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Lecture 12: The Intergalactic Medium

Dr Graham M. Harper

School of Physics, TCD

PY4A04 Senior Sophister

Physics of the Interstellar and

Intergalactic Medium

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What is the Intergalactic Medium (IGM)?

[1] Material left over from when the

galaxies formed – collapsed onto a

cosmic web

[2] Material ejected back into volume

between galaxies by supernovae/star

forming regions (metal rich pollution)

[3] IGM contains 90% of baryonic matter

[4] IGM traces Cold Dark Matter

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Quasar and GRB redshifts: z

z = redshift

obs = observed wavelength

lab = rest (laboratory) wavelength

λ (Lyα) = 121.567 nm (n=2-1)

λ (Lyβ) = 102.572 nm (n=3-1)

1lab

labobsz

21 zlabobs

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Quasars & GRB: light probes

Quasar=Quasi-Stellar Object (QSOs)

Central engine of an Active Galaxy

Active Galactic Nuclei – accreting super-

massive black holes

Light sources to observe foreground

absorption

Constant search for oldest (highest z)

quasars

Faint in optical – bright in radio

Positional reference frame for astronomy

Parallax?

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What is the low z IGM?

What is the redshift of this Quasar?

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Spectrum QSO 1422+231

10m W. H. Keck Observatory

Mauna Kea (4145m)

18 hours spectrum (Keck I)

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Cloud statistics

Voigt profiles of individual H Lyman α lines are used to model

absorption: permits measurement of column density and

temperatures (104 K)of the H I clouds

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The Lyman-α forest

[1] Nearby quasars are

not that helpful – few IGM

hydrogen clouds

[2] Large z quasars permit

a statistics analysis

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Quasars shifted back to rest frame (for

comparison)

Statistical trends

Cosmological evolution of the

column-density fits well with

observations for an ionising

background dominated by QSOs

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Cosmic Web: simulations

Gas density contours in a cosmological

Box size 4x4 Mpc (Cen & Ostriker 1994)

Image: Volker Springel Millennium

simulation

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Gunn-Peterson Trough

Predicted by James E. Gunn and

Bruce Peterson over 2 decades

before observed

Gunn, J.E. & Peterson, B.A. 1965, ApJ,

142, 1633 "On the Density of Neutral

Hydrogen in Intergalactic Space".

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Epoch of Reionization

Neutral hydrogen is a very

efficient scatterer near the

Lyman lines (lower level n=1)

- else gas is transparent

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Gunn-Peterson – Lyα and Lyβ troughs

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Epoch of re-ionization

What is the redshift

of the epoch of re-

ionization?

Re-ionization phase

similar to the evolving

H II region phase

already studied

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Warm Hot Intergalactic Medium (WHIM)

Hydrogen does not trace all the material

– if it is ionized.

Cosmic Origins Spectrograph (COS)

10x more sensitive STIS (Hubble)

COS built to search for O IV formed at

T~ 100,000 K (when hydrogen is

ionized)

H I & O VI distributions are different

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Pollution consistent with simulations

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C IV absorbers

Intracluster medium (ICM)

Largest bound gravitational structures, e.g Coma, Virgo

Hot plasma seen in X-rays 107-8 K – why?

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Hypothesis – shock heated to virial temperature

Virial temperature

For a star

Virial temperature not well defined for galaxies and clusters

MMW ~ 3x1011 solar mass, RMW ~ 17 kpc: TMW~ 106 K

kR

mGMT

pgas

gas12

~ KkR

mGMT

sun

psun

Sun

6102~12

~

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Hypothesis – gas shock heated to virial temperature

Virial temperature

MMW ~ 3x1011 solar mass, RMW ~ 17 kpc

Mcluster ~7x1014 solar mass, Rcluster ~ 1 Mpc

Cools slowly (like shock heated plasma in supernovae)

gas flows into centre, density increases, gas cools

Not observed – the cooling flow problem

Many suggestions on why we do not see it

KkR

mGMT

pcluster

virial

7104~12

~

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The Sunyaev-Zeldovich Effect

Inverse Compton scattering

First order CMB gains as much as it

loses when scattering of hot electrons

Second order net statistical gain

Net decrease in CMB in cm-radio

Can be used to explore ICM

independent of redshift (fractional

change)

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210~2

dln

cm

kT

I

IeT

e

e

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