Showing papers by "Joel R. Primack published in 2003"

The Dark Side of the Halo Occupation Distribution

Abstract: We analyze the halo occupation distribution (HOD), the probability for a halo of mass M to host a number of subhalos N, and two-point correlation function of galaxy-size dark matter halos using high-resolution dissipationless simulations of the concordance flat LCDM model. The halo samples include both the host halos and the subhalos, distinct gravitationally-bound halos within the virialized regions of larger host systems. We find that the first moment of the HOD, (M), has a complicated shape consisting of a step, a shoulder, and a power law high-mass tail. The HOD can be described by a Poisson statistics at high halo masses but becomes sub-Poisson for (M). We find that ~M^b with b~1 for a wide range of number densities, redshifts, and different power spectrum normalizations. This formulation provides a simple but accurate model for the halo occupation distribution found in simulations. At z=0, the two-point correlation function (CF) of galactic halos can be well fit by a power law down to ~100/h kpc with an amplitude and slope similar to those of observed galaxies. At redshifts z>~1, we find significant departures from the power-law shape of the CF at small scales. If the deviations are as strong as indicated by our results, the assumption of the single power law often used in observational analyses of high-redshift clustering is likely to bias the estimates of the correlation length and slope of the correlation function.

A New Non-Parametric Approach to Galaxy Morphological Classification

Abstract: We present two new non-parametric methods for quantifying galaxy morphology: the relative distribution of the galaxy pixel flux values (the Gini coefficient or G) and the second-order moment of the brightest 20% of the galaxy's flux (M20). We test the robustness of G and M20 to decreasing signal-to-noise and spatial resolution, and find that both measures are reliable to within 10% at average signal-to-noise per pixel greater than 3 and resolutions better than 1000 pc and 500 pc, respectively. We have measured G and M20, as well as concentration (C), asymmetry (A), and clumpiness (S) in the rest-frame near-ultraviolet/optical wavelengths for 150 bright local "normal" Hubble type galaxies (E-Sd) galaxies and 104 0.05 < z < 0.25 ultra-luminous infrared galaxies (ULIRGs).We find that most local galaxies follow a tight sequence in G-M20-C, where early-types have high G and C and low M20 and late-type spirals have lower G and C and higher M20. The majority of ULIRGs lie above the normal galaxy G-M20 sequence, due to their high G and M20 values. Their high Gini coefficients arise from very bright nuclei, while the high second-order moments are produced by multiple nuclei and bright tidal tails. All of these features are signatures of recent and on-going mergers and interactions. We also find that in combination with A and S, G is more effective than C at distinguishing ULIRGs from the "normal" Hubble-types. Finally, we measure the morphologies of 45 1.7 < z < 3.8 galaxies from HST NICMOS observations of the Hubble Deep Field North. We find that many of the z $\sim$ 2 galaxies possess G and A higher than expected from degraded images of local elliptical and spiral galaxies, and have morphologies more like low-redshift single nucleus ULIRGs.

Status of cold dark matter cosmology

Abstract: Cold Dark Matter (CDM) has become the standard modern theory of cosmological structure formation. Its predictions appear to be in good agreement with data on large scales, and it naturally accounts for many properties of galaxies. But despite its many successes, there has been concern about CDM on small scales because of the possible contradiction between the linearly rising rotation curves observed in some dark-matter-dominated galaxies vs. the 1/ r density cusps at the centers of simulated CDM halos. Other CDM issues on small scales include the very large number of small satellite halos in simulations, far more than the number of small galaxies observed locally, and problems concerning the angular momentum of the baryons in dark matter halos. The latest data and simulations have lessened, although not entirely resolved, these concerns. Meanwhile, the main alternatives to CDM that have been considered to solve these problems, self-interacting dark matter (SIDM) and warm dark matter (WDM), have been found to have serious drawbacks.

Damped Lyman alpha systems and galaxy formation models – II. High ions and Lyman-limit systems

Abstract: We investigate a model for the high-ionization state gas associated with observed damped Lyman a systems, based on a semi-analytic model of galaxy formation set within the paradigm of hierarchical structure formation. In our model, the hot gas in haloes and subhaloes is assumed to be in a multiphase medium which gives rise to CIV absorption, while the low-ionization state gas is associated with the cold gas in galaxies. The model matches the distribution of C iv column densities if we assume that the hot gas has a mean metallicity log C/H = -1.5, which is the observed mean metallicity of damped systems. The same model then leads naturally to kinematic properties that are in good agreement with the data, for both the low- and high- ionization state gas. We examine the contribution of both hot and cold gas to subdamped systems (N H 1 > 4 × 10 1 9 cm - 2 ) and suggest that the properties of these systems can be used as an important test of the model. We expect that sub-DLA systems will generally be composed of a single gas disc and thus predict that they should have markedly different kinematics from the damped systems. We also find that the frequency of absorbers drops dramatically for column densities below 4 × 10 1 9 cm - 2 . These results are a consequence of our model for damped Lyman a systems and we believe they are a generic prediction of multicomponent models. Finally, we find that hot halo gas produces less than one-third of Lyman-limit systems at a redshift of 3. We model the contribution of mini-haloes (haloes with virial velocities ≤35 km s - 1 ) to Lyman-limit systems and find that they may contain as much gas as is observed in these systems. However, if we adopt realistic models of the gas density distribution we find that these systems are not a significant source of Lyman-limit absorption. Instead we suggest that uncollapsed gas outside of virialized haloes is responsible for most of the Lyman-limit systems at high redshift.

Flat-Cored Dark Matter in Cuspy Clusters of Galaxies

Abstract: Sand, Treu, & Ellis (2002) have measured the central density profile of cluster MS2137-23 with gravitational lensing and velocity dispersion and removed the stellar contribution with a reasonable M/L. The resulting dark matter distribution within r<50 kpc was fitted by a density cusp of r^{-beta} with beta=0.35. This stands in an apparent contradiction to the CDM prediction of beta~1, and the disagreement worsens if adiabatic compression of the dark matter by the infalling baryons is considered. Following El-Zant, Shlosman & Hoffman (2001), we argue that dynamical friction acting on galaxies moving within the dark matter background counters the effect of adiabatic compression by transfering the orbital energy of galaxies to the dark matter, thus heating up and softening the central density cusp. Using N-body simulations of massive solid clumps moving in clusters we show that indeed the inner dark matter distribution flattens (with beta approx 0.35 for a cluster like MS2137-23) when the galaxies spiral inward. We find as a robust result that while the dark matter distribution becomes core-like, the overall mass distribution preserves its cuspy nature, in agreement with X-ray and lensing observations of clusters.

Summary talk: How serious are the problems faced by CDM: cusps, thin disks, and halo substructure

Abstract: This is a summary of the discussion in Panel I of IAU Symposium 220 on Dark Matter in Galaxies. The panel topics and panellists were as follows -- Cusps Theory: Julio Navarro; Cusps Observations: Erwin de Blok, Rob Swaters; Substructure Observations: Mario Mateo; Substructure Theory: James Taylor, Shude Mao. In addition to the talks by the panellists and the discussion among them, there was a good deal of discussion with the audience. Despite the differences of opinion expressed I think a consensus emerged, and I try to summarize it here.

Summary talk: How serious are the problems faced by CDM: Cusps, thin disks, and halo substructure

Abstract: This is a summary of the discussion in Panel I of IAU Symposium 220 on Dark Matter in Galaxies. The panel topics and panellists were as follows -- Cusps Theory: Julio Navarro; Cusps Observations: Erwin de Blok, Rob Swaters; Substructure Observations: Mario Mateo; Substructure Theory: James Taylor, Shude Mao. In addition to the talks by the panellists and the discussion among them, there was a good deal of discussion with the audience. Despite the differences of opinion expressed I think a consensus emerged, and I try to summarize it here.

The Masses of Lyman Break Galaxies

Abstract: Data on galaxies at high redshift, identified by the Lyman-break photometric technique, can teach us about how galaxies form and evolve. The stellar masses and other properties of such Lyman break galaxies (LBGs) depend sensitively on the details of star formation. In this paper we consider three different star formation prescriptions, and use semi-analytic methods applied to the now-standard ΛCDM theory of hierarchical structure formation to show how these assumptions about star formation affect the predicted masses of the stars in these galaxies and the masses of the dark matter halos that host them. We find that, within the rather large uncertainties, recent estimates of the stellar masses of LBGs from multi-color photometry are consistent with the predictions of all three models. However, the estimated stellar masses are more consistent with the predictions of two of the models in which star formation is accelerated at high redshifts z > ∼ 3, and of these models the one in which many of the LBGs are merger-driven starbursts is also more consistent with indications that many high redshift galaxies are gas rich. The clustering properties of LBGs have put some constraints on the masses of their host halos, but due to similarities in the halo occupation of the three models we consider and degeneracies between model parameters, current constraints are not yet sufficient to distinguish between realistic models.

The Origin and Distribution of Angular Momemtum in Galaxies

Abstract: Cold Dark Matter with a large cosmological constant (LambdaCDM) appears to fit large scale structure observations well. Of the possible small scale problems, the Central Cusps and Too Many Satellites problems now appear to be at least partly solved, so Angular Momentum has become the most serious remaining CDM problem. There are actually at least two different angular momentum problems: A. Too much transfer of angular momentum to the dark halo to make big disks, and B. Wrong distribution of spedific angular momentum to make spiral galaxies, if the baryonic material has the same angular momentum distribution as the dark matter. The angular momentum of dark matter halos, and presumably that of the galaxies they host, appears to arise largely from the orbital angular momentum of the satellites that they accrete. Since the dark and baryonic matter behave very differently in such accretion events, it is possible that the resulting angular momentum distribution of the baryons is different from that of the dark matter, as required to make the sort of galactic disks that are observed. The latest hydrodynamical simulations give some grounds for hope on this score, but much higher resolution simulations are needed.