extragalactic distance scale astr 2120...
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Extragalactic Distance ScaleASTR 2120
Sarazin
Test #2Monday, April 6, 75 minute limit, start by 5 pmCollab Tests & Quizzes tabHave pencils, paper, calculatorYou may not consult the text, your notes, or
any other materials or any personYou may use a 3x5 card with equations~2/3 Quantitative Problems (like homework
problems)~1/3 Qualitative Questions
Multiple Choice, Short Answer, Fill In the Blank questionsNo essay questions
Test #2 (Cont.)Material:
Chapters 16-20, 22 (at least parts)Stellar Evolution, Stellar Remnants, General
Relativity (quantitative only), Black Holes, Death of Stars, Planetary Nebulae, Supernovae, Pulsars, Compact Binaries, Interstellar Gas & Dust, Star Formation, the Milky Way, Normal Galaxies, Extragalactic Distance Scale, Clustering of Galaxies
Homeworks 5-8Know pc, AU, , , , H0No problem set week of March 31 - April 6 to allow
study for test
M¤
L¤
R¤
Test #2 (Cont.)Equation/Formula Card:You may use one 3x5 inch index card with equations
and formulae written on both sides.Make 6x5 inch, one side only, easier to scan or photoDO NOT LIST pc, AU, M¤, L¤, R¤, or H0 (Hubble
Constant)DO NOT LIST the stellar classifications OBAFGKMDO NOT INCLUDE ANY QUALITATIVE MATERIAL
(text, etc.)
Test #2 (Cont.)
Review Session:Discussion sessionFriday, April 3, 2-3 pm
Extragalactic Distance ScaleASTR 2120
Sarazin
Distance = hardest measurement problem in astronomyParallax only works to nearest starsDistance Ladder:
1. Find nearby objects, distances known2. Calibrate properties3. Use to measure distances on larger scales4. Repeat steps 1-3
Properties:Standard candles: calibrate luminosity L, measure flux
f = L/(4pd2)Standard rulers: calibrate diameter D, measure apparent
diameter q (radians) = D/dNote: 1 Mpc = 1 million pc = 1 thousand kpc
Distances to Galaxies
Cosmic Distance Ladder
Cosmic Distance Ladder
Hubble
Mega Masers
NGC 4258 Megamaser
Some galaxies with Super Massive Black Holes at their centers have accretion disks with molecular gas
Molecular masers (= lasers in radio) in diskDisk big enough to image with VLBAMolecular clouds move fast enough to detect proper
motion, μ = v / dMasers = lines: Use Doppler effect to measure velocity vSolve for distance d(Jim Braatz, NRAO)
Mega Masers
Mega Masers
NGC 4258 Megamaser
Cosmic Distance Ladder
Slipher (1910’s): spectra of galaxies almost all redshifted, galaxies are moving away from us
Hubble Expansion
Hubble (1928): measured radial velocity vr from Doppler shifts, distances using Cepheids and other estimates
Hubble Expansion
Hubble (1928): measured radial velocity vr from Doppler shifts, distances using Cepheids and other estimates
Found linear relationshipvr µ d (distance)
vr = H0 d
H0 = Hubble constant(but actually not constant)
Hubble Expansion
Measured Hubble Constant vs. Time
H0
vr = H0 d
Hubble Key Project on Cepheids, WMAP, Planck, Type Iasupernova, other techniques
H0 = 71 km/sec/Mpc (±4%)
MEMORIZE!
Hubble Expansion
Reminder - Redshift
€
z ≡ λobs −λemλem
redshift
z > 0 moving away, redshiftz < 0 moving closer, blueshiftvr ≡ radial velocity, speed moving away
1+ z = 1+ vr /c1- vr /c
vr ≈ cz for z <<1
vr = H0 d
H0 = 71 km/sec/Mpc (±4%)
Breaks the tyranny of the distance ladder, allows direct determination of distance, once calibrated
Doppler effect - independent of distance (as a measurement)
Hubble Expansion
Cosmic Distance Ladder
Limitations:1) Galaxies move around
vpec ≲ 1000 km/sHubble expansion accurate ifvr >> vpec
2) Relativistic & cosmological effectsOccur when vr ~ c, which also implies time
when light left ~ age of Universe
1000 km/s << vr << c = 3 x 105 km/s
15 Mpc << d << 3000 Mpc
Hubble Expansion
us
Must extend distance ladder this far to calibrate Hubble law
Clustering of GalaxiesASTR 2120
Sarazin
2MASS Survey of Galaxies
Clustering of Galaxies
Slice of skyRA vs. Redshift
Redshift removes projection
Clustering of Galaxies
“Great Wall”
Gravity pulls things together• Universe started with small density fluctuations in
normal and dark matter• Gravity stronger in denser regions• Gravity pulls things together, increases density
contrast
Why is Universe Clustered?
Fixed Co-Moving VolumeExpands with Universe
• Galaxy pairs• Galaxy groups• Rich galaxy clusters
Spectrum of Clustering
Local Group
Milky Way & Andromeda (M31) and ~ 30 of their closest friends
Clusters of Galaxies
• Largest relaxed systems in Universe
• ~2 Mpc in radius
• ~100 bright galaxies, 1000’s of faint galaxies
• ~1015 M8 total mass• Centers ~6000 x average
density in Universe
X-ray ImageMost atoms in Universe in hot intergalactic gas
Optical vs. X-rayOptical X-ray
Coma cluster
General Relativity: Light affected by gravityRedshiftCurvature of photon paths
Gravitational lensing
Gravitational Lensing
Gravity = not an optician!Highly distorted, elongated images
Bottom of wine glass
Gravitational Lensing in Clusters
Gravitational Lensing in Clusters
End of Material for Test 2
Dark MatterASTR 2120
Sarazin
“Bullet” Cluster of Galaxies - Dark Matter Lab
Dark Matter
Large objects in the Universe have more gravity than can be accounted for by their visible matterDark Matter~85% of mass in UniverseWhat is it?
Cluster Mergers:Test of Dark Matter vs. Modified Gravity
• Gas behind DM ≈ Galaxies• DM = location of gravity• Gas = location of most
baryons• Whatever theory of gravity,
not coming from where baryons are
Require dark matter(not MOND or modified gravity)
In principle, could be anything large but not a star (avocados?) Astrophysically plausible:
• Stellar mass black holes?• If made by death of stars, too many
supernovae, too many heavy elements
What is Dark Matter?
In principle, could be anything large but not a star (avocados?) Astrophysically plausible:
• Stellar mass black holes?• Planets, brown dwarfs, etc. MACHOs = MAssive
Compact Halo Objects
What is Dark Matter?
Detect MACHOs from Milky Way due to gravitational lensing of background stars
MACHO Project
Detect MACHOs from Milky Way due to gravitational lensing of background stars• background stars = Large Magellanic Cloud
(LMC)
MACHO Project
Detect MACHOs from Milky Way due to gravitational lensing of background stars• background stars = Large Magellanic Cloud
(LMC)• Detect lensing brightening, same in all colors
MACHO Project
Detect MACHOs from Milky Way due to gravitational lensing of background stars• background stars = Large Magellanic Cloud
(LMC)• Detect lensing brightening, same in all colors
• Found expected number of brown dwarfs, planets, binary stars
• No MACHO dark matter
MACHO Project
In principle, could be anything large but not a star (avocados?) Astrophysically plausible:
• Stellar mass black holes?• Planets, brown dwarfs, etc. MACHOs
What is Dark Matter?
Big Bang nucleosynthesis correctly gives composition of ordinary matter in Universe today• Hydrogen, helium, trace amounts of 2H, 3He• Gives very accurate measurement of amount of
ordinary matter in Universe at t ~ 1 sec• Too little to be dark matter (but agrees with amounts of
matter in clusters of galaxies)
General Problem with Baryonic Dark Matter
In principle, could be anything large but not a star (avocados?) Astrophysically plausible:
• Stellar mass black holes?• Planets, brown dwarfs (MACHOs)• Weakly interacting, stable elementary particles
made in Big Bang• Gravity but no light or other interactions• Neutrinos, axions, photinos, weakinos, …
Is Dark Matter non-interacting particles?Dwarf galaxies suggest self-interacting DM?
What is Dark Matter?
Mergers: Test of Gravitational Physics
Gas behind DM ≈ Galaxies
Bullet Cluster
1E0657-56
Markevitch et al. 2004 Clowe et al. 2004
Image = galaxies
Red = X-rays = gas
Blue = lensing mass = gravity
Mergers:Test of Collisional Dark Matter
• Gas behind DM ≈ Galaxies• Gas collisional fluid• Galaxies collisionless
particles• Limit on self-collision cross-
section of DM
σ/m (DM) ≲ 0.5 cm2/g < 5 cm2/g required forcores in dwarf galaxies
Made in Big Bang• Hot Dark Matter
• v ~ c when formed• Example: neutrinos• Problem: hard to get to cluster, form superclusters ®
clusters ® galaxies ® stars• But, galaxies old, structure appears to grow small to large
• Cold Dark Matter• v << c when formed• Example: axions, lightest supersymmetric particle, …• Structure grows hierarchically, small to large• But, what particle(s) makes up dark matter?
Particle Dark Matter
• LHC (Large Hadron Collider)• Produce some of the (so far hypothetic) particle
candidates, constrain properties
Hope for the Future
• LHC (Large Hadron Collider)• Direct detection on Earth with cryogenic detectors
• Detect dark matter particles from Milky Way Halo• Detect motion of Earth through Dark Matter Halo
Hope for the Future
• LHC (Large Hadron Collider)• Direct detection on Earth with cryogenic detectors• Detect annihilation or decay radiation from Dark
Matter particles in nearby galaxies• Particle + antiparticle or decay® 2 photons• Gamma rays or X-rays
Hope for the Future Present?
Fermi XMM
Fermi Gamma-ray line, 130 GeVfrom center of Milky Way and clusters of galaxies??Annihilation of DM particles and anti-particles
Lightest super-symmetric particle??
Was the DM Particle Been Detected?
X + X→ γ + γ
mXc2 =130 GeV
XMM X-ray line, 3.56 keV, from Andromeda and clusters of galaxies??Decay of DM particle
Sterile neutrino ??
X→ γ + γ
mXc2 = 7.1 keV