Voids in the Simulated Local Universe

We use simulations of the formation and evolution of the galaxy population in the Local Universe to address the issue of whether the standard theoretical model succeeds in producing empty regions as large and as dark as the observed nearby ones. We follow the formation of galaxies in an LCDM universe and work with mock catalogues which can resolve the morphology of LMC sized galaxies, and the luminosity of objects 6 times fainter. We look for a void signature in sets of virialized haloes selected by mass, as well as in mock galaxy samples selected according to observationally relevant quantities, like luminosity, colour, or morphology. We find several void regions with diameter 10 Mpc/h in the simulation where gravity seems to have swept away even the smallest haloes we were able to track. We probe the environment density of the various populations and compute luminosity functions for galaxies residing in underdense, mean density and overdense regions. We also use nearest neighbour statistics to check possible void populations, taking $L_{*}$ spirals as reference neighbours. Down to our resolution limits, we find that all types of galaxies avoid the same regions, and that no class appears to populate the voids defined by the bright galaxies.Comment: 14 pages, 6 figures. Submitted to MNRAS. A high-resolution version of Figure 1 and galaxy populations analysed here are available at http://www.mpa-garching.mpg.de/NumCos/CR/Voids

The Correlation Function of Clusters of Galaxies and the Amplitude of Mass Fluctuations in the Universe

We show that if a sample of galaxy clusters is complete above some mass threshold, then hierarchical clustering theories for structure formation predict its autocorrelation function to be determined purely by the cluster abundance and by the spectrum of linear density fluctuations. Thus if the shape of the initial fluctuation spectrum is known, its amplitude $\sigma_8$ can be estimated directly from the correlation length of a cluster sample in a way which is independent of the value of $\Omega_0$. If the cluster mass corresponding to the sample threshold is also known, it provides an independent estimate of the quantity $\sigma_8\Omega_0^{0.6}$. Thus cluster data should allow both $\sigma_8$ and $\Omega_0$ to be determined observationally. We explore these questions using N-body simulations together with a simple but accurate analytical model based on extensions of Press-Schechter theory. Applying our results to currently available data we find that if the linear fluctuation spectrum has a shape similar to that suggested by the APM galaxy survey, then a correlation length $r_0$ in excess of 20\mpch for Abell clusters would require $\sigma_8>1$, while r_0<15\mpch would require $\sigma_8<0.5$. With conventional estimates of the relevant mass threshold these imply \Omega_0\la 0.3 and \Omega_0\ga 1 respectively.Comment: Latex, 25 pages (including 8 PS figures). The PS-file of the paper is also available via anonymous ftp at: ftp://ibm-3.mpa-garching.mpg.de/pub/jing/xicc.ps . Submitted to MNRAS. In the replaced version, a typo in Eq.(1a) is fixe

Stellar Masses and Star Formation Histories for 80,000 Galaxies from the Sloan Digital Sky Survey

SIGLEAvailable from: http://www.mpa-garching.mpg.de / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

Where are the first stars now?

SIGLEAvailable from: http://www.mpa-garching.mpg.de / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

The nature and observability of protogalaxies

I discuss recent theoretical work on the formation and evolution of galaxies paying particular attention to the ability of current models to make detailed comparisons with observations of the galaxy population both nearby and at high redshift. These models suggest that much (perhaps most) start formation in the universe took place in objects that are already detected in deep galaxy samples. In addition, they predict that systems with large star formation rates are unlikely to be much more abundant in the past than they are at present. Recent data show that the star formation rate in the nearby universe is, in fact, a substantial fraction of that required to make all the stars seen in galaxies, and that the observed abundance of objects forming stars at rates in excess of 10 M_sun/yr is approximately the same at redshifts of 1.25 and 3.25 as it is at z=0. Both the epoch of galaxy formation and ''typical'' protogalaxies may aready have been observed but not recognised. Thermal emission from dust in such protogalaxies could be detected by a large millimeter array, and molecular line observations could explore the dynamical state of the gas in the more massive systems. (orig.)Invited review: ESO workshop on large millimeter arrays. November 1995, 38 refs.Available from TIB Hannover: RR 4697(958) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

On the dynamics of the Sagittarius dwarf galaxy

We use numerical simulations to test the feasibility of the suggestion by Ibata et al. (1994) that the excess population of stars which they discovered in the Sagittarius region may be disrupted remains of a dwarf spheroidal galaxy. We find that a Fornax-like model for the pre-disruption system can indeed reproduce the data. However, the galaxy must be on a relatively short period orbit with a pericentre of about 10 kpc and an apocentre of about 52 kpc, giving a current transverse velocity of 255 km/s and a period of #propor to# 760 Myr. Furthermore, disruption must have occurred predominantly on the last pericentric passage rather than on the present one. The data are consistent with transverse motion either towards or away from the Galactic Plane. These results depend primarily on the rotation curve of the Galaxy and are insensitive to the mass distribution in its outer halo or to the mass of its disk. (orig.)Available from TIB Hannover / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Dark halo mergers and the formation of a universal profile

We argue that a universal density profile for dark matter halos arises as a natural consequence of hierarchical structure formation. It is a fixed point in the process of repeated mergers. We present analytic and numerical arguments for the emergence of a particular form of the profile. At small radii, the density should vary as r&quot;-&quot;#alpha#, with #alpha# determined by the way the characteristic density of halos scales with their mass. If small halos are dense, then #alpha# is large. The mass-density relation can be related to the power spectrum of initial fluctuations, P(k), through 'formation time' arguments. Early structure formation leads to steep cusps. For P(k) #approx# k&quot;n we find #alpha# #approx =# 3(3 + n)/(5 + n). The universal profile is generated by tidal stripping of small halos as they merge with larger objects. (orig.)30 refs.Available from TIB Hannover: RR 4697(994) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Building up the stellar Halo of the galaxy

We study numerical simulations of satellite galaxy disruption in a potential resembling that of the Milky Way. Our goal is to assess whether a merger origin for the stellar halo would leave observable fossil structure in the phase-space distribution of nearby stars. We show how mixing of disrupted satellites can be quantified using a coarsegrained entropy. Although after 10 billion years few obvious asymmetries remain in the distribution of particles in configuration space, strong correlations are still present in velocity space. We give a simple analytic description of these effects, based on a linearised treatment in action-angle variables, which shows how the kinematic and density structure of the debris stream changes with time. By applying this description we find that a single satellite of current luminosity 10&quot;8 L_sun disrupted 10 Gyr ago from an orbit circulating in the inner halo (mean apocentre #propor to#12 kpc) would contribute about #propor to#30 kinematically old streams with internal velocity dispersions below 5 km s&quot;-&quot;1 to the local stellar halo. If the whole stellar halo were built by disrupted satellites, it should consist locally of 300-500 such streams. Clear detection of all these structures would require a sample of a few thousand stars with 3-D velocities accurate to better than 5 km s&quot;-&quot;1. Even with velocity errors several times worse than this, the expected clumpiness should be quite evident. We apply our formalism to a group of stars detected near the NGP, and derive an order of magnitude estimate for the initial properties of the progenitor system. (orig.)Available from TIB Hannover: RR 4697(1128) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

An analytic model for the gravitational clustering of dark matter haloes

We develop a simple analytic model for the gravitational clustering of dark haloes. The statistical properties of dark haloes are determined from the initial density field (assumed to be Gaussian) through an extension of the Press-Schechter formalism. Gravitational clustering is treated by a spherical model which describes the concentration of dark haloes in collapsing regions. We test this model against results from a variety of N-body simulations. The autocorrelation function of dark haloes in such simulations depends significantly on how haloes are identified. Our predictions agree well with results based on algorithms which break clusters into subgroups more efficiently than the standard friends-of-friends algorithm. The agreement is better than that found by assuming haloes to lie at the present positions of peaks of the linear density field. We use these techniques to study how the distribution of haloes identified at a given redshift and having circular velocities #upsilon#_c = #upsilon#_c&quot;*(z) (i.e. mass equal to the characteristic nonlinear mass M&quot;* at that redshift) are very weakly correlated with the linear density field or among themselves. As a result of dynamical evolution, however, the present-day correlations of these haloes are similar to those of the mass. Haloes with lower #upsilon#_c are biased toward regions with negative overdensity, while those with higher #upsilon#_c are biased toward regions with positive overdensity. Among the haloes identified at any given epoch, those with higher circular velocities are more strongly correlated today. Among the haloes of given circular velocity, those at higher redshifts are also more strongly clustered today. In the 'standard CDM' model, haloes with #upsilon#_c = 200 km s&quot;-&quot;1 and identified at redshift z &gt;or#approx# 2 have present-day autocorrelation comparable to that of normal galaxies in the real universe. (orig.)SIGLEAvailable from TIB Hannover / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

The abundance and clustering of dark haloes in the standard #LAMBDA#CDM cosmogony

SIGLEAvailable from: http://www.mpa-garching.mpg.de / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman