University of California, Santa Cruz

About: University of California, Santa Cruz is a education organization based out in Santa Cruz, California, United States. It is known for research contribution in the topics: Galaxy & Population. The organization has 15541 authors who have published 44120 publications receiving 2759983 citations. The organization is also known as: UCSC & UC, Santa Cruz.

The Dark Side of the Halo Occupation Distribution

Abstract: We analyze the halo occupation distribution (HOD) and two-point correlation function of galaxy-size dark matter halos using high-resolution dissipationless simulations of the concordance flat ΛCDM 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 HOD, the probability distribution for a halo of mass M to host a number of subhalos N, is similar to that found in semianalytic and N-body+gasdynamics studies Its first moment, NM, has a complicated shape consisting of a step, a shoulder, and a power-law high-mass tail The HOD can be described by Poisson statistics at high halo masses but becomes sub-Poisson for NM 4 We show that the HOD can be understood as a combination of the probability for a halo of mass M to host a central galaxy and the probability to host a given number Ns of satellite galaxies The former can be approximated by a steplike function, while the latter can be well approximated by a Poisson distribution, fully specified by its first moment The first moment of the satellite HOD can be well described by a simple power law Ns ∝ Mβ with β ≈ 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 fitted by a power law down to ~100 h-1 kpc with an amplitude and slope similar to those of observed galaxies The dependence of correlation amplitude on the number density of objects is in general agreement with results from the Sloan Digital Sky Survey At redshifts z 1, we find significant departures from the power-law shape of the CF at small scales, where the CF steepens because of a more pronounced one-halo component The departures from the power law may thus be easier to detect in high-redshift galaxy surveys than at the present-day epoch They can be used to put useful constraints on the environments and formation of galaxies 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 dangerous and is likely to bias the estimates of the correlation length and slope of the correlation function

Massive Black Holes as Population III Remnants

Abstract: Recent numerical simulations of the fragmentation of primordial molecular clouds in hierarchical cosmogonies have suggested that the very first stars (the so-called Population III) may have been rather massive. Here we point out that a numerous population of massive black holes (MBHs)—with masses intermediate between those of stellar and supermassive holes—may be the end product of such an episode of pregalactic star formation. If only one MBH with m• 150 M☉ formed in each of the "minihalos" collapsing at z ≈ 20 from 3 σ fluctuations, then the mass density of Population III MBHs would be comparable to that of the supermassive variety observed in the nuclei of galaxies. Since they form in high-σ rare density peaks, relic MBHs are predicted to cluster in the bulges of present-day galaxies as they become incorporated through a series of mergers into larger and larger systems. Dynamical friction would cause 50 (m•/150 M☉)1/2 such objects to sink toward the center. The presence of a small cluster of MBHs in galaxy nuclei may have several interesting consequences associated with it, such as tidal captures of ordinary stars (likely followed by disruption), MBH capture by the central supermassive black hole, and gravitational wave radiation from such coalescences. Accreting pregalactic MBHs may be detectable as ultraluminous, off-nuclear X-ray sources.

Long γ-ray bursts and core-collapse supernovae have different environments

Abstract: When massive stars exhaust their fuel, they collapse and often produce the extraordinarily bright explosions known as core-collapse supernovae. On occasion, this stellar collapse also powers an even more brilliant relativistic explosion known as a long-duration γ-ray burst. One would then expect that these long γ-ray bursts and core-collapse supernovae should be found in similar galactic environments. Here we show that this expectation is wrong. We find that the γ-ray bursts are far more concentrated in the very brightest regions of their host galaxies than are the core-collapse supernovae. Furthermore, the host galaxies of the long γ-ray bursts are significantly fainter and more irregular than the hosts of the core-collapse supernovae. Together these results suggest that long-duration γ-ray bursts are associated with the most extremely massive stars and may be restricted to galaxies of limited chemical evolution. Our results directly imply that long γ-ray bursts are relatively rare in galaxies such as our own Milky Way.

Propagule dispersal distance and the size and spacing of marine reserves

Abstract: This study compiled available information on the dispersal distance of the propagules of benthic marine organisms and used this information in the development of criteria for the design of marine reserves. Many benthic marine organisms release propagules that spend time in the water column before settlement. During this period, ocean currents transport or disperse the propagules. When considering the size of a marine reserve and the spacing between reserves, one must consider the distance which propagules disperse. We could find estimates of dispersal distance for 32 taxa; for 25 of these, we were also able to find data on the time the propagules spent dispersing. Dispersal distance ranged from meters to thousands of kilometers, and time in the plankton ranged from minutes to months. A significant positive correlation was found between the log-transformed duration in the plankton and the log-transformed dispersal distance ( r 5 0.7776, r 2 5 0.60, df 5 1, 25, P 5 0.000); the more time propagules spend in the water column the further they tend to be dispersed. The frequency distribution of the log-transformed dispersal distance is bimodal (kurtosis 52 1.29, t 52 4.062, P , 0.001) with a gap between 1 and 20 km. Propagules that dispersed ,1 km spent less time in the plankton (,100 h), or if they remained in the plankton for a longer period, they tended to remain in the waters near the bottom. Propagules that dispersed .20 km spent more than 300 h in the plankton. The bimodal nature of the distribution suggests that evolutionary constraints may reduce the likelihood of evolving mid-range dispersal strategies (i.e., dispersal between 1 and 20 km) resulting in two evolutionarily stable dispersal strategies: dispersal , 1k m or.;20 km. We suggest that reserves be designed large enough to contain the short-distance dispersing propagules and be spaced far enough apart that long-distance dispersing propagules released from one reserve can settle in adjacent reserves. A reserve 4-6 km in diameter should be large enough to contain the larvae of short-distance dispersers, and reserves spaced 10- 20 km apart should be close enough to capture propagules released from adjacent reserves.

Marine defaunation: Animal loss in the global ocean

Abstract: BACKGROUND: Comparing patterns of ter- restrial and marine defaunation helps to place human impacts on marine fauna in context and to navigate toward recovery. De- faunation began in ear- nest tens of thousands of years later in the oceans than it did on land. Al- though defaunation has been less severe in the oceans than on land, our effects on marine animals are increasing in pace and impact. Humans have caused few complete extinctions in the sea, but we are responsible for many ecological, commercial, and local extinctions. Despite our late start, humans have already powerfully changed virtually all major marine ecosystems. ADVANCES: Humans have profoundly de- creased the abundance of both large (e.g., whales) and small (e.g., anchovies) marine fauna. Such declines can generate waves of ecological change that travel both up and down marine food webs and can alter ocean ecosystem functioning. Human harvesters have also been a major force of evolutionary change in the oceans and have reshaped the genetic structure of marine animal popula- tions. Climate change threatens toaccelerate marine defaunation over the next century. The high mobility of many marine animals offers some increased, though limited, ca- pacity for marine species to respond to cli- mate stress, but it also exposes many species to increased risk from other stressors. Be- cause humans are intensely reliant on ocean ecosystems for food and other ecosystem ser- vices, we are deeply affected by all of these forecasted changes. Three lessons emerge when comparing the marine and terrestrial defaunation ex-