1. Galaxies Galaxies are immense star systems.
Each galaxy is an immense swarm of interstellar hundreds of millions to hundreds of billions of stars and fast clouds of interstellar gas.
The stars move along their own orbit, all held together by the force of gravity of the rest of the matter of the galaxy.

2. Types of Galaxies
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3. Galaxies
Hubble introduced a classification scheme which separates most galaxies into elliptical, normal spiral, and barred spiral categories, and then sub-classifies these categories with respect to properties, such as
the amount of flattening for elliptical galaxies and the
nature of the arms for spiral galaxies.
The galaxies that do not fit into these categories are classified separately as irregular galaxies.

5. Spiral Galaxies More than 50% of observed galaxies are spiral galaxies
Properties of spiral galaxies
Typical size range is 109 to 1012 solar masses, with 1011 being the average
The spiral structure is associated with active star birth, with hot, young blue and blue-white stars in these regions….this makes the spiral arms very visible.
The nucleus tends toward more red in color, indicating the presence of more older stars
Rich in gas and dust

6. M81 – Type Sb Spiral

7. NGC
Type
Sc Spiral

8. M95 – Type SBa Barred Spiral

9. NGC 1365 – Type SBb Barred Spiral

10. Elliptical Galaxies
In elliptical galaxies, the Hubble sequencing of E0, E1, E2….E7 relates to how much flattening there is, with virtually none in an E0, and E7 being very elongated.
The classification uses the apparent ellipticity, referring to its appearance on the celestial sphere, not its actual shape.

11. Elliptical Galaxies Properties of Elliptical Galaxies
Size ranges from 107 to 1013 solar masses, so the smallest may be barely larger than a globular cluster, to some of the largest objects in the universe.
Motion of stars in these galaxies is much more random than in spiral galaxies
There is less evidence of younger stars, gas or dust in these galaxies than in spirals

12. Leo 1 – Type E1 (dwarf) Elliptical

13. M110 – Type E6 Elliptical

14. Irregular Galaxies
About 3% of galaxies cannot be classified as spiral or elliptical and are classified as irregular, having little symmetry to their structure.
Properties of irregular galaxies
Masses of 108 to 1010 solar masses
Although Hubble’s original classification just lumped them all into one category, we now separate them into normal irregular and peculiar irregular
Peculiar irregular look unusual in some way
Some have been tidally distorted by interactions with other galaxies
Some have violent internal processes occurring
Some have unusual appearance due to obscuring dust

15. Small Magellanic Cloud – Irregular Galaxy

16. M82 – Peculiar Irregular Galaxy

17. Interacting Galaxies
Galaxies sometimes appear to interact and affect one another, causing obvious distortions to one or both, and may have significant connections to how galaxies evolve over time.
Arp 273 (next slide) in the Andromeda constellation
The top galaxy is being significantly tidally distorted, while the lower one is not noticeably affected.

19. The bright blue stars at the edges are regions of star birth triggered by the supernova moves outward.

20. The nuclei are the orange centers; the tails are believed to have been formed by tidal distortion; at least 1000 new stars are forming
Antennae Galaxies

21. Interacting Galaxies
Although most galaxies are moving away from one another, some are moving toward each other – the Andromeda Galaxy is moving toward the Milky Way Galaxy, and a collision is expected in about 3 billion years.

22. Interacting Galaxies
The Milky Way shows a past history of mergers as well.
Streams of stars in the Galaxy’s halo, which appear to be debris from small galaxies that fell into the galaxy and were torn apart.
In the bulge of the Milky Way, the stars show a wide range of ages – how if the system was formed at one time?
Galactic cannibalism, the process of a large galaxy capturing and absorbing smaller galaxies, would explain both of these things.

24. Interacting Galaxies
But if the galaxies are colliding, how do some end up as spirals, others are elliptical, etc….some blue, some red….
Computer models have suggested that if enough stars form at once, the supernova explosions of many massive stars drive gas out of the galaxy, resulting in an elliptical galaxy. With no gas to form new stars, the galaxy grows redder over time.
If the collision leaves quite a bit of gas so that new stars can continue to form, the galaxy stays blue, the gas becomes the disc of the spiral and the elliptical becomes the bulge.

25. Interacting Galaxies
But if the elliptical turns into a spiral if there is a lot of gas in the system, where do the arms come from?
Perhaps a density wave travels around a galaxy’s disk, compressing gas clouds and triggering star formation in a spiral pattern.
Maybe the galaxy’s rotation stretches the regions of star formation into elongated arcs.

26. Active Galaxies
Some galaxies show evidence of extremely violent processes occurring within.
Non-thermal emissions in the radio to x-ray frequencies
Jets and unusual structures associated with the appearance of the galaxy
Sometimes the activity is a result of interactions between galaxies, but sometimes, the activity appears to be internal to the galaxy.
These are called active galaxies.

27. Active Galaxies
Seyfert galaxies (named for the astronomer who identified them) are usually spiral galaxies with very bright (almost starlike) nuclei.
They exhibit a strong continuum from the IR thru the xray frequencies of the spectrum
Brightness fluctuates, causing changes in appearances, sometimes over just a few minutes.
Information on Seyfert galaxies –
NGC7742

28. Active Galaxies
Radio galaxies are usually elliptical and exhibit a jet structure from compact nuclei.
Generally exhibit two lobes of radio frequency emission which may extend for millions of light years
Usually the lobes are symmetrically located on opposite sides of the galaxy
Some of the energy comes from the visible body of the galaxy and tiny nuclear region, less than a light year across

29. M87

31. Active Galaxies
In the 1960s, objects that were emitting radio waves but were identified as stars, were determined to have very unusual optical spectra.
They determined that these objects were actually Doppler shifted away by a large amount – they were moving away from us.
They were thought to be nearby, so it never occurred to scientists that they would be moving away from us so fast.

32. Only 1% of quasars have detectable radio emission.
Active Galaxies
These unusual objects would be called Quasistellar Radio Sources (star-like radio sources) or quasars.
They later found many similar objects that did NOT emit radio waves and called these objects Quasistellar Objects or QSOs.
Now they are all grouped together as quasars.
Only 1% of quasars have detectable radio emission.

33. Active Galaxies
First quasar discovered and the one with the greatest apparent brightness (with its jet) 3C273

34. Active Galaxies
Quasars are very luminous, very distant, active galaxies.
Huge redshift indicates great distance (greater the redshift, the farther away it is.
The farthest ones we have seen are more than 10 billion light years from Earth.
To be that far away and as bright as they are they must be extremely luminous.
Typical quasar is about 1000x more luminous than the Milky Way
Quasars fluctuate in brightness, with large changes over the course of a few hours, indicating extremely small cores. (smaller than a fraction of a light year in some cases!)

35. Active Galaxies
Quasars have the power output of trillions of stars packed into a volume smaller than the Solar System.
This is nuts, so astronomers have looked for any other possibility….
Maybe the large redshift is caused by gravitational redshift instead, and they are actually closer by – which would explain why they are so luminous. But that has other problems.
So scientists have accepted that quasars are what they seem, as incredible as they are.

36. This image of PG is what settled the question – a normal spiral galaxy surrounding a quasar at its core, proving that a quasar does act as a nucleus of an active galaxy.

37. Active Galaxies
Quasars are thought to be powered by supermassive rotating black holes at their centers.
They are believed to be the nuclei of galaxies, but because they are so far away, only the very bright nuclei are usually visible, although some jets and faint galaxy structures have begun to be observed.

38. Active Galaxies
99% of quasars are non-emitting. The prevailing theory is that they turn on when there is matter to feed the black hole engine at their center, and turn off when not feeding.
Hubble has shown evidence of quasars in interacting galaxies, so perhaps they turn off when all matter has been consumed in their original galaxy, and turn back on when their host galaxy interacts with another

39. Supermassive Black Holes
Until recently, evidence of supermassive black holes has been circumstantial.
Pictures from Hubble and Doppler shift have given us more direct evidence.

40. Supermassive Black Holes
M87 - the right side is coming toward the Earth at approx 550 km/s, and the left side is moving away from Earth at about the same speed.
Indicates a huge gravitational field, much larger than the stars there could account for.
This is what you would expect from a supermassive black hole, with part of the matter falling forever into the black hole, and part being ejected by jets as seen in this Messier object.

41. Supermassive Black Holes
Blue shift indicates velocities up to 400 km/s toward us.
The simplest explanation – an accretion disc is feeding a black hole
M84 has an active nucleus, emitting jets of particles that are strong rf sources; its radial velocities suggest a mass of 300 solar masses for the black hole

42. White holes????
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