Massive Neutrinos and Galaxy Formation

We report the most recent results from high-resolution numerical simulations of structure formation in two flat cold+hot dark matter models with neutrino mass densities \onu=0.2 and 0.3. We find that structure forms too late in all CDM+HDM models with \onu>0.2 to account for the amount of dense neutral gas in high-redshift damped Lyman-$\alpha$ systems. The \onu=0.2 model at $z\approx0$ provides a better match to observations than the pure CDM model.Comment: 5 pages including 3 figures, uuencoded compressed postscript. To appear in the Proceedings of the 5th Annual Astrophysics Conference in Maryland "Dark Matter

Cosmological Perturbation Theory in the Synchronous vs. Conformal Newtonian Gauge

We present a systematic treatment of the linear theory of scalar gravitational perturbations in the synchronous gauge and the conformal Newtonian (or longitudinal) gauge. We first derive the transformation law relating the two gauges. We then write down in parallel in both gauges the coupled, linearized Boltzmann, Einstein and fluid equations that govern the evolution of the metric perturbations and the density fluctuations of the particle species. The particle species considered include cold dark matter (CDM), baryons, photons, massless neutrinos, and massive neutrinos (a hot dark matter or HDM candidate), where the CDM and baryon components are treated as fluids while a detailed phase-space description is given to the photons and neutrinos. The linear evolution equations presented are applicable to any $\Omega=1$ model with CDM or a mixture of CDM and HDM. Isentropic initial conditions on super-horizon scales are derived. The equations are solved numerically in both gauges for a CDM+HDM model with $\Omega_{\rm cold}=0.65,$ $\Omega_{\rm hot}=0.3$, and $\Omega_{\rm baryon}=0.05$. We discuss the evolution of the metric and the density perturbations and compare their different behaviors outside the horizon in the two gauges. In a companion paper we integrate the geodesic equations for the neutrino particles in the perturbed conformal Newtonian background metric computed here. The purpose is to obtain an accurate sampling of the neutrino phase space for the HDM initial conditions in $N$-body simulations of the CDM+HDM models.Comment: 35 pages, AAS LaTeX v3.0, figures and/or postscript available by anonymous ftp to arcturus.mit.edu, Caltech GRP-375; MIT-AT-94-01; IASSNS-AST-94/

Schechter vs. Schechter: Sub-Arcsecond Gravitational Lensing and Inner Halo Profiles

Sub-arcsecond lensing statistics depend sensitively on the inner mass profiles of low-mass objects and the faint-end slopes of the Schechter luminosity function and the Press-Schechter mass function. By requiring the luminosity and mass functions to give consistent predictions for the distribution of image separation below 1'', we show that dark matter halos with masses below 10^12 M_sun cannot have a single type of profile, be it the singular isothermal sphere (SIS) or the shallower ``universal'' dark matter profile. Instead, consistent results are achieved if we allow a fraction of the halos at a given mass to be luminous with the SIS profile, and the rest be dark with an inner logarithmic slope shallower than -1.5 to compensate for the steeper faint-end slope of the mass function compared with the luminosity function. We quantify how rapidly the SIS fraction must decrease with decreasing halo mass, thereby providing a statistical measure for the effectiveness of feedback processes on the baryon content in low-mass halos.Comment: 13 pages, 4 figures. CLASS lensing data added; minor revisions. ApJL in pres

Thermal and photochemical reactions of large metallaborane clusters and functionalizations of boron nitride nanosheets

The thermal and photochemical pathways of several metallaborane compounds and the functionalization of boron nitride nanosheets have been investigated. The nanosheets have potential applications in Dye Sensitized Solar Cells.Chapters 1 and 3 describe the thermal and photochemical pathways for two metalladecaboranes, [nido -6-Mn(CO) 3 B 9 H 13 ][NMe 4 ] (1.1) and [6-C5Me5-nido-6-RhB9H13] (3.1). Isomerization or cage closure reactions were observed depending on the reaction conditions and the transition metal employed. Characterized products include the firstnido-5 metalladecaborane and ahypercloso-metalladecaborane containing a group 7 transitional metal, compound1.3 and1.4, respectively. Other findings include the facile acid catalyzed cyclic oxonium functionalization of complex1.1, the role of complex3.1 as the catalyst for the [2+2+2] cycloadditions of terminal alkynes, and the formation ofhypercloso-rhodadecaborane3.3 which is the firsthypercloso-metalladecaborane containing the metal rhodium. Chapter 2 describes investigations of the photochemical reactions between metallaboranes and unsaturated molecules. The complex 1.1was found to photoreact with both terminal and internal alkynes to afford the carborane with general formula [7-R-8-R\u27-7,8-nido-C2B9H10][NMe4] (2) in good yield. This represents a novel method for generating the substituted nido-C2B9 complex. Besides alkynes, the investigation of the photoreactions of other unsaturated species with the complex 1.1 were found to generate the [arachno-B9H14]- (2.4) and [nido-B9H12]- (2.5) anions in different relative ratios, depending on the nature of the unsaturated molecules. The complex1.1 was also found to act as an efficient catalyst for isocyanate cyclodimerization under photo-irradiation conditions, representing the first example of a metallaborane complex to catalyse the cyclodimerization of isocyanate. In Chapter 4, an effective and simple noncovalent functionalization and solubilization of boron nitride nanosheets (BNNSs) has been achieved by using polythiophene or polyvinylpyrrolidone as a functionalization polymer with BNNSs. The BNNSs form strong π-π stacking interactions with the polythiophenes and were found to act as a bandgap tuning tool for polythiophene derivatives. A BNNS-polythiophne-TiO2 hybrid nanomaterial was achieved by the covalent binding between TiO2 nanoparticle surfaces and the BNNS-polythiophene unit containing a carboxylic acid functional group. This nanomaterial was then further incorporated into a Dye Sensitized Solar Cells (DSSCs) and the observed enhancement of the performance of the cells is discussed