physics of the interstellar and intergalactic medium · 2016. 1. 11. · 1 lecture 12: the...
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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
~
19
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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