Characteristic Scales in Galaxy Formation

Recent data, e.g. from SDSS and 2dF, reveal a robust bi-modality in the distribution of galaxy properties, with a characteristic transition scale at stellar mass M_*~3x10^{10} Msun (near L_*), corresponding to virial velocity V~100 km/s. Smaller galaxies tend to be blue disks of young populations. They define a "fundamental line" of decreasing surface brightness, metallicity and velocity with decreasing M_*, which extends to the smallest dwarf galaxies. Galaxies above the critical scale are dominated by red spheroids of old populations, with roughly constant high surface brightens and metallicity, and they tend to host AGNs. A minimum in the virial M/L is obtained at the same magic scale. This bi-modality can be the combined imprint of several different physical processes. On smaller scales, disks are built by cold flows, and supernova feedback is effective in regulating star formation. On larger scales, the infalling gas is heated by a virial shock and star formation can be suppressed by AGN feedback. Another feedback mechanism -- gas evaporation due to photo-ionization -- may explain the existence of totally dark halos below V~30 km/s. The standard cooling barriers are responsible for the loose upper and lower bounds for galaxies: 10 < V < 300 km/s.Comment: 10 pages, 5 figures, LaTeX (svmult.cls, physprbb.sty), to be published in ESO/USM/MPE Venice Workshop on Multiwavelength Mapping of Galaxy Formation and Evolution, eds. R. Bender and A. Renzini. Also http://phys.huji.ac.il/~dekel/scales/scales.ps.g

Dynamics of Cosmic Flows

CONTENTS: 1. INTRODUCTION 2. GRAVITATIONAL INSTABILITY 3. MEASURING PECULIAR VELOCITIES 4. ANALYSIS OF OBSERVED PECULIAR VELOCITIES 5. PREDICTED MOTIONS FROM THE GALAXY DISTRIBUTION 6. TESTING BASIC HYPOTHESES 7. THE INITIAL FLUCTUATIONS 8. THE VALUE OF OMEGA 9. DISCUSSION: ARE THE HYPOTHESES JUSTIFIED? REFERENCESComment: A review for Ann. Rev. of Astron. and Astrophys. 32 (October 1994). 46 pages, 10 figures built-in, compressed and uuencoded PostScript (1.58 Mbyte). (Also available by anonymous ftp from venus.huji.ac.il as pub/dekel/reviews/annrev9.uu, or text and figures separately as annrev7.uu with fig*.ps) (HUJI-AST-94-002

Cosmological Implications of Large-Scale Flows

Cosmological implications of the observed large-scale peculiar velocities are reviewed, alone or combined with redshift surveys and CMB data. The latest version of the POTENT method for reconstructing the underlying three-dimensional velocity and mass-density fields is described. The initial fluctuations and the nature of the dark matter are addressed via statistics such as bulk flow and mass power spectrum. The focus is on constraining the mass density parameter $\Omega$, directly or via the parameter $\beta$ which involves the unknown relation between galaxies and mass. The acceptable range for $\Omega$ is found to be $0.4-1.0$. The range of $\beta$ estimates is likely to reflect non-trivial features in the galaxy biasing scheme, such as scale dependence. Similar constraints on $\Omega$ and $\Lambda$ from global measures are summarized.Comment: 40 pages, 15 figures, LaTex (aaspp4.sty), to appear in the proceedings of the 3rd ESO-VLT Workshop on ``Galaxy Scaling Relations: Origins, Evolution and Applications", ed. L. da Costa (Springer

Cosmic Flows 99: Conference Summary

I address the following issues: All bulk velocity measurements (but one) are consistent with our standard gravitational instability theory. New accurate data and reconstruction methods allow high-resolution dynamical analysis nearby, revealing Virgo, Ursa Major and Fornax as attractors. Large peculiar-velocity surveys enable robust reconstruction of the dynamical fields on the Great-Attractor scale. A decomposition of the velocity field into its local and tidal components indicates the presence of big perturbations further away. Cluster velocities start exploring very large scales, revealing Coma, Shapely and other mass enhancements, and constraining a possible local Hubble bubble. Supernovae type Ia (SNIa) are very promising for cosmic flow analysis. Peculiar velocities do provide unique valuable constraints on cosmological parameters, e.g., 0.3<Omega_m<1 (95% confidence) independent of biasing. Jointly with other data they can confine other parameters such as Omega_Lambda, h, sigma_8, n, and the biasing. Nontrivial features of the biasing scheme can explain much of the span of estimates for beta. Quantitative error analysis is essential in our maturing field; every method ought to be calibrated with suitable mock catalogs, that are offered as benchmarks.Comment: 8 pages LaTeX, 8 embedded figures. paspconf.sty. Higher quality figs from ftp://alf.fiz.huji.ac.il/pub/dekel/vic99/ as adekel*_l.ps.gz (*=1-8). In "Cosmic Flows: Towards an Understanding of Large-Scale Structure", eds S. Courteau, M.A. Strauss, & J.A. Willick, ASP Conf. Serie

Galaxy Biasing: Nonlinear, Stochastic and Measurable

I describe a general formalism for galaxy biasing (Dekel & Lahav 1998) and its application to measurements of beta (=Omega^0.6/b), e.g. via direct comparisons of light and mass and via redshift distortions. The linear and deterministic relation g=b*d between the density fluctuation fields of galaxies g and mass d is replaced by the conditional distribution P(g|d) of these as random fields, smoothed at a given scale and at a given time. The mean biasing and its nonlinearity are characterized by the conditional mean =b(d)*d and the local scatter by the conditional variance s_b^2(d). This scatter arises from hidden effects on galaxy formation and from shot noise. For applications involving second-order local moments, the biasing is defined by three natural parameters: the slope b_h of the regression of g on d (replacing b), a nonlinearity parameter b_t, and a scatter parameter s_b. The ratio of variances b_v^2 and the correlation coefficient r mix these parameters. The nonlinearity and scatter lead to underestimates of order b_t^2/b_h^2 and s_b^2/b_h^2 in the different estimators of beta, which may partly explain the range of estimates. Local stochasticity affects the redshift-distortion analysis only by limiting the useful range of scales. In this range, for linear stochastic biasing, the analysis reduces to Kaiser's formula for b_h (not b_v) independent of the scatter. The distortion analysis is affected by nonlinearity but in a weak way. Estimates of the nontrivial features of the biasing scheme are made based on simulations and toy models, and a new method for measuring them via distribution functions is proposed.Comment: 13 pages, LaTeX (conf_iap.sty enclosed), 2 figures (Postscript), to appear in the XIV IAP meeting on Wide Field Surveys in Cosmology, eds. Y. Mellier & S. Colombi (Gif-sur-Yvette: Editions Frontieres

Virial shocks in galactic haloes?

We investigate the conditions for the existence of an expanding virial shock in the gas falling within a spherical dark-matter halo. The shock relies on pressure support by the shock-heated gas behind it. When the radiative cooling is efficient compared to the infall rate the post-shock gas becomes unstable; it collapses inwards and cannot support the shock. We find for a monoatomic gas that the shock is stable when the post-shock pressure and density obey gamma effective>10/7, with gamma effective begin the time depended equivalent to the adiabatic index. We express the effective gamma in terms of r, u and rho at the shock to obtain a simple condition for shock stability. This result is confirmed by hydrodynamical simulations, using an accurate spheri-symmetric Lagrangian code. When the stability analysis is applied in cosmology, we find that a virial shock does not develop in most haloes that form before z ~ 2, and it never forms in haloes less massive than a few 10^11 solar masses. In such haloes the infalling gas is never heated to the virial temperature, and it does not need to cool radiatively before it drops into a disc. Instead, the gas collapses at T ~ 10^4K directly into the disc. This should have nontrivial effects on the star-formation rate and on the gas removal by supernova-driven winds. Instead of radiating soft x rays, this gas would emit lyman alpha thus helping explain the low flux of soft x-ray background and the lyman alpha emitters observed at high redshift.Comment: 17 pages, 13 eps figs, latex, mn2e, accepted by MNRAS - minor additions to the discussio

An analytic solution for the minimal bathtub toy model: challenges in the star-formation history of high-z galaxies

We study the minimal ``bathtub" toy model as an analytic tool for capturing key processes of galaxy evolution and identifying robust successes and challenges in reproducing observations at high redshift. The source and sink terms of the continuity equations for gas and stars are expressed in simple terms from first principles. The assumed dependence of star-formation rate (SFR) on gas mass self-regulates the system into a unique asymptotic behavior, which is approximated by an analytic quasi-steady-state solution (QSS). We address the validity of the QSS at different epochs independent of earlier conditions. At high z, where the accretion is assumed to consist of gas only, the specific SFR is robustly predicted to be sSFR = [(1+z)/3]^{5/2} Gyr^{-1}, slightly higher than the cosmological specific accretion rate, in agreement with observations at z=3-8. The gas fraction is expected to decline slowly, and the observations constrain the SFR efficiency per dynamical time to epsilon=0.02. The stellar-to-virial mass ratio f_sv is predicted to be constant in time, and the observed value requires an outflow mass-lading factor of eta=1-3, depending on the penetration efficiency of gas into the galaxy. However, at z=2, where stars are also accreted through mergers, the simplest model has an apparent difficulty in matching observations. The model that maximizes the sSFR, with the outflows fully recycled, falls short by a factor 3 in sSFR, and overestimates f_sv. With strong outflows, the model can reproduce the observed f_sv but at the expense of underestimating the sSFR by an order of magnitude. We discuss potential remedies including a bias due to the exclusion of quenched galaxies.Comment: 14 pages, 3 figure

Stochastic Nonlinear Galaxy Biasing

We propose a general formalism for galaxy biasing and apply it to methods for measuring cosmological parameters, such as regression of light versus mass, the analysis of redshift distortions, measures involving skewness and the cosmic virial theorem. The common linear and deterministic relation g=b*d between the density fluctuation fields of galaxies g and mass d is replaced by the conditional distribution P(g|d) of these random fields, smoothed at a given scale at a given time. The nonlinearity is characterized by the conditional mean =b(d)*d, while the local scatter is represented by the conditional variance s_b^2(d) and higher moments. The scatter arises from hidden factors affecting galaxy formation and from shot noise unless it has been properly removed. For applications involving second-order local moments, the biasing is defined by three natural parameters: the slope b_h of the regression of g on d, a nonlinearity b_t, and a scatter s_b. The ratio of variances b_v^2 and the correlation coefficient r mix these parameters. The nonlinearity and the scatter lead to underestimates of order b_t^2/b_h^2 and s_b^2/b_h^2 in the different estimators of beta (=Omega^0.6/b_h). The nonlinear effects are typically smaller. Local stochasticity affects the redshift-distortion analysis only by limiting the useful range of scales, especially for power spectra. In this range, for linear stochastic biasing, the analysis reduces to Kaiser's formula for b_h (not b_v), independent of the scatter. The distortion analysis is affected by nonlinear properties of biasing but in a weak way. Estimates of the nontrivial features of the biasing scheme are made based on simulations and toy models, and strategies for measuring them are discussed. They may partly explain the range of estimates for beta.Comment: 28 pages, LaTeX (aaspp4.sty), 1 figure (2 ps files), minor revision

Phase-Space Structure & Substructure of Dark Halos

A method is presented for computing the 6-D phase-space density f(x,v) and its PDF v(f) in an N-body system. It is based on Delaunay tessellation, yielding v(f) with a fixed smoothing window over a wide f range, independent of the sampling resolution. It is found that in a gravitationally relaxed halo built by hierarchical clustering, v(f) is a robust power law, v(f) f^{-2.5 \pm 0.05}, over more than 4 decades in f, from its virial level to the current resolution limit. This is valid for halos of different sizes in the LCDM cosmology, indicating insensitivity to the initial-fluctuation power spectrum as long as the small-scale fluctuations were not completely suppressed. By mapping f in position space, we find that the high-f contributions to v(f) come from the "cold" subhalos within the parent halo rather than the halo central region and its global spherical profile. The f in subhalos near the halo virial radius is more than 100 times higher than at the halo center, and it decreases gradually with decreasing radius. This indicates phase mixing due to mergers and tidal effects involving puffing up and heating. The phase-space structure provides a sensitive tool for studying the evolution of subhalos during the buildup of halos. One wishes to understand why the substructure adds up to the universal power law in v(f). It seems that the f^{-2.5} behavior is related to the hierarchical clustering process and is not a general result of violent relaxation.Comment: 12 pages, LaTeX using newpasp.sty, to be published in Satellites and Tidal Streams, eds. D. Martinez-Delgado & F. Prada, ASP Conference Serie

Cosmological Parameters and Power Spectrum from Peculiar Velocities

The power spectrum of mass density fluctuations is evaluated from the Mark III and the SFI catalogs of peculiar velocities by a maximum likelihood analysis, using parametric models for the power spectrum and for the errors. The applications to the two different data sets, using generalized CDM models with and without COBE normalization, give consistent results. The general result is a relatively high amplitude of the power spectrum, with \sigma_8 \Omega_m^{0.6} = 0.8+/-0.2 at 90% confidence. Casting the results in the \Omega_m - \Omega_\Lambda plane, yields complementary constraints to those of the high-redshift supernovae, together favoring a nearly flat, unbound and accelerating universe with comparable contributions from \Omega_m and \Omega_\Lambda. Further implications on the cosmological parameters, arising from a joint analysis of the velocities together with small-scale CMB anisotropies and the high-redshift supernovae, are also briefly described.Comment: 8 pages, 3 figures. To appear in Proceedings of the Cosmic Flows Workshop, Victoria, B.C., Canada, July 1999, eds. S. Courteau, M. Strauss, and J. Willic