Astronomy 101 Lecture 23, Apr. 23 2003

Normal Galaxies – (Chapter 24 in text)

Our galaxy, the Milky Way, has a mass of 1011 suns. We see many more such galaxies in the universe, about 40 billion in all. We are not unique!!

The ‘nearby ’ Andromeda galaxy is similar to ours and has pronounced spiral arms. The somewhat more distant NGC2997 shows a similar structure, seen here face on.

Others, like M49, are elliptical in shape.

What are the types of galaxies? How are they distributed in space? How do they form and evolve?

Still others are quite irregular in shape.

Spiral galaxies:

Spiral galaxies have a basic disk shape with the spiral arms in the flat disk. There is a central core containing the bulk of the stars, visible here as bright hubs (99% of the light) from which the spirals radiate. And like the Milky Way, there is a sparsely populated halo of stars.

Spirals come with differing structures of arms ranging from very tightly wound (type Sa) to rather loosely wound (type Sc).

Milky Way is typical in size for spirals – about 30 kpc across the disk.

Type Sa Type Sb Type Sc

The arms appear bluish white due to the young, bright O and B stars that produce most of the light (recall that O, B stars have a short Main Sequence lifetime and very large luminosity. So the spiral arms are young.

Spiral galaxies:Seen edge on, the central bulge is apparent. The ‘Sombrero galaxy’ spiral arms are seen edge-on as a dark band, due to the presence of gas and dust, characteristic of star-forming regions.

The central bulge is largest for Type Sa and smallest for Type Sc. The Type Sc galaxies have the largest concentrations of gas and dust.

Barred spiral galaxies also exist, with similar variations in tightness of spiral arms and size of central bulge (Types SBa, SBb, SBc). The spiral arms project from a central ‘bar’ rather than a central ellipsoid.

Elliptical Galaxies:

Galaxies without spiral arms are seen with large variations in size – ranging from giant ellipticals with more than 1012 solar masses (10 times more massive than Milky Way) and a few Mpc (Mega parsecs) across, to dwarf ellipticals with a few million solar masses and 1 kpc across. There are about 10 times as many dwarf ellipticals as giants.

Elliptical galaxies are labelled E1, E2, … E7, according to their eccentricity (departure from spherical) with E1 being nearly spherical.

Type E3

Type E2

The elliptical galaxies contain very little dust and gas, so are not actively making new stars. They resemble the halo of the Milky Way and other spirals.

Irregular galaxies:

Many galaxies have rather non-uniform structure and are called Irregulars. The most famous are the Large and Small Magellenic Clouds that are the nearest neighbor galaxies to the Milky Way (about 50 kpc from the center of our galaxy). Seen from the Southern Hemisphere, they resemble bright clouds several degrees across, and clearly visible to the naked eye.

The Large Magellenic Cloud contains the famous supernova SN1987a seen via neutrinos on earth in 1987.

Magellenic clouds as seen by eye

Irregular galaxies

Some irregulars show signs of violent activity or disruption by collisions. There are many young O and B stars present and much gas and dust, indicating that star formation is going on.

Quite possibly, these irregulars are the result of collisions between galaxies, leaving the fragmented remains behind.

Edwin Hubble, who first discovered galaxies external to ours, provided the classification of their types used today. The ‘tuning fork’ diagram was thought perhaps to be evidence for evolution of galaxies from one type to another – but this seems incorrect. There is no evidence for evolution of spirals into ellipticals or vice versa.

How do the different types of galaxies differ in the amount of dust, gas and young stars? How do spiral arms, central bulge and halo differ?

Mapping the galaxies and the large scale structure of the universe requires methods for measuring distances deep into space.

1. Cepheid variable stars have Period of oscillation that depends on Luminosity. Allows distance measurement to 25 Mpc (to galaxies in our neighborhood). It was Cepheid variables that enabled Hubble to deduce that there were galaxies outside our own in the 1920’s.

All the deep space distance measurements rely on knowing the true brightness or luminosity of an object by some means, and obtaining distance from measuring the apparent brightness on earth:

Apparent brightness ~ Luminosity / distance2

or L = 4d2 Iapp

L

period

2. Tully Fisher method relates the rotational velocity of a galaxy to its absolute luminosity. Use Doppler shift of the 21 cm (radio) spectral line from atomic hydrogen. Radio emission from opposite sides of galaxy are shifted oppositely (to red or blue), broadening the line. Works to about 200 Mpc.

3. Type Ia supernovae have a fixed peak luminosity (they come from 1.4 solar mass carbon stars) so are standard candles. Can be used for distance measurements out to 1 Gpc. (1 Giga parsec=1000 Mpc)

How do astronomers determine distances to very distant galaxies?

Mapping the location of galaxies shows us that they live in clusters of 10’s to 1000’s of galaxies, bound together by their gravitational attraction. Probably the clusters reflect the primordial clumping of matter in the very early universe.

Coma cluster (all but the bright blue spot are galaxies)

Magnified view shows even more galaxies in a ‘small’ region of space.

Our Milky Way galaxy is part of the Local Group – a cluster of 45 galaxies. Milky Way and Andromeda are the two largest galaxies in Local Group; the rest are bound by gravity to Milky Way or Andromeda, and the two portions are gravitationally bound rather like a binary star system.

Local group is about 2 Mpc across.

Milky Way

Andromeda

Virgo cluster, about 18 Mpc from us, is much larger than the Local Group –

2500 galaxies in a space about 3 Mpc across.

What holds the clusters of galaxies together – why don’t the individual galaxies wander off into intergalactic space?

Clusters are found in superclusters – associations of thousands to a million individual galaxies. Our local supercluster, containing the Local Group, Virgo cluster and many others, is 50 Mpc across.

And even these superclusters have structure – bands of rich clusters such as the ‘Great Wall’

Gravity binds these structures together.

On the largest scale, the universe is far from uniform.

View of the universe from our vantage point

As far as we can see into deepest space (here with Hubble telescope in satellite), we see clusters of galaxies !

We started this course with Ptolemaic idea that earth is at the center of the universe.

Then with Copernicus thought the universe centered on the sun.

Then with Shapley, we learned the sun is near the periphery of the Milky Way.

Now Hubble tells us that our Milky Way is not at the center, and is just one of billions of galaxies.

We are pretty insignificant!

Galactic collisions and mergers

Galaxies are not as isolated as stars:

Andromeda and Milky Way are separated by about 1000 kpc and are about 30 kpc across – the ratio of separation to diameter is about 30. Magellenic clouds are much closer to Milky Way.

Like two basketballs on opposite sides of a court (remember there are 45 ‘basketballs’ and ‘baseballs’ in the Local Group!)

Sun and Alpha Centauri (nearest star to us) are separated by about 4.3 light years; diameter of sun is 1.4 million km. Ratio of separation to diameter of stars in our neighborhood is nearly 10 million.

Like two basketballs, one in New York and the other in California

So, relatively speaking there is a lot more empty space between stars than between galaxies.

Thus, collisions between stars are very rare, but collisions of galaxies happen much more often !

When galaxies collide, their individual stars don’t bang into each other, but the gravitational forces tend to disrupt the initial galaxies. Galaxy collisions have been studied in supercomputer simulations to see what results.

Sometimes the colliding galaxies can ‘stick’ together to form a larger one.

Sometimes the two galaxies pass through each other, but modify their shapes, for example making spiral arms where none existed before.

Galactic Merger

simulation

observed

When looking at the very distant galaxies (out to 1000 Mpc) away and measure their velocities relative to us using the Doppler shift of spectral lines Hubble made an amazing discovery: The more distant the galaxy (known from standard candles), the larger the red shift.

This implies that galaxies are moving away from us with speeds that increase with distance from us.

940

600

330

230

18 Mpc

Cosmological red shift

velo

city

distance

Mpc

km/s

The distant galaxies are all receding from us with a velocity that increases in proportion to their distance (Hubble Law):

v = H0 d

H0 is called the Hubble constant.

A plot of velocity and distance for some galaxies. The increase in v with d is clear, but there is some scatter due to ‘proper motion’ of a galaxy relative to its neighbors.

Discovery of the cosmological red shift is one of the most important discoveries of the 20th century

Hubble Law:

v = H0 d

When v is measured in km/s and d is Mpc , H0 = 70 (km/s)/Mpc

(today’s best measurement of Hubble constant: book has H0 = 65 (km/s)/Mpc)

Does this mean that our Milky Way has ‘bad breath’ and all galaxies are rushing away from us?

No, we now understand that the universe as a whole is expanding so that every galaxy is receding from every other galaxy!

For the most distant objects in the universe, we can use the Hubble expansion to estimate distance – measure the red shift to get the velocity of recession and calculate distance

d = v/H0

For the furthest objects in the universe, we can’t use supernovae, Cepheid variable, Tully Fisher to get distance. Explain how the cosmological red shift can be used to estimate distance? How is H0

determined?

Measuring the mass of Galaxies (similar to discussion of Milky Way mass in lecture and recitation)

Use Kepler’s Third Law for a small mass (a star) orbiting a large mass M:

P2 = a3/M

M

v a

Get P from v (Doppler shift): 2a = vP, so P ~ (a/v)

and thus (a/v)2 ~ a3/M or v2 ~ M/a or v ~ √(M/a)

Ifinside the galaxy disk, mass increases as we go further out in radius with M ~ a, expect v ~ constant.

If outside the galaxy, expect mass is all inside the orbit radius a; M is a constant as orbit size increases, expect v ~ 1 / √a

v

a

Expect measured velocity to stay about constant out to edge of galaxy, then decrease like 1/√a a0

edge of galaxy

P is period, a is semimajor axis (radius of orbit) and M is the total mass inside the orbit (we neglect the small mass of the orbiting star compared to the rest of the galaxy.

We see no decrease in velocity v beyond the visible edge of the galaxy ! It seems that there is matter outside the visible stuff, that is exerting a gravitational force.

There is Dark Matter around all galaxies! The total amount of dark matter is about 50 times what we see in visible stars, and about 5 times the total mass of gas, dust and stars.

Its not burnt out stars; it is mostly not neutrinos; we can’t understand it as a bunch of black holes.

Its something new! It may well be new forms of matter that can be found in labs on earth soon.

approximate visible edge of galaxies