分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： Kardashev Type III civilisations have by definition energy requirements that are likely to generate strong excess emission in the mid-infrared (MIR) that is associated with the waste heat they generate. For those civilisations that capture starlight via Dyson sphere like constructions, the Optical to MIR flux ratio of the host galaxies is also expected to be unusually low. Since a wide range of galaxy types adhere to the infrared-radio correlation (IRC), galaxies hosting Type III civilisations should also strongly deviate from this relation. Radio data can therefore play a crucial role in breaking the degeneracy between the effects of dust obscuration in a galaxy and the signature of an energy-intensive civilisation. We have used the newly released LoTSS-DR1 value-added catalogue to create a sample of 16,367 $z < 0.2$ sources with radio and MIR flux densities, optical photometry and (photometric) redshifts. We calculated the IRC parameter $q^{{\rm22~\mu m}}_{{\rm 150~MHz}}$ = log($S_{150~{\rm MHz}}$/$S_{22~{\rm \mu m}}$) and identified 21 sources with exceptionally high values of $q^{{\rm22~\mu m}}_{{\rm 150~MHz}}$, an indication of MIR emission enhanced by a factor of $\sim$ 10. Out of the 21 high $q$-value sources, 4 sources have very red MIR colors, and appear to have relatively low optical/MIR ratios. Two of the 4 sources are not well known in the literature, they are considered as potential hosts of Type III civilisations. These sources deserve further study and investigation. Finally, we note that extending our analysis to the full LoTSS survey area can place very strong constraints on the incidence of Type III civilisations in the universe.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： Some studies suggest that the dust temperatures ($T_\mathrm{d}$) in high-redshift ($z\gtrsim 5$) Lyman break galaxies (LBGs) are high. However, possible observational bias in $T_\mathrm{d}$ is yet to be understood. Thus, we perform a simple test using random realizations of LBGs with various stellar masses, dust temperatures, and dust-to-stellar mass ratios, and examine how the sample detected by ALMA is biased in terms of $T_\mathrm{d}$. We show that ALMA tends to miss high-$T_\mathrm{d}$ objects even at total dust luminosity. LBGs are, however, basically selected by the stellar UV luminosity. The dust-temperature bias in a UV-selected sample is complicated because of the competing effects between high $T_\mathrm{d}$ and low dust abundance. For ALMA Band 6, there is no tendency of high-$T_\mathrm{d}$ LBGs being more easily detected in our experiment. Thus, we suggest that the observed trend of high $T_\mathrm{d}$ in $z\gtrsim 5$ LBGs is real. We also propose that the 450 $\mu$m band is useful in further clarifying the dust temperatures. To overcome the current shallowness of 450 $\mu$m observations, we examine a future Antarctic 30-m class telescope with a suitable atmospheric condition for wavelengths $\lesssim 450~\mu$m, where the detection is not confusion-limited. We find that, with this telescope, an $L_\mathrm{IR}$-selected sample with $\log(L_\mathrm{IR}/\mathrm{L}_{\odot})>11$ is constructed for $z\gtrsim 5$, and detection in the intermediate-$M_\star$ (stellar mass) range [$9<\log (M_\star /M_{\odot})<9.5$] is much improved, especially at high $T_\mathrm{d}$.
分类： 物理学 >> 普通物理:统计和量子力学,量子信息等 提交时间： 2017-11-10
摘要： From the overlap lattice quark propagator calculated in the Landau gauge, we determine the quark chiral condensate by fitting operator product expansion formulas to the lattice data. The quark propagators are computed on domain wall fermion configurations generated by the RBC-UKQCD Col-laborations with Nf = 2 + 1 flavors. Three ensembles with different light sea quark masses are used at one lattice spacing 1/a = 1.75(4) GeV. We obtain ψ¯ψMS(2 GeV) = (−305(15)(21) MeV)3 in the SU(2) chiral limit.
分类： 物理学 >> 核物理学 提交时间： 2016-09-02
摘要： We propose a novel type of interpolating field operators, which manifests the hybrid-like configuration that the charm quark-antiquark pair recoils against gluonic degrees of freedom. A heavy vector charmonium-like state with a mass of 4.33(2)GeV is disentangled from the conventional charmonium states in the quenched approximation. This state has affinity for the hybrid-like operators but couples less to the relevant quark bilinear operator. We also try to extract its leptonic decay constant and give a tentative upper limit that it is less than one tenth of that of J/ψ, which corresponds to a leptonic decay width about dozens of eV. The connection of this state with X(4260) is also discussed.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： The 80-cm azimuthal telescope is newly mounted at Yaoan Station, Purple Mountain Observatory in 2018. The astrometric performance of the telescope is tested in the following three aspects. (a) The geometric distortion of its CCD attached. It is stable in both a single epoch and multi epochs. Eight distortion solutions are derived over about one year. The maximum values range from 0.75 to 0.79 pixel and the median values range from 0.14 to 0.16 pixel. (b) The limit magnitude of stars. About 20.5 magnitude (Gaia-G) stars can be detected with Johnson-V filter exposured in 300 seconds. The astrometric error of about 20.5 magnitude stars is estimated at 0.14 arcsec using the fitted sigmoidal function. (c) The astrometric accuracy and the precision of stacked fast-moving faint object. 24 stacked frames of the potentially hazardous asteroid (PHA) (99942) Apophis are derived on April 14 and 15, 2021 (fainter than 18 mag) based on the ephemeris shifts. During data reduction, the newest Gaia EDR3 Catalog and Jet Propulsion Laboratory Horizons ephemeris are referenced as theoretical positions of stars and Apophis, respectively. Our results show that the mean (O-C)s (observed minus computed) of Apophis are -0.018 and 0.020 arcsec in right ascention and declination, and the dispersions are estimated at 0.094 and 0.085 arcsec, respectively, which show the consistency of the stacked results by Astrometrica.
分类： 光学 >> 量子光学 提交时间： 2023-02-19
摘要： Weyl points are the degenerate points in three-dimensional momentum space with nontrivial topological phase, which are usually realized in classical system with structure and symmetry designs. Here we proposed a one-dimensional layer-stacked photonic crystal using anisotropic materials to realize ideal type-II Weyl points without structure designs. The topological transition from two Dirac points to four Weyl points can be clearly observed by tuning the twist angle between layers. Besides, on the interface between the photonic type-II Weyl material and air, gappless surface states have also been demonstrated in an incomplete bulk bandgap. By breaking parameter symmetry, these ideal type-II Weyl points at the same frequency would transform into the non-ideal ones, and exhibit topological surface states with single group velocity. Our work may provide a new idea for the realization of photonic Weyl points or other semimetal phases by utilizing naturally anisotropic materials.
分类： 物理学 >> 核物理学 提交时间： 2016-09-14
摘要： The ρ meson leptonic decay constant at the physical pion mass is calculated with chiral fermions. Overlap valence quarks are used on the L3×T=483×96 Nf=2+1 domain-wall fermion configurations with light sea quarks at the physical point and the spatial extension of 5.5 fm. The mass of the vector meson ρ is determined to be mρ=775.9±6.0±1.8 MeV, and the valence πρ sigma term is extracted to be σ(val)πρ=9.82(6) MeV from Feynmann-Hellman theorem. By adopting a new method to reduce the noise of the wall-wall correlation functions of hadrons, we obtain the ρ leptonic decay constant fρ=208.5±5.5±0.9 MeV, which is in very good agreement with the experimental value fρ=209.4±1.5 MeV derived from the τ decay τ→ρντ.
分类： 物理学 >> 核物理学 提交时间： 2016-09-14
摘要： Topological charge susceptibility χt for pure gauge SU(3) theory at finite temperature is studied using anisotropic lattices. The over-improved stout-link smoothing method is utilized to calculate the topological charge. Near the phase transition point we find a rapid declining behavior for χt with values decreasing from (188(1)MeV)4 to (67(3)MeV)4 as the temperature increased from zero temperature to 1.9Tc which demonstrates the existence of topological excitations far above Tc. The 4th order cumulant c4 of topological charge, as well as the ratio c4/χt are also investigated. Results of c4 show step-like behavior near Tc while the ratio at high temperature agrees with the value as predicted by the diluted instanton gas model.
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