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PH607 – 1 Galaxies: Major Question: How did our Galaxy form? Monolithic or Hiearchical? Top-down or Bottom-up?

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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 large-scale 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

A good internet reference site is:


and a very complete set of (more advanced) notes at:

Web sites containing images of Nebulae and Galaxies: and


Definition. A galaxy is a self-gravitating system composed of an interstellar medium, stars, and dark matter.


Interstellar Medium Stars Dark Matter

molecular gas main-sequence stars black holes dust brown dwarfs massive black holes warm gas (104 K) giant stars stable neutral particles hot gas (106 K) supergiant stars Machos magnetic fields white dwarf stars WIMPS cosmic rays neutron stars **************************************************

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.

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.

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.

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:

Distant: collective light of stars

(most distant galaxy: UDFy-38135539, redshift of 8.6, just 600 million years after the Big Bang)

Nearby: emission from massive, luminous stars can be resolved.

Edge-on: scattered light from cold dust, thermal emission from warm dust


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

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:

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.

  • 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 assumes uniform distributed, same absolute magnitudess, 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.

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

  • 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.

  • Both Shapley and Kapteyn 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.

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"?

1920: 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 3000 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.

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…

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.

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.

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.

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,

  • 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), metal-poor

  • 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 Intermediate Pop II

Orbits: Circular Elongated Very elliptical

Shape spiral arms disk spherical/halo

Thickness(pc) 120 400 2000

Metals (%) 3-4 0.4-2 0.4 or less

Mass (M) 2x109 5x1010 2x1010

Age (yr) 108 109 1010

Typical objects:

Open clusters, Sun Globular Clusters

HII regions,

OB stars

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.

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.



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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