Download Distant galaxies and quasars The ages of things Light

Distant galaxies and quasars
Prof Andy Lawrence
Astronomy 1G 2011-12
The ages of things
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Rock and Roll
Science
Civilisation
Early Hominids
50 years
400 years
5000 years
5 million years
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Pleiades
60 million years
Life on Earth
3.8 billion years
Earth
4.5 billion years
Globular Clusters 12 billion years
Universe
14 billion years*
* see later lectures
Were things different in the past ?
Astronomy 1G 2011-12
Light-travel time
• Sun D=1AU → 8.3 minutes
• Alpha Centauri D= 1.338 pc → 4.365 years
– 1 "light year" = c x 3600 x 24 x 365.25 = 9.461x1015m
– 1 pc = 3.086e16m so 1 pc = 3.262 lt yr
• M31 D=778 kpc → 2.5 million years
• Virgo cluster D=18 Mpc → 60 million years
– redshift z=0.004
• Coma cluster D=99 Mpc → 300 million years
– redshift z=0.024
• Quasar 3c273 D=658 Mpc → 2.1 billion years
– redshift z=0.158
– 15% of the age of the Universe !!!
Observations of distant objects allows
us to study the history of the Universe
Observing the early universe
Naively, half the age of the Universe
would correspond to D=2145 Mpc,
and with V=cz=H0D to z=0.52
However, the expansion of the
universe has not been
constant.. see later
Sparke and Gallagher CUP 2007
"Lookback time" vs redshift
shown to left for various
cosmological models. Details not
important... confirms that halfway
back is roughly z=0.5 and
observations at z>3 show
Universe when it was only
10-20% of its current age.
Geometrical vs Physical effects
• in the past, the universe :
– was smaller
– was maybe expanding at different rate
– was more curved; Euclidean formulae don't work
• so flux vs distance, number counts, angular
size etc all more subtle
• put aside until cosmology section
• concentrate on physical effects
• did galaxies look different ?
High redshift galaxies
• In the past galaxies were on average
bluer : smaller : messier
• This has only become clear in recent years
with Hubble Space Telescope deep images
• It is consistent with the idea that galaxies
have formed from the merger of sub-units
Hubble Ultra Deep Field
shows large numbers of
faint galaxies which are
on average smaller and
look more peculiar than
current day galaxies
2003 NASA/ESA/STScI
Mugshots of individual distant galaxies
From random
piece of sky
Selected as
distant radio
sources
Best, Longair, and Roettgering, MNRAS 292, 758 (1997)
Comparison of galaxies
at different distances
Motorway pile-up !
Rich clusters of galaxies
As usual for astronomical objects,
note that small ones are common
and big ones are rare
Rosati, Borgani, and Norman 2002 AnnRev 40 539
No per unit volume
• Massive, rich clusters of galaxies
were probably rarer in the past
• This result has been
controversial over the last twenty
years .. but latest studies seem
to confirm it is correct
• The best way to find massive
clusters is by X-ray surveys...
they stand out very clearly
X-ray luminosity of cluster
Recent cluster results
Number per unit volume
Vikhlinin et al 2009 ApJ 692 1033
Not much
difference in
smaller clusters
When universe
was half its
current age,
massive clusters
were rarer by a
factor of several
Mass of cluster
Evolution of quasars
• Quasars (the most luminous Active Galactic Nuclei, or
AGN) have evolved dramatically over cosmic time
• Because they are easy to see to large distances, this
(unlike galaxy evolution) has been clearly known for several
decades
• In the past they were both much more numerous and
substantially more luminous
The most luminous
quasars were several
hundred times more
common at z=2 than
they are today
adapted from Shaver et al. 1996 Nature 384, 449
This is actually much
more striking if plotted in
linear number density
against linear time since
the Big Bang...
The first third of the history of the Universe was the
age of quasars ... we now see only a faint leftover
History of star formation
By adding up starlight from blue stars / IR from deep samples, we can now
estimate the total rate of star formation in the Universe (regardless which
galaxies it is taking place in ... This, like the quasar evolution, shows a
dramatic peak at earlier times ... but somewhat later than the quasars
The Inter Galactic Medium (IGM)
Bill Keel website http://www.astr.ua.edu/keel/agn/forest.html
Some absorption lines seen
in quasars are due to small
Hydrogen gas clouds
between us and the quasar
High redshift
quasars show
hugely more of
these ... this is
known as the
"Lyα forest"
The increase
happens both
because of the
longer sight line
and because the
IGM evolves