Download PH607 – Galaxies

PH607 – 1 - Galaxies 1
The Milky Way: course begins by considering the largescale structure of our own Galaxy.
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
Galaxies come in many shapes and colours
1. WHAT IS A GALAXY?
Definition. A galaxy is a self-gravitating system
composed of an interstellar medium, stars, and dark
matter.
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Bigger than a star cluster, smaller than a cluster of galaxies.
Compact condensation of baryons near the center of dark matter
halos.
These halos are at least ~10 x more massive ~10 x more extended
than the luminous baryons at the centre.
Most stars / cold gas live in galaxies
Galaxies cover a large range in (stellar) mass:
cD galaxies L∗ =1013 solar luminosities
Milky Way L∗ =1010
Dwarf galaxies L∗ =107
Ultra-faint gal‘s L∗ =103 (Milky Way satellites)
MASS_TO_LUMINOSITY RATIOS ?
Ingredients:Interstellar Medium
Stars
Dark Matter
molecular gas
main-sequence stars
black holes
dust
brown dwarfs
massive BHs
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warm gas(10 K) giant stars
stable neutral particles
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hot gas(10 K)
supergiant stars
Machos
magnetic fields
white dwarf stars
WIMPS
cosmic rays
neutron stars
SO WHAT IS DARK MATTER?
Galaxies: Major Question: How did our Galaxy form?
Monolithic or Hiearchical? Top-down or Bottom-up?
Components: Almost 90% of its mass cannot be
accounted for (the "dark matter" problem).
Cold Dark Matter: bottom-up approach (accretion, merger)
Hot Dark Matter: Top-down (fragmentation)
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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.
Candidates. 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.
Many theories which seek to extend our current understanding of
particle physics predict new particles, often with masses
comparable to those of the W and Z bosons (around 100 GeV 100 times the mass of a proton).
In many cases the lightest of these new particles is stable, never
decaying to lighter particles. These are extremely interesting
candidates to comprise the universe's dark matter.
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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Also, in contrast to MACHOs, there are no known stable particles
within the standard model of particle physics that have all the
properties of WIMPs. The particles that have little interaction with
normal matter, such as neutrinos, are all very light, and hence
would be fast moving or hot.
Hot dark matter would smear out the large scale structure of
galaxies and thus is not considered a viable cosmological model.
This would lead to top-down structure formation.
Dark matter candidates motivated by these problems include
weakly interacting massive particles (WIMPs), superWIMPs, light
gravitinos, hidden dark matter, sterile neutrinos, and axions.
2. What do we SEE as a Galaxy?
Depends on distance, dust, orientation:
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Distant galaxies: collective light of stars
(most distant galaxy: UDFy-38135539, redshift of 8.6, just 600
million years after the Big Bang) but not confirmed! See:
http://arxiv.org/abs/1301.4477
z8_GND_5296 is a dwarf galaxy at a redshift of 7.5 (Lyman-alpha
emission confirmed)
700 million years after Big Bang
4Gpc away (NOW at 13.8 billion years, 9.2Gpc away )
Nearby galaxies : 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? The Units …….
Size: Kiloparsecs………. 1 parsec = 3.262 light years
Or about 200,000 AU
Mass: 1 million ….. 1 billion …. 1 trillion solar masses
Age:
100 million years to 10 billion years
Speed: Hundreds of kilometres/second for small clusters
Thousands of km/s rich clusters
Distances: Megaparsec (local) out to 1000 Megaparsec
Universe: 15,000 Megaparsecs = 15Gpc
Mass: 100 billion galaxies of mass 100 billion solar masses
Luminosity: magnitudes (optical passbands) or fluxes (Jansky)
Andromeda: apparent magnitude is 3.44
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absolute mag (at 10pc) is -21
(mag: negative logarithmic base 2.5, Sun is -26.75 app mag , 4.83
abs. mag))
4. A Historical Timeline of Galaxy Studies
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)
All human cultures have named it:
a Celestial River
a Celestial Road or Path
Our words "Galaxy" and "Milky Way" are derived from
Greek and Latin:
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Greek: Galaxias kuklos = "Milky Band"
Latin: Via Lactea = "Road of Milk"
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.
‘’For the Galaxy is nothing else than a congeries of
innumerable stars distributed in clusters."
PHILOSOPHY speculated:
1750: Thomas Wright proposes that the Milky Way is a
thin spherical shell of stars.

The Sun is located inside the shell about midway
between the inner and outer edges.
1755 Immanuel Kant speculates: Lens-shaped disk of
stars rotating about its centre with no special place for
the Sun.
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
Other "nebulae" are distant, rotating milky ways like
ours. "Island Universes" like our Milky Way

SCIENCE then begun:
The Herschels' Star Gauges
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.
This model assumes:
1. uniform distributed,
2. same absolute magnitudes,
3. no ISM.

Herschel's model of the Milky Way obtained from "star gauges"
along many (683) lines of sight in the Galaxy. The Sun is the
yellow star to the right of centre.
DUST: Herschel, Kapteyn and Shapley were unaware of the
presence of dust in the Galaxy which causes extinction and
reddening of starlight.
The Optical View (above) is dominated by emission from stars and
dust
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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 centre.


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 Shapley and Kapteyn were wrong because of
interstellar extinction;



Shapley took a falsely calibrated P-L relation
because of extinction. Stars are actually closer
Kapteyn was looking into the Galactic plane – only
nearby stars were observed.
Interstellar space is filled with gas and dust
 This dust absorbs and scatters starlight passing
through it,
This makes more distant objects look fainter than they
otherwise would be if there were no interstellar dust.
If left unaccounted for, it can lead to serious
overestimates of Luminosity Distances.

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"?
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1920: Nebulae:- The Shapley-Curtis Debate
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.
Curtis believed that the spiral nebulae are galaxies
like our own lying at distances ranging from 150 kpc
(M31) to 3,000 kpc for the most distant systems.
Shapley believed the spirals were part of our Galaxy
(Island v. Nebular Hypothesis)
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.
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Measurements to the centre of the Milky Way have varied
greatly from 8.5±0.5 kpc to 7.9±0.2 kpc.
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…
DISC: The most prominently visible part of our
galaxy is its thin disc. The disc is about
50,000 parsecs in diameter, but only about
600 parsecs thick. In the near infrared:
Stars in the disc 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).
Population I. In the jargon of astronomers, these
young stars, rich in heavy elements, are called
``Population I'' stars.
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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:
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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.
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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 unobscured 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.
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The globular clusters surrounding our galaxy are part of
the halo, and are very old, with ages greater than 10
billion years.
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:


atomic hydrogen gas,
giant molecular clouds,
and some high luminosity

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)
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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), metal-poor
Colour: Several different fundamental properties affect
observed colour:
 Metallicity (metal poor stars are bluer than metal
rich stars)
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 Age (younger stars are bluer)
 Dust (makes stars redder)
Property
Pop I
Orbits:
Circular
Shape
spiral arms
Thickness(pc)
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Metals (%)
3-4
Mass (M )
2x109
Age (yr)
108
Intermediate
Elongated
disk
400
0.4-2
5x1010
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Typical objects:
Open clusters,
HII regions,
OB stars
Sun
Pop II
Very elliptical
spherical/halo
2000
0.4 or less
2x1010
1010
Globular Clusters
The ISM: Galactic dust
Our galaxy contains several 107 M of dust.
The dust is mostly found concentrated in a very thin layer
(~100 pc thick) in the galactic plane.
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
The ISM: 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 H I, temperature is ~ 100K, and the
average temperature of the molecular and the solid (dust)
material is ~ 15K.
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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: the spin-flip
transition.
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.
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Multi-wavelength view of the Galactic Plane
Radio comtinuum: The majority of the bright emission seen in the
image is from hot, ionized regions (free-free), or is produced by
energetic electrons moving in magnetic fields (synchrotron)
21cm: Atomic neutral hydrogen (note extended
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 1: 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.
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Gamma rays 2: The bright, compact sources near Galactic
longitudes 185°, 195°, and 265° are 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.
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