arXiv subjects

Feng Gao

Publications and source records attributed to Feng Gao.

296 records · Page 17Linked to original sources

arXiv2015

The Megamaser Cosmology Project. VII. Investigating disk physics using spectral monitoring observations

We use single-dish radio spectra of known 22 GHz H$_2$O megamasers, primarily gathered from the large dataset observed by the Megamaser Cosmology Project, to identify Keplerian accretion disks and to investigate several aspects of the disk physics. We test a mechanism for maser excitation proposed by Maoz & McKee (1998), whereby population inversion arises in gas behind spiral shocks traveling through the disk. Though the flux of redshifted features is larger on average than that of blueshifted features, in support of the model, the high-velocity features show none of the predicted systematic velocity drifts. We find rapid intra-day variability in the maser spectrum of ESO 558-G009 that is likely the result of interstellar scintillation, for which we favor a nearby ($D \approx 70$ pc) scattering screen. In a search for reverberation in six well-sampled sources, we find that any radially-propagating signal must be contributing $\lesssim$10% of the total variability. We also set limits on the magnetic field strengths in seven sources, using strong flaring events to check for the presence of Zeeman splitting. These limits are typically 200--300 mG ($1σ$), but our most stringent limits reach down to 73 mG for the galaxy NGC 1194.

astro-ph.GA↗

arXiv2014

Effects of Side Groups on Kinetics of Charge Carrier Recombination in Dye Molecule-Doped Multilayer Organic Light-Emitting Diodes

The kinetics of the charge carrier recombination in dye molecule-doped multilayer organic light-emitting diodes (OLEDs) was quantified by transient electroluminescence (EL). Three sets of dye molecules, such as derivatives of naphthalimide and stilbene, were used as dopants in light-emission layer. Although the devices show almost the same EL spectra for each set of molecules, they show very different EL efficiency. The difference in EL efficiency was attributed to the difference in charge carrier recombination, as revealed by transient EL. The recombination coefficient (γ) was determined from the long-time component of the temporal decay of the EL intensity after a rectangular voltage pulse was turned off. It was found that γ and EL efficiency were both strongly dependent on the molecular structures of the dopants, and the donor groups and π-conjugated structure guaranteed high γ and EL efficiency in OLEDs.

cond-mat.mtrl-sci↗

arXiv2014

High burden of private mutations due to explosive human population growth and purifying selection

Recent studies have shown that human populations have experienced a complex demographic history, including a recent epoch of rapid population growth that led to an excess in the proportion of rare genetic variants in humans today. This excess can impact the burden of private mutations for each individual, defined here as the proportion of heterozygous variants in each newly sequenced individual that are novel compared to another large sample of sequenced individuals. We calculated the burden of private mutations predicted by different demographic models, and compared with empirical estimates based on data from the NHLBI Exome Sequencing Project and data from the Neutral Regions (NR) dataset. We observed a significant excess in the proportion of private mutations in the empirical data compared with models of demographic history without a recent epoch of population growth. Incorporating recent growth into the model provides a much improved fit to empirical observations. This phenomenon becomes more marked for larger sample sizes. The proportion of private mutations is additionally increased by purifying selection, which differentially affect mutations of different functional annotations. These results have important implications to the design and analysis of sequencing-based association studies of complex human disease as they pertain to private and very rare variants.

q-bio.PE↗

arXiv2013

Neutral genomic regions refine models of recent rapid human population growth

Human populations have experienced dramatic growth since the Neolithic revolution. Recent studies that sequenced a very large number of individuals observed an extreme excess of rare variants, and provided clear evidence of recent rapid growth in effective population size, though estimates have varied greatly among studies. All these studies were based on protein-coding genes, in which variants are also impacted by natural selection. In this study, we introduce targeted sequencing data for studying recent human history with minimal confounding by natural selection. We sequenced loci very far from genes that meet a wide array of additional criteria such that mutations in these loci are putatively neutral. As population structure also skews allele frequencies, we sequenced a sample of relatively homogeneous ancestry by first analyzing the population structure of 9,716 European Americans. We employed very high coverage sequencing to reliably call rare variants, and fit an extensive array of models of recent European demographic history to the site frequency spectrum. The best-fit model estimates ~3.4% growth per generation during the last ~140 generations, resulting in a population size increase of two orders of magnitude. This model fits the data very well, largely due to our observation that assumptions of more ancient demography can impact estimates of recent growth. This observation and results also shed light on the discrepancy in demographic estimates among recent studies.

q-bio.PE↗

arXiv2013

Contiguous relations and summation and transformation formulas for basic hypergeometric series

By using contiguous relations for basic hypergeometric series, we give simple proofs of Bailey's $_4ϕ_3$ summation, Carlitz's $_5ϕ_4$ summation, Sears' $_3ϕ_2$ to $_5ϕ_4$ transformation, Sears' ${}_4ϕ_3$ transformations, Chen's bibasic summation, Gasper's split poised $_{10}ϕ_9$ transformation, Chu's bibasic symmetric transformation. Along the same line, finite forms of Sylvester's identity, Jacobi's triple product identity, and Kang's identity are also obtained.

math.CO↗

arXiv2011

Constraining the Flaring Region of Sagittarius A* By 1.3mm VLBI Measurements

We use a model of an accretion flow coupled with an emergent flare to interpret the latest 1.3mm VLBI measurements for Sagittarius A*. The visibility data constrained the distances from the flare center to the black hole center as $d_{\rm EW}\lesssim20{\rm R_g}$ and $d_{\rm NS}\lesssim80{\rm R_g}$ in the East-West and North-South directions, respectively. If interpreted by the hot-spot model, the flare was preferred to pass in front of the black hole at a radius much larger than $d_{\rm EW}$. If interpreted by the episodic jet launched from a nearly edge-on hot accretion flow, the flare was preferred to be ejected with $θ_{\rm j}\gtrsim40^\circ$ off the black hole rotating axis. This method can be generalized to help us understand future sub-millimeter VLBI observations, and study the millimeter/sub-millimeter variabilities in the vicinity of the Galactic Center supermassive black hole.

astro-ph.GA↗

arXiv2008

Phase-sensitive Manipulations of Squeezed Vacuum Field in an Optical Parametric Amplifier inside an Optical Cavity

Squeezed vacuum field can be amplified or deamplified when it is injected, as the signal beam, into a phase-sensitive optical parametric amplifier (OPA) inside an optical cavity. The spectral features of the reflected quantized signal field are controlled by the relative phase between the injected squeezed vacuum field and the pump field for the OPA. The experimental results demonstrate coherent phenomena of OPA in the quantum regime, and show phase-sensitive manipulations of quantum fluctuations for quantum information processing.

quant-ph↗

arXiv2006

Further Results on Active Magnetic Bearing Control with Input Saturation

We study the low-bias stabilization of active magnetic bearings (AMBs) subject to voltage saturation based on a recently proposed model for the AMB switching mode of operation. Using a forwarding-like approach, we construct a stabilizing controller of arbitrarily small amplitude and a control-Lyapunov function for the AMB dynamics. We illustrate our construction using a numerical example.

math.OC↗