AY202a Galaxies & Dynamics Lecture 16: Galaxy Groups & Clusters

Galaxies Are Not Randomly Distributed in Space Fundamental discovery (and debate) in the 20th century is that Galaxies cluster. On small scales: Binaries ~100 kpc intermediate scales: Groups ~1 Mpc larger scales: Clusters few Mpc even larger scales: Superclusters 10’s Mpc largest seen: Voids + Cosmic Web ~50 Mpc

Messier’s map: What do you see?

History Clustering of nebulae known since the first maps. 1930’s Hubble declares that the universe is “Sensibly Uniform” -- i.e. clusters rare Zwicky studies clusters and finds them common with clustering occurring on all scales Shapley describes a supercluster 1948 Palomar Schmidt telescope goes into operation to produce the National Geographic Palomar Observatory Sky Survey (POSS) 1956 George Abell produces first catalog of clusters

Abell Catalog 2712 Clusters discovered on the POSS plates  > -30 o mostly high |b| Abell defined a size, R A = 1.72’/z Abell Radius ~ 1.5 h -1 Mpc (~ 2.1 Mpc for H 0 = 70) Richness defined in terms of the number of galaxies within m 3 to m = = clusters in AC 1 = = clusters 2 = = cluster Abell’s original estimate of the cluster space density was /Mpc 3 What is SD with today’s surveys?

Abell 1185 Cuillandre CFHT

Coma Cluster Spitzer+SDSS

Distance classes based on tenth brightest galaxy magnitude: 1: mag ~ : mag ~ : mag ~ Why the 10 th brighest? 4: mag ~ : mag ~ : mag ~ Eventually Abell, Corwin & Olowin (1989) added the rest of the sky from the southern (SRC) Schmidt at Coonabarabran. ACO catalog now 4076 clusters total, all sky. Many other cluster catalogs Zwicky + 10,000 clusters, northern sky Sersic, Duus & Newell, Noonan, Dressler, APM survey, SDSS in optical X-ray: Piccinotti 35 Clusters HEAO A (1978), now ROSAT surveys also identified via Radio Sources (3C clusteers, etc) + some deep surveys

Magnitude- Redshift for Abell (Postman et al. ’85) Scott effect? M 10 correlates with richness Richness correlates with distance (how?)

Galaxy Groups The Local Group first identified by Lundmark, Hubble & friends 2 Big galaxies (M31 + MW), 5 or 6 mid-sized galaxies (M33, LMC,SMC, M32, IC1613, NGC6822, and dwarf Companions.

M33 R. Hammar

LMC 30 Doradus

SMC

NGC6822 IC10 IC1613 Fornax Dwarf 0.5 degree

Mass of the LG Kahn & Woltjer’s timing argument (1952) v (M31-MW) = -118 km/s has solutions of the form r = R max /2 (1 – cos  ) for  = 0 to  t = ( ) 1/2 (  - sin  ) d 2 r - GM total dt 2 r 2 = and assume r=0 at t=0 R max 3 8GM total

r increases from 0 to R max and then decreases as  cycles. Relative velocity is v = = / = ( ) 1/2 ( ) combine to give and assuming t = 14 Gyr, r = 770 kpc, we can solved for , and then the mass of the Local Group. dr dr dt 2GM total sin  dt d  d  R max (1 - cos  ) v t sin  (  - sin  ) r (1 – cos  ) 2 =

Other Galaxy Groups Many other galaxy groups identified on the sky and in initial surveys --- nearest M81, Sculptor, Centaurus, etc. and eventually many nearby clusters (Virgo, Fornax, Centaurus, Pegasus, Perseus, etc. --- note that many of these are not in Abell’s catalog! Why?) First systematic group catalog goes to deVaucouleurs (1975) who identified galaxy groups “by eye” on the sky by associating galaxies by morphology, position and a few redshifts

Stefan’s Quintet

Seyfert’s Sextet

DeVaucouleurs’ galaxy group list from Vol. IX of Stars and Stellar Systems.

De Vaucouleurs’ catalog Nearest Groups

De Vaucouleurs’ Groups 30.2 < DM < 31.2 (LSC Center)

The Local Neighborhood (Tully)

Hickson Compact Groups (1978)

Hickson 68

HCG 62 in x-rays HCG 87 optical

3-D Groups First real attempt to get a large homogeneous set of galaxy groups in 3-D by Huchra & Geller (1982, 1983) who derived the redshift-space Percolation algorithm a.k.a. Friends-of-Friends To correct for the redshift dependent selection function, define limits such that  D < D fiducial [  φ(L) dL /  φ(L) dL ] Where M lim = m lim log (V fiducial /H 0 ) and M pair = m lim log ((V 1 + V 2 ) /2H 0 ) -∞ -∞ M pair M lim

And  V = | V 1 - V 2 | < V lim (V 1,V 2,m 1,m 2 ) with two choices, either fix  V or scale it as  D. Then select D lim and V lim as needed for the sample you have.  RSA Sample

“Groupy” Flow Chart Start with any galaxy and look for friends, then look around friends for more until you don’t find any more. Then go to the next unsearched galaxy and start over…. Hhhhh kkkkkkkkkkk

Different D lim ‘s correspond to different density enhancements relative to the mean number density of galaxies in the sample. You can also ask if the crossing times are reasonable t v = 3/5 3/2 r H /  Virial crossing time. What should it be?

Density contours around Virgo: 2 = dash dot 20 = dash 100 = solid HG ‘82

Other checks  vs z CfA groups GH ‘83 Other checks

Most recent large group catalogs: Eke et al. 2004; Padilla et al PIGGs = 2dF Percolation Inferred Galaxy Group Survey  clustering of group, bias factor Crook et al MRS Survey  group masses, M/L, 

2dF 2PIGS

2MRS Sample (raw)

2MRS Sample (filled )

2MRS Group Selection Number of groups found f

2MRS Groups

3 largest 2MRS Groups Virgo, Fornax/Eridanus, Perseus-Pisces  /  =12 80

2MRS Group Mass Function

2MASS Galaxy Groups δρ/ρ = 12 δρ/ρ = σ P (km/s) R PV (Mpc) log M V /L K Log M P /L K Ω M,V 0.14+/ /-0.02 Ω M,P 0.23+/ / V=Virial Estimator P = Projected Mass