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PH607 – Galaxies, General Relativity &
Cosmology
Reading Material
Carroll & Ostlie,
An Introduction to Modern Astrophysics, 2nd edition
Plan:
1: Historical perspective. The Milky Way. size, morphology,
ingredients, populations, dark matter, Galactic centre.
kinematics.
2: Structure of Galaxies, History, Hubble classification of
galaxies, types. Mass and dynamics of galaxies, elliptical
galaxies, mass distribution of spherical galaxy, spiral
galaxies, mass distribution. Tully-Fisher, Faber-Jackson
relations. Luminosity functions.
3: Galactic evolution, interaction, mergers, starbursts,
formation
4: Structure of the Universe:Overview, galaxies, clusters,
Hubble’s Law, redshift.
5: Active galactic Nuclei, Quasars, Jets. Superluminal
motions.
6: General Relativity, Black Holes, Lensing
7: Cosmology
8: The Early Universe, Inflation
9: A workshop
10 A test (final lecture period)
Assignments?
Galaxies: Major Question: How did our Galaxy form?
Monolithic or Hiearchical? Top-down or Bottom-up?
The aim of this course is to explore the continuing evolution of
the universe. The scales examined will range from the structure
of individual galaxies up to the geometry of the universe as a
whole.
The course begins by considering the large-scale structure of our
own Galaxy, the Milky Way.
Components: Almost 90% of its mass cannot be accounted for
(the "dark matter" problem).
The Local Group: It then goes on to consider how the Milky
Way fits in with what we see in other galaxies, and what the
morphologies of these systems tell us about their life histories.
Evolution: Galaxies are not isolated, especially in the past……..
AGN: The course then discusses active galaxies, which far
outshine our own. The different types of active galaxies --Seyferts, radio galaxies and BL Lac objects --- will be studied in
some detail. These studies suggest a coherent model in which all
these objects are ordinary galaxies whose excess luminosity is
powered by central supermassive black holes.
The course then turns to quasars, whose redshifts imply that
they lie at huge distances from us. These objects are probably
extreme active galaxies, although alternative interpretations of
their properties will also be presented.
A good internet reference site is:
http://nedwww.ipac.caltech.edu/level5/
and http://nedwww.ipac.caltech.edu/
and a very complete set of (more advanced) notes at:
http://www.astr.ua.edu/keel/galaxies/
Web sites containing of pictures of Nebulae and Galaxies can be
found at: http://seds.lpl.arizona.edu/messier/Messier.html and
http://astro.princeton.edu/~frei/galaxy_catalog.html
Definition? A galaxy is a self-gravitating system composed of
an interstellar medium, stars,and dark matter.
Interstellar Medium
molecular gas
dust
warm gas (104 K)
hot gas (106 K)
magnetic fields
cosmic rays
Stars
Dark Matter
main-sequence stars
black holes
brown dwarfs
massive black holes
giant stars
stable neutral particles
supergiant stars
???
white dwarf stars
???
neutron stars
Structure of the Universe: from beginning to end, requires
GR to understand spacetime. Here we shall apply GR to
black holes, the early universe and the present Universe.
A Historical Timeline of Galaxy studies
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As we look out into the night sky, we see an enormous
number of stars fairly uniformly distributed across the sky
Additionally, on a clear, DARK night we see the Milky
Way – a faint band of light cut by a dark rift stretching
around the sky
In 1610, Galileo (1564-1642) pointed his telescope at the
Milky Way and discovered it could be resolved into
“innumerable” faint stars – thus it is not a “celestial fluid”
but is a stellar system
1750: Thomas Wright proposes that the Milky Way is a
stellar disk system.
1755 Immanuel Kant speculates that there may exist
"Island Universes" like our Milky Way.
1785 William & Caroline Herschel studies star counts
along several hundred lines of sight in the galaxy.
Concluded that Sun lies near the center of a flattened,
roughly elliptical system which is five times wider in the
direction of the plane (assumes uniform distributed, same
absolute mags, no ISM).
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Herschel's model of the Milky Way obtained from "star gauges" along
many lines of sight in the Galaxy. The Sun is the yellowr star to the right
of centre. Herschel, Kapteyn and Shapley were unaware of the presence
of dust in the Galaxy which causes extinction and reddening of starlight .
f
The Optical View (above) is dominated by emission from
stars and extinction by dust.
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1845 William Parsons, Earl of Rosse, using a 72-inch
home constructed telescope in Ireland with a metal mirror
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(size unsurpassed until the 100-inch Mount Wilson
telescope in 1917)
He discovers the "Spiral Nebulae" (Messier 51),
speculates that they may be Kant's Island Universes.
1912 Vesto Slipher at Lowell Observatory observes
brighter spiral nebulae spectroscopically. Spectra show
emission lines from hot gas, absorption lines from stars.
Radial velocities are nearly all positive with values up to
several hundred km/s - later to be determined to be due to
the expansion of the Universe.
1915: Harlow Shapley (1885-1972) used RR Lyrae
variables found in globular clusters (evolved Population
II stars). Assumed that the clusters were uniformly
distributed in space. Every RR Lyrae variable star has a
luminosity of about L = 80 Lsun. Shapley estimated that
the Sun was 15 kpc from the galactic center.
1901-1922 Jacobus C. Kapteyn (1851-1922) made
extensive star counts from photographic plates to
determine the structure of the Milky Way. His model has
become known as Kapteyn's Universe. A flattened
spheroidal system, with the Sun only 650 parsecs from
the centre. Why is it so heliocentric??
Kapteyn's Universe; the Sun is slightly off centre.
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Both were wrong because of interstellar extinction;
Kapteyn was looking into the Galactic plane – only nearby
stars observed. Shapley took a falsely calibrated P-L
relation because of extinction.
1917 G. W. Ritchey observes novae in spiral nebulae.
Unable to reconcile the "nova" S Andromedae observed in
1885 in the Andromeda nebula with other novae in spiral
nebulae - perhaps it is a particularly powerful nova - a
"supernova"?
1920: The Shapley-Curtis Debate
Curtis believed that the spiral nebulae are galaxies like our
own lying at distances ranging from 150 kpc (M31) to 3000
kpc for the most distant systems. Shapley believed the spirals
were part of our Galaxy.
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1923 Edwin Hubble resolves the disks of two nearby spiral
galaxies (M31 and M33) into stars
He discovers Cepheid Variable stars in Messier 31 - the
Great Nebula in Andromeda, estimating its distance as
nearly 0.3 Mpc (modern value is about 0.7 Mpc), well
outside our Galaxy. Hubble’s Law.
Best estimates today – Sun is 8 kpc from Galactic center,
with diameter of 50 kpc. The Sun's place in the Milky
Way is crucial to various galactic calculations.
Measurements to the center of the Milky Way have
varied greatly from 8.5±0.5 kpc to 7.9±0.2 kpc (one of
the most recent measurements in 2005).
The orbital speed is 217 km/s, i.e. 1 light-year in ca. 1400 years,
and 1 AU in 8 days. It would take the solar system about 225250 million years to complete one orbit ("galactic year"), and so
is thought to have completed about 20-25 orbits during its
lifetime.
The Material in our Galaxy: The Stars…
DISK: The most prominently visible part of our galaxy
is its thin disk. The disk is about 50,000 parsecs in
diameter, but only about 600 parsecs thick –
approximately.
Stars in the disk are fairly rich in heavy elements. This
indicates that they are young stars, made from recycled
gas into which planetary nebulae and supernovae have
dumped heavy elements (carbon, oxygen, iron, and so
forth). In the jargon of astronomers, these young stars,
rich in heavy elements, are called ``Population I'' stars.
Note: the Galactic plane lies at an angle of 63 degrees to the
Celestial/Equatorial plane. The North galactic Pole lies at
Dec 12h 51m, RA 27 degrees.
BULGE: At the centre is a relatively small central bulge. The bulge is
about 2000 parsecs in diameter.
Some stars in the bulge are young population I stars. Other stars are
``Population II'' stars, meaning that they are relatively old, and are poor
in heavy elements, having been created before the interstellar gas had
been seriously polluted with elements heavier than helium.
A good view of the central bulge of our Galaxy is given by the nearinfrared picture below, which also shows the disk extending to either
side. The picture was obtained using the COBE satellite. Infrared images
show stellar emission relatively un-obscured by dust, allowing us to
obtain a clear overall view of our galaxy for the first time:
[Image credit: NASA & the Cosmic Background Explorer]
HALO: Surrounding both the disk and bulge is an enormous
spherical halo. The halo is about 100,000 parsecs in diameter,
twice the diameter of the disk.
The stars in the halo are widely scattered. (The Milky Way that
we see in the sky is made of disk stars, not halo stars).
The stars in the halo are all population II stars, very old and
containing few heavy elements. The globular clusters
surrounding our galaxy are part of the halo, and are very old,
with ages greater than 10 billion years. (That is, in the globular
clusters, all stars more massive than the Sun have evolved off
the main sequence.)
Spiral Arms. The disk contains stars, gas, and dust, and displays
spiral arms.
Maps of hydrogen gas reveal that the gas is not spread evenly throughout
the disk, but is concentrated in a few spiral arms. (The Sun is located in
the Orion arm.)
The spiral arms of our Galaxy contain a large fraction of:
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atomic hydrogen gas,
giant molecular clouds,
hot O and B stars.
Since spiral arms contain giant molecular clouds (the material from which
stars are made), and also contain O and B stars (newly made, short-lived
stars), it is apparent that spiral arms are where star formation takes
place.
Because O and B stars are very luminous, spiral arms are very prominent
in snapshots of galaxies similar to our own. For instance, the picture
below is of a galaxy called the Whirlpool Galaxy (also known by its
catalogue number of M51). Note the two long, bright arms spiraling
outward from its bulge. (HST image)
Stellar Populations:
Population I: objects closely associated with spiral arms –
luminous, young hot stars (O and B), Cepheid variables,
dust lanes, HII regions, open clusters, metal-rich
Population II: objects found in spheroidal components of
galaxies (bulge of spiral galaxies, ellipticals) – older,
redder stars (red giants), metal-poor
Note several different fundamental properties affect
observed color:
 Metallicity (metal poor stars are bluer than metal rich
stars)
 Age (younger stars are bluer)
 Dust (makes stars redder)
Property
Population
Orbits
Circular
Shape
spiral arms
Thickness(pc)
120
Metals (%)
3-4
Mass (Msun)
2x109
Age (yr)
108
Typical objects
Open clusters,
HII regions,
OB stars
Iintermediate Population II
Elongated
Very elliptical
disk
spherical/halo
400
2000
0.4-2
0.4 or less
10
5x10
2x1010
109
1010
Sun
Globular Clusters
Globular clusters, RR Lyrae stars
Galactic dust
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Our galaxy contains ~several 107 M of dust.
o The dust is mostly found concentrated in a very thin
layer (~100 pc thick) in the galactic plane, although
thin clouds of dust termed "cirrus" can also be found
well away from the plane of the Galaxy.
View of the Galaxy showing the main features
Galactic Atomic & Molecular Hydrogen
Hydrogen gas is a major constituent of the Milky Way --- there
is ~5 109 M of atomic hydrogen in our galaxy – also known as
HI, is ~ 100K, and the average temperature of the molecular and
the solid (dust) material is ~ 15K
In 1944, van de Hulst pointed out that there is a hyperfine
transition in hydrogen in which the relative spins of the proton
and electron change direction:
This transition is observable via the 21cm radiation that it
produces, and has proved crucial in developing our
understanding of the galactic rotation curve. These radio waves
are too long in wavelength to be absorbed by dust, so they
provide an excellent way of peering through the dust
Multi-wavelength view of the Galactic Plane
Age
The age of the Galaxy is currently estimated to be about 13.6 billion (109)
years, which is nearly as old as the Universe itself.
This estimate is based on Very Large Telescope measurements of the
beryllium content of two stars in globular cluster NGC 6397. This
allowed astronomers to deduce the elapsed time between the rise of the
first generation of stars in the entire Galaxy and the first generation of
stars in the cluster, at 200 million to 300 million years.
By including the estimated age of the stars in the globular cluster (13.4 ±
0.8 billion years), they estimated the age of the Galaxy at 13.6 ± 0.8
billion years.
Detailed structure
Observed structure of the Milky Way's spiral arms
As of 2005, the Milky Way is thought to comprise a large barred spiral
galaxy of Hubble type SBbc (loosely wound barred spiral) with a total
mass of about 1012 solar masses, comprising 200-400 billion stars.
A BARRED SPIRAL: It was only in the 1980s that astronomers began to
suspect that the Milky Way is a barred spiral rather than an ordinary
spiral, which observations in 2005 with the Spitzer Space Telescope have
since confirmed, showing that the galaxy's central bar is larger than
previously suspected .
The galaxy's bar is thought to be about 27,000 light years long, running
through the center of the galaxy at a 44±10 degree angle to the line
between our sun and the center of the galaxy. It is composed primarily of
red stars, believed to be ancient.
The bar is surrounded by a ring called the "5-kpc ring" that
contains a large fraction of the molecular hydrogen present in the
galaxy and most of the Milky Way's star formation activity.
Observed and extrapolated structure of the spiral
arms
Each spiral arm describes a logarithmic spiral (as do the arms of all
spiral galaxies) with a pitch of approximately 12 degrees.
There are believed to be four major spiral arms which all start at the
Galaxy's center. These are named as follows, according to the image at
right:
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2 and 8 – 3 kpc and Perseus Arm
3 and 7 - Norma and Cygnus Arm (Along with a newly discovered
extension - 6)
4 and 10 - Crux and Scutum Arm
5 and 9 - Carina and Sagittarius Arm
There are at least two smaller arms or spurs, including:
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11 - Orion Arm (which contains the solar system and the Sun - 12)
Outside of the major spiral arms is the Outer Ring or Monoceros Ring, a
ring of stars around the Milky Way, which consists of gas and stars torn
from other galaxies billions of years ago.
Summary: The galactic disk is surrounded by a spheroid halo of old
stars and globular clusters. While the disk contains gas and dust
obscuring the view in some wavelengths, the halo does not. Active star
formation takes place in the disk (especially in the spiral arms, which
represent areas of high density), but not in the halo. Open clusters also
occur primarily in the disk.
Recent discoveries; extended structure.
1. The Andromeda Galaxy (M31) extends much further than previously
thought. The disk of the Milky Way extends further is a clear possibility
and is supported by evidence of the newly discovered Outer Arm
extension of the Cygnus Arm.
2. With the discovery of the Sagittarius Dwarf Elliptical Galaxy came
the discovery of a ribbon of galactic debris as the polar orbit of
Sagittarius and its interaction with the Milky Way tears it apart.
Similarly, with the discovery of the Canis Major Dwarf Galaxy , a ring
of galactic debris from its interaction with the Milky Way encircles the
galactic disk.
3. On January 9, 2006 Mario Juric and others announced that the Sloan
Digital Sky Survey of the northern sky has found a huge and diffuse
structure within the Milky Way that does not seem to fit within our
current models. The collection of stars rises close to perpendicular to the
plane of the spiral arms of the Milky Way. The proposed likely
interpretation is that a dwarf galaxy is merging with the Milky Way.
This galaxy is tenatively named the Virgo Stellar Stream and is found in
the direction of Virgo about 30,000 light years away.
Spiral galaxies in general:
 Spirals are complex systems, more complex than ellipticals
 Wide range in morphological appearance
 Fine scale details – bulge/disk ratios, structure of
arms, resolution into knots, HII regions, etc.
 Wide range in stellar populations – old, intermediate,
young, and currently forming
 Wide range in stellar dynamics:
 “cold” rotationally supported disk stars
 “hot” mainly dispersion supported bulge & halo
stars
 Significant interstellar medium (ISM)
Kinematics: Spiral Galaxy Rotation Curve
As is typical for many galaxies, the distribution of mass in the Milky Way
is such that the orbital speed of most stars in the galaxy does not depend
strongly on its distance from the center. Away from the central bulge or
outer rim, the typical stellar velocity is between 210 and 240 km/s. Hence
the orbital period of the typical star is directly proportional only to the
length of the path travelled.
This is unlike the solar system where different orbits are also expected
to have significantly different velocities associated with them.
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Like the Milky Way, external spiral galaxies are supported against
collapse by their rotation. (c.f. elliptical galaxies, which are not).
By using the Doppler shifts in spectral lines to measure galaxies'
line-of-sight velocity as a function of position, we can measure
their rotation curves (speed of material following circular orbits
around the centre of the galaxy as a function of radius). We can
derive this quantity from:
o 21cm emission from atomic hydrogen
Optical emission lines from hotter gas
o Optical absorption lines from the stellar component
Most rotation curves look very similar:
o
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For our own galaxy, it is possible to obtain the "rotation curve" --- the
circular velocity as a function of radius --- of the inner part of the Milky
Way using the line-of-sight velocity of atomic hydrogen as measured
from the Doppler shift in its 21cm emission.
Combining the results from these methods, we obtain the following
estimate for the Milky Way rotation curve:
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Although the enclosed mass, M(r), continues to grow apparently
without limit, the enclosed luminosity, L(r), tends to a finite
limit as we reach the edge of the luminous material in the galaxy.
There must therefore be significant amounts of dark matter which
continue to contribute to M(r) out to very large radii.
o Out to the furthest point measured, typical galaxies have a
luminosity of L ~ 1010 L , and a typical enclosed mass of M
~ 1011 M .
 The "mass-to-light ratio" M / L is hence ~ 10 solar
units.
90% of the material in the galaxy is dark!
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Example:
The Sun moves at about 220 km s-1 in a circular orbit around the centre of
the Galaxy, like almost all the stars near the Sun. We can assume that all
the matter is at the Galactic centre, a not too bad approximation.
Let the speed be V0 , the mass of the Galaxy be MG and the distance of
the Sun from the Galactic centre be R0. Then the centrifugal force due to
rotational speed must balance the gravitational force due to the mass of
the Galaxy.
GMG/R02 = V02/R0
where G is the gravitational constant. Hence
MG = V02 R0/G
Substituting values of 8 kpc and 220 km s-1 for the Sun and G = 0.00430
M (km s-1)2 / pc, we get
MG = 1011 M
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However, measurements of the rotation of the outer edge of the Milky
Way show that the stars out there also rotate at 220 km s-1, out to about
20 kpc.
Thus, within a radius of 20 kpc we get a mass of
MG = 2x1011 M
If the light distribution of the Galaxy were proportional to the mass
distribution, then the two mass estimates above would imply that the
amount of light emitted by the 8 kpc region would be the same as the
region from 8 - 20 kpc... whereas measurements show that the 8 kpc
region emits about 10 times more light than the 8 - 20 kpc region.
The major conclusion is that the distribution of emitted light is not
necessarily the same as the underlying distribution of matter.
The Galactic Centre
The nucleus of the Milky Way contains a complex of gas, dust,
stars, supernova remnants, magnetic filaments, and, almost
certainly, a massive black hole at the very center; it lies in the
direction of Sagittarius, around R.A. 17h 46m and Dec. -28° 56'.
The galactic centre harbours a compact object of very large mass,
strongly suspected to be a supermassive black hole. Most galaxies are
believed to have a supermassive black hole at their centre.
Ever since black holes were suggested as the power sources for Active
Galactic Nuclei (AGNs) such as Seyfert Galaxies and QSOs, we have
speculated on whether the center of our galaxy might contain a black hole
Because the center of the Milky Way is by far the closest galaxy nucleus,
we can study details that will remain indistinguishable in other galaxies
for a long time..
"Galactic Center" here will mean the central ~10 parsecs of the Galaxy.
1) the stellar population including evidence for star formation
there in the last 50 million years or even less
2) interstellar material including both ionized gas (HII regions)
and molecular clouds which orbit the Center in a ring with
an inner radius of about 2 pc. Hot dust is also observed.
3) strong magnetic fields (milliGauss) as compared to
elsewhere in the Galaxy
4) a compact radio source called SgrA* which is quite unlike
any another radio source in the Galaxy.
5) radial velocities and proper motions of both stars and gas
which imply the existence of a large, unseen, compact
object. Large means a mass=~2.5x106MSun.
6) The discovery that the radio source SgrA* corresponds to the
dynamical center of the Milky Way and coincides with the
large, dark mass has lead to the realization that SgrA* is a
black hole, albeit a puzzling one:
7)
Lying dead center in the Galaxy is the Sagittarius A Complex,
which is believed to be associated with a black hole, material in
orbit around this object, and a nearby supernova remnant.
Surrounding the galactic center are narrow threads known as
nonthermal filaments (NTFs), the most prominent of which are
called the Arc, the Pelican, and the Snake. These seem to consist
of magnetic flux tubes filled with relativistic electrons, beaming
synchrotron radiation, that have been swept up from adjacent
molecular clouds and hurled along the field lines at incredible
speeds. Another unusual structure in the nucleus is catalogued as
359.1-00.5 and appears to be a superbubble with a cluster of as
many
as
200
newborn
stars
at
its
heart.