Clusters & Super ClustersLarge Scale Structure

Clusters & Super ClustersLarge Scale Structure

Chapter 22Chapter 22

~Half of all galaxies are in:• clusters (mostly Es and S0s; mass > few times 1014 to 1015 Msun) or

• groups (less dense; more Sp and Irr; less than 1014 Msun)

Clusterscontain 100s to 1000s of gravitationally bound galaxiesTypically ~few Mpc acrossCentral Mpc contains 50 to 100 luminous galaxies (L > 2 x 1010 Lsun)Abell’s catalogs (1958; 1989) include 4073 rich clusters

Coma Cluster

Nearest clusters are Virgo and Fornax (containing 1000’s of galaxies; d=15-20 Mpc)

Richer cluster, Coma, at d=100 Mpc and several Mpc across, contains ~10,000 galaxies

Clusters filled with hot gas (T=107 – 108 K making them X-ray bright)

Galaxy Clusters & Groups

smaller than clustersContain less than ~100 galaxiesLoosely (but still gravitationally) bound Contain more spirals and irregular galaxies than clusters

“The Local Group”

Groups

Compare relative sizes of groups and clusters

Projected density of galaxies in a cluster drops as r1/4 (similar to surface brightness of elliptical galaxies)Crossing time in a typical cluster (galaxy moving at 1000 km/s, cluster size 1 Mpc) 109 yearsThus, clusters must be gravitationally bound systems and have possibly had enough time to “relax”

M = 2 x 1015 Msun

If clusters are relaxed systems, we can use the virial theorem to estimate their masses

M = 7.5(σ2Rh/G) eq. 20.20 & 22.5

Now galaxies, rather than stars, are the masses whose line-of-sight velocities we measure.

For Coma cluster, σ= 880 km/s and Rh = 1.5 Mpc, what is mass?

Luminous matter does not make up this massLB ~ 8 x 1012 LB,sun

M/LB ~ 250 Msun/LB,sun

Adding up mass in DM halos of spiral galaxies still not enoughLook for mass in hot, intracluster gas – Temp =107 to 108KEstimate gas mass from diffuse X-ray emission

Significant mass in gas – can be up to 10 times stellar massDynamical (virial) measurements indicate this accounts for about 10% to 20% of the mass...

Clusters have a Dark Matter problem too...

Mass appears to be contained in individual galaxy halos that extend further than we can measure

Clusters have their own Dark Matter halos as well

M/L ratios for clusters is ~200:1

Example of dark matter evidence in clusters (and the exotic nature of DM) The Bullet Cluster

Galaxy Mergers

How common is it for objects to run into each other in space? Derive the time between collisions for an object with size R, velocity v in an environment with number density n t = 1/(nv4πR2)

For Coma: galaxy sizes are ~ 1.3 x 1011 R, v ~ √3 x 880 km/s = 1500 km/s, n ~ 3.5 x 10-16 pc-3 gives

t = 17 Gyr or 1.2 Ho-1

(galaxy has on average a 50/50 chance of colliding)*Note that when galaxies collide, individual stars do not normally

collide due to their tiny cross-sections

Depending on conditions, galaxies may interact but not merge. Even small interactions can cause an increase in the entropy of the stellar system (i.e. thickens spiral disk).

Many clusters have a central dominant or cD galaxy at their center (e.g. M87 in Virgo)

•contain multiple nuclei•could come from merger of central galaxies•galactic “cannibalism”

Numerical n-body simulations reveal what happens to the stars and gas when two galaxies collide and merge.

Are there structures larger than clusters? YES

Local Supercluster - 106 galaxies about 30 Mpc across

•Can’t get mass with virial theorem

•Crossing times are too large, systems are not relaxed – just now collapsing

•In addition to superclusters, large scale structure of galaxies reveals large voids

Redshift surveys of distant galaxies reveal the 3-d large-scale structure in the Universe

•Galaxies appear to sit on 3-d surfaces (e.g. bubbles, sponges); structures are flattened along these surfaces

•Voids are ~50 Mpc across and more spherical

•Survey magnitude limit appears as galaxy “thinning” beyond z=0.15, but we an assume this structure continues…

Other redshift surveys:

Where does the structure come from?

Top-down: Large scale structures form first (superclusters, voids) followed by smaller structures forming out of the matter

Bottom-up: Small scale structures (i.e. galaxies) form first and then come together to form larger scale structures.

Which is it?

Millennium SimulationCompares large galaxy surveys with simulations designed to model the data

•Assumes cold dark matter dominates Universe

•N-body simulation with particles interacting gravitationally

•1010 particles mapped from early times in the Universe to the present in cubes 700 Mpc on a side

Galaxies

Dark Matter

The simulation shows that structure forms more along the lines of the “bottom-up” model (i.e. galaxies form first), but that these form in the already over-dense regions of the dark matter distribution.

Redshift z=0 (t = 13.6 Gyr)

Redshift z=1.4 (t = 4.7 Gyr)

Redshift z=5.7 (t = 1.0 Gyr)

Redshift z=18.3 (t = 0.21 Gyr)

Galaxy Luminosity FunctionA census of galaxies over a large enough region of space gives the number density of galaxies as a function of luminosity

Φ is the number density of galaxies with luminosity between L and dL

The “knee” of the distribution (where the exponential drop off in the LF occurs) is at L* = 2 x 1010 Lsun ~ LMW

LF is weighted towards dim galaxies with α = -1.2

If we integrate Φ(L) weighted by L we get the luminosity density of the observable universe

~ 40 watt light bulb inside sphere of 1 AU radius!