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<jats:title>Abstract</jats:title> <jats:p>We investigate the fine-structure [C <jats:sc>ii</jats:sc>] line at 158 <jats:italic>μ</jats:italic>m as a molecular gas tracer by analyzing the relationship between molecular gas mass (<jats:italic>M</jats:italic> <jats:sub>mol</jats:sub>) and [C <jats:sc>ii</jats:sc>] line luminosity (<jats:italic>L</jats:italic> <jats:sub>[C <jats:sc>II</jats:sc>]</jats:sub>) in 11,125 <jats:italic>z</jats:italic> ≃ 6 star-forming, main-sequence galaxies from the <jats:sc>simba</jats:sc> simulations, with line emission modeled by the Simulator of Galaxy Millimeter/Submillimeter Emission. Though most (∼50%–100%) of the gas mass in our simulations is ionized, the bulk (&gt;50%) of the [C <jats:sc>ii</jats:sc>] emission comes from the molecular phase. We find a sublinear (slope 0.78 ± 0.01) <jats:inline-formula> <jats:tex-math> <?CDATA $\mathrm{log}{L}_{[{\rm{C}}\,{\rm\small{II}}]}\mbox{--}\mathrm{log}{M}_{\mathrm{mol}}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>log</mml:mi> <mml:msub> <mml:mrow> <mml:mi>L</mml:mi> </mml:mrow> <mml:mrow> <mml:mo stretchy="false">[</mml:mo> <mml:mi mathvariant="normal">C</mml:mi> <mml:mspace width="0.25em" /> <mml:mi mathsize="small" mathvariant="normal">II</mml:mi> <mml:mo stretchy="false">]</mml:mo> </mml:mrow> </mml:msub> <mml:mo>–</mml:mo> <mml:mi>log</mml:mi> <mml:msub> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>mol</mml:mi> </mml:mrow> </mml:msub> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac5cbaieqn1.gif" xlink:type="simple" /> </jats:inline-formula> relation, in contrast with the linear relation derived from observational samples of more massive, metal-rich galaxies at <jats:italic>z</jats:italic> ≲ 6. We derive a median [C <jats:sc>ii</jats:sc>]-to-<jats:italic>M</jats:italic> <jats:sub>mol</jats:sub> conversion factor of <jats:italic>α</jats:italic> <jats:sub>[C <jats:sc>II</jats:sc>]</jats:sub> ≃ 18 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>/<jats:italic>L</jats:italic> <jats:sub>⊙</jats:sub>. This is lower than the average value of ≃30 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>/<jats:italic>L</jats:italic> <jats:sub>⊙</jats:sub> derived from observations, which we attribute to lower gas-phase metallicities in our simulations. Thus, a lower, luminosity-dependent conversion factor must be applied when inferring molecular gas masses from [C <jats:sc>ii</jats:sc>] observations of low-mass galaxies. For our simulations, [C <jats:sc>ii</jats:sc>] is a better tracer of the molecular gas than CO <jats:italic>J</jats:italic> = 1–0, especially at the lowest metallicities, where much of the gas is <jats:italic>CO-dark</jats:italic>. We find that <jats:italic>L</jats:italic> <jats:sub>[C <jats:sc>II</jats:sc>]</jats:sub> is more tightly correlated with <jats:italic>M</jats:italic> <jats:sub>mol</jats:sub> than with star formation rate (SFR), and both the <jats:inline-formula> <jats:tex-math> <?CDATA $\mathrm{log}{L}_{[{\rm{C}}\,{\rm\small{II}}]}\mbox{--}\mathrm{log}{M}_{\mathrm{mol}}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>log</mml:mi> <mml:msub> <mml:mrow> <mml:mi>L</mml:mi> </mml:mrow> <mml:mrow> <mml:mo stretchy="false">[</mml:mo> <mml:mi mathvariant="normal">C</mml:mi> <mml:mspace width="0.25em" /> <mml:mi mathsize="small" mathvariant="normal">II</mml:mi> <mml:mo stretchy="false">]</mml:mo> </mml:mrow> </mml:msub> <mml:mo>–</mml:mo> <mml:mi>log</mml:mi> <mml:msub> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>mol</mml:mi> </mml:mrow> </mml:msub> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac5cbaieqn2.gif" xlink:type="simple" /> </jats:inline-formula> and <jats:inline-formula> <jats:tex-math> <?CDATA $\mathrm{log}{L}_{[{\rm{C}}\,{\rm\small{II}}]}\mbox{--}\mathrm{log}\,\mathrm{SFR}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>log</mml:mi> <mml:msub> <mml:mrow> <mml:mi>L</mml:mi> </mml:mrow> <mml:mrow> <mml:mo stretchy="false">[</mml:mo> <mml:mi mathvariant="normal">C</mml:mi> <mml:mspace width="0.25em" /> <mml:mi mathsize="small" mathvariant="normal">II</mml:mi> <mml:mo stretchy="false">]</mml:mo> </mml:mrow> </mml:msub> <mml:mo>–</mml:mo> <mml:mi>log</mml:mi> <mml:mspace width="0.25em" /> <mml:mi>SFR</mml:mi> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac5cbaieqn3.gif" xlink:type="simple" /> </jats:inline-formula> relations arise from the Kennicutt–Schmidt relation. Our findings suggest that <jats:italic>L</jats:italic> <jats:sub>[C <jats:sc>II</jats:sc>]</jats:sub> is a promising tracer of the molecular gas at the earliest cosmic epochs.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We perform 2.5-dimensional general relativistic radiation magnetohydrodynamics simulations of black hole accretion disks and disk winds in the range of mass accretion rate from 0.1 to 10<jats:sup>4.5</jats:sup> times the Eddington limit. In this paper, we compare the results of the INAZUMA code, in which the frequency-integrated time-dependent radiation transfer equation is solved in order to evaluate the Eddington tensor, with those of the first momentum (M1) approximation method. In both methods, accretion disks and disk winds appear, and there is no remarkable difference in accretion rate, outflow rate, or luminosity. However, the significant difference in the radiation field appears around the rotation axis. In the M1 method, the radial component of the radiation flux tends to be amplified owing to unphysical radiation collisions. Such an enhancement of the outward radiation flux does not appear in INAZUMA. Also, the problem of radiation not reaching the rotation axis occurs with M1, but not with INAZUMA. Our results indicate that the radiation transfer equation should be solved to obtain the accurate radiation field in the optically thin region around the rotation axis.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Over the past decade, rest-frame color–color diagrams have become popular tools for selecting quiescent galaxies at high redshift, breaking the color degeneracy between quiescent and dust-reddened star-forming galaxies. In this work, we study one such color–color selection tool—the rest-frame <jats:italic>U</jats:italic> − <jats:italic>V</jats:italic> versus <jats:italic>V</jats:italic> − <jats:italic>J</jats:italic> diagram—by employing mock observations of cosmological galaxy formation simulations. In particular, we conduct numerical experiments assessing both trends in galaxy properties in <jats:italic>UVJ</jats:italic> space and the color–color evolution of massive galaxies as they quench at redshifts <jats:italic>z</jats:italic> ∼ 1–2. We find that our models broadly reproduce the observed <jats:italic>UVJ</jats:italic> diagram at <jats:italic>z</jats:italic> = 1–2, including (for the first time in a cosmological simulation) reproducing the population of extremely dust-reddened galaxies in the top right of the <jats:italic>UVJ</jats:italic> diagram. However, our models primarily populate this region with low-mass galaxies and do not produce as clear a bimodality between star-forming and quiescent galaxies as is seen in observations. The former issue is due to an excess of dust in low-mass galaxies and relatively gray attenuation curves in high-mass galaxies, while the latter is due to the overpopulation of the green valley in <jats:sc>simba</jats:sc>. When investigating the time evolution of galaxies on the <jats:italic>UVJ</jats:italic> diagram, we find that the quenching pathway on the <jats:italic>UVJ</jats:italic> diagram is independent of the quenching timescale, and instead dependent primarily on the average specific star formation rate in the 1 Gyr prior to the onset of quenching. Our results support the interpretation of different quenching pathways as corresponding to the divergent evolution of post-starburst and green valley galaxies.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Simulations indicate that the inflow of gas of star-forming galaxies is almost coplanar and corotating with the gas disk, and that the outflow of gas driven by stellar winds and/or supernova explosions is preferentially perpendicular to the disk. This indicates that the galactic gas disk can be treated as a <jats:italic>modified</jats:italic> accretion disk. In this work, we focus on the metal enhancement in galactic disks in this scenario of gas accretion. Assuming that the star formation rate surface density (Σ<jats:sub>SFR</jats:sub>) is of exponential form, we obtain the analytic solution of gas-phase metallicity with only three free parameters: the scale length of Σ<jats:sub>SFR</jats:sub> (<jats:italic>h</jats:italic> <jats:sub>R</jats:sub>), the metallicity of the inflowing gas, and the mass-loading factor defined as the wind-driven outflow rate surface density per Σ<jats:sub>SFR</jats:sub>. According to this simple model, the negative gradient of gas-phase metallicity is a natural consequence of the radial inflow of cold gas that is continuously enriched by in situ star formation as it moves toward the disk center. We fit the model to the observed metallicity profiles for six nearby galaxies chosen to have well-measured metallicity profiles extending to very large radii. Our model can well characterize the overall features of the observed metallicity profiles. The observed profiles usually show a floor at the outer regions of the disk, corresponding to the metallicity of inflow gas. Furthermore, we find the <jats:italic>h</jats:italic> <jats:sub>R</jats:sub> of Σ<jats:sub>SFR</jats:sub> inferred from these fits agree well with independent estimates from Σ<jats:sub>SFR</jats:sub> profiles, supporting the basic model.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Type I X-ray bursts (XRBs) are powered by thermonuclear burning on proton-rich unstable nuclides. The construction of burst models with accurate knowledge of nuclear physics is required to properly interpret burst observations. Numerous studies that have investigated the sensitivities of burst models to nuclear inputs have commonly extracted the strength of the NiCu cycle in the <jats:italic>rp</jats:italic> process, determined by the <jats:sup>59</jats:sup>Cu(<jats:italic>p</jats:italic>,<jats:italic>α</jats:italic>)<jats:sup>56</jats:sup>Ni and <jats:sup>59</jats:sup>Cu(<jats:italic>p</jats:italic>,<jats:italic>γ</jats:italic>)<jats:sup>60</jats:sup>Zn thermonuclear reaction rates, as critical in the determination of reaction flow in the burst. In this study, the strength of the cycle at the XRB temperature range was estimated based on published experimental data. The nuclear properties of the compound nucleus <jats:sup>60</jats:sup>Zn were evaluated for the <jats:sup>59</jats:sup>Cu(<jats:italic>p</jats:italic>,<jats:italic>α</jats:italic>)<jats:sup>56</jats:sup>Ni and <jats:sup>59</jats:sup>Cu(<jats:italic>p</jats:italic>,<jats:italic>γ</jats:italic>)<jats:sup>60</jats:sup>Zn reaction rate calculations. Monte Carlo rate calculations were conducted to include the large uncertainties of nuclear properties in the calculations. In the current work, a weak NiCu cycle is expected, whereas the rates adopted by the previous studies suggest a strong NiCu cycle. Model simulations were performed with the new rates to assess the impact on Type I XRBs. The results show that the estimated cycle strength does not strongly influence the model predictions of the burst light curve or synthesized abundances.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We analyze the slow periodicities identified in burst sequences from FRB 121102 and FRB 180916 with periods of about 16 and 160 days, respectively, while also addressing the absence of any fast periodicity that might be associated with the spin of an underlying compact object. Both phenomena can be accounted for by a young, highly magnetized, precessing neutron star that emits beamed radiation with significant imposed phase jitter. Sporadic narrow-beam emission into an overall wide solid angle can account for the necessary phase jitter, but the slow periodicities with 25%–55% duty cycles constrain beam traversals to be significantly smaller. Instead, phase jitter may result from variable emission altitudes that yield large retardation and aberration delays. A detailed arrival time analysis for triaxial precession includes wobble of the radio beam and the likely larger, cyclical torque resulting from the changes in the spin–magnetic moment angle. These effects will confound identification of the fast periodicity in sparse data sets longer than about a quarter of a precession cycle unless fitted for and removed as with orbital fitting. Stochastic spin noise, likely to be much larger than in radio pulsars, may hinder detection of any fast periodicity in data spans longer than a few days. These decoherence effects will dissipate as sources of fast radio bursts age, so they may evolve into objects with properties similar to Galactic magnetars.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>This paper reports the first possible evidence for the development of the Kelvin–Helmholtz (KH) instability at the border of coronal holes separating the associated fast wind from the slower wind originating from adjacent streamer regions. Based on a statistical data set of spectroscopic measurements of the UV corona acquired with the UltraViolet Coronagraph Spectrometer on board the SOlar and Heliospheric Observatory during the minimum activity of solar cycle 22, high temperature–velocity correlations are found along the fast/slow solar wind interface region and interpreted as manifestations of KH vortices formed by the roll-up of the shear flow, whose dissipation could lead to higher heating and, because of that, higher velocities. These observational results are supported by solving coupled solar wind and turbulence transport equations including a KH-driven source of turbulence along the tangential velocity discontinuity between faster and slower coronal flows: numerical analysis indicates that the correlation between the solar wind speed and temperature is large in the presence of the shear source of turbulence. These findings suggest that the KH instability may play an important role both in the plasma dynamics and in the energy deposition at the boundaries of coronal holes and equatorial streamers.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present a case study for the global extreme-ultraviolet (EUV) wave and its chromospheric counterpart the <jats:italic>Moreton-Ramsey Wave</jats:italic> associated with the second X-class flare in Solar Cycle 25 and a halo coronal mass ejection (CME). The EUV wave was observed in the H<jats:italic>α</jats:italic> and EUV passbands with different characteristic temperatures. In the 171 Å and 193/195 Å images, the wave propagates circularly with an initial velocity of 600–720 km s<jats:sup>−1</jats:sup> and a deceleration of 110–320 m s<jats:sup>−2</jats:sup>. The local coronal plasma is heated from log(<jats:italic>T/K</jats:italic>) ≈ 5.9 to log(<jats:italic>T/K</jats:italic>) ≈ 6.2 during the passage of the wave front. The H<jats:italic>α</jats:italic> and 304 Å images also reveal signatures of wave propagation with a velocity of 310–540 km s<jats:sup>−1</jats:sup>. With multiwavelength and dual-perspective observations, we found that the wave front likely propagates forwardly inclined to the solar surface with a tilt angle of ∼53°.2. Our results suggest that this EUV wave is a fast-mode magnetohydrodynamic wave or shock driven by the expansion of the associated CME, whose wave front is likely a dome-shaped structure that could impact the upper chromosphere, transition region, and corona.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Based on observations from the Interface Region Imaging Spectrograph and Hinode, we analyze the thermodynamic evolution of the supra-arcade fan (SAF) in the 2017 September 10 flare. The SAF presents discontinuous characters during the rising process, indicating a nonuniform process of magnetic reconnection in the solar eruption. The intensity peaks of the high-temperature spectral lines (Fe <jats:sc>xxi</jats:sc> 1354.08 Å, Fe <jats:sc>xxiii</jats:sc> 263.76 Å, and Fe <jats:sc>xxiv</jats:sc> 255.10 Å) basically correspond to the valley of the Doppler velocity and Doppler width. The temperature and density increase spatially at the upper boundary of the SAF. These results indicate that a compressed interface may exist in the SAF, where the plasma environment shows remarkable changes in density, temperature, and turbulence. In view of the fact that the height of the SAF is close to the hard X-ray source, we conclude that the interface could be related to termination shocks (TSs), taking into account the synthetic spectral profiles obtained from numerical experiments. In turn, the variations of the spectral profiles might be useful tools for identifying TSs from EUV spectral observations.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We take advantage of a state-of-art semianalytic model of galaxy formation, and the model presented by Contini &amp; Gu, to investigate the mass distribution of brightest cluster galaxies (BCGs) and intracluster light (ICL) by addressing two points: (1) the region of transition between a BCG-dominated distribution and an ICL-dominated one, and (2) the relation between the total BCG+ICL mass and the ICL mass alone. We find the transition radius to be independent of both BCG+ICL and halo masses, with an average of 60 ± 40 kpc, in good agreement with previous observational measurements, but given the large scatter it can be considered as a sort of physical separation between the two components only on a cluster scale. From the analysis of the <jats:italic>M</jats:italic> <jats:sub>ICL</jats:sub>–<jats:italic>M</jats:italic> <jats:sub>BCG+ICL</jats:sub> relation, we construct a method able to extract the ICL mass directly from knowledge of the BCG+ICL mass. Given the large scatter in low-mass systems, such a method under/overpredicts the true value of the ICL in a significant way, up to a factor of three in the worst cases. On the other hand, for <jats:inline-formula> <jats:tex-math> <?CDATA $\mathrm{log}{M}_{\mathrm{BCG}+\mathrm{ICL}}\gt 12$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>log</mml:mi> <mml:msub> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>BCG</mml:mi> <mml:mo>+</mml:mo> <mml:mi>ICL</mml:mi> </mml:mrow> </mml:msub> <mml:mo>&gt;</mml:mo> <mml:mn>12</mml:mn> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac57c4ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> or <jats:inline-formula> <jats:tex-math> <?CDATA $\mathrm{log}{M}_{\mathrm{Halo}}\gt 14$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>log</mml:mi> <mml:msub> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>Halo</mml:mi> </mml:mrow> </mml:msub> <mml:mo>&gt;</mml:mo> <mml:mn>14</mml:mn> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac57c4ieqn2.gif" xlink:type="simple" /> </jats:inline-formula> (where masses are in units of <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>), the difference between the true value and the one extracted from the <jats:italic>M</jats:italic> <jats:sub>ICL</jats:sub>–<jats:italic>M</jats:italic> <jats:sub>BCG+ICL</jats:sub> relation ranges between ±30%. We therefore suggest this relation as a reliable test for observational works aiming to isolate the ICL from the BCG, for systems hosted by haloes on a cluster scale.</jats:p>