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The Intergalactic Medium: The Cosmic Web of Matter Connecting Galaxies. Kenneth Sembach Space Telescope Science Institute. Outline. Brief Introduction What is the Intergalactic Medium and Why is it Important? Spectroscopy of the Intergalactic Medium

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the intergalactic medium the cosmic web of matter connecting galaxies

The Intergalactic Medium:The Cosmic Web of Matter Connecting Galaxies

Kenneth Sembach

Space Telescope Science Institute



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

K. Sembach - NSN Telecon


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


K. Sembach - NSN Telecon


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.

K. Sembach - NSN Telecon


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.

K. Sembach - NSN Telecon


1 cc

Milky Way


Keep going!

3 million light years

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 Sun’s 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!

K. Sembach - NSN Telecon


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.

K. Sembach - NSN Telecon


Redshift = 0

(1024 h-1 Mpc)3


104 K

105 - 106 K

108 K

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

K. Sembach - NSN Telecon


How Does Matter Get Out of Galaxies?

Red: Galaxies Green: Metals Blue: 105-107 K gas


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.

K. Sembach - NSN Telecon


Sometimes to study the Universe on large scales, it is necessary to consider what is happening on very small scales.

So, let’s take a look at atoms for a moment….

K. Sembach - NSN Telecon


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.

K. Sembach - NSN Telecon


Types of Spectra

Figure reproduced from Universe by Freedman and Kaufmann


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

K. Sembach - NSN Telecon



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?

K. Sembach - NSN Telecon



K. Sembach - NSN Telecon


Collapse and sum spectrum in this direction

Plot intensity versus wavelength



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.

K. Sembach - NSN Telecon

spectroscopy with hubble


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.

K. Sembach - NSN Telecon

extracting information
Extracting Information
  • How do we extract information about the gas from the spectral lines?

QuestionInformationObservable 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? QuantityStrengths of lines
      • How fast? Velocity Wavelengths of lines
      • Where is it? Location (redshift) Wavelengths of lines

K. Sembach - NSN Telecon


The light at the wavelength of this line is the color it is when it is

absorbed (in this case, yellow).

This same line is shifted to redder wavelengths when observed by someone moving away from the absorber.

The faster the recession, the greater the redshift.



Wavelength l

Redshift of Spectral Lines


K. Sembach - NSN Telecon


Recession velocity of galaxies





Redshift and Cosmic Expansion

Hubble’s Law

vr = H0 d

vr = velocity of recession

d = distance

H0 = Hubble’s 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.

K. Sembach - NSN Telecon


Measuring the Redshifts of Intergalactic Gas Clouds with Hubble

STIS = Space Telescope Imaging Spectrograph

K. Sembach - NSN Telecon

a hubble spectrum is a beautiful thing
A Hubble Spectrum is a Beautiful Thing!

Hubble spectrum of quasar H1821+643

K. Sembach - NSN Telecon


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

K. Sembach - NSN Telecon


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?

K. Sembach - NSN Telecon


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

K. Sembach - NSN Telecon


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.

K. Sembach - NSN Telecon


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.

K. Sembach - NSN Telecon


Cosmic web absorption features

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!


COS Science Themes

What is the large-scale structure of matter in the Universe?

How did galaxies form out of the intergalactic medium?

What types of galactic halos and outflowing winds do star-forming galaxies produce?

How were the chemical elements for life created in massive stars and supernovae?

How do stars and planetary systems form from dust grains in molecular clouds?

What is the composition of planetary atmospheres and comets in our Solar System (and beyond)?

K. Sembach - NSN Telecon


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 Earth’s atmosphere makes this problem impossible for terrestrial telescopes

K. Sembach - NSN Telecon