March 21, 2006Astronomy 20101 Chapter 27 The Evolution and Distribution of Galaxies What happens to galaxies over billions of years? How did galaxies form?

March 21, 2006 Astronomy 2010 1

Chapter 27

The Evolution and Distribution of Galaxies

What happens to galaxies over billions of years?How did galaxies form?

Do they evolve over time?How are the galaxies distributed, and why?


Distant Galaxies• Taking advantage of the constant speed of light, we can look back in time to see how galaxies have evolved.

• The most distant galaxies we can see are the oldest.

• Using the Hubble Telescope, astronomers can see back more than 10 billion years, to see galaxies soon after they’d formed.

• The earliest galaxies tend to be small and irregularly shaped.


HST Deep Field• This picture was taken by pointing the Hubble telescope at a tiny region of space, empty of nearby stars or galaxies.

• It required combining many hours of observations to make this picture.

• More than 10,000 galaxies are found in this picture!


Small Galaxies in Deep Field

• These are some of the most distant galaxies yet seen.


Looking Back in Time


Mergers and Collisions• If the earliest galaxies were small and irregular, how did the large, structured galaxies we have today form?

• It is believed that the first galaxies combined -- either through mergers or collisions.– The difference between a merger and a collision is all in the “attitude”.

– Violent mergers are called collisions.– The merger of a larger galaxy with a smaller one is called cannibalism.


Galaxies Can Collide• The collision of two galaxies is evidenced in some distinctive patterns.

Colliding Galaxies

The Galaxy that Collided with the Cartwheel Galaxy


A Ring Galaxy



Star Birth in Distant Galaxy

Blue color is evidence of Otype stars. O stars have shortlives.


Collisions Initiate New Stars


Cartwheel GalaxyNote theblue stars(O type).

Known asa starburst.


The Mice


One Galaxy From Two• NGC 6240• Note the two nuclei visible in the center.

• Probably the collision of two spiral galaxies.

• Large infrared output indicative of heating of dust clouds at the center of the galaxy.


Star Formation Peaked Early

• Measurements and predictions suggest that the rate of star formation peaked around 3 or 4 billion years after the big bang.


Formation of Galaxies• Bottom up picture of galaxy formation.

• Small, irregular clusters of stars pull together under gravity to form more structured galaxies.

• Galaxies are pulled together by gravity to form clusters and superclusters.


The Local Group• Our Milky Way is part of a cluster called the Local Group (an imaginative name, yes?).

• The Local Group spreads over a region about 3 million LY in radius.

• The Local Group has two large spirals, one small spiral, two ellipticals, 13 irregulars, and 14 dwarf ellipticals.

• There may be more irregular and dwarf ellipticals.


The Local Group• Three dimensional view of the Local Group.

• The Milky Way is at the center due to our choice.


Virgo Cluster• Closest large cluster to us • Moderately-rich • Called Virgo Cluster because it is in the direction of the Virgo constellation.

• It has many hundreds of galaxies (mostly spirals and irregulars) distributed into an irregular shape about 10 million light years across.

• It is about 49 to 59 million light years from us.


Virgo Cluster (cont’d)

The center of the Virgo cluster has three giant ellipticals, M 87 (bottom left corner) and M 84, M 86 (center right). M 86 may be a lenticular S0 type instead.

• Some ellipticals in the central part of the cluster

• Includes giant elliptical at the center (M87) that has become so large by gobbling up nearby galaxies.

• Total mass of the Virgo cluster is large enough that its gravity pulls nearby groups of galaxies (including the Local Group) toward it.


Local Supercluster


Mass Near Milky Way


Superclusters• The clustering phenomenon does not stop with galaxies.

• Galaxy clusters “attract” each other in superclusters of 10s to 100s of clusters.

• Mutual gravity binds them together into long filaments (thin, string-like structures) 300 to 900 million LY long, 150 to 300 million LY wide, and 15 to 30 million LY thick on average.

• Discovery of these huge structures made recently from years of taking Doppler shifts of thousands of galaxies.

• Doppler shifts of the galaxies converted to distances using Hubble Law.

• Between the filamentary superclusters are HUGE voids with very few (if any) galaxies.

• Voids are typically 150 million light years across.


The Coma Cluster

• Hickson Compact Group 87

• group of galaxies in orbit about each other


Mapping the Universe• Two pioneers in the mapping of the structure of the universe are Margaret Geller and John Huchra.

• Have taken thousands of spectra of galaxies along thin pie-shaped slices of the sky.


Universe Map• Would take much too long to take spectra of galaxies in every direction in space,

• Map the universe in slices.

• This slice map took about 15 years of data-collection to generate!

• Geller and Huchra's have shown surprising results about the large-scale structure of the universe.

• Other groups of astronomers have joined their efforts to map the universe.


Superclusters and Voids• Arrangements of superclusters and voids look like a bunch of soap bubbles or swiss cheese with the galaxies on the borders of the huge holes.

• What produces the long thin strands of clusters around the huge bubbles of empty space?

• Obviously, gravity is the force at work, but how has it worked to produce these structures?

• Dark matter must play a significant role but how it does that is not known.


The Cosmological Principle• The Sloan Digital Sky Survey is making a more complete map of several slices of the sky.

• One thing we learn is that the universe looks similar in all directions.

• This is embodied in the “Cosmological Principle” which says that universe is “isotropic” and “homogeneous”.


Summary• The earliest galaxies were small, irregular clusters of stars that have merged over time to produce the large, structured galaxies we see today.

• Evidence for the merger and collision of galaxies is visible.

• Galaxies tend to clump into clusters and supercluster, leaving large empty regions of space called voids.

• From the large scale distribution of galaxies we deduce the Cosmological Principle, that the universe is isotropic and homgeneous.

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March 21, 2006Astronomy 20101 Chapter 27 The Evolution and Distribution of Galaxies What happens to galaxies over billions of years? How did galaxies form?