Galaxies (continued)

Artist's Conception

Take a Giant Step Outside the Milky Way

Example (not to scale)

90% of Matter in Milky Way is Dark Matter

Gives off no detectable radiation. Evidence is from rotation curve:

RotationVelocity (AU/yr)

Solar System Rotation Curve: when almost all mass at center, velocity decreases with radius ("Keplerian")

R (AU)

10

5

1

1 10 20 30

Curve if Milky Way ended where radiating matter pretty much runs out.

observed curve

Milky Way Rotation Curve

Not enough radiating matter at large R to explain rotation curve => "dark" matter!

Dark matter must be about 90% of the mass!

Mass of Milky Way

6 x 1011 solar masses within 40 kpc of center.

Composition unknown. Probably mostly exotic particles that hardly interact with ordinary matter at all (except gravity). Small fraction may be brown dwarfs, dead white dwarfs.

Most likely it's a dark halo surrounding the Milky Way.

The Variety of Galaxy Morphologies

Galaxy Classification

Spirals Ellipticals Irregulars

barred unbarred E0 - E7 Irr I Irr IISBa-SBc Sa-Sc "misshapen truly spirals" irregular

Hubble’s 1924 "tuning fork diagram"

bulge less prominent,arms more loosely wrapped

Irr

disk and large bulge, but no spiral

increasing apparent flatness

Still used today. We talk of a galaxy's "Hubble type"

Milky Way is an SBbc, between SBb and SBc.

What the current structure says about a galaxy’s evolution isstill active research area.

Ignores some notable features, e.g. viewing angle for ellipticals, number of spiral arms for spirals.

bulge less prominent,arms more loosely wrapped

Irr

disk and large bulge, but no spiral

increasing apparent flatness

Messier 81 – Sa galaxy Messier 101 – Sc galaxy

Sa vs. Sc galaxies

Irr I vs. Irr II

Irr I (“misshapen spirals”) Irr II (truly irregular)

Large Magellanic Cloud Small Magellanic Cloud

These are both companion galaxies of the Milky Way.

bar

poor beginningsof spiral arms

Similar to halos of spirals, but generally larger, with many more stars. Stellar orbits are like halo star orbits in spirals.Stars in ellipticals also very old, like halo stars.

Orbits in a spiral

An elliptical

Ellipticals

A further distinction for ellipticals and irregulars:

Giant vs. Dwarf

1010 - 1013 stars 106 - 108 stars 10's of kpc across few kpc across

Dwarf Elliptical NGC 205

Spiral M31

Dwarf Elliptical M32

In giant galaxies, the average elliptical has more stars than the average spiral, which has more than the average irregular.

What kind of giant galaxy is most common?

Spirals - about 77%Ellipticals - 20%Irregulars - 3%

But dwarfs are much more common than giants.

"Star formation history" also related to Hubble type:

amount of star formation

time (billions of years)

Ellipticals formed all their stars early, no gas left. Stars are old, red, dim.

14 (now)1

Irregulars have a variety of star formation histories.

Spirals still have star formation, and gas.Luminous, massive, short-lived stars make spirals bluer than ellipticals

amount of star formation

time (billions of years)1 14 (now)

How Far Away are Galaxies?

For "nearby" (out to 20 Mpc or so) galaxies, use a very bright class of variable star called a "Cepheid".

luminosity

time (days or weeks)

Cepheid star in galaxy M100 with Hubble. Brightness varies over a few weeks.

averageluminosity

From Cepheids in Milky Way star clusters (with known distances), it was found that period (days to weeks) is related to average luminosity.

So measure period of Cepheid in nearby galaxy, this gives star's average luminosity. Measure average apparent brightness. Now can determine distance to star and galaxy.

Has been used to find distances to galaxies up to 25 Mpc.

(average)

In 1920's, Hubble used Cepheids to find distances to galaxies. Showed that redshift or recessional velocity is proportional to distance:

V = H0 x D (Hubble's Law)

velocity (km / sec) Distance (Mpc)

Hubble's Constant (km / sec / Mpc)

Or graphically. . . Current estimate:

H0 = 65 -75 km/sec/Mpc

If H0 = 70 km/sec/Mpc, a

galaxy at 1 Mpc moves away from us at 70 km/sec,etc.

Get used to these huge distances!

Milky Way

30 kpc

Milky Way to Andromeda

Milky Way to Virgo Cluster

17 Mpc

700 kpc

Structures of Galaxies

Groups

A few to a few dozen galaxies bound together by their combined gravity.

No regular structure to them.

The Milky Way is part of the Local Group of about 30 galaxies, including Andromeda.

About 20 dwarfs in Local Group found since 2004, most with Sloan Digital Sky Survey in NM!

Clusters

Larger structures typically containing thousands of galaxies.

Center of Virgo Cluster of about 2500 galaxies Center of the Hercules Cluster

Galaxies orbit in groups or clusters just like stars in a stellar cluster.

Most galaxies are in groups or clusters.

Another group

Superclusters

Recognizable structures containing clusters and groups. 10,000's of galaxies.

The Local Supercluster consists of the Virgo Cluster, the Local Group and many other groups.

50 Mpc

To search for structure on larger scales, need a new method of finding distances beyond about 25 Mpc.

One is the Tully-Fisher Relation. Please read how this works.

The second is:

Hubble's Law

In 1912, Slipher used spectra of "spiral nebulae" to find essentially all of them are receding from us, that is, show“redshifted” spectral lines. This is before we knew they wereoutside the MW.

Spectra of galaxies in clusters of increasing distance

Calcium absorptionlines

In 1920's, Hubble used Cepheids to find distances to galaxies. Showed that redshift or recessional velocity is proportional to distance:

V = H0 x D (Hubble's Law)

velocity (km / sec) Distance (Mpc)

Hubble's Constant (km / sec / Mpc)

Or graphically. . . Current estimate:

H0 = 65 -75 km/sec/Mpc

If H0 = 70 km/sec/Mpc, a

galaxy at 1 Mpc moves away from us at 70 km/sec,etc.

So get spectrum of a galaxy, measure its redshift, convert it to a velocity, and determine distance.

Hubble's Law now used to unveil Large Scale Structure of the universe. Result: empty voids surrounded by shells or filaments, each containing many galaxies and clusters. Like a froth.

Results from a mid 1980's survey.

Assumes H0 = 65

km/sec/Mpc. Note how scale of structure depends on this.

The localuniverse

300

Mpc

Galaxy Interactions and Mergers

Galaxies sometimes come near each other, especially in groups and clusters.

Large tidal force can draw stars and gas out of them => tidal tails in spirals.

Galaxy shapes can become badly distorted.

Galaxies may merge.

Some giant ellipticals may be mergers of two or more spirals.

Since ellipticals have old stars, most such mergers must have occurred long ago. All gas must have been rapidly turned into stars.

We do see this in many interactions and mergers today - "starbursts": unusually high rates of star formation at centers of merging pairs

These galaxies may be 10-100 times as luminous as Milky Way, fora few x 108 years.

Interactions and mergers are simulated by computers.

See MA Ch 16 Study Area forsimulations of interacting and merging galaxies

How do Galaxies Form?

Old idea: a single large collapsing cloud of gas, like Solar Nebula.

New idea: observations indicate that "sub-galactic" fragments of size several hundred parsecs were the first things to form. Hundreds might merge to form a galaxy.

Deep Hubble image. Small fragments contain several billion stars each. May merge to form one large galaxy. Looking back 10 billion years.

600 kpcFew hundred pc

Schematic of galaxy formation

Subsequent mergers of large galaxies also important for galaxy evolution. Large galaxies continue to swallow small ones today: “galactic cannibalism”.

The Milky Way is still accreting dwarf galaxies

Artist’s impression oftidally stripped stream of starsfrom Sag. dwarf. Predicted insimulations. Later foundobservationally.

Tidally stripped stars from a small galaxy orbiting NGC 5907

In some starbursts, supernova rate so high that the exploded gas combinesto form outflow from disk.

Sometimes a galaxy may pass right through another one, creating a ring galaxy.

Hubble image of The “Cartwheel” galaxy

VLA observations show a bridge of atomic gas connecting Cartwheel and a more distant galaxy.

Another Ring Galaxy: AM 0644-741

Cosmology

The Study of the Universe as a Whole

What is the largest kind of structure in the universe? The ~100-Mpc filaments, shells and voids? On larger scales, things look more uniform.

600 Mpc

The Cosmological Principle

On the largest scales, the universe is roughly homogeneous (same at all places) and isotropic (same in all directions). Laws of physics same.

Given no evidence of further structure, assume:

Hubble's Law might suggest that everything is expanding away from us, putting us at center of expansion. Is this necessarily true?

(assumes H

0 = 65

km/sec/Mpc)

If there is a center, there must be a boundary to define it => a finite universe. If we were at center, universe would be isotropic (but only from our location) but not homogeneous:

UsFinite volume of galaxies expanding away from us into...what, empty space?Then part of universe hasgalaxies and part doesn’t.

But if we were not at center, universe would be neither isotropic nor homogeneous:

Us

So if the CP is correct, there is no center, and no edge to the Universe!

Best evidence for CP comes from Cosmic Microwave Background Radiation (later).

The Big Bang

All galaxies moving away from each other. If twice as far away from us, then moving twice as fast (Hubble's Law). So, reversing the Hubble expansion, all separations go back to zero. How long ago?

H0 gives rate of expansion. Assume H

0 = 75 km / sec / Mpc. So galaxy at

100 Mpc from us moves away at 7500 km/sec. How long did it take to move 100 Mpc from us?

time =

=

= 13 billion years

distancevelocity100 Mpc7500 km/sec

(Experts note that this time is just ).

The faster the expansion (the greater H0), the shorter the time to get to

the present separation.

1H

0

Big Bang: we assume that at time zero, all separations were infinitely small. Universe then expanded in all directions. Galaxies formed as expansion continued.

But this is not galaxies expanding through a pre-existing, static space. That would be an explosion with a center and an expanding edge.

If CP is correct, space itself is expanding, and galaxies are taken along for the ride. There is no center or edge, but the distance between any two points is increasing.

A raisin bread analogy provides some insight:

But the bread has a center and edge. Easier to imagine having no center or edge by analogy of universe as a 2-d expanding balloon surface (this isonly one possible analogy for our universe’s geometry):

(To understand what it would be like in a 2-d universe, read Flatland by Edwin Abbott: www.ofcn.org/cyber.serv/resource/bookshelf/flat10 )

Now take this analogy "up one dimension". The Big Bang occurred everywhere at once, but "everywhere" was a small place.

If all distances increase, so do wavelengths of photons as they travel and time goes on.

When we record a photon from a distant source, its wavelength will be longer. This is like the Doppler Shift, but it is not due to relative motion of source and receiver. This is correct way to think of redshifts of galaxies.

The Cosmic Microwave Background Radiation (CMBR)

A prediction of Big Bang theory in 1940's. "Leftover" radiation from early, hot universe, uniformly filling space (i.e. isotropic, homogeneous). Predicted to have perfect black-body spectrum.

Photons stretched as they travel and universe expands, but spectrum always black-body. Wien's Law: temperature decreases as wavelength of brightest emission increases => was predicted to be ~ 3 K now.