Outline: Galaxy groups & clusters
Transcript of Outline: Galaxy groups & clusters
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Outline: Galaxy groups & clusters
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Outline: Gravitational lensing
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Galaxy groups and clusters I
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Galaxy groups and clusters II
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Cluster classification
Increasing rareness
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Intermission: What are you looking at?
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Brightest Cluster Galaxies
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Galaxy content
Many S / SB Many E / S0
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The Butcher-Oemler effect
+ = ?
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Galaxy groups & clusters in our backyard
• Groups:
• Clusters:
• Superclusters: Local group
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Galaxy groups & clusters in our backyard II
Virgo cluster & M87 (lower left) with foreground objects masked
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The Laniakea Supercluster Local Group
Virgo SuperclusterLaniakea Supercluster
• Laniakea: ”immeasurable heaven” in Hawaiian • 100 000 galaxies and 300-500 groups and clusters over 160 Mpc
– total mass ∼1017 M
https://www.youtube.com/watch?v=rENyyRwxpHo
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Compact groups
Stephan’s Quintet
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Intermission: Group or cluster?
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Gas in groups and clusters
X-ray gas, T=107—108 K
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Why does the gas glow?
e-
e- p
p
p
e-
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Why is the gas so hot?
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Why do the galaxies move so fast?
G~ grav
2 RvM
The virial theorem:
Gravitational radius
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Where does the gas come from?
Gas in the Coma cluster
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Mass estimates
)(He TnnL Λ=
Number densities
Depends on the radiation process
)()(
)(2
Tdrd
rGr
mkrM
p
B ρρµ
−=<
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The Sunyaev-Zeldovich effect I
e- e-
e-
Galaxy cluster with ionized gas
CMBR
Observer
Slightly blueshifted
CMBR
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The Sunyaev-Zeldovich effect II
The S-Z effect is an important tool for cosmology!
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• Lensing – basic stuff: What? Why? Where?
• What do you need it for? Want to probe the source, the lens, or the Universe
Gravitational lensing
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Overdensities of matter along line of sight →
• Magnification
• Distorted morphology
• Shift in apparent position
• Multiple images
• Delays in time signals
Lensing – quick overview I
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Magnification
Lensing – quick overview II
Intrinsic source size Apparent source size (boosted due to lensing)
Surface brightness conserved (as long as the whole source experiences the same magnification)
Increased size + conserved surface brighness → increased apparent flux
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Intermission: What magnification?
Intrinsic size Lensed size
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Distorted morphology
Lensing – quick overview III
Intrinsic source morphology/orientation/parity
Apparent source morphology/orientation/parity
Stretched, curved and mirror-flipped!
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Shift in apparent positions
Lensing – quick overview IV
The mass of the Sun shifts the apparent positions of stars close to the limb
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Multiple images
Lensing – quick overview V
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Delays in time signals
Lensing – quick overview VI
Longer path length & Shapiro time delay (clocks running slow in strong gravitational fields) → outburst delayed
Observer Lens
Source
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• Magnification → Can detect sources too faint to be seen otherwise
• Multiple images, distortions time delays → Probes of structure and dust reddening along line(s) of sight
• Testing gravity & cosmology
Lensing – A tool…
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A couple of examples: •The flux you measure doesn’t directly reflect the
intrinsic luminosity • Can standard candles (e.g. type Ia supernovae) always
be trusted? •Cosmic Microwave Background Radiation
(CMBR) maps distorted
… and a nuisance
Intrinsic CMBR Lensed CMBR
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Different types of lensing I: Strong lensing
Strong lensing: Multiple images, large distortions, high magnifications Very rare!
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Different types of lensing II: Weak lensing
Weak lensing: Mild distortions, small magnifications Very common!
Strong lensing
Weak lensing
Strong lensing
Weak lensing
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Unlensed Lensed
Cosmic shear
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Technological challenges for weak lensing
Weak lensing distorts the ellipticities of sources at the ~1% level - very difficult to measure!
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Intermission: Strong or weak lensing?
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Different types of lensing III: Microlensing Microlensing is a special, time-dependent case of strong lensing. There’s also nanolensing, attolensing, femtolensing…
The angle between images is at the microarcsecond level if the lens has the mass of a star or planet Unresolvable with current telescopes → Observer sees just one image!
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•Glass lenses are chromatic
•Graviational lenses are achromatic • But note: GL may still alter the colour profiles of
extended sources experiencing non-uniform magnification
Gravitational lensing is achromatic
Unlensed source Lens magnifies red area
Total colour becomes redder
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Strong lensing: Multiply-imaged quasars I
Multiply-imaged Quasar
Lens galaxy (with dark halo)
Observer
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Multiply-imaged quasars II: Measuring the Hubble parameter
Depends on lens model Measured
Angular size distances - Depend on cosmology (mostly H0)
Time delay
∫Φ= dzrcDD
DSL
LS )(2)( 2θψ
3D gravitational potential (depends on density profile of lens)
Projected gravitational potential
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Multiply-imaged quasars III: Dust extinction
Colour differences between images → Extinction law measurement at high z
Quasar
Lens galaxy with dark halo
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Microlensing in multiply-imaged quasars as as a probe of stars in the lens galaxy
Quasar
Intrinsic quasar variability
Star
Lens galaxy Observer
Microlensing peak superposed on intrinsic variability
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Strong lensing in clusters I
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Lensing as gravitational telescopes
Galaxy cluster
Observer
µ = 1
Magnification µ ~ 10-100
Lensing makes background objects brighter/bigger by a factor µ, but also zooms in on a volume that is smaller by the same amount → Very rare types of objects may be impossible to detect this way
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Strong lensing in clusters II
Galaxy cluster Magnification map
The magnification attains its highest value along a narrow strip – the critical line
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Strong lensing in clusters III Giant arc
×=<
Gpc1"30101.1)( L
2
solar14
arcDMM arcθθ
Giant arcs can be used to assesss: • Enclosed mass • Cluster shape • Density profile (through arc curvature vs. θarc)
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Dark matter mapping – 2D X-ray gas (believed to dominate baryon budget)
Overall matter distribution (dark matter) from weak lensing
The bullet cluster
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Dark matter mapping – 3D
Dark matter tomography in the COSMOS survey based on weak lensing
z=0
z=1
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Magnification bias
True flux-limited distribution around massive foreground
object
Observed flux-limited distribution around massive foreground object
A flux-limited survey: Containing objects with fluxes higher than a certain magnitude threshold