Certificate in Astronomy: Galaxies

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Certificate in AstronomyGalaxies Quasars

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

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Text Book Chapters

• Mainly 5 6
• Also includes elements of 7 8
• Next 2 lectures include elements that will be
  covered in more detain in Module 4 Cosmology.

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Lectures 11 12 Galaxy Formation Cosmology

• A Cosmological Model
• Cosmic History
• The standard Big Bang model
• General Theory of Relativity
• Big Bang Nucleosynthesis
• Inflation
• Observing Galaxy Formation
• Can we observe galaxies forming?
• The Universe as a laboratory
• Theory of Galaxy Formation
• Monolithic -vs- hierarchical collapse
• Does light follow mass?
• Simulations of galaxy formation Ellipticals -vs-
  spirals.

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Formulation of the Standard Hot Big Bang Model

• 1915 Einsteins General Theory of Relativity
  meant that we could discuss the evolution of the
  Universe in terms of physically.
• 1922 Friedmann found solutions to Einsteins
  equations to conclude that the Universe was
  either expanding or contracting.
• 1929 Hubble observed the recession of galaxies
  due to the expansion of the Universe
• 1946 Gamov predicted from nucleosynthesis that
  the Universe must begin in a very hot dense phase
  and that the Universe should be filled with
  microwaves.

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Cosmic History

• The Universe is split into two broad eras -
  radiation-dominated era and the matter-dominated
  era
• Before t10-6 s
• All that could exist under such high temperatures
  and densities was a soup of sub-atomic particles
  and radiation.

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Cosmic History

• Series of notable epochs in the evolution of the
  Universe
• tlt 10-6s Quark soup anti-matter and matter
  annihilate releasing photons of radiation
• t10-6s Formation of protons and neutrons (p
  same as ionised Hydrogen)

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Cosmic History

• Series of notable epochs in the evolution of the
  Universe
• t lt 1min Formation of atomic nuclei of H, D, He
  some Li (known as Nucleosynthesis)
• During this epoch radiation scatters of free
  electrons thus the Universe is opaque to us.
• 400,000yrs gt t gt 15mins The Universe has
  expanded and cooled to allow electrons to join
  with nuclei making atoms (recombination).
• Number of free electrons reduced and so photons
  no longer collide with electrons decoupling
• Universe becomes transparent once again (when all
  atomic _at_ 400,000 yrs)
• Matter domination begins

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Surface of last scattering

• The point at which the Universe becomes neutral
  is where radiation no longer scatters of free
  electrons.
• Thus there is a sphere the surface of which is
  where the radiation last scattered of the free
  electrons.
• Surface of last scattering

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The Radiation Cosmic Microwave Background
Radiation

• The radiation that we have referred to so far is
  the CMBR.
• Since the epoch of last scattering the radiation
  has not interfered with matter and so provides a
  test bed for the properties of the early Universe.

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Einsteins General Theory of Relativity

• Einsteins formulated the field equations
• Using the field equations we can derive the rate
  of expansion of the universe
• In a spatially flat universe (K0)

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Big Bang Nucleosynthesis (BBN)

• Light elements (D, He, Li) were produced in the
  first few minutes of the Big Bang.
• 3mins after Big Bang the temperature of the
  Universe rapidly cooled and p and n collided to
  produced D.
• BBN predicts that of the mass in the Universe
• 25 He
• 0.01 D
• lt0.01 Li

• These results depend critically on the density of
  baryons at the epoch of nucleosynthesis.
• The fact that He nowhere exists at gt 23 is
  strong evidence that the Universe went through an
  early hot phase.

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Problems with the Standard Model

• Flatness Problem
• In order for the observed geometry of the
  Universe to be maintained as the Universe expands
  and evolved the conditions in the early Universe
  must be very finely tuned
• Otherwise the Universe would collapse or expand
  very quickly before any structure could form.
• Horizon Problem
• We observe that the Universe is homogeneous and
  isotropic.
• For this to be the case information must have
  been transmitted to and from all regions of the
  Universe - this is impossible because as the
  Universe expands the information would have to
  travel faster than the speed of light!

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The Solution Inflation

• The problems of the standard model arises from
  the fact that in the past the Universe was
  decelerating in its expansion. (velocity
  decreases)
• Inflation is a phase in the very early Universe
  of extremely rapid expansion
• After inflation the Universe continues according
  to the standard Hot Big Bang model.

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The Solution Inflation

• Flatness Problem
• The extremely rapid expansion means that (aH)
  increases so O-1? 0.
• Therefore O ? 1
• Horizon Problem
• Regions were causally connected in the period of
  inflation and rapid expansion.

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Observing Galaxy Formation

• Ideally we would like to observe galaxies as they
  form and evolve!
• Problems
• Time taken for a galaxy to form or evolve many
  100,000s and billions of years.
• Cannot really predict where a galaxy would form
• Distant galaxies are extremely faint
• Solutions?
• Use the constant speed of light
• Statistical methods

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Observing Galaxy Formation

• What would we like to know?
• Sizes as a function of time?
• Brightness as a function of time?
• And therefore how many stars are being born in
  the past compared with today?
• Type as a function of time?
• Distribution of types and how that evolves with
  the Universe?
• The relation between normal galaxies and active
  galaxies
• Do all galaxies have supermassive black holes?
• Are there differences between the galaxies that
  live in clusters and those that exist in the
  field?
• ..

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Observing Galaxy Formation

• Task to find the most distant galaxies in the
  Universe and understand their properties.
• Naturally there are problems
• Selection effects
• Automatically choosing the brightest galaxies
  otherwise we wouldnt be able to see them -
  quasars.
• Does this mean that all normal galaxies evolve
  from quasars?
• Or can we just not observe normal galaxies due to
  their low brightness.

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Brief Theoretical Interlude

• Theory 1 Monolithic collapse
• 60 70s
• Rapid gravitational collapse ? large galaxies ?
  huge bursts of star formation at early epochs
  Galaxies rapidly exhausting their fuel leaving
  behind a collection of old red stars.
• Hierarchical structure formation.
• Universe in which smallest mass collapses first
  and merges in to progressively larger structures.

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Observing Star Formation

• How do we observe galaxies at high redshifts (z)?
• Method
• To investigate galaxies at zgt1, look for expected
  strong features in the spectra of galaxies.

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Observing High-z Galaxies

• At very high redshifts the sharp break in
  wavelength at the lyman break is moved into the
  ultra-violet (UV).
• This method enables us to distinguish between
  nearby extremely red objects and high redshift
  galaxies.
• If a galaxy is truly distant the image will
  disappear in the UV.

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UV Dropouts
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Narrow Band Ly-a Observations

• SFR of high redshift galaxies will help to
  confirm the current hierarchical theory of
  structure formation.
• Lots of small, high redshift galaxies of low
  luminosity ? Hierarchical collapse
• Large galaxies with high SFR ? Monolithic
  collapse
• Hu, McMahon Cowie have made observations, using
  the 10m Keck Telescope, of a galaxy at z5.74
  (Universe 1 Gyr)
• To calculate the SFR of this galaxy they used
  narrowband imaging of the Ly-a peak region of the
  spectrum

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Hu et al. Observations
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1. CIRPASS
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Telescopes Instrumentation

• Dr Lisa Wright,
• Astrophysics and Cosmology Back to the Future

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Galaxy Morphologies

• Giavalisco et al. presented deep HST images of 19
  Lyman-break galaxies, 6 of which we know have
  2.8ltzlt3.4.
• They find that light in most of these galaxies is
  concentrated in a central lump. Spheroids
• However the galaxies are also surrounded by low
  surface brightness gaseous regions, suggesting
  systems in interaction. Hierarchical merging?

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Galaxy Morphologies
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Evidence of Hierarchical Merging
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The quest is on

• Even though observations have improved hugely
  over the last two decades they still do not
  present us with the definitive answer to galaxy
  formation.
• What we would like is the distribution of galaxy
  types as we look back in time.
• Selection effects make this almost impossible to
  investigate.
• As telescopes continue to improve however we will
  approach the answer.

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What Do We Understand So Far? Simulating Galaxy
Formation

• The range of scales that are important in galaxy
  formation makes detailed simulations difficult.
• Simulations of the formation of elliptical
  galaxies have proved relatively simple compared
  with the formation of disc or spiral galaxies.
  Why?
• We need to understand in more detail the
  mechanisms of star formation and how early star
  formation in a galaxy affects later star
  formation.

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Forming Disc Galaxies

• Simple theoretical model of galaxy formation
• Dark matter halos provide the potential wells in
  which the material of the galaxy collects and
  collapses.
• The gas and dust conserves its angular momentum
  during the collapse to a disc. (Fall Efstathiou
  80)
• Galaxies form from the inside out, thus growing
  with time.
• Cooling and angular momentum catastrophes.
• Efficient cooling at early epochs causes the gas
  to collapse into the centres of the dark halos.
  (White Rees 78, White Frenk 91)
• Gaseous cores subsequently merge and lose angular
  momentum to their surrounding halo. (Navarro,
  Frenk White 95)
• Disc galaxies which are too small and have too
  little angular momentum. (Navarro Steinmetz 97)

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Structure Formation Animation
50 Mpc / 155 million lt yr
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Catastrophe Solutions

• Need to prevent the gas from collapsing to form
  stars at very early epochs when the dark matter
  halos are still experiencing many major mergers.
• Prevent all the gas from cooling by heating the
  reservoir.
• E.g. Supernovae and supernovae driven winds
• Maybe we need to tweak the cosmological model
• Warm dark matter
• Artificially suppress star formation

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Dark Matter

• The temperature of the dark matter sets a lower
  limit on the mass of the first objects to form.
• Initially hot dark matter in the form of
  neutrinos was favoured. The smallest structures
  that are able to form in a universe of hot dark
  matter is 16 Mpc (3 million lt yrs) across.
• This led to the idea that galaxies may form from
  large, super-cluster size structures. Galaxy
  formation theory 1.
• Alternatively in a cold dark matter universe the
  first structures to form are smaller than the
  size of observed galaxies and so galaxies form
  from the mergers of smaller objects
• Galaxy formation theory 2.

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Dark Matter and Fluctuations
Galaxy Halo Scales
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Our Simulation Details 1(Wright, Efstathiou
Eke)

• Cosmology
• ?m 0.3, ?? 0.7,
• H0 65 kms-1Mpc-1,
• Artificially suppress cooling until z1.
• Simple (though unrealistic) model of stellar
  feedback.
• Provides a scheme to test numerical effects on
  the formation of the galaxy.
• Star formation criteria
• Gaseous region has to be collapsing
• ?crit gt 7 x 10-23 kgm-3

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Our Simulation Details 2

• Lbox 5 Mpc NDM Ngas 583195112
• ? Mgas 9 x 106 M
• ? Spatial resolution lt 1 kpc
• Using 40 GRAPE 3-A processors and Sun
  workstation,
• CPU time 3 months, z 24 ? z 0.36
• Currently only 1 simulation in sample.
• 5 Simulations at resolution, NDM Ngas 503
• 25 Simulations at resolution, NDM Ngas 343

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The GRAPEs

• The IoA grape system 5 GRAPE 3A boards each
  containing 8 processors 40 processors.
• GRAPE (GRAvity PipE) chips are hardwired to
  calculate a plummer gravitational force law
• Calculates the forces on 40 particles by up to
  131,072 in any one function call.
• GRAPE returns forces, potentials and neighbour
  lists.

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The Story So Far Spiral Galaxies
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Disc Galaxy Formation Simulation
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Numerical Comparisons
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The Future

• Masses of distant galaxies are a key piece of
  information required to tie together observable
  luminous matter and simulated dark matter.
• One of the other challenging aspects of deducing
  the history of galaxies is establishing the
  evolutionary links between samples of galaxies
  observed at different epochs
• Galaxy morphologies
• Chemical contents
• Although computing power and star formation
  mechanisms may improve our current theoretical
  models, large gains will be achieved in
  observational astronomy and the development of
  the next generation of telescopes.