Transcript Slide 1

Galactic Groupings and Active Galactic Nuclei
Topics
Clusters and superclusters;
Giant Elliptical Formation
Starburst and other explosive galaxies;
Seyferts, BL Lacs, and Quasars;
Radio Galaxies
Motivation
Clusters tell us more about dark matter.
The denizens in the distant reaches of our Universe are bizarre
beyond compare.
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Field vs. Cluster Galaxies
Field Galaxies
– Lone galaxies in intergalactic space are called
“field galaxies;”
– The biggest field galaxies are usually large spirals.
Groups and clusters
– Most galaxies are in collections called groups and
clusters;
– 100s to 1000s of galaxies;
– 3 Mpc or more across;
– The Local Group contains 30 or so galaxies,
including the Milky Way Galaxy;
– The biggest cluster galaxies are usually ellipticals.
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Galaxy Superclusters and Bigger
Superclusters
– Associations of groups and clusters, 60-160(+?) Mpc;
– Laniakea, 160 Mpc, 105 galaxies;
– Virgo Supercluster is 33 Mpc, 50,000 galaxies.
Chains, sheets, and voids
– May extend up to one billion light-years across.
Even larger scales
– The universe finally takes on a more uniform look;
– Giant clusters of quasars are inexplicable (1.3 BLY);
– Huge Large Quasar Group is 4 BLY across!;
– Hercules-Corona Borealis Great Wall is also too large (10
GLY) for current models.
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Getting Weird: cD Galaxies
Theme
– Looking further away, we look back in time;
– Galaxies formed within the first 3 billion years;
– The three major galactic types appeared after that.
cDs (Galactic Cannibals)
– Elliptical galaxies dominate the central regions of
rich clusters;
– Central-dominant galaxies and galactic
cannibalism.
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Getting Weird: Starburst Galaxies
A small percentage of present-day galaxies are forming
stars at an unsustainable rate.
Thought to be another consequence of intergalactic
collisions. (M81 seems to have disturbed M82.)
Starbursts may be a mechanism to create ellipticals, by
using up all the cool gas, and allowing supernova or
galactic winds to blow away the rest.
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Getting Weird: Seyfert Galaxies
About 1% of nearby galaxies have extremely bright
nuclei.
Radio interferometry tells us that the bright core source is
less than 3 LY across.
Emission lines include narrow ones, and broad ones that
vary in intensity rapidly.
The conventional thought is that the cores are
supermassive black holes, between 107M and 108M,
and the broad lines are thermally broadened by the hot,
variable, embedded accretion disk.
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Supermassive black holes???
Astronomers and physicists have been forced into accepting them
because nothing else fits observations and physics. Only a
supermassive black hole could explain ultra-compact objects with
such enormous gravities and outrageous luminosities.
As material falls towards the black hole, collisions convert kinetic
energy into heat. The black hole functions to convert gravitational
potential energy into kinetic energy, and then into radiation.
Energy (and high velocity matter) is expelled out of the rotational
poles—the only direction of easy escape.
Q1: What keeps the beam collimation mechanism from eroding?
Q2: How do supermassive black holes form in the first place?
Q3: What determines if a black hole is active or not?
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Getting Weird: Radio Galaxies
Almost invariably a large elliptical galaxy (N.B. ellipticals seem to
have the largest black holes).
The center is an active region only a few light-years across, shooting
out two oppositely directed jets of ionized gas, at near the speed of
light.
At the end of the jets are the lobes, the lobes are up 107 LY long
(N.B. the Milky Way is 105 LY long.
The radio lobes are the result of the jets ramming into the
intergalactic gas, heating, and then spreading out.
Many quasars also have jets and radio lobes.
Many active nuclei of radio galaxies are obscured by donut-shaped
rings of dark molecular clouds.
Radio galaxies with jets pointing at us are called BL Lac objects.
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Getting Weird: Quasars
Extremely bright galactic objects that are so far (up to
28×109Billion LY away), they look like tiny point objects.
They are variable in visible and (especially) X-rays, with periods
of a few hours.
Note: brightness variation periods → size scales.
A wide range of radiation emitted; they are like powerful radio
galaxies, with luminosities exceeding 1000 Milky Way Galaxies
They are thought to be black holes about 106-109M, with an
active accretion disk.
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Evidence for Supermassive Black Holes
Note:
Matter falls into a supermassive black hole.
Gravitational potential energy of matter is converted into
kinetic energy which then heats the gas.
The inward-spiraling matter heats, then emits intense
radiation.
So, the most compelling evidence is produced by matter
just before it falls into the black hole.
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Active Galactic Nuclei Power Source Model
To account for a quasar’s luminosity, the central black hole
must consume 1 M per year or more.
The broad spectrum of an active galactic nucleus is the
result of the varied temperature structure in and around the
disc.
– Hot gas in and above the disc produce x-rays and UV;
– Radiation ionizes the surrounding interstellar gas,
leading to visible light;
– Dust grains in molecular clouds encircling the nucleus
emit infrared radiation;
– Fast-moving electrons give radio emissions.
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