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Register a new userGalaxy formation in dissipationless N-body models
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Eelco van Kampen
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We present a method of including galaxy formation in dissipationless N-body simulations. Galaxies that form during the evolution are identified at several epochs and replaced by single massive soft particles. This allows one to produce two-component models containing galaxies and a background dark matter distribution. We applied this technique to obtain two sets of models: one for field galaxies and one for galaxy clusters. We tested the method for the standard CDM scenario for structure formation in the universe. A direct comparison of the simulated galaxy distribution to the observed one sets the amplitude of the initial density fluctuation spectrum, and thus the present time in the simulations. The rates of formation and merging compare very well to simulations that include hydrodynamics, and are compatible with observations. We also discuss the cluster luminosity function.
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Accurate modeling of galaxy formation in a hierarchical, cold dark matter universe requires the use of sufficiently high-resolution merger trees to obtain convergence in the predicted properties of galaxies. When semi-analytic galaxy formation models are applied to cosmological N-body simulation merger trees, it is often the case that those trees have insufficient resolution to give converged galaxy properties. We demonstrate a method to augment the resolution of N-body merger trees by grafting in branches of Monte Carlo merger trees with higher resolution, but which are consistent with the pre-existing branches in the N-body tree. We show that this approach leads to converged galaxy properties.
We use high-resolution dissipationless simulations of the concordance flat LCDM model to make predictions for the galaxy--mass (GM) correlations and compare them to the recent SDSS weak lensing measurements.We use a simple observationally motivated scheme to assign luminosities and colors to the halos.This allows us to closely match the selection criteria used to define observational samples.The simulations reproduce the observed GM correlation function and its observed dependencies on luminosity and color.The luminosity dependence of the correlation function is primarily determined by the changing relative contribution of central and satellite galaxies at different luminosities. The color dependence of the GM correlations reflects the difference in the typical environments of blue and red galaxies. We also find agreement between the predicted and observed cross-bias, b_x=b/r,at all probed scales.The GM correlation coefficient, r, is close to unity on scales >1/h Mpc.The cross bias is thus expected to measure the actual bias of galaxy clustering on these scales.The aperture mass-to-light ratio is independent of galaxy color.The aperture mass scales approximately linearly with luminosity at L_r>10^{10}h^{-2} Lsun, while at lower luminosities the scaling is shallower: L_r^{0.5}. We show that most of the luminous galaxies (M_r<-21) are near the centers of their halos and their GM correlation function at r<100/h kpc can therefore be interpreted as the average dark matter density profile of these galaxies. We find that for galaxies in a given narrow luminosity range, there is a broad and possibly non-gaussian distribution of halo virial masses. Therefore, the average relation between mass and luminosity derived from the weak lensing analyses should be interpreted with caution.
We examine two extreme models for the build-up of the stellar component of luminous elliptical galaxies. In one case, we assume the build-up of stars is dissipational, with centrally accreted gas radiating away its orbital and thermal energy; the dark matter halo will undergo adiabatic contraction and the central dark matter density profile will steepen. For the second model, we assume the central galaxy is assembled by a series of dissipationless mergers of stellar clumps that have formed far from the nascent galaxy. In order to be accreted, these clumps lose their orbital energy to the dark matter halo via dynamical friction, thereby heating the central dark matter and smoothing the dark matter density cusp. The central dark matter density profiles differ drastically between these models. For the isolated elliptical galaxy, NGC 4494, the central dark matter densities follow the power-laws r^(-0.2) and r^(-1.7) for the dissipational and dissipationless models, respectively. By matching the dissipational and dissipationless models to observations of the stellar component of elliptical galaxies, we examine the relative contributions of dissipational and dissipationless mergers to the formation of elliptical galaxies and look for observational tests that will distinguish between these models. Comparisons to strong lensing brightest cluster galaxies yield median M*/L_B ratios of 2.1+/-0.8 and 5.2+/-1.7 at z=0.39 for the dissipational and dissipationless models, respectively. For NGC 4494, the best-fit dissipational and dissipationless models have M*/L_B=2.97 and 3.96. Comparisons to expected stellar mass-to-light ratios from passive evolution and population syntheses appear to rule out a purely dissipational formation mechanism for the central stellar regions of giant elliptical galaxies.
We construct the Numerical Galaxy Catalog ($ u$GC), based on a semi-analytic model of galaxy formation combined with high-resolution N-body simulations in a $Lambda$-dominated flat cold dark matter ($Lambda$CDM) cosmological model. The model includes several essential ingredients for galaxy formation, such as merging histories of dark halos directly taken from N-body simulations, radiative gas cooling, star formation, heating by supernova explosions (supernova feedback), mergers of galaxies, population synthesis, and extinction by internal dust and intervening HI clouds. As the first paper in a series using this model, we focus on basic photometric, structural and kinematical properties of galaxies at present and high redshifts. Two sets of model parameters are examined, strong and weak supernova feedback models, which are in good agreement with observational luminosity functions of local galaxies in a range of observational uncertainty. Both models agree well with many observations such as cold gas mass-to-stellar luminosity ratios of spiral galaxies, HI mass functions, galaxy sizes, faint galaxy number counts and photometric redshift distributions in optical pass-bands, isophotal angular sizes, and cosmic star formation rates. In particular, the strong supernova feedback model is in much better agreement with near-infrared (K-band) faint galaxy number counts and redshift distribution than the weak feedback model and our previous semi-analytic models based on the extended Press-Schechter formalism. (Abridged)
We generate mock galaxy catalogues for a grid of different cosmologies, using rescaled N-body simulations in tandem with a semi-analytic model run using consistent parameters. Because we predict the galaxy bias, rather than fitting it as a nuisance parameter, we obtain an almost pure constraint on sigma_8 by comparing the projected two-point correlation function we obtain to that from the SDSS. A systematic error arises because different semi-analytic modelling assumptions allow us to fit the r-band luminosity function equally well. Combining our estimate of the error from this source with the statistical error, we find sigma_8=0.97 +/- 0.06. We obtain consistent results if we use galaxy samples with a different magnitude threshold, or if we select galaxies by b_J-band rather than r-band luminosity and compare to data from the 2dFGRS. Our estimate for sigma_8 is higher than that obtained for other analyses of galaxy data alone, and we attempt to find the source of this difference. We note that in any case, galaxy clustering data provide a very stringent constraint on galaxy formation models.
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