Galaxies and the Universe - Large-Scale Structure

Galaxies and the Universe - Large-Scale Structure - 1/15/12...

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Unformatted text preview: 1/15/12 Gala ie and he Uni e e - La ge-Scale S c e Large-Scale Structure de Vaucouleurs long argued for the physical reality of a flattened distribution of nearby galaxies centered on the traditional Virgo cluster, extending well past our distance from the center - the Local or Virgo Supercluster, extent 50+ Mpc. Its reality now seems established on dynamical as well as density grounds. At larger distances, galaxy counts by themselves suggested that clusters sometimes group into superclusters or "clouds": as shown by Shane and Wirtanen (1954 AJ 59, 285, courtesy of the AAS), These same data were reanalyzed by Seldner et al 1977 (AJ 82, 249) to produce the higher- resolution gray- scale map familiar from poster use (but which scanned so poorly for the ADS that most of the structure disappeared). The evidence for superclusters was reviewed by Oort 1983 (ARA&A 21, 373). One revealing way to see them is a set of sky maps sliced by redshift interval - see, for example, the ones in Fairall, Large-Scale St ruct ures in t he Univ erse (Wiley- Praxis 1997). Clumps and groups of Abell clusters also exist, so that these have been used as tracers of the galaxy distribution to large distances - Abell 1961 (the year, not the cluster - AJ 66, 607), Murray et al 1978 (ApJLett 219, L89); Tully, Nearb Gala ies At las. These structures, if real, must persist in redshift as well as angular coordinates. Redshift surveys are now available for Abell clusters (since velocity dispersions require multiple objects anyway), for Zwicky galaxies and their southern counterparts from the CfA and SSRS groups; and for selected deep "postage- stamp" areas. The galaxy distribution is often shown in angle- redshift wedge diagrams. These frequently show intricate structure - clouds, superclusters, filaments, sheets, voids... as shown in the famous "Slice of the Universe" by de Lapparent et al 1986 (ApJLett 302, L1, courtesy AAS): There is even evidence of structure extending on scales .a . a.ed /keel/gala ie /la ge cale.h ml =0.1 (Tully 1986 ApJ 303, 25 and Nearb Gala ies At las): 1/6 1/15/12 Gala ie and he Uni e e - La ge-Scale S c e a d e a a ge ca e f he f i g ab ac (BAAS 1990, 22, 753). Thi ca ed a bi f a i , i g f he ei diffe e di ec i h ha he a a e e i dici aa a ifac f i g h gh a fe hee i e c e ad e hi g ab he ed hif pe e. The 2DF e ee ha e fi a f d a i i c e ie a a f ac i fa G c ( h be ). I i i a ha a e ed d ce a a ic a ca e a a egi ch a ge ha hi ca e; e i e ha e h ce he i e hich c d be de ec ed. I fac , a e f ha i d i i acc d i h a e f ac a di ib i i hich a ic a ca e e g h i ig ifica , a a i fac b gh i Ma de b ' a ife Fo m, Chance, and Dimen ion i h e ci a i i ib e e i he ga a di ib i . The ef e, i i ea i a ee he he he e i i fac a ca e hich ce d no cc (500 M c , c fi a i ca e f he ha he 2dF g a i g ha d bea hi e ). I hi he e he c gica i ci e' if i a d i a e e? S aigh f a d c gica che e d edic eii he ca e f e ba i ha c d ha e eached ig ifica a i de b , b af e eei g he i f i fa i h h g e ba i ce h gh be i ib a ge, ha i g da a a a bea ha i g .a . a.ed /keel/gala ie /la ge cale.h ml e de ... 2/6 1/15/12 Gala ie and he Uni e e - La ge-Scale S G , , c e - -- Z ( - .E .T , ) ( , ) .S .S .T , .F A. H - ' The E ol ing Uni erse ( . D. H ), " " H ( .A , H .A .a , , H . a.ed /keel/gala ie /la ge cale.h ml , . V ; D 'A , - -G " , H .L 150 / .A " , H - 1998), , , P ,K , "G W" ,G &B 1996 AJ 112, 1780). " , G" ( 3/6 1/15/12 Gala ie and he Uni e e - La ge-Scale S c e The e ha e been claim (Keel e al. 1999 AJ 118, 2547, fo one) ha c e a high ed hif a e being een a o befo e na o nd, ba ed on he a io of in e nal eloci di pe ion o e pec ed H bble dep h fo ea onable geome ie . T na o nd and collap e ma ell happen on diffe en i e cale a diffe en ime . Thi i a ef l con e o in od ce como ing coordinate s , a e of coo dina e hich follo he e pan ion of he co mic cale fac o o ha p oce e d e onl o he H bble e pan ion di appea . In hi a , a cl e ill ho onl collap e o a d a ela ed a e, ince he ini ial e pan ion in ph ical pace e l p el f om he H bble flo . Va io egion a o nd a cl e ma be in diffe en a e a he ame ime - in fac , all a e. E en local di ance/ ed hif da a ho ha he e i a la ge egion i h ema ic infall o a d he Vi go cl e . A ho n in he CAIRNS e (one efe ence i Rine e al. 2003, AJ 126, 2152, f om hich he follo ing pic e i ed co e of he AAS), ich cl e a e gene all o nded b a d namicall di inc egion of infall, comple e i h a na o nd adi i ible in he angle- ed hif lice of pha e pace. In one dimen ion, he e egion can be de ec ed (a lea in p inciple) a S- haped pe ba ion o he H bble ed hif - di ance ela ion, he e he e a e ema ic eaming mo ion in f on of and behind ich cl e . .a . a.ed /keel/gala ie /la ge cale.h ml 4/6 1/15/12 Gala ie and he Uni e e - La ge-Scale S c e Along with large- scale clustering structure, redshift surveys have revealed its inverse - voids with dimensions 100 Mpc devoid of luminous galaxies. They also occur in the single dimension probed by QSO absorption lines using the Lyman α forest, so the deficit isn't only in fraction of gas converted to stars in galaxies (though there are complexities introduced by the ionization of intergalactic matter due to QSOs and starburst galaxies, and by confusion of spatial and velocity structure along a single available line of sight). Voids pose particular challenges to theories of galaxy formation. Why do they exist with relatively sharp edges, when dark matter presumably doesn't sustain such strong boundaries? A very general consideration is forming large scale structure is that material must move at a velocity of order R/t H to clear a void of radius R in a Hubble time, and if this process is all gravitational, the density contrast has to have been able to accelerate the material to this velocity. Starting with the uniformity shown by the microwave background, it is not all obvious that there was enough density structure to do this. One popular answer to this apparent mismatch of structures, as seen in galaxies and the plausible density field, is biased galax y f ormat ion - the notion that collapse to a galaxy requires a certain threshold density, which is almost never reached by fluctuations in the valleys of larger- scale density dips. The following cartoon depicts a density slice through a complicated density field with power on a wide range of scales, and the idea of a bias threshold producing clusters, groups, and voids out of proportion to the local density. It is customary to define a biasing parameter b which relates fluctuations in the overall mass and light distributions. Thus, for a galaxy overdensity (defined statistically) δ and mass overdensity δM, we have = b M , while the power spectra scale as b2. Since in a simple approximation, the growth rate of mass perturbations scales with the mass contribution to the cosmic density as Ω M0.6, one often encounters the combination (called the redshift distortion parameter) =Ω M0.6b in studies trying to solve derived density fields and galaxy distributions for b. This has the interesting property that structure amplitudes in redshift space are enhanced by a factor (1+ ) over those in linear coordinate space. Hamilton's review (reference above) is a nice compendium of the density- velocity field connection, showing that values of .a . a.ed /keel/gala ie /la ge cale.h ml 5/6 1/15/12 Gala ie and he Uni e e - La ge-Scale S c e from 0.5 to 1 are allowed by existing surveys. To confuse matters somewhat, some papers use a bias parameter that runs inversely to this convention. Analysis of the 2dF galaxy survey suggests that bias is not important on scales 5- 30 Mpc (Verde et al. 2002 MNRAS 335, 432), with a fitted linear bias parameter of 1.04 ± 0.11 and quadratic term - 0.05 ± 0.08. On these scales (at least), both angular and redshift clustering suggest that mass does trace light. Biasing may still matter for galaxy formation, if not so much for the current galaxy distribution. Physically, one might imagine biasing to arise either as a threshold effect operating on the density field which is the superposition of components with a wide range of spatial frequencies (k in P(k )), or on a column- density criterion if star formation can disrupt galaxy formation in lower- density regions when its energy input drives the gas to expand. Nongravitational ways to explain voids have been explored, though biassing has driven them from favor (Ostriker and Cowie 1981 ApJL 243, L17). The "soap- bubble" or foamlike geometry revealed by large- scale redshift surveys has given rise to mathematical treatments of the walls and voids. A particularly popular description has been Vo onoi e ela ion, a way of generating the minimum- area set of surfaces dividing a given set of generating points (i.e. void centers). This division in two dimensions gives the set of line segments with minimum total length that separates the generating points into their own regions bounded by polygonal tesselations, with the obvious generalization to three dimensions. In two dimensions, the Voronoi tile around a given generating point is the smallest polygon produced by lines bisecting the radius vector between the generating point and all other tile generators, which is to say the boundary containing points closer to the generating point than to any other in the set. For those interested, IDL has a routine to compute the Voronoi tesselation of 2D coordinate lists. « Groups and clusters of galaxies | Galaxy interactions and mergers Course Home | Bill Keel's Home Page | Image Usage and Copyright Info | UA Astronomy k eel@bildad.a . a.ed Ls cags 920 at hne: /09 .a . a.ed /keel/gala ie /la ge cale.h ml 20009 6/6 ...
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