Search

Using the photon-ion merged-beams technique at a synchrotron light source, we have measured relative cross sections for single and up to five-fold photoionization of Fe2+ ions in the energy range of 690-920 eV. This range contains thresholds and resonances associated with ionization and excitation of 2p and 2s electrons. Calculations were performed to simulate the total absorption spectra. The theoretical results show very good agreement with the experimental data, if overall energy shifts of up to 2.5 eV are applied to the calculated resonance positions and assumptions are made about the initial experimental population of the various levels of the Fe2+([Ar]3d6) ground configuration. Furthermore, we performed extensive calculations of the Auger cascades that result when an electron is removed from the 2p subshell of Fe2+. These computations lead to a better agreement with the measured product-charge-state distributions as compared to earlier work. We conclude that the L-shell absorption features of low-charged iron ions are useful for identifying gas-phase iron in the interstellar medium and for discriminating against the various forms of condensed-phase iron bound to composite interstellar dust grains.

Line intensity mapping (LIM) serves as a potent probe in astrophysics, relying on the statistical analysis of integrated spectral line emissions originating from distant star-forming galaxies. While LIM observations hold the promise of achieving a broad spectrum of scientific objectives, a significant hurdle for future experiments lies in distinguishing the targeted spectral line emitted at a specific redshift from undesired line emissions originating at different redshifts. The presence of these interloping lines poses a challenge to the accuracy of cosmological analyses. In this study, we introduce a novel approach to quantify line–line cross-correlations (LIM-LLX), enabling us to investigate the target signal amid instrumental noise and interloping emissions. For example, at a redshift of z ∼ 3.7, we observed that the measured auto-power spectrum of C ii 158 exhibited substantial bias, from interloping line emission. However, cross-correlating C ii 158 with CO(6–5) lines using an FYST-like experiment yielded a promising result, with a signal-to-noise ratio of ∼10. This measurement is notably unbiased. Additionally, we explore the extensive capabilities of cross-correlation by leveraging various CO transitions to probe the tomographic Universe at lower redshifts through LIM-LLX. We further demonstrate that incorporating low-frequency channels, such as 90 and 150 GHz, into FYST’s EoR-Spec-like experiment can maximize the potential for cross-correlation studies, effectively reducing the bias introduced by instrumental noise and interlopers.

Weak-line quasars (WLQs) are a notable group of active galactic nuclei (AGNs) that show unusually weak UV lines even though their optical-UV continuum shapes are similar to those of typical quasars. The physical mechanism for WLQs is an unsolved puzzle in the AGN unified model. We explore the properties of UV emission lines by performing extensive photoionization calculations based on Cloudy simulations with different spectral energy distributions (SEDs) of AGNs. The AGN continua are built from several observational empirical correlations, where the blackbody emission from the cold disk, the power-law emission from the hot corona, and a soft X-ray excess component are considered. We find that the equivalent width (EW) of C iv from our models is systematically lower than observational values if the component of soft X-ray excess is neglected. The EW will increase several times and is roughly consistent with the observations after considering the soft X-ray excess component as constrained from normal type I AGNs. We find that the UV lines are weak for QSOs with quite large black hole mass (e.g., M _BH > 10 ^9 M _⊙ ) and weak soft X-ray emission due to the deficit of ionizing photons. As an example, we present the strength of C iv based on the multiband SEDs for three nearby weak-line AGNs, where the weaker soft X-ray emission normally predicts the weaker lines.

High-resolution spectra observed by the Solar X-ray spectrometer on board the Hinotori mission are revisited. Flat crystals slightly offset to the satellite spin axis produce automatic spectral scans for emission lines emerging from highly charged iron ions in solar flares every half-spin time period. All the downlinked data of the mission are converted to FITS format and major flare spectral data are revived as IDL save files in ISAS/DARTS. Based on these data sets, single-temperature fits are performed for the emission line complex of highly charged iron ions in the wavelength range of 1.75–1.95 Å and compared with theoretical predictions. Synthetic spectra with single electron temperatures estimated from j / w line-intensity ratios fit fairly well for Fe xxiv and Fe xxiii lines in the wavelength range of 1.85–1.88 Å, while intensity ratios of Fe xxv lines ( x , y , z ) and the inner-shell excitation line of Fe xxiv ( q ) to the Fe xxv resonance line ( w ) have systematic excesses. Empirical relations for the observed line ratios are derived. Ion fractions of Fe ^+25 /Fe ^+24 estimated by intensity ratios of Ly α /w in the temperature range of log T _e =7.25–7.45 are consistent with values in ionization equilibrium, and the remaining excesses of the Fe xxv line ratios may suggest problems with the atomic parameters or atomic modeling.

The initial density distribution provides a basis for understanding the complete evolution of cosmological density fluctuations. While reconstruction in our local Universe exploits the observations of galaxy surveys with large volumes, observations of high-redshift galaxies are performed with a small field of view and therefore can hardly be used for reconstruction. Here, we propose reconstructing the initial density field using the H i 21 cm and CO line intensity maps from the epoch of reionization. Observations of these two intensity maps provide complementary information on the density field—the H i 21 cm field is a proxy of matter distributions in the neutral regions, while the CO line intensity maps are sensitive to the high-density, star-forming regions that host the sources for reionization. Technically, we employ the conjugate gradient method and develop the machinery for minimizing the cost function for the intensity mapping observations. Analytical expressions for the gradient of cost function are derived explicitly. We show that the resimulated intensity maps match the input maps of mock observations using semi-numerical simulations of reionization with an rms error ≲7% at all stages of reionization. This reconstruction is also robust with an rms error of ∼10% when an optimistic level of shot noise is applied to the CO map or white noise at the level of ≲10% of the standard deviation is applied to each map. Our proof-of-concept work demonstrates the robustness of the reconstruction method, thereby providing an effective technique for reconstructing the cosmological initial density distribution from high-redshift observations.

Line intensity mapping (LIM) can provide a powerful means to constrain the theory of gravity and the nature of dark energy at low and high redshifts by mapping the large-scale structure over many redshift epochs. In this paper, we investigate the potential of the next generation ground-based millimeter-wavelength LIM surveys in constraining several models beyond ΛCDM, involving either a dynamic dark energy component or modifications of the theory of gravity. Limiting ourselves to two-point clustering statistics, we consider the measurements of auto-spectra of several CO rotational lines (from J = 2−1 to J = 6−5) and the [C ii ] fine structure line in the redshift range of 0.25 < z < 12. We consider different models beyond ΛCDM, each one with different signatures and peculiarities. Among them, we focus on Jordan–Brans–Dicke and axion-driven early dark energy models as examples of well-studied scalar-tensor theories acting at late and early times, respectively. Additionally, we consider three phenomenological models based on an effective description of gravity at cosmological scales. We show that LIM surveys deployable within a decade (with ∼10 ^8 spectrometer hours) have the potential to improve upon the current bounds on all considered models significantly. The level of improvements range from a factor of a few to an order of magnitude.

Mapping of multiple lines such as the fine-structure emission from [C ii ] (157.7 μ m), [O iii ] (52 and 88.4 μ m), and rotational emission lines from CO are of particular interest for upcoming line intensity mapping (LIM) experiments at millimeter wavelengths, due to their brightness features. Several upcoming experiments aim to cover a broad range of scientific goals, from detecting signatures of the epoch of reionization to the physics of star formation and its role in galaxy evolution. In this paper, we develop a semianalytic approach to modeling line strengths as functions of the star formation rate (SFR) or infrared luminosity based on observations of local and high- z galaxies. This package, LIMpy (Line Intensity Mapping in Python), estimates the intensity and power spectra of [C ii ], [O iii ], and CO rotational transition lines up to the J levels (1–0) to (13–12) based both on analytic formalism and on simulations. We develop a relation among halo mass, SFR, and multiline intensities that permits us to construct a generic formula for the evolution of several line strengths up to z ∼ 10. We implement a variety of star formation models and multiline luminosity relations to estimate the astrophysical uncertainties on the intensity power spectrum of these lines. As a demonstration, we predict the signal-to-noise ratio of [C ii ] detection for an EoR-Spec-like instrument on the Fred Young Submillimeter Telescope. Furthermore, the ability to use any halo catalog allows the LIMpy code to be easily integrated into existing simulation pipelines, providing a flexible tool to study intensity mapping in the context of complex galaxy formation physics.

Deuterium fractionation in molecules is known as one of the most powerful tools to study chemical processes during star and planet formation. Among various interstellar molecules, methanol often shows very high deuterium fractionation. It is the most abundant saturated organic molecule and is known as a parent species to form more complex organic molecules. However, deriving the abundance of deuterated methanol suffers from the uncertainty in the intrinsic line intensities ( S μ ^2 ) of CH _3 OH isotopologues. Due to their floppy nature, theoretical evaluation of the S μ ^2 values is not straightforward, particularly for asymmetric-top asymmetric-frame isotopologues such as CH _2 DOH. In this study, we have measured the line frequencies and their intensities for CH _2 DOH in the millimeter-wave region from 216 to 264 GHz by using an emission-type millimeter and submillimeter-wave spectrometer. For the a -type J = 5 − 4 transition, the derived S μ ^2 values are 13%–27% larger than those theoretically calculated, except for the transitions of K _a = 2 for e _0 and K _a = 1 for e _1 affected by avoided level crossing. For b -type transitions, significant systematic differences are found between theoretical and experimental S μ ^2 values. The results of the present study enable us to accurately derive from observations the CH _2 DOH abundances, which are essential for understanding deuterium fractionation in various sources.

We have determined absorption oscillator strengths for rotational transitions of the C ^2 Π(0)- A ^2 Σ ^+ (0) band of NO, which has been detected in the upper atmosphere of Venus. The molecular quantum defect orbital method has been used in the calculation of the electronic transition moment and the Rydberg–valence interaction between the C ^2 Π( v = 0) and the B ^2 Π ( v = 7) states has been considered. Considerable deviations from normal intensity distribution of the rotational lines have been found at J ≤ 5.5. As far as we know, the present line f -values are reported here for the first time. These data may be useful in the interpretation of the observations obtained by the Visible and Infrared Thermal Imaging Spectrometer instrument on the Venus Express spacecraft.

LaO bands are found in the spectra of cool S-type stars. The bands of the A ^2 Π–X ^2 Σ ^+ transition with $v^{\prime} \leqslant 3$ and v ″ ≤ 4 are rotationally analyzed, providing spectroscopic constants for the A ^2 Π state. Line strengths are calculated using an ab initio transition dipole moment function, and radiative lifetimes for the A ^2 Π state have also been computed. A line list for the A ^2 Π–X ^2 Σ ^+ transition of LaO is provided and can be used to determine LaO stellar abundances.

Line-intensity mapping (LIM) is a promising technique to constrain the global distribution of galaxy properties. To combine LIM experiments probing different tracers with traditional galaxy surveys and fully exploit the scientific potential of these observations, it is necessary to have a physically motivated modeling framework. As part of developing such a framework, in this work, we introduce and model the conditional galaxy property distribution (CGPD), i.e., the distribution of galaxy properties conditioned on the host halo mass and redshift. We consider five galaxy properties, including the galaxy stellar mass, molecular gas mass, galaxy radius, gas-phase metallicity, and star formation rate (SFR), which are important for predicting the emission lines of interest. The CGPD represents the full distribution of galaxies in the five-dimensional property space; many important galaxy distribution functions and scaling relations, such as the stellar mass function and SFR main sequence, can be derived from integrating and projecting it. We utilize two different kinds of cosmological galaxy simulations, a semi-analytic model and the IllustrisTNG hydrodynamic simulation, to characterize the CGPD and explore how well it can be represented using a Gaussian mixture model (GMM). We find that with just a few (approximately three) Gaussian components, a GMM can describe the CGPD of the simulated galaxies to high accuracy for both simulations. The CGPD can be mapped to LIM or other observables by constructing the appropriate relationship between galaxy properties and the relevant observable tracers, which will be discussed in future works.

The hydrogen Balmer decrement (e.g., H α /H β ) is widely adopted as an indicator of the internal reddening of active galactic nuclei (AGNs). This is challenged by some low-luminosity AGNs and changing-look AGNs (CLAGNs), which have steep Balmer decrement but without strong evidence for absorption. We compile a sample of normal AGNs and CLAGNs with a wider distribution of bolometric Eddington ratio ( λ _Edd = L _bol / L _Edd ) and find a strong negative correlation between H α /H β and λ _Edd , which suggests that the Balmer decrement is also accretion-rate dependent. We further explore the Balmer decrement based on the photoionization model using the Cloudy code by considering spectral energy distribution from the accretion disk with different accretion rates (e.g., disk/corona and truncated disk at high and low Eddington ratios, respectively). Both the standard disk and truncated disk predict a negative correlation of H α /H β – λ _Edd , where the relation is steeper in the case of the truncated disk. The negative correlations are also explored in two single CLAGNs. The measured negative correlation of H α /H β – λ _Edd is mainly caused by the lower responsivity $(d\mathrm{log}{L}_{\mathrm{line}}/d\mathrm{log}{L}_{\mathrm{cont}})$ in H α relative to that in H β due to the larger optical depth in the former. We propose that the steep Balmer decrements in low Eddington ratio AGNs (e.g., some Seyferts 1.5–1.9 and CLAGNs) are not simply caused by absorption but mainly caused by the relatively low flux of ionizing photons.

Methanol (CH _3 OH) is an abundant interstellar species and is known to be an important precursor of various interstellar complex organic molecules. Among the methanol isotopologues, CH _2 DOH is one of the most abundant isotopologues and it is often used to study the deuterium fractionation of CH _3 OH in interstellar medium. However, the emission lines of CH _2 DOH can sometimes be optically thick, making the derivation of its abundance unreliable. Therefore, observations of its presumably optically thin ^13 C substituted species, ^13 CH _2 DOH, are essential to overcome this issue. In this study, the rotational transitions of ^13 CH _2 DOH have been measured in the millimeter-wave region from 216 GHz to 264 GHz with an emission-type millimeter- and submillimeter-wave spectrometer by using a deuterium and ^13 C enriched sample. The frequency accuracy of measured ^13 CH _2 DOH is less than a few kHz, and the relative line intensity error is less than 10% in most of the frequency range by taking advantage of the wide simultaneous frequency-coverage of the emission-type spectrometer. These results offer a good opportunity to detect ^13 CH _2 DOH in space, which will allow us to study the deuterium fractionation of CH _3 OH in various sources through accurate determination of the CH _2 DOH abundance.