The Intergalactic Medium: The Cosmic Web of Matter Connecting Galaxies

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The Intergalactic MediumThe Cosmic Web of
Matter Connecting Galaxies

• Kenneth Sembach
• Space Telescope Science Institute

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Outline

1. Brief Introduction
2. What is the Intergalactic Medium and Why is it
   Important?
3. Spectroscopy of the Intergalactic Medium
4. Hubble - Current Status and Future Prospects
5. Questions from you (and hopefully some satisfying
   answers!)

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Where is the Material that Forms Stars and
Galaxies?

• Interstellar medium
• Gas and dust between stars inside galaxies
• Circumgalactic medium
• Gas and dust outside but near galaxies
• Intergalactic medium
• Gas and dust between galaxies

?
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The Mass/Energy Budget of the Universe

• Even though ordinary matter accounts for only a
  small fraction of the mass of the Universe, it is
  the only form of matter that is directly
  observable.
• About 50 of ordinary matter has yet to be
  accounted for in the present-day Universe. It is
  hidden (or missing) in the form of tenuous
  intergalactic material.

The rest of this presentation concentrates on
ordinary matter, where it is located, and how it
is studied.
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Where is the Ordinary Matter?

• Most of the ordinary matter in the Universe is in
  the intergalactic medium.
• Galaxies contain less than 10 of the ordinary
  matter.
• Gas near galaxies (in clusters and groups)
  accounts for about 30.
• Another 10-20 has been identified in the
  intergalactic medium.
• The remaining 50 is believed to be in the form
  of hot, ionized intergalactic gas.
• The intergalactic medium provides the raw
  materials needed to build galaxies, stars,
  planets, and life.
• The intergalactic gas is hard to detect because
  it is so tenuous.
• It has such a low density that it is not yet
  possible to image it.

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What is the Density of the Intergalactic Medium?

• Air has a density of 3x1019 molecules per cubic
  centimeter.
• This is about 30 billion billion molecules.
• 1 cubic centimeter is about the size of a sugar
  cube.
• The Suns photosphere has a density of about 109
  atoms per cc.
• This is a much better vacuum than can be produced
  in any laboratory.
• The interstellar medium has a density of about 1
  atom per cc.
• Take the air particles in a box the size of a
  sugar cube and stretch the cube in one dimension
  33 light years to get the same density!
• The intergalactic medium has a density of about
  1/100,000 atom per cc.
• Take the box and stretch it 3 million light
  years, or about 4 times further than the
  Andromeda galaxy!

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Evolution of the Cosmic Web of Matter
Simulation by Volker Springel (MPIA) If this
does not play automatically from your computer,
go to the still pictures on the next slide (slide
8).

• The intergalactic gas evolves with time under
  the influence of gravity.
• Large-scale gaseous structures collapse into
  sheets and filaments.
• Shocks in the collapsing structures heat the
  intergalactic gas to high temperatures.

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Evolution of the Cosmic Web of Matter
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A Representation of What the Cosmic Web Might
Look Like Now
Much of the gas is at temperatures of 100,000 to
1,000,000 degrees (greenish colors in figure).
Clusters of galaxies form at the intersections of
the filaments where the gas is hottest (bluish
colors in figure).
Figure from Kang et al. 2004
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How Does Matter Get Out of Galaxies?
Red Galaxies Green Metals Blue 105-107 K
gas
M82
A slice of the cosmic web
Credit X-ray NASA / CXC / JHU /
D.Strickland Optical NASA / ESA / STScI / AURA
/ The Hubble Heritage Team IR NASA /
JPL-Caltech / Univ. of AZ /C. Engelbracht
Cen Ostriker (1999)
Galaxies power strong winds that blow dust, gas,
and heavy elements into the intergalactic medium.
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Sometimes to study the Universe on large scales,
it is necessary to consider what is happening on
very small scales. So, lets take a look at
atoms for a moment.
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Bohr Model of the Hydrogen Atom

• A negatively charged electron orbits the
  positively charged proton in one of several
  possible energy levels n.
• When the electron moves to a lower energy level
  (preferred), the atom emits a photon of light
  with energy DE and wavelength l.
• If the atom absorbs a photon of energy DE, the
  electron can move to a higher energy level if the
  energy separation of the levels equals DE.
• Each element, whether simple like Hydrogen or
  complex like Iron, has a unique set of energy
  levels.

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Spectroscopy

• Spectroscopy is the technique that allows us to
  disperse light into its constituent colors and
  determine the energy levels of atoms and
  molecules.

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Cosmic Barcodes

• Each element has its own unique set of spectral
  lines.
• The sequence of lines is determined by the energy
  levels populated within the atom or molecule.
• These series of lines can be used to identify the
  chemical composition of the gas causing the
  absorption.
• Pop quiz! What elements are present in this
  spectrum?

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Answer
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Decoding the Information in a Spectrum

• Astronomers convert two-dimensional spectra
  (below) into one-dimensional plots of intensity
  versus wavelength.
• This allows precise line wavelengths, shapes, and
  strengths to be measured easily.
• The line parameters contain information about the
  physical properties of the absorbing material.

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A Portion of an Astronomical Spectrum
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Spectroscopy with Hubble

• Hubble has obtained spectra of many astronomical
  objects
• Complementary to imaging information
• A spectrum information
• What is it? Chemical composition
• What state? Molecular/atomic/ionic
• Hot hot? Temperature
• How much? Quantity
• How fast? Velocity
• Where is it? Location (redshift)

• The ultraviolet spectral region is loaded with
  information about atoms and molecules in their
  ground (lowest) and excited (higher) states.

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Extracting Information

• How do we extract information about the gas from
  the spectral lines?
• Question Information Observable quantity
• What is it? Chemical composition Pattern of
  lines
• What state? Molecular/atomic/ionic Pattern of
  lines
• How hot? Temperature Widths of lines
• How much? Quantity Strengths of lines
• How fast? Velocity Wavelengths of lines
• Where is it? Location (redshift) Wavelengths of
  lines

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Redshift of Spectral Lines
Light
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Redshift and Cosmic Expansion
Hubbles Law vr H0 d
vr velocity of recession d distance H0
Hubbles constant
H0 20 kilometers per second per million light
years

• The Universe is expanding in all directions.
• Distant objects move away from us faster than
  nearby objects.
• As a result, distant objects appear redder than
  they would if they were nearby - they are
  redshifted.

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Measuring the Redshifts of Intergalactic Gas
Clouds with Hubble
STIS Space Telescope Imaging Spectrograph
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A Hubble Spectrum is a Beautiful Thing!
Hubble spectrum of quasar H1821643
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Current Hubble Status

• Wide Field Planetary Camera 2 (WFPC2)
• Installed in December 1993
• Operating well
• Near Infrared Camera and Multi-Object
  Spectrometer (NICMOS)
• Installed in February 1997
• Operating well
• Space Telescope Imaging Spectrograph (STIS)
• Installed in February 1997
• Currently disabled
• Advanced Camera for Surveys (ACS)
• Installed in March 2002
• Serious electrical failure on January 27, 2007
• Optical channels are disabled
• Only ultraviolet (solar-blind) channel is
  operational

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Hubble Servicing Mission 4

• Scheduled for Fall 2008 on Shuttle Atlantis

• Two new science instruments
• Wide Field Camera 3 (WFC3) and the Cosmic Origins
  Spectrograph (COS)
• Replacement of one of the three Fine Guidance
  Sensors
• Repair of the Space Telescope Imaging
  Spectrograph
• Replacement of batteries (needed for power during
  orbital night)
• Replacement of gyros (used to determine HST
  pointing)
• Replacement of thermal blankets (used to maintain
  temperature)
• Repair of the Advanced Camera for Surveys?

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New Hubble Science Instruments

• Wide Field Camera 3 (Panchromatic Imaging)
• Two channels cover near-ultraviolet to
  near-infrared wavelengths
• Wide field imaging from 200 to 1000 nm
• Greater sensitivity, wider field of view
• Replaces WFPC2
• Cosmic Origins Spectrograph (Ultraviolet
  Spectroscopy)
• Far-ultraviolet channel (110 nm - 180 nm)
• Improves HST sensitivity by at least 10x
• Near-ultraviolet channel (180 nm - 320 nm)
• Replaces COSTAR

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WFC3 Panchromatic Imaging of Star-Forming Regions

• Ultraviolet observations reveal young stars that
  are flooding their surroundings with intense
  ultraviolet light.

• Infrared observations penetrate deeper into
  regions heavily obscured by dust.

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WFC3 Will Peer into the Hearts of Galaxies

• High angular resolution, great sensitivity and
  multi-wavelength coverage will give WFC3
  unprecedented views into the cores of galaxies.

• WFC3 will observe ultraluminous infrared galaxies
  created by firestorms of star formation after
  galaxy-galaxy collisions.

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COS is Designed to Study the Cosmic Web
COS will greatly increase the number of quasar
sight lines explored by Hubble.
In just a few days, COS can sample as much of the
Universe as all existing STIS observations of
quasars have probed!
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COS and Planets

• COS can record the ultraviolet spectra of
  transiting hot Jupiters fainter than those
  observable with STIS (many more faint stars)
• Ground-based surveys will find 10 transiting
  planets around bright stars (10m) over next 3
  years
• HST should be able to detect atmospheric
  absorption from atoms/molecules in the extended
  atmospheres of these planets
• Scintillation noise in the Earths atmosphere
  makes this problem impossible for terrestrial
  telescopes