Download PH607 – Galaxies

PH607 – Galaxies
Plan:
L1: The Milky Way Galaxy, Historical perspective, size,
morphology, ingredients, kinematics. Overview, galaxies,
clusters, Hubble’s Law, redshift.
L2: 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.
Structure of galaxy,
Interpretation of spiral and elliptical galaxies.
L3: Evolution of Galaxies Distant Galaxies and Clusters.
Galaxy evolution: mergers, interactions. Starbursts, sub-mm
galaxies.
L4: The Galactic Centre, Active Galaxies & Radio Galaxies
L5: Quasars & Unified Models
L6: Tutorial/Examples/Revision
Other Topics: Luminosity function. Distribution of galaxies
in space: local group, clustering, cluster stability - missing
mass.
Galaxy interaction, Starbursts, Infrared, Submillimetre galaxies. Seyfert galaxies, quasars, observational
properties, central black holes and accretion disks.
Aims: 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/
Major Revision Topics:
Types of galaxies and their contents.
Formation/evolutionary scenarios.
Galaxy Luminosity Function. The Schecter function.
Types of active/radio galaxies. The radio emission.
Jet outflows.
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
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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 brighter 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.
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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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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 0.3Mpc (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). 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. The orbital speed is 217
km/s, i.e. 1 light-year in ca. 1400 years, and 1 AU in 8 days.
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Shapley’s estimate too large because he neglected dust
absorption
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
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.
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
catalog number of M51). Note the two long, bright arms spiraling
outward from its bulge.
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)
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 Very Large Telescope to 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 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.
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 have given added dimension to our knowledge of the
structure of the Milky Way.
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:
 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)
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
o Optical emission lines from hotter gas
o Optical absorption lines from the stellar component
Most rotation curves look very similar:
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!