Reconstructing the Initial Density Field of the Local Universe: Method and Test with Mock Catalogs

Our research objective in this paper is to reconstruct an initial linear density field, which follows the multivariate Gaussian distribution with variances given by the linear power spectrum of the current CDM model and evolves through gravitational instability to the present-day density field in the local Universe. For this purpose, we develop a Hamiltonian Markov Chain Monte Carlo method to obtain the linear density field from a posterior probability function that consists of two components: a prior of a Gaussian density field with a given linear spectrum, and a likelihood term that is given by the current density field. The present-day density field can be reconstructed from galaxy groups using the method developed in Wang et al. (2009a). Using a realistic mock SDSS DR7, obtained by populating dark matter haloes in the Millennium simulation with galaxies, we show that our method can effectively and accurately recover both the amplitudes and phases of the initial, linear density field. To examine the accuracy of our method, we use $N$-body simulations to evolve these reconstructed initial conditions to the present day. The resimulated density field thus obtained accurately matches the original density field of the Millennium simulation in the density range 0.3 <= rho/rho_mean <= 20 without any significant bias. Especially, the Fourier phases of the resimulated density fields are tightly correlated with those of the original simulation down to a scale corresponding to a wavenumber of ~ 1 h/Mpc, much smaller than the translinear scale, which corresponds to a wavenumber of ~ 0.15 h\Mpc.Comment: 43 pages, 15 figures, accepted for publication in Ap

Properties of Galaxy Groups in the SDSS: I.-- The Dependence of Colour, Star Formation, and Morphology on Halo Mass

Using a large galaxy group catalogue constructed from the SDSS, we investigate the correlation between various galaxy properties and halo mass. We split the population of galaxies in early types, late types, and intermediate types, based on their colour and specific star formation rate. At fixed luminosity, the early type fraction increases with increasing halo mass. Most importantly, this mass dependence is smooth and persists over the entire mass range probed, without any break or feature at any mass scale. We argue that the previous claim of a characteristic feature on galaxy group scales is an artefact of the environment estimators used. At fixed halo mass, the luminosity dependence of the type fractions is surprisingly weak: galaxy type depends more strongly on halo mass than on luminosity. We also find that the early type fraction decreases with increasing halo-centric radius. Contrary to previous studies, we find that this radial dependence is also present in low mass haloes. The properties of satellite galaxies are strongly correlated with those of their central galaxy. In particular, the early type fraction of satellites is significantly higher in a halo with an early type central galaxy than in a halo of the same mass but with a late type central galaxy. This phenomenon, which we call `galactic conformity', is present in haloes of all masses and for satellites of all luminosities. Finally, the fraction of intermediate type galaxies is always ~20 percent, independent of luminosity, independent of halo mass, independent of halo-centric radius, and independent of whether the galaxy is a central galaxy or a satellite galaxy. We discuss the implications of all these findings for galaxy formation and evolution.Comment: 28 pages, 15 figures. Submitted for publication in MNRA

Evidence for a 3 x 10^8 solar mass black hole in NGC 7052 from HST observations of the nuclear gas disk

We present an HST study of the nuclear region of the E4 radio galaxy NGC 7052, which has a nuclear disk of dust and gas. The WFPC2 was used to obtain B, V and I broad-band images and an H_alpha+[NII] narrow-band image. The FOS was used to obtain H_alpha+[NII] spectra along the major axis, using a 0.26 arcsec diameter circular aperture. The observed rotation velocity of the ionized gas is V = 155 +/- 17 km/s at r = 0.2 arcsec from the nucleus. The Gaussian dispersion of the emission lines increases from sigma = 70 km/s at r=1 arcsec, to sigma = 400 km/s on the nucleus. To interpret the gas kinematics we construct axisymmetric models in which the gas and dust reside in a disk in the equatorial plane of the stellar body. It is assumed that the gas moves on circular orbits, with an intrinsic velocity dispersion due to turbulence. The circular velocity is calculated from the combined gravitational potential of the stars and a possible nuclear black hole (BH). Models without a BH predict a rotation curve that is shallower than observed (V_pred = 92 km/s at r = 0.2 arcsec), and are ruled out at > 99% confidence. Models with a BH of 3.3^{+2.3}_{-1.3} x 10^8 solar masses provide an acceptable fit. NGC 7052 can be added to the list of active galaxies for which HST spectra of a nuclear gas disk provide evidence for the presence of a central BH. The BH masses inferred for M87, M84, NGC 6251, NGC 4261 and NGC 7052 span a range of a factor 10, with NGC 7052 falling on the low end. By contrast, the luminosities of these galaxies are identical to within 25%. Any relation between BH mass and luminosity, as suggested by independent arguments, must therefore have a scatter of at least a factor 10.Comment: 39 pages, LaTeX, with 16 PostScript figures. Submitted to the Astronomical Journal. Postscript version with higher resolution figures available from http://sol.stsci.edu/~marel/abstracts/abs_R22.htm

Observational Evidence for an Age Dependence of Halo Bias

We study the dependence of the cross-correlation between galaxies and galaxy groups on group properties. Confirming previous results, we find that the correlation strength is stronger for more massive groups, in good agreement with the expected mass dependence of halo bias. We also find, however, that for groups of the same mass, the correlation strength depends on the star formation rate (SFR) of the central galaxy: at fixed mass, the bias of galaxy groups decreases as the SFR of the central galaxy increases. We discuss these findings in light of the recent findings by Gao et al (2005) that halo bias depends on halo formation time, in that halos that assemble earlier are more strongly biased. We also discuss the implication for galaxy formation, and address a possible link to galaxy conformity, the observed correlation between the properties of satellite galaxies and those of their central galaxy.Comment: 4 pages, 4 figures, Accepted for publication in ApJ Letters. Figures 3 and 4 replaced. The bias dependence on the central galaxy luminosity is omitted due to its sensitivity to the mass mode

Alignments of galaxies within cosmic filaments from SDSS DR7

Using a sample of galaxy groups selected from the Sloan Digital Sky Survey Data Release 7 (SDSS DR7), we examine the alignment between the orientation of galaxies and their surrounding large scale structure in the context of the cosmic web. The latter is quantified using the large-scale tidal field, reconstructed from the data using galaxy groups above a certain mass threshold. We find that the major axes of galaxies in filaments tend to be preferentially aligned with the directions of the filaments, while galaxies in sheets have their major axes preferentially aligned parallel to the plane of the sheets. The strength of this alignment signal is strongest for red, central galaxies, and in good agreement with that of dark matter halos in N-body simulations. This suggests that red, central galaxies are well aligned with their host halos, in quantitative agreement with previous studies based on the spatial distribution of satellite galaxies. There is a luminosity and mass dependence that brighter and more massive galaxies in filaments and sheets have stronger alignment signals. We also find that the orientation of galaxies is aligned with the eigenvector associated with the smallest eigenvalue of the tidal tensor. These observational results indicate that galaxy formation is affected by large-scale environments, and strongly suggests that galaxies are aligned with each other over scales comparable to those of sheets and filaments in the cosmic web.Comment: 11 pages, 10 figures, accepted for publication in Ap

Linking Early and Late Type Galaxies to their Dark Matter Haloes

(Abridged) Using data from the 2dFGRS we compute the conditional luminosity functions (CLFs) of early- and late-type galaxies. These functions give the average number of galaxies with luminosity L that reside in a halo of mass M, and are a powerful statistical tool to link the distribution of galaxies to that of dark matter haloes. We find that the average mass-to-light ratios of dark matter haloes have a minimum of about 100 Msun/Lsun around a halo mass of about 3x10^{11} Msun. In addition, haloes with M < 10^{10} Msun are virtually devoid of galaxies, while clusters have b_J band mass-to-light ratios in the range 500-1000 Msun/Lsun. Finally, the fact that early-type galaxies are more strongly clustered than late-type galaxies requires that the fraction of late-type galaxies is a strongly declining function of halo mass. We also compute two-point correlation functions as function of both luminosity and galaxy type, and find a weak (strong) luminosity dependence for the late (early) type galaxies. Finally we compare our CLFs with predictions from several semi-analytical models for galaxy formation. As long as these models accurately fit the 2dFGRS luminosity function the agreement with our predictions is remarkably good. This indicates that we have recovered a statistical description of how galaxies populate dark matter haloes which fits nicely within the standard framework for galaxy formation.Comment: Due to a normalization error in the luminosity function provided to us, and used to constrain the models, the normalization of our models was wrong by about 15 percent. In this new version of the paper we use the properly normalized data, which results in small changes to the best-fit model parameters listed in Table 1. All figures in this new version have been updated, though the changes are virtually too small to notice. This version supersedes the version published in MNRA

Spin alignments of spiral galaxies within the large-scale structure from SDSS DR7

Using a sample of spiral galaxies selected from the Sloan Digital Sky Survey Data Release 7 (SDSS DR7) and Galaxy Zoo 2 (GZ2), we investigate the alignment of spin axes of spiral galaxies with their surrounding large scale structure, which is characterized by the large-scale tidal field reconstructed from the data using galaxy groups above a certain mass threshold. We find that the spin axes of only have weak tendency to be aligned with (or perpendicular to) the intermediate (or minor) axis of the local tidal tensor. The signal is the strongest in a \cluster environment where all the three eigenvalues of the local tidal tensor are positive. Compared to the alignments between halo spins and local tidal field obtained in N-body simulations, the above observational results are in best agreement with those for the spins of inner regions of halos, suggesting that the disk material traces the angular momentum of dark matter halos in the inner regions.Comment: 8 pages, 7 figures, accepted for publication in Ap