Showing papers by "Anatoly Klypin published in 2002"

Sloan digital sky survey: Early data release

Abstract: The Sloan Digital Sky Survey (SDSS) is an imaging and spectroscopic survey that will eventually cover approximately one-quarter of the celestial sphere and collect spectra of ≈106 galaxies, 100,000 quasars, 30,000 stars, and 30,000 serendipity targets. In 2001 June, the SDSS released to the general astronomical community its early data release, roughly 462 deg2 of imaging data including almost 14 million detected objects and 54,008 follow-up spectra. The imaging data were collected in drift-scan mode in five bandpasses (u, g, r, i, and z); our 95% completeness limits for stars are 22.0, 22.2, 22.2, 21.3, and 20.5, respectively. The photometric calibration is reproducible to 5%, 3%, 3%, 3%, and 5%, respectively. The spectra are flux- and wavelength-calibrated, with 4096 pixels from 3800 to 9200 A at R ≈ 1800. We present the means by which these data are distributed to the astronomical community, descriptions of the hardware used to obtain the data, the software used for processing the data, the measured quantities for each observed object, and an overview of the properties of this data set.

ΛCDM-based Models for the Milky Way and M31. I. Dynamical Models

Abstract: We apply standard disk formation theory with adiabatic contraction within cuspy halo models predicted by the standard cold dark matter (?CDM) cosmology. The resulting models are confronted with the broad range of observational data available for the Milky Way and M31 galaxies. We find that there is a narrow range of parameters that can satisfy the observational constraints, but within this range, the models score remarkably well. Our favored models have virial masses of 1012 and 1.6 ? 1012 M? for the Galaxy and for M31, respectively, average spin parameters ? ? 0.03-0.05, and concentrations Cvir = 10-17, typical for halos of this mass in the standard ?CDM cosmology. The models require neither dark matter modifications nor flat cores to fit the observational data. We explore two types of models, with and without the exchange of angular momentum between the dark matter and the baryons. The models without exchange give reasonable rotation curves, fulfill constraints in the solar neighborhood, and satisfy constraints at larger radii, but they may be problematic for fast rotating central bars. We explore models in which the baryons experience additional contraction due to loss of angular momentum to the surrounding dark matter. These models produce similar global properties, but the dark matter is only a 25% of the total mass in the central 3 kpc region, allowing a fast rotating bar to persist. According to preliminary calculations, our model galaxies probably have sufficient baryonic mass in the central ~3.5 kpc to reproduce recent observational values of the optical depth to microlensing events toward the Galactic center. Our dynamical models unequivocally require that about 50% of all the gas inside the virial radius must not be in the disk or in the bulge, a result that is obtained naturally in standard semianalytic models. Assuming that the Milky Way is typical, we investigate whether the range of virial masses allowed by our dynamical models is compatible with constraints from the galaxy luminosity function. We find that if the Milky Way has a luminosity MK = -24.0, then these constraints are satisfied, but if it is more luminous (as expected if it lies on the Tully-Fisher relation), then the predicted space density is larger than the observed space density of galaxies of the corresponding luminosity by a factor of 1.5-2. We conclude that observed rotation curves and dynamical properties of normal spiral galaxies appear to be consistent with standard ?CDM.

The origin of angular momentum in dark matter halos

Abstract: We propose a new explanation for the origin of angular momentum in galaxies and their dark halos, in which the halos obtain their spin through the cumulative acquisition of angular momentum from satellite accretion. In our model, the buildup of angular momentum is a random walk process associated with the mass assembly history of the halo's major progenitor. We assume no correlation between the angular momenta of accreted objects. The main role of tidal torques in this approach is to produce the random tangential velocities of merging satellites. Using the extended Press-Schechter approximation, we calculate the growth of mass, angular momentum, and spin parameter λ for many halos. Our random walk model reproduces the key features of the angular momentum of halos found in ΛCDM N-body simulations: a lognormal distribution in λ with an average of ≈ 0.045 and dispersion σλ = 0.56, independent of mass and redshift. The evolution of the spin parameter in individual halos in this model is quite different from the steady increase with time of angular momentum in the tidal torque picture. We find both in N-body simulations and in our random walk model that the value of λ changes significantly with time for a halo's major progenitor. It typically has a sharp increase due to major mergers and a steady decline during periods of gradual accretion of small satellites. The model predicts that, on average, the λ of ~1012 M☉ halos that had major mergers after redshift z = 3 should be substantially larger than the λ of those that did not. Perhaps surprisingly, this suggests that halos that host later forming elliptical galaxies should rotate faster than halos of spiral galaxies.

Constrained Simulations of the Real Universe. II. Observational Signatures of Intergalactic Gas in the Local Supercluster Region

Abstract: We present results of gasdynamics+N-body constrained cosmological simulations of the Local Supercluster region (LSC; about 30 h-1 Mpc around the Virgo cluster), which closely mimic the real universe within 100 Mpc, by imposing constraints from the MARK III catalog of galaxy peculiar velocities. The simulations are used to study the properties and possible observational signatures of intergalactic medium in the LSC region. We find that in agreement with previous unconstrained simulations, ≈30% of the gas in this region is in the warm/hot phase at T ~ 105-107 K and ≈40% in the diffuse phase at T < 105 K in low-density regions. The X-ray emission from the warm/hot gas may represent a small (~5%-10%) but important contribution to the X-ray background observed by the ROSAT All-Sky Survey at energies around 1 keV. The best prospects for detection of the warm/hot intergalactic medium of the LSC located in filaments and in the vicinity of virialized regions of groups and clusters are through absorption in resonant lines of O VII and O VIII in soft X-rays and in the O VI doublet in UV. If intergalactic gas in filaments (ρ/ρ ~ 1-10) is enriched to typical metallicities of 0.05, the column densities of O VI, O VII, and O VIII along a random line of sight near the north Galactic pole, especially near the supergalactic plane, have a significant probability to be in the range detectable by current (FUSE, XMM) and future (Constellation-X) instruments.

Secular bar formation in galaxies with significant amount of dark matter

Abstract: Using high resolution N-body simulations of stellar disks embedded in cosmologically motivated dark matter halos, we study the evolution of bars and the transfer of angular momentum between halos and bars. We find that dynamical friction results in some transfer of angular momentum to the halo, but the effect is much smaller than previously found in low resolution simulations and is incompatible with early analytical estimates. In simulations with millions of particles reaching a resolution of 20-40 pc, the pattern speed may not change over billions of years. Our high resolution models produce bars which are fast rotators, where the ratio of the corotation radius to the bar major semi-axis lies in the range R =1.2-1.7, marginally compatible with observational results. In contrast to many previous simulations, we find that bars are relatively short. As in many observed cases, the bar major semi-axis is close to the exponential length of the disk. The transfer of angular momentum between inner and outer parts of the disk plays a very important role in the secular evolution of the disk and the bar. The bar formation increases the exponential length of the disk by a factor of 1.2 -1.5. The transfer substantially increases the stellar mass in the centre of the galaxy and decreases the dark matter-to-baryons ratio. As the result, the central 2 kpc region is always strongly dominated by the baryonic component. At intermediate 3-10kpc scales the disk is sub-dominant. In summary, realistic models produce bar structure in striking agreement with observational results.

Spatial distribution of galactic halos and their merger histories

Abstract: We use a novel statistical tool, the mark correlation functions (MCFs), to study clustering of galaxy- size halos as a function of their properties and environment in a high-resolution numerical simulation of the CDM cosmology. We applied MCFs using several types of continuous and discrete marks: maximum circular velocity of halos, merger mark indicating whether halos experienced or not a major merger in their evolution history (the marks for halo with mergers are further split according to the epoch of the last major merger), and a stripping mark indicating whether the halo underwent a tidal stripping (i.e., mass loss). We nd that halos which experienced a relatively early ( z> 1) major merger or mass loss (due to tidal stripping) in their evolution histories are over-abundant in halo pairs with separations <3 h 1 Mpc. This result can be interpreted as spatial segregation of halos with dierent merger histories, qualitatively similar to the morphological segregation in the observed galaxy distribution. In addition, we nd that at z = 0 the mean circular velocity of halos in pairs of halos with separations <10 h 1 Mpc is larger than the mean circular velocity vcirc of the parent halo sample. This mean circular velocity enhancement increases steadily during the evolution of halos from z =3t oz = 0, and indicates that the luminosity dependence of galaxy clustering may be due to the mass segregation of galactic dark matter halos. The analysis presented in this paper demonstrate that MCFs provide powerful, yet algorithmically simple, quantitative measures of segregation in the spatial distribution of objects with respect to their various properties (marks). This should make MCFs very useful for analysis of spatial clustering and segregation in current and future large redshift surveys.

Spatial distribution of galactic halos and their merger histories

Abstract: We use a novel statistical tool, the mark correlation functions (MCFs), to study clustering of galaxy-size halos as a function of their properties and environment in a high-resolution numerical simulation of the LambdaCDM cosmology. We applied MCFs using several types of continuous and discrete marks: maximum circular velocity of halos, merger mark indicating whether halos experienced or not a major merger in their evolution history, and a stripping mark indicating whether the halo underwent a tidal stripping. We find that halos which experienced a relatively early (z>1) major merger or mass loss (due to tidal stripping) in their evolution histories are over-abundant in halo pairs with separations < 3 Mpc/h. This result can be interpreted as spatial segregation of halos with different merger histories, qualitatively similar to the morphological segregation in the observed galaxy distribution. The analysis presented in this paper demonstrate that MCFs provide powerful, yet algorithmically simple, quantitative measures of segregation in the spatial distribution of objects with respect to their various properties (marks).