Lecture 28: Galaxies

Don't Panic

Sections

Units, Distance Scales, and Orbits

There are some units and distance scales that it is convenient to note or recapitulate at the start.


Table of Units


     AU  = astronomical unit

     lyr = light-year

     pc  = parsec

     1 AU = 1.496*10**11 m

     1 lyr = 0.3066 pc = 6.324*10**4 AU = 9.460*10**15 m

     1 pc = 3.262 lyr = 3.086*10**16 m

     1 kpc = 10**3 pc

     1 Mpc = 10**3 kpc = 10**6 pc



Table of Distance Scales for Galaxies and the Universe


Object                               Characteristic or Order of Size


Large spiral galaxies: disk          30 kpc diameter (CK-379)

Large spiral galaxies: luminous halo 50--100 kpc radius (FK-566)

Large spiral galaxies: dark halo     100--200 kpc radius (FK-566)

Intergalactic nearest neighbors      1 Mpc

Hubble Length                        4220 Mpc (FK-653)
 (characteristic size of 
  observable universe)

The actual radius of the observable universe is rather tricky to describe and its actual value depends on the assumed cosmological model. We will consider it in IAWL Lecture 31: Cosmology. (See also CL-14,46--47.)

The Hubble Length and the observable universe (CL-47; FK-653).

A general remark, not to be forgotten, is that stars, dust, gas, and presumably dark matter essentially orbit the center of mass of galaxies.

The orbits are, however, constantly being perturbed by interactions with other stars, dust, gas, and, perhaps, dark matter, and so are non-repeating and somewhat chaotic.

Types of Galaxies

Edwin Hubble (1889--1953)

CK-388

No-508--510

The main types are spiral (S), barred spiral (SB), elliptical (E), lenticular (SO and SBO), and irregular (Irr).

Hubble

NOT

He thought that ellipticals evolved into spirals or barred spirals which is NOT at all true.

Spirals have net angular momentum; ellipticals do not and there is no easy means of imparting net angular momentum to them.

In fact collisions and mergers of spirals in rich clusters convert spirals to ellipticals (FK-602).]

Question: Hubble is famous for, among other things, for this discovery announced in about 1924.

The expansion of the universe from observations.
The extragalactic nature of the spiral nebulae.
General relativity (GR).

Answer 2 is right. See No-510.

Hubble announced the discovery of the expansion of the universe in about 1929 (No-523). Einstein completed GR by 1915 ( St. Andrews Mathematics Archives: Einstein biography).

The galaxy types are divided into SUBTYPES.

The classification scheme is best illustrated by a Hubble tuning fork diagram.

Hubble tuning fork diagram (CK-394; FK-585; No-508--509).


Some Galaxy Properties for the Modern (i.e., Nearby) Universe

Property         S and SB galaxies     E galaxies        Irr galaxies


Luminous mass    10**9 -- 4*10**11     10**5 -- 10**13   10**8 -- 3*10**10
 (M_Sun)         (Milky Way 2*10**11)  the small end are
                                       dwarf ellipticals  
                                       < a few 10**6

Luminosity       10**8 -- 2*10**10     3*10**5 --10**11  10**7 -- 10**9
 (L_Sun)

Diameter         5 -- 250              1 -- 200          1 -- 10
 (kpc)

Stellar          arms: young Pop I;    Pop II and old    mostly Pop I
 population       bulge and disk:       old Pop I
                  old Pop I and Pop II
                  halo:  Pop II
 
% observed       77                    20                3
 not counting    numbers of ordinary
 many dwarf      spirals and barred 
 ellipticals?    spirals are comparable.

Color            blue/pink/dark        yellow            blue/pink mostly? 
 depends          yellow in bulge
 mainly
 star ages

Gas and Dust     thick in arms         very little       generally rich
                  and throughout disk

References: FK-565,582,583,585; CK-393.

Note 1: CK-393 says that the number ratio of ordinary spirals to barred spirals is 2:1, but FK-583 says 1:2. The web provides no tie breaker.

Note 2: Neither FK-582 nor CK-393 indicate where lenticular galaxies are included for their tables: are they included as spirals or as ellipticals?

Elliptical (E) Galaxies

References:

CK-393

FK-584

Ellipticals generally have little gas and dust and are mostly composed of Population II and old Population I stars. They have little or no STAR FORMATION.

Question:

Population I stars.
Population II stars.
Horizontal branch stars.

Answer 2 is right.

There are actually supposed to have been Population III stars: these were the very first metal-free stars (except for a little lithium and beryllium): they were probably all massive and exploded early on: none survive.

Question:

elements that tend to lose electrons in chemical reactions: in general, they are malleable (can be beaten into sheets), are ductile (can be stretched into wires), possess metallic luster, are solids at room temperature (except mercury), and are generally good conductors of heat and electricity because they have free electrons.
all elements, except hydrogen and helium.
decorations for the chest.

Answers 1 and 2 (SWT-277) are right. Answer 3 is close.

It astro-jargon answer 2 is right, of course.

Ellipticals

The smallest dwarf ellipticals are comparable to GLOBULAR CLUSTERS in mass, but are less compact I think????.

The dwarf ellipticals are quite transparent due their few stars (less than a few times 10**6) and lack of dust and gas: you can see right through them (FK-584).

The subtypes range from E0 (most round on the sky) to E7 (to most elongated on the sky), but these types are somewhat dependent on their orientation on the sky and are not completely determined by their true 3-dimensional shapes.

Unlike spirals and barred spirals (see below), we have little ability to determine the intrinsic shape independent of orientation of ellipticals.

The stars in ellipticals orbit in all random orientations (FK-585).

Question:

ellipticals

relatively small.
huge.
infinite.

Answer 1 is right.

Each star in an elliptical has its own angular momentum that keeps it from falling to the center. But the summing the random angular momentum components, there is a great deal of cancellation and the net angular momentum will be relatively small compared to spirals where the disk stars rotate in one direction.

Angular momentum is actually a vector quantity, but we will say no more.

elliptical

M87 is the giant elliptical galaxy at the center of the Virgo cluster, the nearest large cluster of galaxies at about 15 Mpc away (FK-593,615).

It is about 100 kpc in diameter and since it is rather spherical it has much more luminous mass than the Milky Way. It probably grew so large by galactic cannibalism.

It is surrounded by a rich system of globular clusters which are fairly clear in the image. (FK-615).

Credit: NOAO/AURA/NSF.

Lenticular (SO and SBO) Galaxies

Reference:

FK-584

The word LENTICULAR means lens-shaped.

The SO (unbarred) and SBO (barred) lenticular galaxies have a bulge and a disk, but NO SPIRAL ARMS.

They are sort of middle case between ellipticals and spirals.

Spiral (S and SB) Galaxies

There may also be isolated arm segments: e.g., the Orion arm segment in the Milky Way which the Sun is just outside of (FK-563).

The arms rotate in the direction you would think: i.e., as if their ends were trailing.

IAWL Lecture 7: Spectra

Doppler effect

GRAND-DESIGN ARMS

FLOCCULENT ARMS

Spiral Arms

spirals probably mostly have luminous halos and dark halos like the Milky Way. The luminous halos are harder to see for remote galaxies, of course, but rotation curves indicate massive DARK HALOS as for the Milky Way (FK-600).

The closest spiral to the Milky Way is the Andromeda galaxy (M31).

The objects are shown in their correct size on the sky. Of course, M31 because of it's low surface brightness never looks like this, except to 2-meter plus telescopes and/or longish exposure images. M31 just looks like a nebula in usual visual observation.

Also the Moon and M31 are never juxtaposed like this since M31 is not sufficiently near the ecliptic.

Recall the Moon's angular diameter is about 0.5 degrees. This sets the size scale for M31

M31 at about 900 kpc is the closest spiral galaxy: it is an Sb (Cox-578).

M31 was missed by Ptolemy (circa 100--175 CE) as a cloudy star in his 2nd century CE catalog of 1022 stars in 1048 constellations: he labeled 6 or 7 stars as cloudy (No-113,402). It was recorded by Al-Sufi (903--986) in his Book of the Fixed Star (No-188,402), but Tycho Brahe (1544--1601) missed it his star catalog (No-299,308,402). Simon Mayr observed it telescopically in 1612 (No-402).

Credit: REU program, N.A.Sharp /NOAO/AURA/NSF.

Ordinary Spiral (S) Galaxies

References:

CK-388

FK-583

Sa: large bulges; tightly wound, broad arms.
Sb: moderate bulges, moderately wound, moderately well-defined arms.
Sc: small bulges, loosely wound, narrow arms.

Note that the spirals can be at any orientation to the Earth observer from face-on to edge-on.

The arms of edge-on spirals are hard to see, but the shape of the bulge (which is correlated with the arms) permits the spiral type to be distinguished.

Here is a famous example of an Sa spiral galaxy.

The Sombrero Galaxy is Sa spiral galaxy seen nearly edge-on: it is tilted 6 degrees from its equatorial plane.

It is in the Virgo cluster at the southern edge.

It is about 9 Mpc from Earth and the disk is about 17 kpc in diameter.

The disk's angular diameter is about 6 arcminutes which is about 1/5 of the Moon's angular diameter

The large bulge is the giveaway for being an Sa galaxy even thought the arms are not easily discerned.

Note the strong dust lane in the disk.

There is a swarm of about 2000 globular clusters in the halo. This about 13 times more than has the Milky Way. These globular clusters, like the Milky Way's, are calculated to be about 10--13 Gyr old.

It is hard to tell which of the star-like objects in the halo are foreground stars in the Milky Way and which are globular clusters: but we know some of them are globular clusters.

Credit: NASA and The Hubble Heritage Team (STScI/AURA). For more information see the SEDS M104 page.

Here is an example of an Sc spiral galaxy.

M51 is actually two galaxies: NGC 5194, a large Sc spiral and a smaller companion NGC 5195 which a sort of barred galaxy.

M51 is about 8.5 Mpc away and 20 kpc across.

The spiral nature of some galaxies (historically nebulae) was first discovered from M51 by the Earl of Rosse in 1845apr at Birr Castle, Parsontown, Ireland using the Leviathan of Parsontown (1.83 m diameter telescope) (CK-366; No-435--437).

Credit: Todd Boroson/NOAO/AURA/NSF. For more information see the SEDS M51 page.

Barred Spiral (SB) Galaxies

References:

CK-392

FK-583

SBa: large bulges; tightly wound, thin arms.
SBb: moderate bulges, moderately wound, arms.
SBc: small bulges, loosely wound, loosely lumpy arms.

Note that the barred spirals (like ordinary spirals) can be at any orientation to the Earth observer from face-on to edge-on.

The arms of edge-on barred spirals are hard to see, but the shape of the bulge (which is correlated with the arms) permits the spiral type to be distinguished.

Here is an example of an SBc spiral galaxy.

M83 is at about 5 Mpc.

Credit: Bill Schoening/NOAO/AURA/NSF . For more information see the SEDS M83 page.

Irregular (Irr) Galaxies

Reference:

FK-585

As their name suggests IRREGULARS are rather random in shape.

Actually, there are two subtypes: Irr I's with some of ordered structure and Irr II's which are badly disordered and may often have resulted from violent collisions with other galaxies (FK-585--586).

IRREGULARS are generally smaller than the large spirals and ellipticals.

They are typically rich in dust and gas, and have young and old stars and STAR FORMATION. The Large Magellanic Cloud (LMC) is the nearest (at about 50 kps) and most famous IRREGULAR: an Irr I (FK-500, 585).

The LMC and SMC (Small Magellanic Cloud) are dwarf irregular galaxies orbiting the Galaxy: they are both Irr I's (FK-585).

The LMC is at about 50 kpc from us, and so is within the Galaxy's dark halo. It was the closest known other galaxy until the Sagittarius dwarf elliptical was discovered in 1994 at about 27 kpc and the Canis Major Dwarf (elliptical) in 2003 at about 13 kpc from the Galaxy center (FK-592).

The bright pink region is the Tarantula Nebula which is a giant H II region.

Both LMC and SMC are unaided-eye objects in the southern Celestial Sphere hemisphere that have always been have always been known to southern observers.

In history, Al Sufi mentioned the LMC in 964 A.D., in his Book of Fixed Stars. The bright Canopes of Amerigo Vespucci are probably the Magellanic Clouds. Magellan on his circumnavigation circa 1519 finally brought them permanently into the historical record.

Credit: NOAO/AURA/NSF. See also the SEDS LMC page.

Question:

being Italian.
astronomy.
his oceanic explorations.
having rather curious first name.

I have no idea.

Who's even heard of the eponym of this country and two continents.

Spiral Arms

The SPIRAL ARMS cannot simply be a fixed set of stars and gas: if they were, the arms would wind tighter and tighter and merge after a few hundred million years (

CK-390

FK-568

FK-565

The winding-up problem for the theory that spiral arms have fixed components.

There are, in fact, believed to be two kinds of SPIRAL ARMS formed by different mechanisms: GRAND-DESIGN ARMS and FLOCCULENT ARMS.

GRAND-DESIGN ARMS AND SPIRAL DENSITY WAVES

The GRAND-DESIGN SPIRALS have well defined arms.

Here is an example of a GRAND-DESIGN SPIRAL, I think????.

M51 is actually two galaxies: NGC 5194, a large Sc spiral and a smaller companion NGC 5195 which a sort of barred galaxy.

M51 is about 8.5 Mpc away and 20 kpc across.

Credit: Todd Boroson/NOAO/AURA/NSF. For more information see the SEDS M51 page.

The arms are marked as we have said above by red or pink H II regions, many hot, young, massive blue stars, and dark dust lanes.

The arms consequently look blue and pink with often with a beady appearance due to the H II regions.

Dust lanes often appear on the trailing edge of the arms and the blue stars and H II regions on the leading edge (Sh-274; FK-569).

The stars and dust clouds in these spiral arms are orbiting more rapidly than the arms which are not fixed structures, but SPIRAL DENSITY WAVES that move more slowly than the stars and clouds that pass through them.

The SPIRAL DENSITY WAVES are somewhat like traffic jams on a highway at the location of a moving work crew. The jam moves forward slowly as cars jam up there, but eventually the cars pass through and speed on ahead.

The SPIRAL DENSITY WAVES are in fact regions of compression of gas and the accompanying star formation regions.

Question:

just a coincidence.
a coincidence, but not just a coincidence.
triggered by the compression of the gas.

Answer 3 is right.

But because such stars and regions live only of order 10 Myr and the orbital periods in galaxies are typically of order hundreds of megayears, the hot young, blue stars, and the H II regions never get far from their formation region in the arms before they become extinct. So they stay part of the arms for all or most of their lives.

One might say the SPIRAL DENSITY WAVES are like waves rippling around in a bucket.

But that analogy cannot be pressed too far since water waves are formed by pressure force and downward gravity.

In spiral galaxies, the gravity is strong between matter components and NOT downward. Pressure forces play a role too.

So the actual SPIRAL DENSITY WAVE MECHANISM is complex (Sh-275ff) and not yet fully worked out (FK-570). But vaguely the waves are self-propagating compressions of gas and gravity perturbations.

Because of the viscosity of the gas, SPIRAL DENSITY WAVES should lose energy (kinetic and gravitational potential energy) to heat energy that eventually is probably mostly radiated away as light energy.

Question:

It gets absorbed by planets.
It gets absorbed by stars.
It radiates out into intergalactic space and, perhaps, never interacts with matter again and redshifts due to the cosmological expansion. In redshifting, the photons lose energy to the expansion of space.

Answer 3 is right.

The expansion of space is the ultimate energy sink for electromagnetic radiation.

My explanation is, I think, the correct one, but text book authors are frequently elusive on fine points. See Bo-25,104 and CL-197.

We take up the expansion of the universe in IAWL Lecture 31: Cosmology.

SPIRAL DENSITY WAVES

CK-392

There are two proposed mechanisms.

The BAR of barred spirals creates an asymmetric gravity field that somehow generates the SPIRAL DENSITY WAVES.
Of course, the energy for the waves must then come ultimately from the rotational kinetic or gravitational potential energy of the galaxy matter, and so some spiraling in toward the center of some matter must happen.
Since barred spirals are in fact only of order half of all spirals (CK-393; FK-583: but the two authors disagree somewhat), there must be a second mechanism.
Interactions between galaxies may also generate SPIRAL DENSITY WAVES at the expense of kinetic or gravitational potential energy one supposes.

FLOCCULENT ARMS AND SELF-PROPAGATING STAR FORMATION

The word FLOCCULENT means woolly or fleecy.

FLOCCULENT SPIRAL GALAXIES have poorly defined, fuzzy arms.

Here is an example of a FLOCCULENT SPIRAL.

M33 is about 0.9 Mpc away and about 10 kpc across.

It is part of the Local Group of Galaxies. The Galaxy, the Andromeda Galaxy (M31), and M33 are the only spiral galaxies in the Local Group. M33 is relatively close to the Andromeda Galaxy (FK-593).

M33 is a flocculent spiral with fuzzy, poorly defined spiral arms thought to be formed by self-propagating star formation (CK-389).

The blue areas get there color from hot, massive, main sequence stars and the reddish areas are H II regions.

Credit: T.A.Rector (NRAO/AUI/NSF and NOAO/AURA/NSF) and M.Hanna (NOAO/AURA/NSF). For more information see the SEDS M33 page.

The mechanism that creates FLOCCULENT ARMS is believed to be self-propagating star formation (CK-390; FK-570).

We have discussed SELF-PROPAGATING STAR FORMATION in IAWL Lecture 21: Star Formation.

In brief this is the story of FLOCCULENT ARMS:

There is a large molecular cloud in which star formation is triggered in one part.
Massive, hot young, blue stars (i.e., OB stars) form. Pressure from their radiation and winds and from ejecta from the supernovae they turn into compress nearby regions of the cloud.
This compression triggers a new generation of star formation adjacent to the original one, where star formation is tending to turn off because the cloud material is being pushed away the same pressure that triggers the adjacent star formation.
Thus star formation self-propagates through the large molecular cloud.
But the cloud is so large that it is differentially revolving around the galaxy center. In most galaxies the orbital speed (e.g., in km/s) is fairly constant, but this means that the angular orbital speed (e.g., in degrees/s) falls off with radius from the galactic center.
Thus, as one goes outward in radius, the cloud material is increasingly trailing.
The cloud is getting wound up and is forming an arm.
But the WINDING-UP PROBLEM never arises, because the cloud, the hot young, blue stars, and the H II regions disperse in a complex way after a few tens of megayears and the orbital periods of the material is of order hundreds of megayears.
Because the star forming regions are NOT created by SPIRAL DENSITY WAVES in this case and these regions ARE messy, the arms created are messy: i.e., FLOCCULENT.

Clusters, Superclusters, and Large-Scale Structure

Galaxies are not the largest structures.

They are often grouped into clusters: POOR CLUSTERS (also called groups) with tens of galaxies and RICH CLUSTERS with thousands of galaxies (CK-396ff; FK-592ff).

There are also isolated FIELD GALAXIES.

The number of galaxies in a cluster is often hard to determine since there are many dwarf elliptical galaxies that are hard to find (FK-594).

The clusters appear to be mostly gravitationally bound systems (FK-594).

Question:

GRAVITATIONALLY BOUND

cannot move indefinitely far apart without some extra kinetic energy. Their mutual gravitational attraction holds them in a group.
are experiencing a mutual gravitational attraction.

Both answers are right, but answer 1 is a more complete answer.

Of course, complex interactions among objects in a gravitationally bound system may impart sufficient kinetic energy to individual members to allow them to escape.

Gravitational perturbations from outside the bound system may also lead to escape.

REGULAR CLUSTERS

IRREGULAR CLUSTERS

FK-593

Poor clusters far outnumber rich ones (FK-592) and most galaxies are NOT in rich clusters????.

I'm not sure what fraction of galaxies are FIELD GALAXIES. A lot???.

The Milky Way belongs to an irregular poor cluster with the inspiring name of the LOCAL GROUP.

A schematic map of the Local Group with 23 of its 40 plus members marked on (FK-593).

The LOCAL GROUP has 40 plus members some of which may still be undiscovered because they are hidden by the dust in the disk of the Milky Way (FK-593).

Question:

Canis Major (the Big Dog).
Canis Minor (the Little Dog).
Draco (the Dragon).
Norma.

Answer 1 is right.

Talk about gifts.

The nearest rich cluster is the irregular Virgo cluster with more than 2000 galaxies. It's center is about 15 Mpc away and its overall size scale is about 3 Mpc and it covers an area of 10 X 12 degrees on the sky (FK-593).

To the lower left is M87, a giant elliptical, which is believed to be close to the dynamical center of the cluster. It is about 100 kpc in diameter (FK-615).

The image has cut-out rectangles a the upper left and lower right.

Credit: NOAO/AURA/NSF. For more information see the SEDS Virgo cluster page.

Clusters can themselves be part of SUPERCLUSTERS that can contain tens of clusters and have a size scale of 50 Mpc (FK-594).

The LOCAL GROUP belongs to the VIRGO SUPERCLUSTER (or Local Supercluster) (CM-402).

The VIRGO SUPERCLUSTER on the sky spreads out and reaches at least to constellations Ursa Major (Big Bear), Virgo, Libra, Centaurus, and Hydra.

Question:

VIRGO SUPERCLUSTER

Virgo.
Hydra.
Norma.

Just judging by the name answer 1.

This is right. The physical center is near the Virgo cluster (CM-402).

But one really didn't have enough information to be sure since there are misnomers in astronomy and the VIRGO SUPERCLUSTER is a big shaggy dog.

NOT

FK-594

Thus, their component clusters and isolated field galaxies will move apart with the expansion of the universe (FK-594). We take up the expansion of the universe in IAWL Lecture 31: Cosmology.

The superclusters themselves seem to lie in FILAMENTS (i.e., strings) and SHEETS that delimit vast VOIDS (FK-596).

The VOIDS are roughly spherical and have diameters of 30 to 120 Mpc (FK-596). The VOIDS may have some hydrogen gas and filaments of dim galaxies.

Galaxies and larger groupings collectively are called the large-scale structure.

The 3-dimensional appearance of the large-scale structure is sudsy or foamy (FK-596; CK-396).

This map shows about 1.6 million galaxies in the nearby universe detected at 2.2 microns in the near infrared.

The brightest galaxies are in blue and thus are mostly nearby.

Faintest galaxies are in red, and thus are mostly relatively far away.

Green and yellow are somewhere in between, but the official caption is not specific.

The color scheme thus gives representation of the 3-dimensional large-scale structure.

The filaments, voids, and sudsy nature of the structure is suggested.

The untrained eye finds clusters, but superclusters are harder to recognize.

There is a dark band that mostly lies on the edge of this image with a spur at the top center. This the band where Milky way disk of star and dust blocks our view. The dark band is just an omission of sources.

Credit: Atlas Image [or Atlas Image mosaic] obtained as part of the Two Micron All Sky Survey (2MASS), a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation. Particular Image Credit: 2MASS/T. H. Jarrett, J. Carpenter, & R. Hurt. The image is released to the public domain.

On size scales much larger than about 200 Mpc, the matter distribution in the observable universe begins to look uniform or, as astronomers often say, HOMOGENEOUS (same in all places) and ISOTROPIC (same in all directions) (CM-446; FK-596).

Every box of about 200 Mpc on a side contains about the same amount of mass and more or less the same features on average.

So there is a size limit to structure it seems: about 200 Mpc.

Remember that the size scale of the observable universe, the Hubble length, is 4220 Mpc. So in a sense, we have to look over a considerable part of the observable universe to find the end of structure.

FRACTAL

A FRACTAL is a thing that is or looks the same on all or many scales at least in some approximation. The branches and roots of trees are approximate FRACTALS.

For example, one can make a FRACTAL by drawing an iteration of 3 branches from each branch.

But the observable universe is NOT a FRACTAL and does have a size limit on its structure.]

large-scale structure

The long answer is beyond our scope.

The short answer is that density fluctuations in the early universe led to gravitational accretions in the particular patterns we know see.

Computational simulations that start with a primordial distribution of density fluctuations and try to reproduce the modern large-scale structure are an ongoing enterprise with some success.

But such computations are demanding and involve many approximations including the treatment of the dark matter and dark energy which we discuss briefly below in the section Galaxy Formation.

Galaxy Formation

GALAXY FORMATION is a complex process and is not fully elucidated although we are learning more and more all the time from modeling and observations particularly of cosmologically remote regions which are timewise part of the early universe (

FK-601--603

A major problem is the dark matter must have played major role in formation since it is overwhelmingly most of the mass. But that role is hard to determine exactly.

What the dark energy does is another rather open question.

Dark matter

observable universe

Some fraction is probably very hot and nearly invisible dilute hydrogen/helium gas between galaxies and galaxy clusters. It should radiate in the X-ray region, but with low detectability---it maybe on the verge of discovery now.

Most of the dark matter may be some kind of exotic, very unreactive particles that are spread through space, but clumped somehow in the dark halos of galaxies.

People are trying to detect these particles and there have been a few hints of detections, but nothing solid so far.

Dark energy is an unknown energy that seems to be accelerating the expansion of the universe. Dark energy is discussed in IAWL Lecture 31: Cosmology.

Both the concepts of dark matter and dark energy may be greatly modified or even dispensed with if relativistic MOND theory turns out to real to some degree or other ( Bekenstein, J. D. 2004, An Alternative to the Dark Matter Paradigm: Relativistic MOND Gravitation, astro-ph/0412652). See IAWL Lecture 25: Black Holes for more on relativistic MOND theory.]

GALAXY FORMATION

The BIG BANG (see IAWL Lecture 31: Cosmology) left fluctuations in the density of the early universe gas as we mentioned above in connection with the large-scale structure.

These fluctuations led to gravitational runaways to PROTOGALAXIES and these merged to make galaxies.

The galaxies formed from the ``the bottom up:'' i.e., from smaller objects rather from ``top down'' (i.e., out of galaxy-sized clouds) as was once theorized.

FK-602

These objects are NOT around in the modern universe and so must have merged to make modern galaxies.

Spirals formed from protogalaxies that were mainly still gas. The gas collapsed into disk according to the process we have discussed many times (see ????). Then most stars formed in the disk.

Computer simulations show that BAR FORMATION is quite natural for spiral galaxies. However, a sufficiently massive dark matter halo will inhibit BAR FORMATION. The numbers of ordinary and barred spirals are comparable (CK-393; FK-583).

In ellipticals the early star formation was faster and occurred before a disk of gas could form.

There is then no energy dissipation mechanism for stars which are point-like and super-rarely collide in a hard sense: gravitational interactions are common but these tend not to dissipate kinetic and gravitational potential energy to heat.

Thus the streaming and dissipation mechanism that leads to disk formation for gas does not occur for stars.

Consequently, the stars in ellipticals stayed in a swarm.

The distinction in formation between spirals and ellipticals probably has to do with the richness of the environment.

Ellipticals are more likely in regions of greater density of galaxies.

Ellipticals can also be made by the collisions of spirals which strip the gas and randomize the star orbits.

Such collisions have evidently happened in rich clusters: distant rich clusters have more spirals than local ones.

Question:

finite travel speed of light.
infinite travel speed of light.
the universal expansion.

In the universe, to look far away is to look long ago.

ellipticals

Rich clusters typically have a very hot intergalactic medium that radiates X-rays: temperatures of order 10**7 and 10**8 K.

Such gas is probably shocked heated during collisions that stripped it from the parent galaxies (FK-596).

In both ellipticals and spirals the star formation rate probably peaked early on in the universe.

The following cartoon illustrates this, but the cartoon simplifies things by ignoring that some spirals have merged to make ellipticals.

A schematic plot of star formation rate in ellipticals and spirals (FK-603)