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