1,550 research outputs found
Determining layer number of two dimensional flakes of transition-metal dichalcogenides by the Raman intensity from substrate
Transition-metal dichalcogenide (TMD) semiconductors have been widely studied
due to their distinctive electronic and optical properties. The property of TMD
flakes is a function of its thickness, or layer number (N). How to determine N
of ultrathin TMDs materials is of primary importance for fundamental study and
practical applications. Raman mode intensity from substrates has been used to
identify N of intrinsic and defective multilayer graphenes up to N=100.
However, such analysis is not applicable for ultrathin TMD flakes due to the
lack of a unified complex refractive index () from monolayer to bulk
TMDs. Here, we discuss the N identification of TMD flakes on the SiO/Si
substrate by the intensity ratio between the Si peak from 100-nm (or 89-nm)
SiO/Si substrates underneath TMD flakes and that from bare SiO/Si
substrates. We assume the real part of of TMD flakes as that of
monolayer TMD and treat the imaginary part of as a fitting
parameter to fit the experimental intensity ratio. An empirical ,
namely, , of ultrathin MoS, WS and WSe
flakes from monolayer to multilayer is obtained for typical laser excitations
(2.54 eV, 2.34 eV, or 2.09 eV). The fitted of MoS has
been used to identify N of MoS flakes deposited on 302-nm SiO/Si
substrate, which agrees well with that determined from their shear and
layer-breathing modes. This technique by measuring Raman intensity from the
substrate can be extended to identify N of ultrathin 2D flakes with N-dependent
. For the application purpose, the intensity ratio excited by
specific laser excitations has been provided for MoS, WS and
WSe flakes and multilayer graphene flakes deposited on Si substrates
covered by 80-110 nm or 280-310 nm SiO layer.Comment: 10 pages, 4 figures. Accepted by Nanotechnolog
Gravitational Waveform and Polarization from Binary Black Hole Inspiral in Dynamical Chern-Simons Gravity: From Generation to Propagation
We calculate the gravitational waveform radiated from spinning black holes
(BHs) binary in dynamical Chern-Simons (dCS) gravity. The equation of motion
(EOM) of the spinining binary BHs is derived based on the modified
Mathisson-Papapetrou-Dixon equation for the spin-aligned circular orbits. The
leading-order effects induced by the dCS theory contains spin-spin interaction
and monopole-quadrupole interaction, which influence both the EOM of the binary
system and corresponding gravitational waveform at the second post-Newtonian
(PN) order (i.e., 2PN order). After reporting the waveforms, we investigate the
polarization modes of gravitational waves (GWs) in dCS theory. None of the
extra modes appears in this theory up to the considered PN order. Moreover,
since the time scale of the binary merger is much smaller than that of the
cosmological expansion, the parity-violating effect of the dCS theory does not
appear in the process of GW generation. However, during the process of GW
propagation, amplitude birefringence, a typical parity-violating effect, makes
plus and cross modes convert to each other, which modifies the gravitational
waveform at 1.5PN order.Comment: 28 page
The Dust Attenuation Scaling Relation of Star-Forming Galaxies in the EAGLE Simulations
Dust attenuation in star-forming galaxies (SFGs), as parameterized by the
infrared excess (IRX ), is found to be tightly
correlated with star formation rate (SFR), metallicity and galaxy size,
following a universal IRX relation up to . This scaling relation can
provide a fundamental constraint for theoretical models to reconcile galaxy
star formation, chemical enrichment, and structural evolution across cosmic
time. We attempt to reproduce the universal IRX relation over using the EAGLE hydrodynamical simulations and examine sensitive
parameters in determining galaxy dust attenuation. Our findings show that while
the predicted universal IRX relation from EAGLE approximately aligns with
observations at , noticeable disparities arise at different stellar
masses and higher redshifts. Specifically, we investigate how modifying various
galaxy parameters can affect the predicted universal IRX relation in comparison
to the observed data. We demonstrate that the simulated gas-phase metallicity
is the critical quantity for the shape of the predicted universal IRX relation.
We find that the influence of the infrared luminosity and infrared excess is
less important while galaxy size has virtually no significant effect. Overall,
the EAGLE simulations are not able to replicate some of the observed
characteristics between IRX and galaxy parameters of SFGs, emphasizing the need
for further investigation and testing for our current state-of-the-art
theoretical models.Comment: 19 pages, 15 figures, accepted for publication in MNRA
The Physical Properties of Star-Forming Galaxies with Strong [O III] Lines at z=3.25
We present an analysis of physical properties of 34 [O III] emission-line
galaxies (ELGs) at z=3.2540.029 in the Extended Chandra Deep Field South
(ECDFS). These ELGs are selected from deep narrow H2S(1) and broad Ks imaging
of 383 arcmin obtained with CFHT/WIRCam. We construct spectral energy
distributions (SEDs) from U to Ks to derive the physical properties of ELGs.
These [O III] ELGs are identified as starburst galaxies with strong [O III]
lines of L([O III]) ~ 10 - 10 erg s, and have stellar
masses of M* ~ 10-10 M and star formation rates of ~
10-210 M yr. Our results show that 24% of our sample galaxies
are dusty with Av > 1 mag and EW(OIII) ~ 70-500 , which are often
missed in optically selected [O III] ELG samples. Their rest-frame UV and
optical morphologies from HST/ACS and HST/WFC3 deep imaging reveal that these
[O III] ELGs are mostly multiple-component systems (likely mergers) or compact.
And 20% of them are nearly invisible in the rest-frame UV owing to heavy dust
attenuation. Interestingly, we find that our samples reside in an overdensity
consisting of two components: one southeast (SE) with an overdensity factor of
~ 41 over a volume of 13 cMpc and the other
northwest (NW) with ~ 38 over a volume of 10 cMpc.
The two overdense substructures are expected to be virialized at z=0 with a
total mass of ~ 1.1 x 10 M and ~ 4.8 x 10 M, and
probably merge into a Coma-like galaxy cluster.Comment: 22 pages, 11 figures, 3 tables. Accepted for publication in Ap
Synchrotron Radiation Dominates the Extremely Bright GRB 221009A
The brightest Gamma-ray burst, GRB 221009A, has spurred numerous theoretical
investigations, with particular attention paid to the origins of ultra-high
energy TeV photons during the prompt phase. However, analyzing the mechanism of
radiation of photons in the MeV range has been difficult because the high
flux causes pile-up and saturation effects in most GRB detectors. In this
letter, we present systematic modeling of the time-resolved spectra of the GRB
using unsaturated data obtained from Fermi/GBM (precursor) and
SATech-01/GECAM-C (main emission and flare). Our approach incorporates the
synchrotron radiation model, which assumes an expanding emission region with
relativistic speed and a global magnetic field that decays with radius, and
successfully fits such a model to the observational data. Our results indicate
that the spectra of the burst are fully in accordance with a synchrotron origin
from relativistic electrons accelerated at a large emission radius. The lack of
thermal emission in the prompt emission spectra supports a
Poynting-flux-dominated jet composition.Comment: 12 pages, 6 figures, 2 tables. Accepted for publication in ApJ
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Genetically Determined Plasma Lipid Levels and Risk of Diabetic Retinopathy: A Mendelian Randomization Study.
Results from observational studies examining dyslipidemia as a risk factor for diabetic retinopathy (DR) have been inconsistent. We evaluated the causal relationship between plasma lipids and DR using a Mendelian randomization approach. We pooled genome-wide association studies summary statistics from 18 studies for two DR phenotypes: any DR (N = 2,969 case and 4,096 control subjects) and severe DR (N = 1,277 case and 3,980 control subjects). Previously identified lipid-associated single nucleotide polymorphisms served as instrumental variables. Meta-analysis to combine the Mendelian randomization estimates from different cohorts was conducted. There was no statistically significant change in odds ratios of having any DR or severe DR for any of the lipid fractions in the primary analysis that used single nucleotide polymorphisms that did not have a pleiotropic effect on another lipid fraction. Similarly, there was no significant association in the Caucasian and Chinese subgroup analyses. This study did not show evidence of a causal role of the four lipid fractions on DR. However, the study had limited power to detect odds ratios less than 1.23 per SD in genetically induced increase in plasma lipid levels, thus we cannot exclude that causal relationships with more modest effect sizes exist
Systematic biases in determining dust attenuation curves through galaxy SED fitting
While the slope of the dust attenuation curve () is found to
correlate with effective dust attenuation () as obtained through spectral
energy distribution (SED) fitting, it remains unknown how the fitting
degeneracies shape this relation. We examine the degeneracy effects by fitting
SEDs of a sample of local star-forming galaxies (SFGs) selected from the Galaxy
And Mass Assembly survey, in conjunction with mock galaxy SEDs of known
attenuation parameters. A well-designed declining starburst star formation
history is adopted to generate model SED templates with intrinsic UV slope
() spanning over a reasonably wide range. The best-fitting
for our sample SFGs shows a wide coverage, dramatically differing from the
limited range of for a starburst of constant star formation. Our
results show that strong degeneracies between , , and in
the SED fitting induce systematic biases leading to a false --
correlation. Our simulation tests reveal that this relationship can be well
reproduced even when a flat -- relation is taken to build the
input model galaxy SEDs. The variations in best-fitting are dominated
by the fitting errors. We show that assuming a starburst with constant star
formation in SED fitting will result in a steeper attenuation curve, smaller
degeneracy errors, and a stronger -- relation. Our findings
confirm that the -- relation obtained through SED fitting is
likely driven by the systematic biases induced by the fitting degeneracies
between , , and .Comment: 21 pages, 13 figures, accepted for publication in the MNRAS, Comments
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Understanding the Universal Dust Attenuation Scaling Relation of Star-Forming Galaxies
Star-forming galaxies (SFGs) adhere to a surprisingly tight scaling relation
of dust attenuation parameterized by the infrared excess (IRX=), being jointly determined by the star formation rate (SFR),
galaxy size (), metallicity (/Z) and axial ratio ().
We examine how these galaxy parameters determine the effective dust attenuation
and give rise to the universal IRX relation, utilizing a simple two-component
star-dust geometry model in which dust in the dense and diffuse interstellar
medium (ISM) follows exponential mass density profiles, connected with but not
necessarily identical to the stellar mass profiles. Meanwhile, empirical
relations are adopted to link galaxy properties, including the gas--star
formation relation, the dust-to-stellar size relation, as well as the
dust-to-gas ratio versus metallicity relation. By fitting a large sample of
local SFGs with the model, we obtain the best-fitting model parameters as a
function of metallicity, showing that the two-component geometry model is able
to successfully reproduce the dependence of IRX on SFR, , at
given /Z, as well as the dependence of power-law indices on
metallicity. Moreover, we also retrieve constraints on the model geometry
parameters, including the optical depth of birth clouds (BCs), BC-to-total dust
mass fraction, BC covering factor of UV-emitting stars, and star-to-total dust
disc radius ratio, which all evolve with galaxy metallicity. Finally, a
consistent picture of how the star-dust geometry in SFGs evolves with galaxy
metallicity is discussed.Comment: 20 pages, 10 figures, published in MNRAS (2024, Volume 528, Issue 1,
pp.658-675); A PHTHON package IRX_TAU_TOT is available at
https://github.com/LvZF/irx_tau_tot/ to calculate the total dust optical
depth of a galaxy with given metallicity and best-fitting geometry parameter
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