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
• 1.5 -1.6
• Chapters of Introduction to Modern Astrophysics
• 22.2 22.3 (mathematics can be ignored)

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Aside Excitation -vs- Ionisation

• H - e- ? H

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Lecture 4 Evolution of the Milky Way

• Stellar Populations
• History of thought
• Stellar Abundances
• Measuring abundances
• Galaxy Formation Evolution
• Galactic Evolution by component
• Sagittarius Dwarf Galaxy

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Stellar Populations

• 1944, Walter Baade
• Noticed 2 distinct stellar systems Pop I and Pop
  II
• Pop I
• Disc stars and stars in spiral arms
• Pop II
• Stars within the bulge and the halo

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Stellar Populations

• Pop I
• Associated with the gas and the dust in the disc
• Includes young stars that must have formed fairly
  recently
• Pop II
• No stars younger than 10 billion years old
• More spherical distribution

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Stellar Populations

• Baade
• Correlation between the location of the stars and
  the concentration of the heavy elements
  (C,N,O,Fe) seen in their spectra.
• Abundances
• Pop I relatively high fractional abundance of
  heavy elements (2)
• Pop II lt 1 concentration of heavy elements

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Population Summary
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Stellar Populations

• Individual facts were all known before Baade
• He noticed that the facts formed a consistent
  picture
• Understanding of the formation and evolution of
  the Milky Way

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Eggen, Linden-Bell Sandage

• ELS 1962
• Showed that it is possible to study galactic
  archaeology using stellar abundances and stellar
  dynamics.
• Studied the motions of high velocity stars
• Discovered that as the abundance decreased orbit
  energies and eccentricities increased but angular
  momentum decreases.

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Abundances

• 1978 Searle Zinn
• Noted that galactic globular clusters have a wide
  range of metal abundances independent of radius.
• Suggested that it could be explained by a halo
  built up over an extended period from independent
  fragments with masses 108 M?

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Abundances

• Fe/H ?
• 10 Fe/H ? Abundance of Sun
• 10-1 0.1 Fe abundance in Sun
• Star with Fe/H-1 has 10 of the Iron of the
  Sun if they have the same amount of hydrogen.

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Measuring Abundances

• For very bright stars (e.g. Sun), use
  high-resolution spectra to measure Z directly
  (sum over most metals).
• For fainter stars, measure any element with
  strong lines (e.g. Mg, Fe) and assume that all
  metals enrich at the same rate
• And with same relative abundances as for Sun
• Z MFe / Mtot x Mall metals / MFe?

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Element Spectra
Sulphur
Helium
Iron
Lithium
Oxygen
Aluminium
Calcium
Carbon
Argon
Nitrogen
Sodium
Neon
Magnesium
Krypton
Xenon
Silicon
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Element Spectra

• Star spectra have absorption lines
• Gas spectra have emission lines
• Galaxy spectra can show both
• Metal-rich/poor stars have stronger/weaker metal
  lines relative to H

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Measuring Abundances

• Spectra Line strengths (equivalent
  widths)
• Astrophysics Stellar atmosphere models
• Physics Laboratory
  calibrations
• 
  Fe/H

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Galaxy Formation Evolution

• Eggen
• Identify debris from small fragments of merged
  material by looking at stellar dynamics.
• Extend this idea to the galactic disc
• Fossil information in the detailed stellar
  distributions of chemical elements.

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Structure Evolution

• Like most spiral galaxies, ours has several
  recognisable structural components that probably
  appeared at different stages in the galaxy
  formation process.
• These components will retain different kinds of
  signatures of their formation.

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Theory of Galaxy Evolution

• Monolithic Collapse -vs- Hierarchical Clustering

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Formation Evolution

• 12-14 billion yrs ago
• Milky Way was an amorphous cloud of gas and dust
  that began to collapse under gravity
• Primordial gas cloud contained virtually no heavy
  elements only H and He
• Gas cloud fragmented in to smaller clouds
• Population II
• Probably formed in clusters

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Initial Star Formation

• Globular clusters in the halo are perhaps
  residues of these first star clusters
• Once the clusters are formed they move in orbits
  similar to the chaotic motions of the collapsing
  gas cloud

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Initial Star Formation

• Population II
• Included massive stars ? Supernovae
• Supernovae produce heavy elements and return them
  to the interstellar gas
• ? as primordial gas cloud collapsed it gained an
  increasing concentration of heavy elements
  produced by 1st generation of stars

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Continued Evolution

• As gas cloud collapsed in began to rotate and
  consequently flatten
• Thin disc we see today

Gravity
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Collapse Evolution
Gravity
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Collapse Evolution
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Evolution

• Pop II stars
• Continue in their random motions as they formed
  before the gas collapse began its rotation
  settled into a disc.
• Gas cloud collapsed into a disc in lt 1 billion
  yrs
• No new stars formed in the halo for gt 10 billion
  years
• No gas left from which they could form
• Only stars left in the halo are low mass main
  sequence stars with lifetimes gt 10 billion years
  old
• Also left stellar remnants like Black Holes,
  Neutron Stars and White Dwarfs - all of which are
  difficult to detect

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Accretion
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Galactic Centre
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Population I Evolution

• Pop 1 stars
• Formed in the rotating gas disc which is peppered
  with heavy elements from the 1st generation SN
• Star formation ongoing therefore concentration of
  heavy elements continues to rise in the disc
• Sun has fractional abundance of heavy elements
  2 - it continues to circle in the disc as it was
  formed from circling gas within the disc

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Heavy Element Abundance

• Present concept that pop II low in heavy
  elements, pop I high in heavy elements
• Not so clear cut
• Gradation of heavy element abundance
• Stars in outer halo have lowest abundance
• Bulge higher
• Stars in inner disc highest
• Consistent with the picture presented

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Caveat

• Need to understand the details of collapsing gas
  clouds and star formation to truly understand the
  formation of the Milky Way

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Signatures of Galaxy Formation

• 3 major epochs
• Dark matter virialises -
• ltK.E.gtt -1/2 ltP.E.gtt
• could be a time of intense star formation but
  need not be, as evidenced by the existence of
  very thin pure disc galaxies
• Baryons form the disc and the bulge
• Ongoing epoch of the formation of objects within
  the disc and accretion of objects from the
  environment of the galaxy, both leaving some
  long-lived relics.

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Galactic Evolution by Component

• Bulge
• Spiral bulges are usually assumed to be old, but
  this is poorly known even for our own.
• It is found that while there is a wide spread,
  abundances are closer to the older stars of the
  metal rich thin disc than to the very old metal
  poor stars in the halo and globular clusters.
• Images of other disc galaxies shows that bulge
  formation is not an essential element of the
  formation process of disc galaxies.

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Galactic Evolution by Component

• Disc
• Sometimes separable into two components Thin
  disc and thick disc
• Thin disc contains almost all of the baryonic
  angular momentum.
• Many disc galaxies show a second fainter
  component with a larger scale height Thick disc.

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Galactic Picture
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Galactic Evolution by Component

• Disc
• Sometimes separable into two components Thin
  disc and thick disc
• Thin disc contains almost all of the baryonic
  angular momentum.
• Many disc galaxies show a second fainter
  component with a larger scale height Thick disc.
• Milky Way has a significant thick disc component
  1kpc high
• Stars are significantly more metal poor than the
  stars of the thin disc
• Galactic thick disc currently believed to arise
  from heating of the early stellar disc by
  accretion events or minor mergers.

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Galactic Evolution by Component

• Stellar Halo
• Contains metal-poor globular clusters and field
  stars.
• Mass is about 1 of total stellar mass 109 M?
• Current view
• Galactic halo formed at least partly through the
  accretion of small metal-poor satellites that
  underwent independent chemical evolution before
  merging.
• Real possibility of identifying groups of stars
  that originate from common progenitor satellites.
• Most of this accretion occurred long ago although
  we do still see such accretion events taking
  place now!

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Galactic Cannabalism

• Galaxy satellites
• Most large galaxies like our Milky Way are
  surrounded by satellite companion galaxies.
• These companion galaxies are small (typically a
  few kiloparsecs or less in size) and low in mass
  (107 - 109 solar masses, or 0.1 - 10 the mass
  of the big galaxy).
• They come in a variety of types, from irregular
  galaxies like the Large Magellenic Cloud, a
  companion of the Milky Way, to dwarf elliptical
  companions like M32 and NGC 205, which accompany
  the nearby Andromeda galaxy.

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Galactic Cannabalism

• Galaxy satellites
• The companion will slowly spiral in nearer and
  nearer to the center of the parent.
• As it moves inwards, the gravitational tidal
  force of the parent strips stars from the
  companion, slowly "eating away" the low mass
  companion.
• An example of this is the recently discovered
  Sagittarius dwarf galaxy in our own Milky Way.

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Galactic Cannabalism

• Galaxy satellites
• The companion will slowly spiral in nearer and
  nearer to the center of the parent.
• As it moves inwards, the gravitational tidal
  force of the parent strips stars from the
  companion, slowly "eating away" the low mass
  companion.
• An example of this is the recently discovered
  Sagittarius dwarf galaxy in our own Milky Way.
• Ultimately, the companion will be absorbed by the
  parent galaxy, either by stripping it completely
  of stars or else by accreting it into the centre
  of the parent once the companion's orbit has
  completely decayed.

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Galactic Archaeology

• RAVE (Radial Velocity Experiment)
• Will measure the radial velocities, metallicities
  and abundances of gt 100,000 stars.

• Selecting stars from thick disc, halo, solar
  neighbourhood thin disc.
• Goals
• Investigate kinematic transition between thin and
  thick discs and halo above the galactic plane

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