Certificate programme in Science: Astronomy Core Module 2 Galaxies

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Certificate programme in Science Astronomy (Core
Module 2)Galaxies Quasars

• Dr Lisa Jardine-Wright,
• Institute of Astronomy, Cambridge University

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Extra Information

• Chapters of Galaxies Cosmology
• 1.3 1.4
• Chapters of Introduction to Modern Astrophysics
• 22.2 22.3 (mathematics can be ignored)

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Lecture 3 Rotation of the Milky Way

• Observing the structure of the Milky Way
• Milky Way in other wavebands
• Tracing Spiral structure
• Rotation curves
• Milky Way Rotation Curve
• Milky Way Mass

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2.i Electromagnetic Spectrum
Radio 10km - 30cm
Far-IR 1mm - 6µm
Visible 800nm 390nm
X-rays 1nm 6pm
Microwaves 30cm 1mm
Mid-IR 6µm - 3µm
UV 390nm 1nm
?-rays 6pm gt
Near-IR 3µm 800nm
km 103m
cm 10-2m
µm 10-6m
mm 10-3m
nm 10-9m
pm 10-12m
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Milky Way in Different Wavebands
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-10

• Xray
• Observed by the ROSAT satellite.
• X-ray emission is detected from hot, shocked gas
• Colour variations show variation in absorption or
  temperatures of the emitting gas.

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Milky Way in Different Wavebands
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• Optical
• Due to the obscuring effect of the interstellar
  dust, the light is primarily from stars within a
  1000 lt yrs of the Sun.
• Dark patches are due to absorbing clouds of gas
  and dust
• Which can be seen in the molecular hydrogen and
  infrared maps as emitting regions.

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Milky Way in Different Wavebands
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• Near Infrared
• Most of the emission at this wavelength is from
  relatively cool giant K stars in the disk and the
  bulge of the Milky Way.
• Interstellar dust does not strongly obscure
  emission at this wavelength. Why?

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Milky Way in Different Wavebands
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• Mid-Infrared
• Most emission from complex molecules called
  polycyclic aromatic hydrocarbons commonly found
  in coal and interstellar gas clouds.
• Red giant stars planetary nebulae produce the
  small bright spots in this map.

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Milky Way in Different Wavebands
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• Infrared
• Most of the emission is thermal from interstellar
  dust warmed by starlight, including regions of
  star formation within the interstellar clouds.

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Milky Way in Different Wavebands
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• Molecular Hydrogen (H2)
• Most of the emission is thermal from interstellar
  dust warmed by starlight, including regions of
  star formation within the interstellar clouds.

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Milky Way in Different Wavebands
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• Radio (2.7 GHz)
• Bright emission seen in this image is from hot,
  ionised regions, or is produced from energetic
  electrons moving in magnetic fields.

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Milky Way in Different Wavebands
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• Atomic Hydrogen (H)
• Traced the gas of the interstellar medium. This
  gas is organised in to large diffuse clouds with
  sizes up to hundreds of light years across.

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Milky Way in Different Wavebands
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90
180
180
0
270

• Radio (407 MHz)
• Emission from electrons moving through the
  magnetic fields, between stars, at nearly the
  speed of light.
• Supernova shock waves accelerate the electrons to
  such high speed, producing lots of emission near
  these sources

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8.5
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2. Tracing Spiral Structure

• Gas
• 99 of the ISM is gas
• 90 is atomic hydrogen (HI) or molecular
  hydrogen (H2) or excited atomic hydrogen, H
  (HII)
• 10 is Helium
• HI H2 emits radiation at radio wavelengths
• HII (H) emits radiation at visible wavelengths

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2. Tracing Spiral Structure

• HII gas
• HII regions are hot, hydrogen emission nebulae
  that glow from the fluorescence of hydrogen
  atoms.
• UV light from hot O B stars ionised the HI gas
• H UV light ? H e-
• H e- ? H Visible Light
• Light emitted by ionised gas is red,
  specifically, 6563Å

• Fluorescent bulbs work same principle
• Mercury vapour produces UV light which ionises
  the gas in the bulb

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2. Tracing Spiral Structure
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2. Tracing Spiral Structure

• O B stars, HII gas
• O B stars only found in regions of star
  formation as they are young stars
• Do not live long or move far from where they were
  born.
• HII regions are convenient for mapping the
  structure of the galaxy because they are large
  luminous regions.
• Spiral pattern
• Most of the gas in the galaxy is not ionised as O
  B stars are rare therefore they are good
  tracers of spiral structure.

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2. Tracing Spiral Structure

• O B stars
• Dust limits how far we are able to see
• Using the brightest stars in the galaxy (O B
  stars) we can map out to 1.5 kpc (5000 lt yrs)

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2. Tracing Spiral Structure

• HII regions
• Enables us to map a little further out in the
  galaxy as they also emit in the radio and so we
  can see through the dust.

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2. Tracing Spiral Structure

• Atomic Hydrogen
• Most hydrogen in the galaxy is in cold atomic
  form
• (T 100 ? 3000K)
• 1944 Hendrick van de Hulst predicted that cold HI
  gas should emit energy at a specific radio
  wavelength.
• Atomic hydrogen at large distances from O B
  stars is in its ground state

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2. Tracing Spiral Structure

• Atomic Hydrogen
• Some of the atomic hydrogen will exist in a state
  where the spin of its electron is parallel to
  that of the proton.

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2. Tracing Spiral Structure

• The hydrogen in the parallel state will
  eventually move to the ground state.
• Releases energy at a wavelength 21.1cm
• Frequency 1420.4 MHz
• 21cm line is not blocked by dust!

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2. Tracing Spiral Structure

• 21cm Radiation
• Intensity of the 21cm emission depends on the
  density of HI along the line-of-sight
• The more HI the brighter the emission
• Need the distance to each clump of atomic
  hydrogen gas
• Observe the galaxy in different directions
• 3-d picture.

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2. Tracing Spiral Structure

• Jan Oort
• 1920s noticed that stars closer to the galactic
  centre move faster than more distant ones
• Differential rotation
• Rotation of the Milky Way would assist
  astronomers in discovering its structure
• Use radio wavelengths to map the spiral structure
  of the Milky Way

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2. Tracing Spiral Structure

• Assumption
• Gas clouds move in the plane of the disc on
  nearly circular orbits.
• Doppler shifts

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Doppler Shifted 21cm
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2. Tracing Spiral Structure

• Molecular Hydrogen
• Can be traced through the radio emission of the
  related molecule, CO.
• Or alternatively GMCs can be viewed through its
  microwave and infra-red emission.

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2. Tracing Spiral Structure

• Combining the CO map with the known rotation
  curve of the galaxy.
• Positions of the molecular clouds.

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Bar or no Bar?

• The first conclusive evidence that the Milky Way
  was actually a barred galaxy was produced in
  1991.
• Blitz Spergel, 1991

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Bar or no Bar?

• The first conclusive evidence that the Milky Way
  was actually a barred galaxy was produced in
  1991.
• Blitz Spergel, 1991
• Lopez-Corredorra, 2001 provided evidence as to
  the exact shape of the bar.
• Concluded that the bar looks like that of the
  barred spiral M95

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M95
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Sun
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3. Rotation Curves

• Solid Body Rotation
• Wheel, record, CD
• Particles further from the centre must have a
  higher speed.

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3. Rotation Curves

• Task
• Plot the rotation curves for the planets given
  their distance from the Sun and their time of
  orbit
• Rotation curve
• Plot velocity (of planet) against radius (of
  planet from the Sun
• What do we notice?

Mercury
Venus
Mars
Earth
Orbital velocity of planet
Radius from Sun
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3. Rotation Curves
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3. Rotation Curves

• Solid Body Rotation
• Wheel, record, CD
• Particles further from the centre must have a
  higher speed.
• Keplerian Rotation
• Planets around the Sun
• Closer planets have the highest speeds
• Why?

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4. Milky Way Rotation Curve

• Using the Doppler shift of the 21cm line we can
  plot the rotation curve of the Galaxy

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4. Milky Way Rotation Curve
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5. Milky Way Mass
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Lecture 3 Summary

• Spiral structure of the Milky Way can be mapped
  using three different observations.
• O B stars with HII
• Atomic hydrogen, HI
• Molecular hydrogen, H2 through CO.
• There is strong evidence that the Milky Way as a
  bar at is centre.
• The rotation curve of the galaxy is measured
  using the Doppler shifts of stars and gas clouds
  at different distances from the Sun.
• There is insufficient luminous material in the
  outer parts of the galaxy to account for the
  rotational velocity of the stars in these regions