http://www.nasa.gov/press/2014/january/nasa-and-esa-space-telescopeshelp-solve-mystery-of-ultra-compact-burned-out/#.U0fwLFcXKSo
• Through the research, astronomers have determined the
compact ellipticals voraciously consumed the gas
available for star formation, to the point they could not
create new stars, and then merged with smaller galaxies
to form giant ellipticals. The stars in the burned-out
galaxies were packed 10 to 100 times more densely than
in equally massive elliptical galaxies seen in the nearby
universe today, and that surprised astronomers,
according to Toft.
http://www.nasa.gov/press/2014/january/nasa-and-esa-space-telescopeshelp-solve-mystery-of-ultra-compact-burned-out/#.U0fwLFcXKSo
• One group of galaxies is the compact ellipticals. The other group
contains galaxies that are highly obscured with dust and undergoing
rapid star formation at rates thousands of times faster than
observed in the Milky Way. Starbursts in these dusty galaxies likely
were ignited when two gas-rich galaxies collided. These galaxies
are so dusty that they are almost invisible at optical wavelengths,
but they shine bright at submillimeter wavelengths, where they were
first identified nearly two decades ago by the Submillimeter
Common-User Bolometer Array (SCUBA) camera on the James
Clerk Maxwell Telescope in Hawaii.
http://www.nasa.gov/press/2014/january/nasa-and-esa-space-telescopeshelp-solve-mystery-of-ultra-compact-burned-out/#.U0fwLFcXKSo
• When Toft's team compared the samples of the two galaxy
populations, it discovered an evolutionary link between the compact
elliptical galaxies and the submillimeter galaxies. The observations
show that the violent starbursts in the dusty galaxies had the same
characteristics that would have been predicted for progenitors to the
compact elliptical galaxies. Toft's team also calculated the intense
starburst activity inside the submillimeter galaxies lasted only about
40 million years before the interstellar gas supply was exhausted.
http://www.jpl.nasa.gov/images/spitzer/20140129/spitzer20140129-full.jpg
By 1944
• Sagittarius A – Galactic nucleus
• Cassiopeia A – Supernova Remnant
– Both inside of Milky Way
• Cygnus A - Where ??
Cygnus A
• First Photographed by Baade and Minkowski in 1951
• Spectrum showed strong emission lines
• Normally galaxies show absorption lines as light gets
absorbed by star atmospheres.
• Emission lines were strongly redshifted by 5.6% ( z =
0.056)
• Amount of Luminosity for Cygnus A is 107
times the amount of luminosity for an
ordinary galaxy
• 1960 Sandage discovered a star at the
location of a strong radio source 3C 48
• Also contained Strong Emission lines
• Unable to identify chemical composition
1962
• 3C 273 Discovered
• Has a jet coming off one side
• Again Chemical Composition could not be figured out
1963 – Break Through!!
• Schmidt took another look at 3C 273 ‘s
Spectrum
• He realized there were four emission lines
with the same pattern as four lines in the
Balmer portion of the Hydrogen spectrum
• Lines were Red-Shifted!!!
• Recognizing that, distances could be
determined and these objects are outside
of Milky Way
• 3C 273 was determined to have a redshift
of z = 0.158
• 3C 48 was determined to have even larger
redshift of z = 0.367
• Because of their strong Radio emission
and star like appearance, these objects
were called Quasi-Stellar Radio Sources
and later this was shortened to Quasars
• Later there were discovered High Redshift
objects that were quiet in Radio Emissions
These were called initially Quasi-stellar
Objects - QSO’s
• Now Quasars apply to both
• ~10% of Quasars are Radio-loud
• More than 10,000 Quasars are known
today, all have star like appearances
• All have large Redshifts 0.06 < z < 5.8,
Most are at least z = 0.3 (fairly far away)
Where have Quasars gone?
• Nearest Quasar is ~250 Mpc (~800 Million ly)
• No new Quasars in almost a billion years
• Quasars initially quite prevalent, peaked
about 10 Billion years ago and the numbers
have been declining since
Quasars are Incredibly Luminous
• Quasar Luminosities range from 1038 to 1042 W
• Milky Way’s Luminosity is 1037 W
• Radiation from Galaxies is mostly a mixture of
normal star light, hence blackbody radiation
usually from star temps of 2500 to 50,000 K
• Quasars have radiation stronger in the x-rays
and gamma rays implying temps of 100,000 K
• Spikes due to emission lines
• Width of emission lines is broadened due to some gas
clouds move towards us while others move away
Redshift Debate
• 1960’s Arp suggested that the Hubble Law
might not apply to Quasars
• Quasars might actually be closer to us
therefore their luminosity may not be so
large and thus not so unexplainable
• Debate ends in 1970’s when galaxies
seen near quasars have same redshifts
Redshift Debate
• Debate further ended when host galaxies were able to be
imaged
• Difficult to image host galaxy due to galaxy’s relative
dimness compared to quasar
• Redshift of host galaxy matches quasar’s redshift
• Relatively nearby Radio quiet Quasars
tend to be in Spiral Galaxies (z<0.2)
• Radio Loud and more distant Quasars
tend to be in Ellipticals
• Often though host galaxies tend to be
distorted and often have nearby
companion galaxies
• Possible connection between Quasars and
galactic collisions or mergers??
Why Skeptical that Quasars exist??
• Quasars are so much more luminous than
other objects (such as normal Galaxies)
that it just seemed impossible.
• Active Galaxies discovered that bridge the
energy gap between normal Galaxies and
Quasars
• 1943 Seyfert discovered a series of very
luminous Spiral Galaxies. These are now
known as Seyfert Galaxies
Why Skeptical that Quasars exist??
• Quasars are so much more luminous than
other objects (such as normal Galaxies)
that it just seemed impossible.
• Active Galaxies discovered that bridge the
energy gap between normal Galaxies and
Quasars
• 1943 Seyfert discovered a series of very
luminous Spiral Galaxies. These are now
known as Seyfert Galaxies
Seyfert Galaxies
• About as luminous as a weak Quasar
• Little to no radio activity
• Seyfert’s appear to often be the result of colliding galaxies
or are seen in pairs of galaxies which are interacting
• NGC 1275 is actually a colliding Spiral galaxy with an
Elliptical galaxy
Radio Galaxies
•
•
•
•
Radio Galaxies are Elliptical Galaxies
They are Strong in Radio Radiation
Have similar Luminosities as Seyfert Galaxies
First one discovered in1918 by Curtis,
M 87
Types of Radiation from M 87
• Thermal Radiation (Black Body Radiation)
is very prominent in the Central region of
M 87
• Non-Thermal Radiation (Synchrotron
Radiation) is more pronounced in the Jet
Radiation of M 87
• Jet radiation is Polarized, Black Body is
unpolarized but Synchrotron is usually
Polarized
Radio Galaxies
• Radio Galaxies usually have two Radio Lobes that span 5
to 10 times the size of the parent galaxy usually a Giant
Elliptical Galaxy which sits in between the lobes
• These galaxies are often referred to as double radio
sources
Radio Galaxies
• Often found near center of Rich Clusters
• Probably subject to collisions and mergers as Seyfert’s
are
• Energy output is on par with Seyfert’s
• Recently Quasars have been discovered
in between radio lobes (both lobes and
Quasar are highly redshifted)
• Speculation exists that Seyfert Galaxies
may be former Radio Quiet Quasars
• Radio Galaxies are former Radio Loud
Quasars
Blazars
• 1929 BL Lacertae was discovered and
thought to be a variable star due to the
variability of its brightness
• However, the light from the central part
had no features, no emission lines, no
absorption lines
• 1970’s the central light was blocked and
the remainder of the light (fuzz) around the
center showed normal spectral evidence
of an elliptical galaxy
Blazars
• The Light from the center appears to be
polarized synchrotron radiation
• Objects were originally known as BL Lac
objects but now, they are just called
Blazars
• It is now believed that they are double
radio sources (elliptical galaxies) which
are oriented so one of the two jets is
towards us
• Further Evidence of this idea that Jets are
aimed at us comes from apparent Faster
Than Light motion!!
Quasars Vs Blazars
• Quasars generally exhibit lower superluminal speeds
typically (1 to 5) x c
• Blazars typically (5 to 10) x c
• Blazar jets are assumed to be aimed more directly
towards us than Quasar jets are
Active Galaxies
• Seyfert Galaxies – Spirals
• Radio Galaxies – Ellipticals
• Quasars
– Radio Quiet – Spirals
– Radio Loud – Ellipticals
• Blazars
• All have active Galactic Nuclei !!
Limit on Size of Active Objects
• Variations in the intensity of an object can place a limit on
how big it is
• Consider an active object 1 ly in diameter
• It suddenly “Flashes”
• Near side emitted photons reach us
• Middle photons reach us 6 months later
• Far side photons reach us 1 year later
• Intensity increase lasts a full year
• Some Active galaxies flicker in such a way
their volume is ~ 1 light day across
• Energy of a Thousand galaxies being
emitted from a volume the size of our
Solar System !!
• Obvious candidate  Black Hole
Supermassive Black Holes at Galaxy Centers
• As early as 1968, Lynden-Bell proposed
that Black Holes were responsible for
Active Galaxies
• Gravitational energy of gases falling into
Black Holes would provide power for the
immense radiation being seen from these
galaxies
• However these Black Holes are much
larger than the 5 to 10 M() Black Holes in
binary systems
Limits on Luminosity by a Black Hole
• Amount of Radiation from an Accretion disk is
known as the Eddington Limit
• If the Luminosity exceeds this limit, there is too
much radiation pressure present to allow
additional gas to fall inward towards the Black
Hole
• The gases would be pushed outward instead
• With gas pushed away, the luminosity fades
and the pressure is reduced, so gas begins to
fall inward again
Eddington Limit
LEdd
 M 
 L()
 30,000
M 
 () 
• LEdd is the maximum luminosity that can be
radiated by accretion on a compact object
• M is the mass of the compact object
• M() and L () are the Standard Solar Mass
and Solar Luminosity respectively
Quasar 3C 273
• L3C 273 = 3 x 1013 L()
• Assume that this is the
Eddington Limit
LEdd
 M 
 L()
 30,000
M 
 () 
3 10 L()
13
 M 
 L()
 3 10 
M 
 () 
• Mass of Black Hole in 3C 273
9
M

10
M ()
~ 1 Billion Solar Masses
4
• Milky Way Galaxy probably has a Black Hole in its center
of about 3 Million Solar Masses
• Rotational Speeds are not constant in the
core of the galaxy as other models have
predicted
• Nor do they tend toward zero, but rather
there are steep peaks on either side of the
exact center
• These peaks indicate extremely large
mass contained within ~ 5 arcseconds of
galaxy center
• Measurements on M 31 indicate a Black Hole at
its center of 30 Million M() in a volume of 5 pc
• New Techniques using better resolving
telescopes (such as the X-ray Chandra Scope)
have indicated a number of galactic black holes
as they can resolve down to about 0.5
arcseconds
• Quasars not so useful as they are so far away
they have too small an angular size.
• However no other source of energy seems
plausible to create Quasar’s energy output
How Does a Black Hole Explain Active
Galaxies??
• Good Model should explain all facets of
Active Galaxies
– Large Luminosity
– Unusual Spectra
– Variable Light Output
– Strong Energetic Jets
• Unified Model Proposes all Active
Galaxies are different views of same type
of object Supermassive Black Hole with
an Accretion Disk
• Gasses in Accretion Disk follow Kepler’s
third law (conservation of Angular
Momentum) – Gasses on inner orbits
travel faster than gasses on outer orbits
• Faster inner gasses have friction with
slower outer gasses cause outer gasses to
lose kinetic energy and thus fall inward
• As gasses fall inward they are
gravitationally compressed to high
temperatures causing them to glow
(Luminosity)
• Variations in gas density will cause
variations in gravitational compression
which will cause variations in temperatures
and hence variations in Luminosity
(Variations in Active Galaxy Luminosity)
• Inward motion towards center of Accretion
disk is stopped abruptly near hole due to
conservation of Angular Momentum
• Inertia wants rotating objects to move
outward as their rotational speed
increases (pizza dough)
• The point at which the inward force due to
gravity of Black Hole is balanced by this
outward movement due to inertia creates
the inner edge of the Accretion disk
• This inner edge is also where a
shockwave is created due to this sudden
stopping of inward flow
• To relieve the pressure created by this
shockwave outward flows develop as
shown perpendicular to the Accretion disk
• The particles leaving are traveling at high
rates of speed and are thus Relativistic
particles
• These Relativistic Particles are also
charged, hence they create and drag
Magnetic Fields with them
Forming Jets
• Recall with our Sun, differential rotation
rates between the equator and the poles
and the fact that magnetic fields are
anchored somewhat to plasma, caused
twisting of the Sun’s magnetic field
eventually creating sunspots
• In the Accretion Disk Outflows, again
recall the inner part of the accretion disk is
moving more rapidly than the outer parts
thus the magnetic field of the accretion
disk is being distorted into a helix shape
Recent Evidence for Unified Theory
• Hubble Space Telescope views of NGC
4261 Shows two radio lobes extending 15
kpc from the nucleus of the galaxy
• A magnified view shows a disk ~250 pc in
diameter at center of galactic nucleus
• Doppler measured speed of gas and dust
indicate a mass of 1.2 x 109 M() must exist
Types of Seyfert Galaxies
• Type 1 have both broad and narrow emission lines,
usually with narrow weak emission lines superimposed
over the broad lines
• Type 2 have only narrow emission lines
Type 1 Seyfert galaxy, NGC 5548 (to the left)
normal spiral galaxy of similar distance and type, NGC 3277 (on the right).
http://www.astr.ua.edu/gifimages/ngc5548.gif
NGC 3393 Type 2 Seyfert Galaxy
http://nrumiano.free.fr/Fgalax/actives.html
• Type 2 Seyfert due to looking at the
accretion disk more edge on so that only a
small amount of the gas spiraling inward is
seen
• Type 1 Seyfert due to looking at the
accretion disk more along a pole which
would allow more of the turbulent gasses
to be observed
Why No New Quasars?
• Over time the available gas and dust that
could be accreted to a Black Hole is used
up.
• The acretion disk is used up. Unless new
material comes near the black hole, no
more fuel.
• The rest of galaxy is not gone. The
material is just not close to the black hole.
• Galactic Collisions and mergers could
provide new fuel sources