Chapter 22Chapter 22The Origin of StructureThe Origin of Structure

Again, much of the material in this chapter was covered earlier in Chapter 18, although further details are given regarding the location of our Galaxy relative to the local universe.

Chapter 21Chapter 21Modern CosmologyModern Cosmology

Much of the material in this chapter was covered earlier in Chapter 18.

The Orion Belt and Sword region, highly processed, for those who have observed it at the BGO.

The Large Magellanic Cloud (SBm III = giant barred spiral) from UK Schmidt plates. An irregular galaxy sometimes believed to be an incipient barred spiral.

The Small Magellanic Cloud (Im IV-V = irregular dwarf) from UK Schmidt plates. An “inverse C” shape.

The Andromeda Galaxy NGC 224 (M31, Sb I-II = supergiant spiral with large bulge). The companions are

NGC 205 (M110, S0/E5pec) and NGC 221 (M32, E2).

M32

M110

The Triangulum Galaxy NGC 598 (M33, Sc II-III = giant spiral with small bulge). Note the small bulge and

restricted length of the spiral arms.

The Sculptor dwarf spheroidal galaxy (dE) contains only old low-mass stars and no gas or dust.

The Leo II dwarf spheroidal galaxy (dE or dSph).

The Milky Way subsystem of the Local Group.

Where the Local Group stands

relative to the Virgo and Coma clusters of galaxies.

A schematic of the Local Group has dashed lines of constant radius about the system barycentre between

M31 and the Milky Way. Note the spatial proximity of M33 to M31, and the satellites of M31 and the MW.

Other Groups Within 10 Mpc (megaparsecs).Although there are ~40 galaxies (and probably many more of low luminosity) lying within a concentration roughly 1 Mpc across containing the Milky Way, LMC, SMC, M31, and M33, with the NGC 3109/Sextans Group debated with regard to its inclusion, there are a variety of other nearby galaxy groups within 10 Mpc. That includes the Sculptor Group (1.8 Mpc), the M81 Group (3.1 Mpc), the Centaurus Group (3.5 Mpc), the M101 Group (7.7 Mpc), the M66 Group (9.4 Mpc), the M96 Group (9.4 Mpc), and the NGC 1023 Group (9.5 Mpc).The locations of the above groups relative to the Milky Way are depicted in the figure, which is plotted in supergalactic coordinates with the y-axis in the direction of the Virgo Cluster. Note that the local concentrations of galaxies are also associated with regions of low galaxy density, termed voids. That characteristic is also seen on larger scales.

The local concentrations of galaxies, relative to the Milky Way at (0, 0), as plotted in supergalactic cordinates.

Virgo Cluster.The Virgo Cluster is the nearest rich cluster, located on the Virgo/Coma Bernices boundary ~16 Mpc distant, containing ~3,000 galaxies (mostly spirals) or more within a region ~3 Mpc across. The image below contains M86 (top left), M84 (top right), and NGC 4388 (bottom).

Some of the galaxies in the Virgo Cluster actually have blueshifted velocities relative to the Milky Way, because of their large orbital speeds about the cluster barycentre. Many of the spirals in the cluster exhibit evidence of being stripped of gas from their outer regions. X-ray spectra of the cluster centre can be represented by a single-temperature plasma in which the temperature of the intracluster medium decreases with radius from 0′ to 50′, and becomes almost constant beyond the 50′ radius. The metal abundances also decrease with radius from 0′ to 40′, then become constant beyond 40′. The X-ray image of the cluster at right is centred on the peculiar galaxy M87, which appears to dominate the dynamics of the intracluster medium in the Virgo cluster.

Evidence for ram pressure stripping of gas and dust in Virgo cluster galaxies is evident in HST images of the spiral galaxies NGC 4522 (left) and NGC 4402 (right).

Wu & Tremaine (2006) estimate a mass of 2.4 1012 M (2 trillion solar masses ~ 10 Milky Way galaxies) within 32 kpc of M87 according to its globular cluster system.

Note the jet of high-speed particles emanating from the nucleus of M87.

Hubble Space Telescope view of the jet in M87.

The M87 jet at radio wavelengths.

The centre of the Coma cluster of galaxies, a rich cluster containing mainly elliptical and lenticular galaxies.

Distance Indicator Summary

Redshift-distance relation (again).

Slope = Rise/Run

= H0

A cartoon illustration of the cosmological constant.

The 3K microwave background with the Doppler shift removed, as recorded by WMAP.

An example of the concept of inflation in cosmology.

How the “scale factor” changes with

time

The supposed elimination of “antimatter” in the early universe.

Visible evidence for galaxy collisions.

Simulation of a collision between two spiral

galaxies to produce a larger merger product

that will become an elliptical galaxy in this

scenario, since the merging process

triggers a burst of star formation that uses up all of the gas and dust

in the original galaxies.

In the Big Bang model of cosmology, the formation of galaxies like the Milky Way occurred only after several previous stages in which the first stars were formed and

galaxy mergers occurred.

From a paper by British scientist Francis Farley showing how the recession of distant galaxies follows exactly what is calculated using special relativity, without any need for

a missing component such as “dark energy.”

Gravitational Lenses and Time DelaysHow different components of a lensed background object can appear to change in brightness with time, given that the path lengths in the two cases are different.

By 1996 the time delay had been measured in 4 quasars, giving values for the Hubble constant of H0 = 63 ±12 km/s/Mpc, H0 = 42 km/s/Mpc, although another analysis of the same data gave H0 = 60 ±17 km/s/Mpc, H0 = 52 ±11 km/s/Mpc, H0 = 63 ±15 km/s/Mpc, and H0 = 71 ±20 km/s/Mpc. All values are similar to what is implied from galaxy redshifts.

The time delay in separate images A and B of the double QSO 0957+561.

The more usual form of gravitational lensing by a rich cluster of galaxies.

An image of the galaxy ZW 2237+030, Huchra’s Lens, showing the multiple imaging of a background quasar

QSO 2237+0305.

Astronomical TerminologyAntiparticles. A particle sharing all of the attributes of

the original particle but with opposite properties, e.g. negatively-charged electron vs. positively-charged positron, etc.

GUT. An abbreviation for a “grand unified theory,” which unifies all forces into one.

Cluster of galaxies. A group of 50-10,000 galaxies orbiting a common centre of mass.

Supercluster. The term used to describe massive groups of clusters of galaxies occupying an extensive region of space, usually separated by “voids” from adjacent superclusters.

Gravitational lensing. The apparent bending of the light path from a very distant object caused by the gravitational interaction of the light photons with a massive galaxy of cluster of galaxies along the same line of sight.

Sample QuestionsSample Questions

6. What are the main differences between galaxy groups and galaxy clusters?

Answer. A group typically has a few dozen galaxies, dominated by just a few large ones, and a size around a few million light years. A cluster may have up to a hundred or more galaxies and may be a few dozen million light years in size or more.

6. What is the difference between a galaxy cluster and a supercluster? Is our Galaxy part of either? How do we know?

Answer. A supercluster is composed of many clusters, spanning sizes up to many hundreds of millions of light years and with much more mass. Also, clusters are small enough that the galaxies are orbiting each other uniformly, while a supercluster has not had time to “relax” into such a state. We are part of the “local supercluster” that contains the Virgo cluster.