119091917
On the Mass-to-Light Ratio of Large-Scale Structure
We examine the dependence of the mass-to-light (M/L) ratio of large-scale structure on cosmological parameters, in models that are constrained to match observations of the projected galaxy correlation function wp(rp) and the galaxy luminosity function. For a sequence of cosmological models with a fixed, observationally motivated power spectrum shape and increasing normalization σ8, we find parameters of the galaxy halo occupation distribution (HOD) that reproduce wp(rp) measurements as a function of luminosity from the Sloan Digital Sky Survey (SDSS). From these HOD models we calculate the r-band conditional luminosity function Φ(L|Mh), and from this the mean M/L ratio as a function of halo mass Mh. We also use Φ(L|Mh) to populate halos of N-body simulations with galaxies and thereby compute M/L in a range of large-scale environments, including cluster infall regions. For all cosmological models, the M/L ratio in high-mass halos or high-density regions is approximately independent of halo mass or smoothing scale. However, the "plateau" value of M/L depends on σ8 in addition to the obvious proportionality with the matter density parameter Ωm, and it represents the universal value M/L = Ωmρcrit/ρlum only for models in which the galaxy correlation function is approximately unbiased, i.e., with σ8 ≈ σ8g. Our results for cluster mass halos follow the trend (M/L)cl = 577(Ωm/0.3)(σ8/0.9)1.7 h M☉/L☉. Combined with the mean M/L ratio for CNOC galaxy clusters, this relation implies (σ8/0.9)(Ωm/0.3)0.6 = 0.75 ± 0.06. M/L estimates for SDSS clusters and the virial regions of clusters in the CAIRNS survey imply a similar value of σ8Ω, while the CAIRNS estimates for cluster infall regions imply a lower value. These results are inconsistent with parameter values Ωm ≈ 0.3, σ8 ≈ 0.9 favored by recent joint analyses of cosmic microwave background measurements and other large-scale structure data, although they agree with values inferred from the van den Bosch et al. analysis of the 2dF Galaxy Redshift Survey. We discuss possible resolutions of this discrepancy, none of which seems entirely satisfactory. In appendices we present an improved formula for halo bias factors calibrated on our 3603 N-body simulations and an improved analytic technique for calculating the galaxy correlation function from a given cosmological model and HOD.
2232602
PROJECTED THREE-POINT CORRELATION FUNCTIONS AND GALAXY BIAS
The three-point correlation function (3PCF) can now be measured in large galaxy redshift surveys, but in three dimensions its interpretation is complicated by the presence of redshift-space distortions. I investigate the projected 3PCF, where these distortions are eliminated by integrating over the redshift dimension, as is commonly done for the two-point correlation function. The calculation of the projected 3PCF from the real-space, three-dimensional bispectrum is greatly simplified by expanding both quantities in Fourier components, analogous to Szapudi's expansion of the three-dimensional quantities in multipole components. In the weakly nonlinear regime, the bispectrum can be well represented by the first few Fourier components. There is a well-known relation between the reduced 3PCFs of matter and galaxies in the weakly nonlinear regime, which can be used to infer galaxy bias factors if the real-space three-dimensional galaxy correlation functions (two-point and three-point) can be measured. I show that the same relation holds for the reduced projected 3PCFs if these are properly defined. These results should aid determinations of galaxy bias from large redshift surveys by eliminating the complication of redshift-space distortions.
35692275
Constraints on radiative dark-matter decay from the cosmic microwave background
If dark matter decays to electromagnetically interacting particles, it can inject energy into the baryonic gas and thus affect the processes of recombination and reionization. This leaves an imprint on the cosmic microwave background (CMB): the large-scale polarization is enhanced, and the small-scale temperature fluctuation is damped. We use the Wilkinson Microwave Anisotropy Probe (WMAP) three-year data combined with galaxy surveys to constrain radiatively decaying dark matter. Our new limits to the dark-matter decay width are about 10 times stronger than previous limits. For dark-matter lifetimes that exceed the age of the Universe, a limit of zetaGammachi<1.7×10^-25 s^-1 (95% C.L.) is derived, where zeta is the efficiency of converting decay energy into ionization energy. Limits for lifetimes short compared with the age of the Universe are also derived. We forecast improvements expected from the Planck satellite.
14026331
Redshift-space distortions with the halo occupation distribution – I. Numerical simulations
We show how redshift-space distortions of the galaxy correlation function or power spectrum can constrain the matter density parameter Ω m and the linear matter fluctuation amplitude σ 8 . We improve on previous treatments by adopting a fully non-linear description of galaxy clustering and bias, which allows us to achieve the accuracy demanded by larger galaxy redshift surveys and to break parameter degeneracies by combining large-scale and small-scale distortions. Given an observationally motivated choice of the initial power spectrum shape, we consider different combinations of Ω m and σ 8 and find parameters of the galaxy halo occupation distribution (HOD) that yield nearly identical galaxy correlation functions in real space. We use these HOD parameters to populate the dark matter haloes of large N-body simulations, from which we measure redshift-space distortions on small and large scales. We include a velocity bias parameter α v that allows the velocity dispersions of satellite galaxies in haloes to be systematically higher or lower than those of dark matter. Large-scale distortions are determined by the parameter combination β ≡ Ω 0.6 m /b g , where bg is the bias factor defined by the ratio of galaxy and matter correlation functions, in agreement with the linear theory prediction of parameter degeneracy. However, linear theory does not accurately describe the distortions themselves on scales accessible to our simulations. We provide fitting formulas to estimate β from measurements of the redshift-space correlation function or power spectrum, and we show that these formulas are significantly more accurate than those in the existing literature. On small scales, the 'FOG' distortions at projected separations ∼0.1 h -1 Mpc depend on Ω m α 2 v but are independent of as, while at intermediate separations they depend on σ 8 as well. One can thus use measurements of redshift-space distortions over a wide range of scales to separately determine Ω m , σ 8 , and α v .
124560798
Lensing of 21cm Absorption "Halos" of First Galaxies
14336682
Interpreting the Observed Clustering of Red Galaxies at z ~ 3
Daddi et al. have recently reported strong clustering of a population of red galaxies at z � 3 in the Hubble Deep Field-South. Fitting the observed angular clustering with a power law of index � 0.8, they infer a comoving correlation length r0 � 8 h � 1 Mpc; for a standard cosmology, this r0 would imply that the red galaxies reside in rare, M � 10 13 h � 1 Mhalos, with each halo hosting � 100 galaxies to match the number density of the pop- ulation. Using the framework of the halo occupation distribution (HOD) in a CDM universe, we show that the Daddi et al. data can be adequately reproduced by less surprising models, e.g., models with galaxies residing in halos of mass M > Mmin ¼ 6:3;10 11 h � 1 Mand a mean occupation Navg(M) ¼ 1:4(M =Mmin) 0:45 above this cutoff. The resultant correlation functions do not follow a strict power law, showing instead a clear transition from the one-halo-dominated regime, where the two galaxies of each pair reside in the same dark matter halo, to the two-halo-dominated regime, where the two galaxies of each pair are from different halos. The observed high- amplitude data points lie in the one-halo-dominated regime, so these HOD models are able to explain the observations despite having smaller correlation lengths, r0 � 5 h � 1 Mpc. HOD parameters are only loosely constrained by the current data because of large sample variance and the lack of clustering information on scales that probe the two-halo regime. If our explanation of the data is correct, then future observations covering a larger area should show that the large-scale correlations lie below a � � 1:8 extrapolation of the small-scale points. Our models of the current data suggest that the red galaxies are somewhat more strongly clustered than UV-selected Lyman break galaxies and have a greater tendency to reside in small groups. Subject heading gs: galaxies: halos — galaxies: high-redshift — large-scale structure of universe
197531110
Ring Structure and Warp of NGC 5907: Interaction with Dwarf Galaxies
The edge-on, nearby spiral galaxy NGC 5907 has long been used as the prototype of a “noninteracting” warped galaxy. We report here the discovery of two interactions with companion dwarf galaxies that substantially change this picture. First, a faint ring structure is discovered around this galaxy that is likely due to the tidal disruption of a companion dwarf spheroidal galaxy. The ring is elliptical in shape with the center of NGC 5907 close to one of the ring’s foci. This suggests that the ring material is in orbit around NGC 5907. No gaseous component to the ring has been detected either with deep Ha images or in Very Large Array H i 21 cm line maps. The visible material in the ring has an integrated luminosity •10 8 L,, and its brightest part has a color . R 2 I » 0.9 All of these properties are consistent with the ring being a tidally disrupted dwarf spheroidal galaxy. Second, we find that NGC 5907 has a dwarf companion galaxy, PGC 54419, which is projected to be only 36.9 kpc from the center of NGC 5907, close in radial velocity ( km s 21 ) to the giant spiral galaxy. This dwarf DV 5 45 is seen at the tip of the H i warp and in the direction of the warp. Hence, NGC 5907 can no longer be considered noninteracting but is obviously interacting with its dwarf companions much as the Milky Way interacts with its dwarf galaxies. These results, coupled with the finding by others that dwarf galaxies tend to be found around giant galaxies, suggest that tidal interaction with companions, even if containing a mere 1% of the mass of the parent galaxy, might be sufficient to excite the warps found in the disks of many large spiral galaxies. Subject headings: galaxies: individual (NGC 5907, PGC 054419) — galaxies: interactions — galaxies: photometry — galaxies: spiral — radio lines: galaxies