Fraction of galaxies of different types plotted as function of cluster radius and projected density

Galaxy Morphology and Environment

Main relation – E’s more common in clusters than fieldf(E) = fraction of ellipticals f(Sp) = fraction of spirals•regular, symmetric cluster – f(E) = 40%•“ratty”, asymmetric cluster – f(E) = 15% (Oemler 1974)•f(Sp) increases with R in clusters – almost no spirals in cores of clusters

Dressler (1980) – first systematic study of 55 clusters with 6000 galaxies

Concluded that the fundamental relation is morphology-density rather than morphology-radius

However, not easy to say which is more fundamental since in clusters N ~ R-1

Coma cluster – red=E; blue=spirals; green contours= X-rays

These substructure groupings may travel through the cluster together

maintains a similar density for a given galaxy no matter where it is in cluster

helps explain how morphology-density relation can be maintained even as galaxy moves throughout cluster

Many clusters contain significant substructure

Contour plots of projected galaxy density in clustersBar represents 0.24 Mpc at cluster distance (Geller & Beers 1982)

What role does substructure play in dictating galaxy morphology?

Sanroma & Salvador-Sole (1990) – •randomize azimuthal position of galaxy in Dressler’s cluster study but preserve radial position•removes small-scale substructure but preserves large-scale radial variations•found morphology-density relation was similar to “unshuffled” data

Whitmore, Gilmore & Jones (1993) –•f(E) always ~50% in centers of clusters, regardless of varying central density in clusters

Local density is not the strongest deciding factor for morphology Radius more important to dictate morphology

But, local environment must still be an important factor when determining galaxy morphology

Galaxies with a nearby neighbor (within 50 kpc) in outer part of cluster (where f(E)~15%) have f(E)~55%! -> density dictates !?

Thus, both effects (local density and broader environment) play a role....

Half of all galaxies are in clusters (higher density; more Es and S0; more than 1014-1015M) or groups (less dense; more Sp and Irr; less than 1014M)

Clusters contain 100s to 1000s of gravitationally bound galaxies Typically ~few Mpc across Central Mpc contains 50 to 100 luminous galaxies (L > 2 x 1010 L) Abell’s catalogs (1958; 1989) include 4073 rich clusters Both luminous Es and dEs more concentrated in clusters than mid-size Es (?)

Coma Cluster

Nearest rich clusters are Virgo and Fornax (containing 1000’s of galaxies; d=15-20 Mpc) Richer cluster, Coma, at d=70 Mpc and 7 Mpc across Clusters filled with hot gas (T=107 – 108K) X-ray bright – strips away cool gas of infalling galaxies Gas mass is 1:1 to 10:1 with stellar mass!

Galaxy Clusters

•Groups of galaxies are smaller than clusters•Contain less than ~100 galaxies•Loosely (but still gravitationally) bound •Contain more spirals and irregular galaxies than clusters

“The Local Group”

Are there structures larger than clusters? YESLocal Supercluster - 106 galaxies in 106 Mpc3

Redshift surveys (Vr = Ho x distance) 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)

•Voids are ~50 h-1 Mpc across

•Survey mag limit appears as galaxy “thinning” beyond z=0.15

•Local group moving at 600 km/s relative CMB.

•At these speeds, a galaxy would take ~40 Gyr to travel from center to edge of a void. Thus process to remove material from voids took place very early when Universe was more compact…

This map out to about 800 Mpc

Elliptical-like galaxies Star-forming galaxies (e.g. Spirals)

Ellipticals are more clustered than spiral galaxies – morphology-density relation

Measuring Galaxy Clustering – the two point correlation function

Compute the probability that a galaxy is found in each of two small volumes ΔV1 and ΔV2 if number density is n

ΔP = n2 [ 1 + ξ(r12)] ΔV1 ΔV2

If galaxies tend to clump together, the the probability that we have galaxies in both volumes is greater if the separation r12 between the two regions is small.

If ξ(r) > 0 at small r, then galaxies are clustered If ξ(r) < 0 at small r, then galaxies tend to avoid each other

Well fit with function ξ(r) = (r/r0)-γ where γ>0

When r < r0, the correlation length, the prob of finding one galaxy within radius r of another is larger than for a random distribution (i. e. Galaxies are clearly clustered!). Must become negative as r increases and deviates from random distribution.

Where does the structure come from?

• Present-day distribution of galaxies is very lumpy on scales up to 50 h-1 Mpc.

• But, measurements of CMB temperature is very smooth to few parts in 100000.

• CMB produced during time of recombination at z = 1100

• These small irregularities in the matter/radiation field would grow to produce large-scale distributions of matter today.

Where does the structure come from?

Top-down: First largest scale structures form (superclusters, voids) and then smaller structures form out of the matter

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

Which is it?

Compare large galaxy surveys with simulations designed to model the data. One of the largest such simulations is the Millennium Simulation (Croton et al. 2005)

• Assumes cold dark matter dominates Universe (alternative is hot dark matter – light particles like neutrinos rather than heavier CDM particles)

• N-body simulation with particles interacting gravitationally

• 1010 particles mapped from early times in the Universe to the present in cube 500 h-1 Mpc on a side

Galaxies

Dark Matter

The simulations show 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 Formation – Nature, Nurture, or merger?Ellipticals are primarily found in the densest parts of a cluster Odd because stars in Ellipticals are old (several billion yrs), while most clusters of galaxies are not that old – still coming together (e.g. Millenium simulations)

As galaxy forms, how does it know if it will end up in inner or outer part cluster?• maybe E’s are actually younger – stars formed earlier in smaller sub-galaxies• then E’s form through mergers of sub-galaxies in “clumps”• clusters grow by adding these clumps (like groups) where, we will discuss,

mergers occur more easily and could form the ellipticals.

If an elliptical formed from a single gas-cloud, how long would it take to make stars and complete collapse? Can use tff – time that a gas cloud of a given density takes to collapse under gravity less than 0.1 Gyr they can form quickly!

Then, how do E’s get dense, metal-rich centers?• must be assembled from partly gaseous sub-galaxies• some metal-enriched gas from first, biggest stars flows to center and makes

metal-rich stars. Also, gas conserved ang. mtm. and would make disky isophotes.

How does life in the center of a cluster effect a galaxy?•clusters form from agglomeration of smaller group/sub-clumps•in sub-clumps – low relative velocities – mergers more likely•stellar disk destroyed and E is formed•other close encounters “fluff up” galaxy

largest Es have lowest central SB and largest sizeSystems less disturbed would be less luminous, disky Es

But some evidence that many Es are NOT formed by mergersRelations between luminosity, core size, central SB, color (i.e. Fundamental Plane)

Luminosity from total stars and gas assembled over timeColor from last episode of SF and metallicityWhy are they linked? Merging would have to take place on the same timetable for all galaxies of a given luminosity…

Also, if largest Es are formed by multiple mergers, we would not expect to see many in the early Universe – but luminous, red galaxies are common back to z~2 (Universe age of only 5 billion years).