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.

Iron-limited diatom growth and Si:N uptake ratios in a coastal upwelling regime

Abstract: There is compelling evidence that phytoplankton growth is limited by iron availability in the subarctic Pacific1, and equatorial Pacific2 and Southern oceans3. A lack of iron prevents the complete biological utilization of the ambient nitrate and influences phytoplankton species composition in these open-ocean ‘high-nitrate, low-chlorophyll’ (HNLC) regimes4. But the effects of iron availability on coastal primary productivity and nutrient biogeochemistry are unknown. Here we present the results of shipboard seawater incubation experiments which demonstrate that phytoplankton are iron-limited in parts of the California coastal upwelling region. As in offshore HNLC regimes, the addition of iron to these nearshore HNLC waters promotes blooms of large chain-forming diatoms. The silicic acid:nitrate (Si:N) uptake ratios in control incubations are two to three times higher than those in iron incubations. Diatoms stressed by a lack of iron should therefore deplete surface waters of silicic acid before nitrate, leading to a secondary silicic acid limitation of the phytoplankton community. Higher Si:cell, Si:C and Si:pigment ratios in diatoms in the control incubations suggest that iron limitation leads to more silicified, faster-sinking diatom biomass. These results raise fundamental questions about the nature of nutrient-limitation interactions in marine ecosystems, palaeoproductivity estimates based on the sedimentary accumulation of biogenic opal, and the controls on carbon export from some of the world's most productive surface waters.

Accounting for Doing Gender

Abstract: W e're delighted to have "Doing Gender" and its sequelae as the subjects of this symposium. The serious readings of our work by Professors Connell, Jones, Kitzinger, Messerschmidt, Risman, Smith, and Vidal-Ortiz do us honor, and we welcome the chance to address them. We use our response to reflect on, clarify, admit, and expand on what we said originally and what we have said since. As important as the path taken, however, is the theoretical path ahead, and we will comment on that as well.

The Assembly and Merging History of Supermassive Black Holes in Hierarchical Models of Galaxy Formation

Abstract: We assess models for the assembly of supermassive black holes (SMBHs) at the center of galaxies that trace their hierarchical buildup far up in the dark halo merger tree. Motivated by the recent discovery of luminous quasars around redshift z ≈ 6—suggesting a very early assembly epoch—and by numerical simulations of the fragmentation of primordial molecular clouds in cold dark matter (CDM) cosmogonies, we assume that the first seed black holes (BHs) had intermediate masses and formed in (mini)halos collapsing at z ~ 20 from high-σ density fluctuations. As these pregalactic holes become incorporated through a series of mergers into larger and larger halos, they sink to the center because of dynamical friction, accrete a fraction of the gas in the merger remnant to become supermassive, form a binary system, and eventually coalesce. The merger history of dark matter halos and associated BHs is followed by cosmological Monte Carlo realizations of the merger hierarchy from early times until the present in a ΛCDM cosmology. A simple model, where quasar activity is driven by major mergers and SMBHs accrete at the Eddington rate a mass that scales with the fifth power of the circular velocity of the host halo, is shown to reproduce the observed luminosity function of optically selected quasars in the redshift range 1 < z < 5. A scheme for describing the hardening of a BH binary in a stellar background with core formation due to mass ejection is applied, where the stellar cusp proportional to r-2 is promptly regenerated after every major merger event, replenishing the mass displaced by the binary. Triple BH interactions will inevitably take place at early times if the formation route for the assembly of SMBHs goes back to the very first generation of stars, and we follow them in our merger tree. The assumptions underlying our scenario lead to the prediction of a population of massive BHs wandering in galaxy halos and the intergalactic medium at the present epoch and contributing 10% to the total BH mass density, ρSMBH = 4 × 105 M☉ Mpc-3 (h = 0.7). The fraction of binary SMBHs in galaxy nuclei is on the order of 10% today, and it increases with redshift so that almost all massive nuclear BHs at early epochs are in binary systems. The fraction of binary quasars (both members brighter than 0.1L*) instead is less than 0.3% at all epochs. The nuclear SMBH occupation fraction is unity (0.6) at the present epoch if the first seed BHs were as numerous as the 3.5 σ (4 σ) density peaks at z = 20.