[en] The types of errors in CFD simulation can be divided into the two main categories: numerical errors and model errors. Turbulence model is one of the important sources for model errors. In this study, in order to assess the prediction performance of Reynolds-averaged Navier-Stokes (RANS)-based two equations turbulence models for the analysis of flow distribution inside a 1/5 scale-down APR+, the simulation was conducted with the commercial CFD software, ANSYS CFX V. 14. In this study, in order to assess the prediction performance of turbulence models for the analysis of flow distribution inside a 1/5 scale-down APR+, the simulation was conducted with the commercial CFD software, ANSYS CFX V. 14. Both standard k-ε model and SST model predicted the similar flow pattern inside reactor. Therefore it was concluded that the prediction performance of both turbulence models was nearly same. Complex thermal-hydraulic characteristics exist inside reactor because the reactor internals consist of fuel assembly, control rod assembly, and the internal structures. Either flow distribution test for the scale-down reactor model or computational fluid dynamics (CFD) simulation have been conducted to understand these complex thermal-hydraulic features inside reactor

[en] More than 50% of active galactic nuclei (AGNs) are suspected to be red and affected by dust obscuration. Meanwhile, popular spectral diagnostics of AGNs are based on optical or ultraviolet light, making dust obscuration the primary concern for understanding the general nature of AGNs and the supermassive black holes residing in them. To provide a method for investigating properties of dusty AGNs, we derive new black hole (BH) mass estimators based on velocity widths and luminosities of near-infrared (NIR) hydrogen emission lines such as Pα and Pβ, and also investigate the line ratios of these hydrogen lines. To derive the BH mass (M_BH) estimators, we used a sample of 37 unobscured type 1 AGNs with an M_BH range of 10^6.8-10^9.4 M_sun, where M_BH comes from either the reverberation mapping method or single-epoch measurement method using Balmer lines. Our work shows that M_BH can be estimated from the Paschen line luminosities and velocity widths to an accuracy of 0.18-0.24 dex (rms scatter). We also show that the mean line ratios of the Paschen lines and the Balmer lines are Hα/Pα≅9.00, Hβ/Pα≅2.70, which are consistent with case B recombination under a typical AGN broad-line region (BLR) environment. These ratios can be used as reference points when estimating the amount of dust extinction over the BLR for red AGNs. We expect the future application of the new BH mass estimators on red, dusty AGNs to provide a fresh view of obscured AGNs.

[en] We obtained near-infrared spectra of 26 Sloan Digital Sky Survey quasars at $0.7<<!--\; <\; -->z<<!--\; <\; -->2.5$ with reported rest-frame ultraviolet M _BH ∼ 10¹⁰ M _⊙ to critically examine the systematic effects involved with their mass estimations. We find that active galactic nuclei (AGNs) heavier than 10¹⁰ M _⊙ often display double-peaked Hα emission, extremely broad Fe ii complex emission around Mg ii, and highly blueshifted and broadenedC iv emission. The weight of this evidence, combined with previous studies, cautions against the use of M _BH values based on any emission line with a width over 8000 km s⁻¹. Also, the M _BH estimations are not positively biased by the presence of ionized narrow line outflows, anisotropic radiation, or the use of line FWHM instead of σ for our sample, and unbiased with variability, scatter in broad line equivalent width, or obscuration for general type-1 quasars. Removing the systematically uncertain M _BH values, ∼10¹⁰ M _⊙ black holes (BHs) in 1 ≲ z ≲ 2 AGNs can still be explained by anisotropic motion of the broad line region from ∼10^9.5 M _⊙ BHs, although current observations support that they are intrinsically most massive, and overmassive to the host’s bulge mass.

[en] Galaxy mergers are expected to produce multiple supermassive black holes (SMBHs) in close-separation, but the detection of such SMBHs has been difficult. 2MASS J165939.7 + 183436 is a red active galactic nucleus (AGN) that is a prospective merging SMBH candidate owing to its merging features in Hubble Space Telescope imaging and double-peaked broad emission lines (BELs). Herein, we report a Gemini Multi-Object Spectrograph Integral Field Unit observation of a double-peaked broad Hα line of 2MASS J165939.7+183436. Furthermore, we confirm the existence of two BEL peaks that are kinematically separated by 3000 km s⁻¹, with the SMBH of each BEL component weighing at ${10}^{8.92\pm <!--\; ??\; -->0.06}{M}_{\odot <!--\; ???\; -->}$ and ${10}^{7.13\pm <!--\; ??\; -->0.06}{M}_{\odot <!--\; ???\; -->}$, if they arise from independent BELs near the two SMBHs. The BEL components were not separated at >0.″1; however, under several plausible assumptions regarding the fitting of each spaxel, the two components are found to be spatially separated at 0.″085 (∼250 pc). Different assumptions for the fitting can lead to a null (<0.″05) or a larger spatial separation (∼0.″15). Given the uncertainty regarding the spatial separation, various models, such as the disk emitter and multiple SMBH models, are viable solutions to explain the double BEL components. These results will promote future research for finding more multiple SMBH systems in red AGNs, and higher-resolution imaging validates these different models.

[en] Red quasars are thought to be an intermediate population between merger-driven star-forming galaxies in dust-enshrouded phase and normal quasars. If so, they are expected to have high accretion ratios, but their intrinsic dust extinction hampers reliable determination of Eddington ratios. Here, we compare the accretion rates of 16 red quasars at z ∼ 0.7 to those of normal type 1 quasars at the same redshift range. The red quasars are selected by their red colors in optical through near-infrared (NIR) and radio detection. The accretion rates of the red quasars are derived from the Pβ line in NIR spectra, which is obtained by the SpeX on the Infrared Telescope Facility in order to avoid the effects of dust extinction. We find that the measured Eddington ratios (L_b_o_l/L_E_d_d ≃ 0.69) of red quasars are significantly higher than those of normal type 1 quasars, which is consistent with a scenario in which red quasars are the intermediate population and the black holes of red quasars grow very rapidly during such a stage

[en] Activity at the centers of galaxies, during which the central supermassive black hole is accreting material, is nowadays accepted to be rather ubiquitous and most probably a phase of every galaxy's evolution. It has been suggested that galactic mergers and interactions may be the culprits behind the triggering of nuclear activity. We use near-infrared data from the new Infrared Medium-Deep Survey and the Deep eXtragalactic Survey of the VIMOS-SA22 field and radio data at 1.4 GHz from the FIRST survey and a deep Very Large Array survey to study the environments of radio active galactic nuclei (AGNs) over an area of ∼25 deg² and down to a radio flux limit of 0.1 mJy and a J-band magnitude of 23 mag AB. Radio AGNs are predominantly found in environments similar to those of control galaxies at similar redshift, J-band magnitude, and (M_u – M_r ) rest-frame color. However, a subpopulation of radio AGNs is found in environments up to 100 times denser than their control sources. We thus preclude merging as the dominant triggering mechanism of radio AGNs. By fitting the broadband spectral energy distribution of radio AGNs in the least and most dense environments, we find that those in the least dense environments show higher radio-loudness, higher star formation efficiencies, and higher accretion rates, typical of the so-called high-excitation radio AGNs. These differences tend to disappear at z > 1. We interpret our results in terms of a different triggering mechanism for these sources that is driven by mass loss through winds of young stars created during the observed ongoing star formation.

[en] We present 2.5-5.0 μm spectra of 83 nearby (0.002 < z < 0.48) and bright (K < 14 mag) type-1 active galactic nuclei (AGNs) taken with the Infrared Camera on board AKARI. The 2.5-5.0 μm spectral region contains emission lines such as Brβ (2.63 μm), Brα (4.05 μm), and polycyclic aromatic hydrocarbons (3.3 μm), which can be used for studying the black hole (BH) masses and star formation activity in the host galaxies of AGNs. The spectral region also suffers less dust extinction than in the ultra violet (UV) or optical wavelengths, which may provide an unobscured view of dusty AGNs. Our sample is selected from bright quasar surveys of Palomar-Green and SNUQSO, and AGNs with reverberation-mapped BH masses from Peterson et al. Using 11 AGNs with reliable detection of Brackett lines, we derive the Brackett-line-based BH mass estimators. We also find that the observed Brackett line ratios can be explained with the commonly adopted physical conditions of the broad line region. Moreover, we fit the hot and warm dust components of the dust torus by adding photometric data of SDSS, 2MASS, WISE, and ISO to the AKARI spectra, finding hot and warm dust temperatures of ∼1100 K and ∼220 K, respectively, rather than the commonly cited hot dust temperature of 1500 K

[en] The ΛCDM cosmological model successfully reproduces many aspects of the galaxy and structure formation of the universe. However, the growth of large-scale structures (LSSs) in the early universe is not well tested yet with observational data. Here, we have utilized wide and deep optical–near-infrared data in order to search for distant galaxy clusters and superclusters (0.8 < z < 1.2). From the spectroscopic observation with the Inamori Magellan Areal Camera and Spectrograph (IMACS) on the Magellan telescope, three massive clusters at z ∼ 0.91 are confirmed in the SSA22 field. Interestingly, all of them have similar redshifts within Δ z ∼ 0.01 with velocity dispersions ranging from 470 to 1300 km s"−"1. Moreover, as the maximum separation is ∼15 Mpc, they compose a supercluster at z ∼ 0.91, meaning that this is one of the most massive superclusters at this redshift to date. The galaxy density map implies that the confirmed clusters are embedded in a larger structure stretching over ∼100 Mpc. ΛCDM models predict about one supercluster like this in our surveyed volume, consistent with our finding so far. However, there are more supercluster candidates in this field, suggesting that additional studies are required to determine if the ΛCDM cosmological model can successfully reproduce the LSSs at high redshift

[en] We present the first results of our survey for high-redshift quasars at $5\lesssim <!--\; ???\; -->z\lesssim <!--\; ???\; -->5.7$. The search for quasars in this redshift range has been known to be challenging due to the limitations of the filter sets used in previous studies. We conducted a quasar survey for two specific redshift ranges, 4.60 ≤ z ≤ 5.40 and 5.50 ≤ z ≤ 6.05, using multi-wavelength data that include observations made with custom-designed filters, is and iz. Using these filters and a new selection technique, we were able to reduce the fraction of interlopers. Through optical spectroscopy, we confirmed six quasars at 4.7 ≤ z ≤ 5.4 with $-<!--\; ???\; -->27.4<<!--\; <\; -->M_{1450}<<!--\; <\; -->-<!--\; ???\; -->26.4$ that recently were discovered independently by another group. We estimated black hole masses and Eddington ratios of four of these quasars from optical and near-infrared spectra, and found that these quasars are undergoing nearly Eddington-limited accretion that is consistent with the rapid growth of supermassive black holes in luminous quasars at z ∼ 5.

[en] Recent studies suggest that faint active galactic nuclei may be responsible for the reionization of the universe. Confirmation of this scenario requires spectroscopic identification of faint quasars (M_1_4_5_0 > −24 mag) at z ≳ 6, but only a very small number of such quasars have been spectroscopically identified so far. Here, we report the discovery of a faint quasar IMS J220417.92+011144.8 at z ∼ 6 in a 12.5 deg"2 region of the SA22 field of the Infrared Medium-deep Survey (IMS). The spectrum of the quasar shows a sharp break at ∼8443 Å, with emission lines redshifted to z = 5.944 ± 0.002 and rest-frame ultraviolet continuum magnitude M_1_4_5_0 = −23.59 ± 0.10 AB mag. The discovery of IMS J220417.92+011144.8 is consistent with the expected number of quasars at z ∼ 6 estimated from quasar luminosity functions based on previous observations of spectroscopically identified low-luminosity quasars. This suggests that the number of M_1_4_5_0 ∼ −23 mag quasars at z ∼ 6 may not be high enough to fully account for the reionization of the universe. In addition, our study demonstrates that faint quasars in the early universe can be identified effectively with a moderately wide and deep near-infrared survey such as the IMS