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Galactic Groupings and Active Galactic NucleiTopics

Clusters and superclusters;Giant Elliptical FormationStarburst and other explosive galaxies;Seyferts, BL Lacs, and Quasars;Radio Galaxies

MotivationClusters 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 BiggerSuperclusters

– 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: QuasarsExtremely 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.

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.

Evidence for Supermassive Black Holes

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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.

Active Galactic Nuclei Power Source Model

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