512 research outputs found
Formation of High-redshift (z>6) Quasars Driven by Nuclear Starbursts
Based on the physical model of a supermassive black hole (SMBH) growth via
gas accretion in a circumnuclear disk (CND) proposed by Kawakatu & Wada (2008),
we describe the formation of high- () quasars (QSOs) whose BH masses
are M_{BH}> 10^{9} M_{\odot}. We derive the necessary conditions to form QSOs
at z > 6 by only gas accretion: (i) A large mass supply with M_{sup} >
10^{10}M_{\odot} from host galaxies to CNDs, because the final BH mass is only
1-10% of the total supplied mass from QSO hosts. (ii) High star formation
efficiency for a rapid BH growth. We also find that if the BH growth is limited
by the Eddington accretion, the final BH mass is greatly suppressed. Thus, the
super-Eddington growth is required for the QSO formation. The evolution of the
QSO luminosity depends on the redshift z_{i} at which accretion onto a seed BH
is initiated. In other words, the brighter QSOs at z >6 favor the late growth
of SMBHs (i.e., z_{i}=10) rather than early growth (i.e., z_{i}=30). Moreover,
we predict the observable properties and the evolution of QSOs at z >6. In a
QSO phase, there should exist a stellar rich massive CND, whose gas mass is
about 10% of the dynamical mass inside 0.1-1 kpc}. On the other hand, in a
phase where the BH grows (i.e., a proto-QSO phase), the proto-QSO has a gas
rich massive CNDs whose gas mass is comparable to the dynamical mass
(abridged).Comment: 12 pages, 10 pages, accepted by Ap
The Importance of the Purification Step and the Characterization of the Products in the Synthesis of Carbon Nanodots
In the synthesis of carbon nanodots (CNDs), the critical step of the purification from the starting materials and unwanted side products is faced. In the exciting race toward new and interesting CNDs, this problem is often underestimated, leading to false properties and erroneous reports. In fact, on many occasions, the properties described for novel CNDs derive from impurities not completely eliminated during the purification process. Dialysis, for instance, is not always helpful, especially if the side products are not soluble in water. In this Perspective, the importance of the purification and characterization steps, in order to obtain solid reports and reliable procedures, is emphasized
Nuclear Magnetic Resonance Reveals Molecular Species in Carbon Nanodot Samples Disclosing Flaws
Carbon nanodots are currently one of the hot topics in the nanomaterials world, due to their accessible synthesis and promising features. However, the purification of these materials is still a critical aspect, especially for syntheses involving molecular precursors. Indeed, the presence of unreacted species or small organic molecules formed during solvothermal treatments can affect the properties of the synthesized nanomaterials. To illustrate the extreme importance of this issue, we present two case studies in which insufficient purification results in misleading conclusions regarding the chiral and fluorescent properties of the investigated materials. Key to identify molecular species is the use of nuclear magnetic resonance, which proves to be an effective tool. Our work highlights the need to include nuclear magnetic resonance as a standard characterization technique for carbon-based nanomaterials, to minimize the risk of observing properties that arise from molecular species, rather than the target carbon nanodots
Transfer of Axial Chirality to the Nanoscale Endows Carbon Nanodots with Circularly Polarized Luminescence
We report the synthesis, purification and characterization of chiral carbon nanodots starting from atropoisomeric precursors. The obtained atropoisomeric carbon nanodots are soluble in organic solvents and have good thermal stability, which are desirable features for technological applications. The synthetic protocol is robust, as it supports a number of variations in terms of molecular doping agents. Remarkably, the combination of axially chiral precursors and 1,4-benzoquinone as doping agent results in green-emissive carbon dots displaying circularly polarized luminescence. Dissymmetry factors of |3.5|×10−4 are obtained in solution, without the need of any additional element of chirality. Introducing axial chirality expands the strategies available to tailor the properties of carbon nanodots, paving the way for carbon nanoparticles that combine good processability in organic solvents with engineered advanced chiroptical properties
Multiple regimes and coalescence timescales for massive black hole pairs ; the critical role of galaxy formation physics
We discuss the latest results of numerical simulations following the orbital
decay of massive black hole pairs in galaxy mergers. We highlight important
differences between gas-poor and gas-rich hosts, and between orbital evolution
taking place at high redshift as opposed to low redshift. Two effects have a
huge impact and are rather novel in the context of massive black hole binaries.
The first is the increase in characteristic density of galactic nuclei of
merger remnants as galaxies are more compact at high redshift due to the way
dark halo collapse depends on redshift. This leads naturally to hardening
timescales due to 3-body encounters that should decrease by two orders of
magnitude up to . It explains naturally the short binary coalescence
timescale, Myr, found in novel cosmological simulations that follow
binary evolution from galactic to milliparsec scales. The second one is the
inhomogeneity of the interstellar medium in massive gas-rich disks at high
redshift. In the latter star forming clumps 1-2 orders of magnitude more
massive than local Giant Molecular Clouds (GMCs) can scatter massive black
holes out of the disk plane via gravitational perturbations and direct
encounters. This renders the character of orbital decay inherently stochastic,
often increasing orbital decay timescales by as much as a Gyr. At low redshift
a similar regime is present at scales of pc inside Circumnuclear Gas
Disks (CNDs). In CNDs only massive black holes with masses below can be significantly perturbed. They decay to sub-pc separations in
up to yr rather than the in just a few million years as in a smooth
CND. Finally implications for building robust forecasts of LISA event rates are
discussedComment: 13 pages, 3 Figures, Invited Paper to appear in the Proceedings of
the 11th International LISA Symposium, IOP Journal of Physics: Conference
Serie
Large Binocular Telescope view of the atmosphere of GJ1214b
The atmospheric composition and vertical structure of the super-Earth GJ1214b
has been a subject of debate since its discovery in 2009. Recent studies have
indicated that high-altitude clouds might mask the lower layers. However, some
data points that were gathered at different times and facilities do not fit
this picture, probably because of a combination of stellar activity and
systematic errors. We observed two transits of GJ1214b with the Large Binocular
Camera, the dual-channel camera at the Large Binocular Telescope. For the first
time, we simultaneously measured the relative planetary radius
at blue and red optical wavelengths (), thus
constraining the Rayleigh scattering on GJ1214b after correcting for stellar
activity effects. To the same purpose, a long-term photometric follow-up of the
host star was carried out with WiFSIP at STELLA, revealing a rotational period
that is significantly longer than previously reported. Our new unbiased
estimates of yield a flat transmission spectrum extending to shorter
wavelengths, thus confirming the cloudy atmosphere scenario for GJ1214b.Comment: 11 pages, 5 figures, 3 tables. Published in A&A. Minor changes to
reflect the published versio
Luminescent Carbon Nanodots Doped with Gadolinium (III): Purification Criteria, Chemical and Biological Characterization of a New Dual Fluorescence/MR Imaging Agent
Carbon Dots (CDs) are luminescent quasi-spherical nanoparticles, possessing water solubility, high biocompatibility, and tunable chemical and physical properties for a wide range of applications, including nanomedicine and theranostics. The evaluation of new purification criteria, useful to achieve more reliable CDs, free from the interference of artifacts, is currently an object of debate in the field. Here, new CDs doped with gadolinium (Gd (III)), named Gd@CNDs, are presented as multifunctional probes for Magnetic Resonance Imaging (MRI). This new system is a case of study, to evaluate and/or combine different purification strategies, as a crucial approach to generate CDs with a better performance. Indeed, these new amorphous Gd@CNDs display good homogeneity, and they are free from emissive side products. Gd@CNDs (7-10 nm) contain 7% of Gd (III) w/w, display suitable and stable longitudinal relaxivity (r(1)) and with emissive behavior, therefore potentially useful for both MR and fluorescence imaging. They show good biocompatibility in both cellular and in vivo studies, cell permeability, and the ability to generate contrast in cellular pellets. Finally, MRI recording T-1-weighted images on mice after intravenous injection of Gd@CNDs, show signal enhancement in the liver, spleen, and kidney 30 min postinjection
Development of Novel Analysis and Characterization Methods Utilizing Reaction Dynamics in a Separation Capillary
Electrophoretic migration of an analyte in capillary electrophoresis (CE) reflects reaction dynamics of the analyte in solution. In affinity CE, an analyte of interest interacts with a modifier added in the separation buffer in fast equilibrium, and effective electrophoretic mobility of the analyte is contributed from its equilibrium species. Precise measurement of effective electrophoretic mobility allows analyzing the equilibrium. Analysis of equilibria under CE separation possesses several advantages against traditional analyses in homogeneous solution; coexisting substances including impurities and kinetically generated substances are resolved by CE from the equilibrium species of interest. Characteristics of the CE analysis have been applied to analyses of acid-base equilibria of degradable substances and ion-association equilibria in an aqueous solution. Since CE is operated in an open-tubular capillary, it is also suitable for the characterization of carbon nanoclusters such as graphene and carbon nanotube, and measurement of effective electrophoretic mobility helps characterization of nanoclusters. A novel analysis technique of capillary electrophoresis/dynamic frontal analysis (CE/DFA) has also been proposed for the analysis of such reactions as involving equilibria and kinetic reactions. In CE/DFA, kinetically generated product is continuously resolved from the equilibrium species, and a plateau signal would be detected when the reaction rate is constant. Michaelis-Menten constants have successfully been determined through the plateau height by CE/DFA. In this review, analysis and characterization methods utilizing reaction dynamics in a separation capillary are summarized
- …