Superclusters...what are they? They are Superfun!

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Jessica Briggs
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1 Superclusters...what are they? They are Superfun! General background The Local Supercluster Observations Structure at high z Formation theory Current research

2 General background Groups and clusters: Galactic clustering on typical scale 1 Mpc Groups: Small, galaxies Clusters: Large, ~1000 galaxies ~50% of galaxies in group or cluster

General background Groups and clusters: Galactic clustering on typical scale 1 Mpc Groups: Small, 10 50 galaxies Clusters: Large, ~1000 galaxies ~50% of galaxies in group or cluster Supercluster:

3 General background Groups and clusters: Galactic clustering on typical scale 1 Mpc Groups: Small, galaxies Clusters: Large, ~1000 galaxies ~50% of galaxies in group or cluster Supercluster: Cluster of groups/clusters >50% of clusters in Superclusters Zwicky was of course the first to notice very large scale clustering (1937). Proposed by Abell True nature emerging in 1980s: Scale size 50 Mpc, voids of similar size Mass 10^(15) 10^(17) Msun. Separation 100 Mc > 10 million in universe Morphology? In 1989 The Great Wall discovered dimensions 150*50 Mpc but only 5 Mpc thick: Highly flattened structure. Superclusters often thin sheet/filament. Cellular structure of universe: view walls as network interconnecting denser regions which are SC.

4 The whole universe isotropic at large scale 3 GPc

5 Zoom by 10: Structure emerge: Superclusters 300 MPc

7 Large scale structure is dynamically young Calculate free fall time t_ff >> age of universe Implications for the peculiar velocities

8 The Local Supercluster Also known as the Virgo Supercluster Disk/Halo morphology with 2/3 and 1/3 mass resp. 100 groups and clusters. Total mass 10^15 Msun from kinematics this implies a large dark matter content. Centered on the massive Virgo Cluster, 15 Mpc away. Aligned closely to supergalactic plane: Z axis towards b=6 degrees, center toward b=0 Milky Way Z Virgo cluster Young dynamical age: little rotation

9 The whole Local Supercluster engaged in bulk flow in direction towards Centaurus 600 km/s for Local Group. Flow is almost perpendicular to disk The Great Attractor : density abnormality 50 Mpc away towards the Centaurus Supercluster.

10 Also: Virgo centric infall for all groups/clusters 240 km/s for Local Group Note on top of Hubble expansion v_e = H0*D = 70 km/s*15 Mpc = 1000 km/s Groups/clusters still receding It can be shown that the peculiar velocity must be parallell to the density dipole anisotropy v_p = f(omega_0)*sum(mr_hat/r^2) Used to estimate density parameter: Omega_0=0.2

11 Observations 1) Rich cluster surveys 2) Galaxy surveys 3) Velocity field analysis Redshift distance transformation tricky 1) Young dynamical age + flattened geometry > systematic peculiar velocity. Looks thinner than it is. 2) Dense regions at higher virialization > high unsystematic peculiar velocities. Finger of God effect. Independent distance estimation desired. Tully Fisher relationship useful. Baffa(1991) use TF to demonstrate strong (1) effect on the Perseus Pisces Supercluster. Sloan Digital Sky Survey powerful tool for massproduction of redshift mapping.

12 Describing clustering Difficult to statistically capture the walls/filaments Usually use the two point correlation function Describes excess probability of finding a galaxy/cluster at distance r from a given one compared to random. Integration constraint: Means will have anti correlation for some r. Fourier transform is Power Spectrum P(k):

13 Quantification of correlation function Las Campanas Survey: Measure with galaxies as objects Parametrize Get r0=5 Mpc, gamma= 1.8. out to ~30 Mpc. Anti correlation Mpc. Converge to zero r> Mpc: universe becomes isotropic. Also: measure for clusters: Higher degree of clusters in SC than galaxies in clusters > stronger correlation, same slope. r0=22 Mpc. Bahcall finds clustering of Superclusters.

14 Structure at high z Superclustering at early times powerful test for galaxy formation theory. Clustering of quasars identified in 1988, 1990 shown to be associated with superclustering (Bahcall) West(1991) finds radio galaxies at high z not only cluster but have spin axis pointing toward other radio galaxies and quasars. Beads on a string. The clustering of quasars and radio galaxies occurs at a stronger level than for galaxies. 2004: 100 Mpc supercluster detected at z=2.38. Cannot be recreated by hierarchical computer models. In conclusion: Superclustering seem to emerge early and severely tests hierarchical models.

15 Structure at high z Superclustering at early times powerful test for galaxy formation theory. Clustering of quasars identified in 1988, 1990 shown to be associated with superclustering (Bahcall) West(1991) finds radio galaxies at high z not only cluster but have spin axis pointing toward other radio galaxies and quasars. Beads on a string. The clustering of quasars and radio galaxies occurs at a stronger level than for galaxies. 2004: 100 Mpc supercluster detected at z=2.38. Cannot be recreated by hierarchical computer models. In conclusion: Superclustering seem to emerge early and severely tests hierarchical models..

16 Formation theory Superclusters at present day have densities just a few times universal average: Scale independent. At earlier times the regions must have had even smaller pertubations and we can use linear theory for their whole evolution. From standard cosmological theory in the matterdominated phase: Pertubation growth stops at a redshift At recombination:

COBE measured temperature variation of 10^ 5 which translates to density

However, a Omega of smaller than 1 would require larger.

17 COBE measured temperature variation of 10^ 5 which translates to density variation of exactly 10^ 3. However, a Omega of smaller than 1 would require larger. Superclusters emerge in both Hot Dark Matter scenarios and Cold Dark Matter scenarios but at different times. The difference is in the mass of the WIMP particles which affect the Jeans length through the sound speed factor. A mixture seems best able to create structure on both small and large scale within the right time frames. z=1 z=0.5 z=0 The problem is that the density spectrum of dark matter could be different than the density spectrum of luminous matter. (Biassed galaxy formation)

18 Current hot topic Origin of peculiar velocity of Local Supercluster Kocevski/Ebeling, June 2006 ApJ Peculiar velocity of LG revisisted: 630 km/s towards Virgo. Lynden Bell(1988) discovers systematic distortion in PV of 400 galaxies within 40 Mpc bulk flow towards Hydra Centaurus. 570 km/s component at Local Group. Lynden Bell identifies the Great Attractor, a region just behind the Centaurus Wall as responsible, with a mass of 3e16 Msun. However, subsequent searches cannot find luminous matter in these proportions, although results mixed. Kocevski/Ebelin use X ray selected catalogues to analyse contributions to the anisotropy dipole field. Find GA only contributes 44%, Shapley Superclusters at others at Mpc 56%.

20 Great Attractor Shapley and others Perseus Pisces on opposite side Notice that underdensities/voids play as big role as overdensities in creating the anisotropy!

Large-Scale Structure

Large-Scale Structure Evidence for Dark Matter Dark Halos in Ellipticals Hot Gas in Ellipticals Clusters Hot Gas in Clusters Cluster Galaxy Velocities and Masses Large-Scale Distribution of Galaxies 1