More Topics in Astronomy OLLI at UNCA, Spring 2017 Mark Whipple Week 8: The Universe at Large

More Topics in Astronomy Course Outline 1: Eclipses 2: Moons of the Solar System 3: The Sun 4: The Milky Way 5: Dark Matter 6: The Big Bang 7: Dark Energy 8: The Universe at Large

Part One: What are the different types of galaxies? Elliptical

Irregular

Spiral

Some elliptical galaxies

Elliptical galaxies Elliptical galaxies are classified by their visual shape: E0 appear circular, while E1, E2, etc. are increasingly elongated, up to E7. No E8 or E9 galaxies have been seen.

This classification depends not only on the actual shape of the galaxy, but also on the angle from which we observe it.

Elliptical galaxies have the broadest range in size of any galaxy type. The smallest are about 3000 light years across, while the largest span about 700,000 ly. (The Milky Way’s disk has a diameter of about 100,000 ly.) The smallest elliptical galaxies (known as “dwarf ellipticals”) are about the same size and similar in structure to a globular cluster—except that they have a substantial amount of dark matter, which globular clusters do not. Elliptical galaxies also have unusually large supermassive black holes at their centers.

Elliptical galaxies are composed almost entirely of old and mostly smaller stars, and there is very little new star formation taking place in them. This corresponds with the small amount of gas and dust that is found in them, material that would be needed to make new stars. The reason for this is believed to be that the free gas and dust has all fallen into the center, while in spiral galaxies the rotation of the disk keeps this from happening. Many elliptical galaxies are believed to be the result of smaller galaxies merging with each other, where the resulting galaxy resembles neither of the originals. Elliptical galaxies are more common now than they were in the early stages of the Universe after the Big Bang.

Some irregular galaxies

Irregular galaxies Irregular galaxies have no clearly defined shape, probably due to them being spirals or ellipticals that were distorted by another galaxy.

Irregulars (except dwarf irregulars) may have abundant amounts of gas and dust, with plentiful activity of new star formation taking place. The Small Magellanic Cloud (left image) is a dwarf irregular.

Some spiral galaxies Regular spirals

Barred spirals

Spiral galaxies Most of the large galaxies in the observable Universe are spirals; there are no dwarf spirals. Spiral galaxies consist of a central bulge, a flat disk of spiral arms, and a spherical halo. There is inevitably a supermassive black hole at the center.

About two-thirds of all spiral galaxies, including the Milky Way, are barred spirals. This fraction is increasing as the Universe evolves. Spiral galaxies are classified as barred or not-barred, and by the tightness of the winding of the spiral arms.

Face-on view

Angled view

Edge-on view

We see each spiral galaxy from only one perspective, and sometimes it is hard to tell what it “really” looks like from that single angle. Perhaps the most difficult galaxy to “see” is our own Milky Way; since we are inside of it, there is no way to see it from a distance and know how it appears from that perspective. For example, it is only recently that we have determined that the Milky Way is indeed a barred spiral.

What causes spiral arms? This is a very good question, because it would seem that they should not exist. Although stars closer to the center of the galaxy move at about the same speed as stars farther out, they have less distance to travel in each orbit, and therefore finish one “galactic year” sooner. This would mean that spiral arms would tend to twist up tighter and tighter until they would blend together and disappear. So why do we see so many spiral galaxies, and why are the spiral arms so durable?

No farewell to arms? There are two popular explanations for why spiral arms are so prevalent in the Universe: ▪ One is called the density wave theory. It says that there are high density regions that move more slowly than the stars and gas themselves. These “ripples” traveling through the galaxy trigger new star formation in the leading edge of the spiral arms. ▪ The other has to do with supernova explosions, which might trigger shock waves in space that would cause new stars to form in regions of nebular gas. These two theories are not exclusive; they could both be true.

The Tuning Fork Diagram The a, b, and c categories for spiral galaxies indicates how tightly wound the spiral arms are, with a being very tight and c being very loose.

SB are barred spirals, with the same a, b, and c designations. So far there is no evidence that there is any evolution of galaxies from one type to another in this diagram. The variation in galaxies seems to be due to galactic collisions more than any other factor.

And that’s not all There are other classifications of galaxies as well: lenticular, Markarian, peculiar, starburst, and others. There also appears to be a distinct shortage of dwarf galaxies, far below the number predicted by computer simulations. The image here is from the HST Deep Field camera, exposed over many hours in a section of space where there are very few Milky Way stars. Except for those (identifiable by the “X”), almost every object in this image is a galaxy.

When galaxies collide

In contrast to collisions between stars, which are so rare as to be almost nonexistent, collisions between galaxies are relatively common events. This is because the distance between stars compared to their size is vastly greater than the relative distance between galaxies. Here is a computer simulation of the expected collision between the Milky Way and our neighboring Andromeda Galaxy billions of years from now.

Never have I seen a better time for a break

Part B: The Structure of the Universe We know that galaxies are grouped into clusters, and that these clusters are grouped into what are rather unimaginatively called superclusters. The Milky Way is a member of a cluster known as the Local Group, a modest collection of several dozen galaxies of which the Milky Way is second largest, next to Andromeda which is 2.5 million light years away. Both of these galaxies have a set of companion “satellite” galaxies.

The Virgo Cluster If the Local Group is a “modest” size for a cluster, the nearby Virgo Cluster is not. This cluster, centered in the constellation Virgo, contains somewhere between 1000 and 2000 galaxies, whose center is about 54 million light years away.

The most famous member of the Virgo Cluster is known as M87, an enormous elliptical galaxy with a peculiar jet emanating from the central region.

The Virgo Supercluster Both the Local Group and the Virgo Cluster are members, along with about 100 others, of the Virgo Supercluster. The Milky Way is near the center, and the diameter is around 110 million light years.

Laniakea Even the Virgo Supercluster is part of a larger grouping known as Laniakea, first identified in 2014. This collection contains four superclusters, including Virgo, and totals about 100,000 galaxies spread over 500 million light years. Located at the center of this body is something called The Great Attractor…

The Great Attractor Referred to as a “gravitational anomaly,” very little is known about this phenomenon. It seems to be located about 200 million light years away from us, at the center of the Laniakea Supercluster, and affecting the motion of galaxies and galaxy clusters over enormous distances. Due to recent cuts in the scientific research budget, I am unable to provide further details.

The Great Wall Unrelated to the Great Attractor, and also known as the Coma Wall, this is one of the largest structures in the known Universe. It is an enormous “sheet” of galaxies with dimensions 600 x 200 x 16 (in millions of light years) whose nearest point is about 300 M ly from us.

The Size and Structure of the Universe Assumption #1: The Universe is homogeneous. What this means is that on very large scales the Universe is basically the same no matter how far out into space you go. If we are surrounded by galaxies and galaxy clusters, then so is anyone else no matter how far away. One corollary of this assumption is that the Universe has no outer edge; it goes on forever.

Assumption #2: The Universe is isotropic.

This assumption is equivalent to saying that the Universe is the same in all directions. If you look in one direction and see galaxies and galaxy clusters, this will be true in any other direction as well. One corollary of this assumption is that the Universe has no center. Taken together, the assumptions that the Universe is both homogeneous and isotropic is known as the Cosmological Principle. There is also something called the Perfect Cosmological Principle, which states that in addition to being homogeneous and isotropic, the Universe is also the same at all moments in time. The evolution of the Universe since the Big Bang has made this principle discredited among astronomers.

Does the Cosmological Principle imply that the Universe is infinite? Not necessarily. If the Universe is closed (meaning that the curvature of space is such that if you travel far enough in a straight line, you come back to where you started), then the number of galaxies is finite (although ridiculously large!). However, to the extent that we can be sure of this now, it does not appear that we live in a closed Universe, so the number of galaxies does seem to be infinite.

Important note You may sometimes see claims that there is only a finite number of galaxies, with the most popular number being two trillion. What this usually means is the number of galaxies in the observable Universe, which is the space we can see with our telescopes (including those we have not yet built or imagined), and the galaxies that we could theoretically detect. There could be other galaxies that are so far away that the light coming from them has not had time to reach us yet since the Big Bang! Any galaxy more than 14 billion light years away would be undetectable to us.

The 2dF Galaxy Redshift Survey This study took five years of data and was published in 2003. It measured over 200,000 galaxies in a region of space near the galactic poles (so that as little of the Milky Way as possible would be included). In effect, it was measuring a “slice” of the Universe that could be seen on a flat surface.

If we expand the survey out farther, we can see the data start to thin out, as objects become harder to observe. Since that is an expected result, the data does tentatively support the Cosmological Principle (so far!).

Voids Cosmic voids are huge “bubbles” of space where very few galaxies are found, and which seem to be lacking in dark matter as well. They are surrounded by “filaments,” which can be walls, sheets, or threads composed of thousands of galaxies.

Conclusions Galaxies are not distributed randomly in space. They are organized into clusters, which themselves are grouped into superclusters. Galaxy clusters and superclusters seem to be drawn into regions which contain large amounts of dark matter, rather than the reverse.

The Universe does seem to be both homogeneous and isotropic (no outer edge, and no center), but only on the very largest scales (at least a hundred million light years). The Universe does appear to be infinite, although only a finite region of it is observable to us. The age of 13.7 or 13.8 billion years since the Big Bang does seem to be holding up.

The Multiverse Many physicists (including myself) believe in the idea that we live in just one of the infinite Universes that lie parallel to our own. Every instant of time this Universe branches off into new ones, where all possible events take place. Some of these Universes may even have different physical laws than our own: a different speed of light for example. Some of them may have laws that make stars and galaxies impossible. Computer simulations have shown that only a delicate balance of physical constants make conditions for stars, planets, and people; even small changes in those values make stars that don’t shine, galaxies that collapse into black holes, atoms that don’t form into molecules. Either we got incredibly lucky, or…who knows?

And for my class reps: Betsie, David, and Barbara: Many tanks. And that goes for all of you, including my unofficial class rep Diane.