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PH607 – 1 - Galaxies 1
Galaxies: Major Question: How did our Galaxy form?
Monolithic or Hiearchical? Top-down or Bottom-up?
The Milky Way: course begins by considering the largescale structure of our own Galaxy.
Components: Almost 90% of its mass cannot be
accounted for (the "dark matter" problem).
The Local Group: We then go 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……..mergers, cannabalism
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 – a Unification Scheme - in which all these objects
are ordinary galaxies whose excess luminosity is powered
by central supermassive black holes.
Quasars. The course then turns to quasars, whose
redshifts imply that they lie at huge distances from us.
These objects are extreme active galaxies.
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 images of Nebulae and Galaxies:
http://seds.lpl.arizona.edu/messier/Messier.html and
http://astro.princeton.edu/~frei/galaxy_catalog.html
1. WHAT IS A GALAXY?
Definition. A galaxy is a self-gravitating system
composed of an interstellar medium, stars, and dark
matter.
Ingredients:Interstellar Medium
Stars
Dark Matter
molecular gas
main-sequence stars
black holes
dust
brown dwarfs
massive black holes
4
warm gas (10 K)
giant stars
stable neutral particles
6
hot gas (10 K)
supergiant stars
Machos
magnetic fields
white dwarf stars
WIMPS
cosmic rays
neutron stars
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MACHO: Massive astrophysical compact halo object, or
MACHO: astronomical object that might explain the apparent
presence of dark matter in galaxy halos.
A MACHO is a small chunk of normal baryonic matter, which
emits little or no radiation and drifts through interstellar space.
Since MACHOs would not emit any light of their own, they would
be very hard to detect.
MACHOs could be black holes, neutron stars, brown dwarfs,
unassociated planets. White dwarfs and very faint red dwarfs have
also been proposed as candidate MACHOs.
Conclusion: not a high fraction of the dark matter. A MACHO may
be detected when it passes in front of or nearly in front of a star
and the MACHO's gravity bends the light, causing the star to
appear brighter in an example of gravitational lensing known as
gravitational microlensing. NOTE: Big Bang doesn’t produce
enough baryons anyway!
WIMP: Weakly interacting massive particles, or WIMPs, are
hypothetical particles serving as one possible solution to the dark
matter problem.
These particles interact through the weak nuclear force and
gravity, and possibly through other interactions no stronger than
the weak force.
Because they do not interact with electromagnetism they cannot
be seen directly.
Because they do not interact with the strong nuclear force they
do not react strongly with atomic nuclei.
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2. What do we SEE as a Galaxy?
Depends on distance, dust, orientation:
Distant: collective light of stars
Nearby: emission from massive, luminous stars can be
resolved.
Edge-on: scattered light from cold dust, thermal emission
from warm dust
3. HOW DO WE MEASURE GALAXIES?
Size: Kiloparsecs………. 1 parsec = 3.262 light years
Mass: 1 million ….. 1 billion …. 1 trillion solar masses
Age: 100 million years to 10 billion years
Speed: Hundreds of kilometres/second
Distances: Megaparsec (local) out to 1000 Megaparsec
Universe: 15,000 Megaparsecs
Mass: 100 billion galaxies of mass 100 billion solar masses
4. 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(see below)
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.
SCIENCE then begun:
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 yellow 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
(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.
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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 were observed.
Shapley took a falsely calibrated P-L relation because of
extinction. Stars are actually closer.
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.
Present Knowledge
Best estimates today – Sun is 8 kpc from Galactic centre, with
diameter of 50 kpc.
Measurements to the centre 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 Sun’s 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
225-250 million years to complete one orbit ("galactic year"),
and so is thought to have completed about 20-25 orbits during
its lifetime. (Age 13.4-13.6 billion years?)
5. The Components
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.
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 RA 12h 51m 26s, Dec +27d 07m 42.0s.
Illustration of Galactic coordinates:
BULGE: At the centre is a relatively small central bulge. The
bulge is about 2,000 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 bulge is given by the near-infrared 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).
Population II: 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.
Star Formation: 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 image 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 & Colours:
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), metalpoor
Note several different fundamental properties affect
observed colour:
 Metallicity (metal poor stars are bluer than metal
rich stars)
 Age (younger stars are bluer)
 Dust (makes stars redder)
Property
Pop I
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
Intermediate
Pop II
Elongated
disk
400
0.4-2
5x1010
109
Very elliptical
spherical/halo
2000
0.4 or less
2x1010
1010
Sun
Globular Clusters
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.
o 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
H I: 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, temperature is ~ 100K, and the average
temperature of the molecular and the solid (dust) material
is ~ 15K.
21 cm: 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
Radio: The majority of the bright emission seen in the image is from hot, ionized
regions, or is produced by energetic electrons moving in magnetic fields
Near Infrared: Most of the emission at these wavelengths is from relatively cool
giant K stars in the disk and bulge
X-rays: extended soft X-ray emission is detected from hot, shocked gas. At the
lower energies especially, the interstellar medium strongly absorbs X-rays, and
cold clouds of interstellar gas are seen as shadows against background X-ray
emission.
Gamma rays: photon energies greater than 300 MeV. At these extreme energies,
most of the celestial gamma rays originate in collisions of cosmic rays with
hydrogen nuclei in interstellar clouds. The bright, compact sources near Galactic
longitudes 185°, 195°, and 265° indicate high-energy phenomena associated with
the Crab, Geminga, and Vela pulsars, respectively.
From: http://mwmw.gsfc.nasa.gov/mmw_sci.html#dirbe
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.