Transcript ASTR2100 - Saint Mary's University | Astronomy & Physics

Chapter 21
Modern Cosmology
Much of the material in this chapter was covered earlier
in Chapter 18.
Chapter 22
The 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.
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 Delays
How 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 Terminology
Antiparticles. A particle sharing all of the attributes of
the original particle but with opposite properties,
e.g. negatively-charged electron vs. positivelycharged 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 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.