Astronomy 422 Lecture 15: Expansion and Large Scale Structure of the Universe
Key concepts: Hubble Flow Clusters and Large scale structure Gravitational Lensing Sunyaev-Zeldovich Effect
Expansion and age of the Universe Slipher (1914) found that most 'spiral nebulae' were redshifted. Hubble (1929): "Spiral nebulae" are other galaxies. Measured distances with Cepheids Found V=H 0 d (Hubble's Law) V is called recessional velocity, but redshift due to stretching of photons as Universe expands. V=H 0 D is natural result of uniform expansion of the universe, and also provides a powerful distance determination method.
However, total observed redshift is due to expansion of the universe plus a galaxy's motion through space (peculiar motion). For example, the Milky Way and M31 approaching each other at 119 km/s. Hubble Flow : apparent motion of galaxies due to expansion of space. v ~ cz Cosmological redshift: stretching of photon wavelength due to expansion of space. Recall relativistic Doppler shift:
Thus, as long as H0 constant For z<<1 (OK within z ~ 0.1) What is H0? Main uncertainty is distance, though also galaxy peculiar motions play a role. Measurements now indicate H 0 = 70.4 ± 1.4 (km/sec)/mpc. Sometimes you will see For example, v=15,000 km/s => D=210 Mpc = 150 h-1 Mpc.
Hubble time The Hubble time, t h, is the time since Big Bang assuming a constant H 0. How long ago was all of space at a single point? Consider a galaxy now at distance d from us, with recessional velocity v. At time t h ago it was at our location For H 0 = 71 km/s/mpc
Large scale structure of the universe Density fluctuations evolve into structures we observe (galaxies, clusters etc.) On scales > galaxies we talk about Large Scale Structure (LSS): groups, clusters, filaments, walls, voids, superclusters To map and quantify the LSS (and to compare with theoretical predictions), we use redshift surveys. For mapping the 3D distribution of galaxies in space ~1 million galaxies with redshifts measured Galaxies are not distributed randomly, but in coherent structures
Most galaxies are in clusters or groups. Clusters contain hundreds to thousands of galaxies. Cluster Abell 1185 showing a galaxy interaction
Large scale structure of the universe Groups: <~ 50 members 1 h -1 Mpc across Velocity dispersion ~150 km/s Masses ~ 2x10 13 h -1 M
Classifying clusters: 1) rich clusters vs. poor clusters Poor clusters include galaxy groups (few to a few dozen members) and clusters with 100 s of members. Masses are 10 12 to 10 14 solar masses. Rich clusters have 1000 s of members. Masses are 10 15 to 10 16 solar masses. Higher density of galaxies. 2) regular vs. irregular clusters Regular clusters have spherical shapes. Tend to be the rich clusters. Irregular clusters have irregular shapes. Tend to be the poor clusters.
Example: Distribution of galaxies (2500 or so) in the Virgo cluster. It is moderately rich but not very regular. Large extension to the south makes it irregular. Also, these galaxies have velocities offset from the main cluster. This is a whole subcluster that is merging with the main cluster. Despite great age of universe, many clusters are still forming!
There is more mass between galaxies in clusters than within them Abell 2029: galaxies (blue), hot intracluster gas (red) X-ray satellites (e.g. ROSAT, Chandra) have revealed massive amounts of hot (10 7-10 8 K) gas in between galaxies in clusters ( intracluster gas ). A few times more than in stars!
One consequence of intracluster gas: Ram Pressure Stripping As a spiral galaxy moves through a cluster, the gas in the galaxy runs into the intracluster gas, dragging some of it (especially from the outskirts) from the galaxy. Stars not affected they are more like bullets) Virgo cluster galaxy showing stripped atomic gas (contours are 21-cm emission).
What is the origin of intracluster gas? Possibilities: 1) Leftover gas from the galaxy formation process 2) Gas lost from galaxies in tidal interactions, ram pressure stripping, supernova explosions, and jets from active galactic nuclei High density of galaxies in clusters means that tidal interactions are common How could you tell between 1) and 2)?
Take a spectrum! Many lines of elements produced by nucleo-synthesis in stars. Can t be mostly leftover gas. X-ray brightness X-ray frequency
Most mass in clusters is in Dark Matter Recal virial mass relation: Example: Coma Cluster Mass in visible matter (galaxies and intracluster gas) 2 x 10 14 solar masses. Size 3 Mpc. Escape speed then 775 km/s. But typical velocity of galaxy within cluster observed to be 1000 km/s, and many have 1000-2000 km/s! Must be more mass than is visible.
For the Coma cluster (a rich, regular cluster, ~10,000 members): Thus, Dark Matter dominates!
But, are clusters in equilibrium? Can we apply the virial theorem? How does a system reach equilibrium? Interactions distribute energy equally among members. Crossing time is a rough estimate of the time it takes to achieve equilibrium. For Coma, t cross ~ 4 Gyr Compare to t h
Gravitational Lensing Remember from Einstein s Theory of General Relativity that gravity causes light to follow curved paths, e.g. So gravity acts like a lens. Saturn-mass black hole
Clusters of galaxies also bend the light of more distant galaxies seen through them
All the blue images are of the same galaxy! From the lensed galaxy images, you can figure out the total mass of the cluster. Results: much greater than mass of stars and gas => further evidence for dark matter!
The Bullet Cluster Dark matter predicted not to interact with ordinary matter, or itself, except through gravity. Gas clouds, by contrast, can run into each other. A collision of two clusters provides dramatic evidence for dark matter: cluster trajectory red shows hot gas from two clusters, seen with Chandra X-ray observatory. The gas clouds have run into each other, slowing each one down cluster trajectory blue shows inferred distribution of cluster mass from gravitational lensing of background galaxies. The dark matter has gone straight through with no interaction, like the galaxies have.
Can MOND explain this? Recall, MOND asserts that on large scales, a given mass has more gravity than Newton s Law predicts, so less mass is needed to explain large scale accelerations, which may get rid of exotic Dark Matter. So red is still hot intracluster gas clouds slowed down by their collision. But in MOND, blue is ordinary non-radiating matter that has passed through with essentially no collisions to slow it down. However, MOND allows for ordinary Dark Matter, such as dead stellar remnants, known elementary particles (e.g. neutrinos), i.e. something in which collisions are rare, allowing the two concentrations to pass through each other. So MOND reduces amount of DM in clusters, but still need some.
Superclusters Assemblages of clusters and groups The Local (or Virgo) Supercluster
Gregory & Thompson 1978
Pisces-Perseus supercluster
Stickman diagram (de Lapparent et al 1985)
The Great Wall (Schaap et al, 2dF Galaxy Survey)
What is overall texture of large scale structure? Revealed by large redshift surveys. Find the largest structures known: walls and voids. Examples are the Great wall, the Southern wall. Suggests a 'frothy' universe
da Costa et al 1994, ApJ
More Superclusters note the filamentary shapes
Simulating the Universe
From measuring such departures, we ve found: 1) Milky Way and Andromeda approaching each other 2) Local Group s Hubble flow being The image cannot modified be displayed. Your computer may not by have enough Virgo memory to open the image, Cluster or the image may have been corrupted. Restart your computer, and then open the file again. If the red x still appears, you may have to delete the image and then insert it again. 3) Local Supercluster Hubble flow modified by even larger supercluster, named the Great Attractor, 70 Mpc away. Mass estimate: 5x10 16 solar masses (50 times Virgo cluster)!
Where is the Great Attractor? Hard to characterize because it crosses the plane of the Milky Way. Galactic dust obscures our view. Radio survey of galaxies shows more. But implied mass of galaxies much less than thought.
15,000 brightest galaxies
Clusters ~50 members (poor cluster) to 1000's (rich cluster) 6h -1 Mpc across Velocity dispersions of 800-2000 km/s regular or irregular morphology masses ~10 15 h -1 M Superclusters - next hierarchical scale Contain clusters and groups Flattened or filamentary Up to 100 Mpc long Examples: Local Supercluster: Virgo and many groups, incl. Local Group First supercluster discovered outside of Local Supercluster was Coma/A1367
Redshift-space distortions Realize that the 'third' dimension is not really radial distance, but redshift. Related by Hubble expansion, but affected by peculiar velocities. Small scales: random motions cause objects at same distance (e.g. within a cluster) to have slightly different redshifts. This elongates structures along the line of sight. Large scale: Objects fall in towards denser region, making object between us and the overdensity to appear farther away. Net effect: enhance overdensities : redshift-space distortion
Sunyaev-Zeldovich Effect Carlstrom et al. 2002
Sunyaev-Zeldovich effect 4 0 The Sunyaev-Zeldovich effect Photons of the CMB are scattered to higher frequencies by hot electrons in galaxy clusters, causing a brightness decrement at long wavelengths. This is an application of Inverse-Compton scattering. Decrement is proportional to integral of electron pressure through the cluster, or electron density if cluster is isothermal. Electron density and temperature can be estimated from X-ray observations, so the linear scale of the cluster is determined. The Compton y-parameter is the integral of the gas pressure along the line of sight in the non-relativistic limit
Thermal SZ effect 4 1
SZ images 4 2 Reese et al. 2002
Next time: Active Galaxies