arXiv subjects

Kiyotomo Ichiki

Publications and source records attributed to Kiyotomo Ichiki.

At least 37 records · Page 2Linked to original sources

arXiv2023

Boosting the 21 cm forest signals by the clumpy substructures

We study the contribution of subhalos to the 21 cm forest signal. The halos can host the substructures and including the effects of those small scale clumps can potentially boost the 21 cm optical depth in favor of detecting the 21 cm forest signals. We estimate the boost factor representing the ratio of the optical depth due to the subhalo contribution and that due to the host halo alone (without subhalos). Even though the optical depth boost factor is negligible for a small host halo with the mass of order $10^5 M_{\odot}$, the subhalo contribution can enhance the optical depth by an order of magnitude for a host halo of order $10^7 M_{\odot}$. The resultant 21 cm absorption line abundance which is obtained by integrating over the halo mass range relevant for the 21 cm forest signal can be enhanced by up to of order $10\%$ due to the substructures. The larger boost factor for a larger host halo would be of particular interest for the 21 cm forest detection because the the contribution of the larger host halos to the 21 cm forest signals is smaller due to their higher temperature and less abundance than the smaller host halos. The subhalos hence can well help the larger host halos more important for the signal estimation which, without considering the subhalos, may not give appreciable contribution to 21 cm forest signals.

astro-ph.CO↗

arXiv2023

Validating dark energy models using polarised Sunyaev-Zel'dovich effect with large-angle CMB temperature and E-mode polarization anisotropies

The tomography of the polarized Sunyaev-Zeldvich effect due to free electrons of galaxy clusters can be used to constrain the nature of dark energy because CMB quadrupoles at different redshifts as the polarization source are sensitive to the integrated Sachs-Wolfe effect. Here we show that the low multipoles of the temperature and E-mode polarization anisotropies from the all-sky CMB can improve the constraint further through the correlation between them and the CMB quadrupoles viewed from the galaxy clusters. Using a Monte-Carlo simulation, we find that low multipoles of the temperature and E-mode polarization anisotropies potentially improve the constraint on the equation of state of dark energy parameter by $\sim 17$ percent.

astro-ph.CO↗

arXiv2022

Measuring the cosmological density field twice: A novel test of dark energy using the CMB quadrupole

The scattering of cosmic microwave background (CMB) radiation in galaxy clusters induces polarization signals according to the quadrupole anisotropy in the photon distribution at the cluster location. This `remote quadrupole' derived from the measurements of the induced polarization provides an opportunity for reconstructing primordial fluctuations on large scales. We discuss that comparing the local CMB quadrupoles predicted by these reconstructed primordial fluctuations and the direct measurements done by CMB satellites may enable us to test the dark energy beyond cosmic variance limits.

astro-ph.CO↗

arXiv2022

Probing the Early History of Cosmic Reionization by Future Cosmic Microwave Background Experiments

Cosmic Reionization imprints its signature on the temperature and polarization anisotropies of the cosmic microwave background (CMB). Advances in CMB telescopes have already placed a significant constraint on the history of reionization. As near-future CMB telescopes target the maximum sensitivity, or observations limited only by the cosmic variance (CV), we hereby forecast the potential of future CMB observations in constraining the history of reionization. In this study, we perform Markov Chain Monte Carlo analysis for CV-limited E-mode polarization observations such as the LiteBIRD (Light satellite for the studies of B-mode polarization and Inflation from cosmic background Radiation Detection), based on a few different methods that vary in the way of sampling reionization histories. We focus especially on estimating the very early history of reionization that occurs at redshifts $z>15$, which is quantified by the partial CMB optical depth due to free electrons at $z>15$, $τ_{z>15}$. We find that reionization with $τ_{z>15} \sim 0.008$, which are well below the current upper limit $τ_{z>15} \sim 0.02$, are achievable by reionization models with minihalo domination in the early phase and can be distinguished from those with $τ_{z>15} \lesssim 5\times 10^{-4}$ through CV-limited CMB polarization observations. An accurate estimation of $τ_{z>15}$, however, remains somewhat elusive. We investigate whether resampling the E-mode polarization data with limited spherical-harmonic modes may resolve this shortcoming.

astro-ph.CO↗

arXiv2021

Densified pupil spectrograph as high-precision radial velocimetry: From direct measurement of the Universe's expansion history to characterization of nearby habitable planet candidates

The direct measurement of the Universe's expansion history and the search for terrestrial planets in habitable zones around solar-type stars require extremely high-precision radial velocity measures over a decade. This study proposes an approach for enabling high-precision radial velocity measurements from space. The concept presents a combination of a high-dispersion densified pupil spectrograph and a novel telescope line-of-sight monitor. The precision of the radial velocity measurements is determined by combining the spectrophotometric accuracy and the quality of the absorption lines in the recorded spectrum. Therefore, a highly dispersive densified pupil spectrograph proposed to perform stable spectroscopy can be utilized for high-precision radial velocity measures. A concept involving the telescope line-of-sight monitor is developed to minimize the change of the telescope line-of-sight over a decade. This monitor allows the precise measurement of a long-term telescope drift without any significant impact on the Airy disk when the densified pupil spectra are recorded. We analytically derive the uncertainty of the radial velocity measurements, which is caused by the residual offset of the line-of-sights at two epochs. We find that the error could be reduced down to approximately 1 $cm/s$, and the precision will be limited by another factor (e.g., wavelength calibration uncertainty). A combination of the high precision spectrophotometry and the high spectral resolving power could open a new path toward the characterization of nearby non-transiting habitable planet candidates orbiting late-type stars. We present two simple and compact high-dispersed densified pupil spectrograph designs for the cosmology and exoplanet sciences.

astro-ph.IM↗

arXiv2021

Small-scale CMB anisotropies induced by the primordial magnetic fields

The primordial magnetic fields (PMFs) produced in the early universe are expected to be the origin of the large-scale cosmic magnetic fields. The PMFs are considered to leave a footprint on the cosmic microwave background (CMB) anisotropies due to both the electromagnetic force and gravitational interaction. In this paper, we investigate how the PMFs affect the CMB anisotropies on smaller scales than the mean-free-path of the CMB photons. We solve the baryon Euler equation with Lorentz force due to the PMFs, and we show that the vector-type perturbations from the PMFs induce the CMB anisotropies below the Silk scale as $\ell>3000$. Based on our calculations, we put a constraint on the PMFs from the combined CMB temperature anisotropies obtained by Planck and South Pole Telescope (SPT). We have found that the highly-resolved temperature anisotropies of the SPT 2017 bandpowers at $\ell \lesssim 8000$ favor the PMF model with a small scale-dependence. As a result, the Planck and SPT's joint-analysis puts a constraint on the PMF spectral index as $n_B<-1.14$ at 95% confidence level (C.L.), and this is more stringent compared with the Planck-only constraint $n_B<-0.28$. We show that the PMF strength normalized on the co-moving 1 Mpc scale is also tightly constrained as $B_{1\mathrm{Mpc}}<1.5$ nG with Planck and SPT at 95% C.L., while $B_{1\mathrm{Mpc}}<3.2$ nG only with the Planck data at 95% C.L. We also discuss the effects on the cosmological parameter estimate when including the SPT data and CMB anisotropies induced by the PMFs.

astro-ph.CO↗

arXiv2021

21 cm Forest Constraints on Primordial Black Holes

Primordial black holes (PBHs) as part of the Dark Matter (DM) would modify the evolution of large-scale structures and the thermal history of the universe. Future 21 cm forest observations, sensitive to small scales and the thermal state of the Inter Galactic Medium (IGM), could probe the existence of such PBHs. In this article, we show that the shot noise isocurvature mode on small scales induced by the presence of PBHs can enhance the amount of low mass halos, or minihalos, and thus, the number of 21 cm absorption lines. However, if the mass of PBHs is as large as $M_{\rm PBH}\gtrsim 10 \, M_\odot$, with an abundant enough fraction of PBHs as DM, $f_{\rm PBH}$, the IGM heating due to accretion onto the PBHs counteracts the enhancement due to the isocurvature mode, reducing the number of absorption lines instead. The concurrence of both effects imprints distinctive signatures in the number of absorbers, allowing to bound the abundance of PBHs. We compute the prospects for constraining PBHs with future 21 cm forest observations, finding achievable competitive upper limits on the abundance as low as $f_{\rm PBH} \sim 10^{-3}$ at $M_{\rm PBH}= 100 \, M_\odot$, or even lower at larger masses, in unexplored regions of the parameter space by current probes. The impact of astrophysical X-ray sources on the IGM temperature is also studied, which could potentially weaken the bounds.

astro-ph.CO↗

arXiv2021

Constraining the luminosity function of active galactic nuclei through the reionization observations in the SKA era

Ultraviolet (UV) and X-ray photons from active galactic nuclei (AGNs) can ionize hydrogen in the intergalactic medium (IGM). We solve radiative transfer around AGNs in high redshift to evaluate the 21-cm line emission from the neutral hydrogen in the IGM and obtain the radial profile of the brightness temperature in the epoch of reionization. The ionization profile extends over 10 [Mpc] comoving distance which can be observed in the order of 10 [arcmin]. From estimation of the radio galaxy number counts with high sensitivity observation through the Square Kilometre Array (SKA), we investigate the capability of parameter constrains for AGN luminosity function with Fisher analysis for three evolution model through cosmic time. We find that the errors for each parameter are restricted to a few percent when AGNs are sufficiently bright at high redshifts. We also investigate the possibility of further parameter constraints with future observation beyond the era of SKA.

astro-ph.GA↗

arXiv2020

An Iterative Reconstruction Algorithm for Faraday Tomography

Faraday tomography offers crucial information on the magnetized astronomical objects, such as quasars, galaxies, or galaxy clusters, by observing its magnetoionic media. The observed linear polarization spectrum is inverse Fourier transformed to obtain the Faraday dispersion function (FDF), providing us a tomographic distribution of the magnetoionic media along the line of sight. However, this transform gives a poor reconstruction of the FDF because of the instrument's limited wavelength coverage. The current Faraday tomography techniques' inability to reliably solve the above inverse problem has noticeably plagued cosmic magnetism studies. We propose a new algorithm inspired by the well-studied area of signal restoration, called the Constraining and Restoring iterative Algorithm for Faraday Tomography (CRAFT). This iterative model-independent algorithm is computationally inexpensive and only requires weak physically-motivated assumptions to produce high fidelity FDF reconstructions. We demonstrate an application for a realistic synthetic model FDF of the Milky Way, where CRAFT shows greater potential over other popular model-independent techniques. The dependence of observational frequency coverage on the various techniques' reconstruction performance is also demonstrated for a simpler FDF. CRAFT exhibits improvements even over model-dependent techniques (i.e., QU-fitting) by capturing complex multi-scale features of the FDF amplitude and polarization angle variations within a source. The proposed approach will be of utmost importance for future cosmic magnetism studies, especially with broadband polarization data from the Square Kilometre Array and its precursors. We make the CRAFT code publicly available.

astro-ph.IM↗

arXiv2020

21cm forest probes on the axion dark matter in the post-inflationary Peccei-Quinn symmetry breaking scenarios

We study the future prospects of the 21cm forest observations on the axion-like dark matter when the spontaneous breaking of the global Peccei-Quinn (PQ) symmetry occurs after the inflation. The large isocurvature perturbations of order unity sourced from axion-like particles can result in the enhancement of minihalo formation, and the subsequent hierarchical structure formation can affect the minihalo abundance whose masses can exceed ${\cal O}(10^4) M_{\odot}$ relevant for the 21cm forest observations. We show that the 21cm forest observations are capable of probing the axion-like particle mass in the range $10^{-18}\lesssim m_a \lesssim 10^{-12}$ eV for the temperature independent axion mass. For the temperature dependent axion mass, the zero temperature axion mass scale for which the 21cm forest measurements can be affected is extended further to as big as of order $10^{-6}$ eV.

astro-ph.CO↗

arXiv2020

Current status of space gravitational wave antenna DECIGO and B-DECIGO

Deci-hertz Interferometer Gravitational Wave Observatory (DECIGO) is the future Japanese space mission with a frequency band of 0.1 Hz to 10 Hz. DECIGO aims at the detection of primordial gravitational waves, which could be produced during the inflationary period right after the birth of the universe. There are many other scientific objectives of DECIGO, including the direct measurement of the acceleration of the expansion of the universe, and reliable and accurate predictions of the timing and locations of neutron star/black hole binary coalescences. DECIGO consists of four clusters of observatories placed in the heliocentric orbit. Each cluster consists of three spacecraft, which form three Fabry-Perot Michelson interferometers with an arm length of 1,000 km. Three clusters of DECIGO will be placed far from each other, and the fourth cluster will be placed in the same position as one of the three clusters to obtain the correlation signals for the detection of the primordial gravitational waves. We plan to launch B-DECIGO, which is a scientific pathfinder of DECIGO, before DECIGO in the 2030s to demonstrate the technologies required for DECIGO, as well as to obtain fruitful scientific results to further expand the multi-messenger astronomy.

gr-qc↗

arXiv2020

Constraints on nature of ultra light dark matter particles with 21cm forest

The ultra-light scalar fields can arise ubiquitously, for instance, as a result of the spontaneous breaking of an approximate symmetry such as the axion and more generally the axion-like particles. In addition to the particle physics motivations, these particles can also play a major role in cosmology by contributing to dark matter abundance and affecting the structure formation at sub-Mpc scales. In this paper, we propose to use the 21cm forest observations to probe the nature of ultra-light dark matter. The 21cm forest can probe much smaller scales than the Lyman-$α$ forest, that is, $k\gtrsim 10\mathrm{Mpc}^{-1}$. We explore the range of the ultra-light dark matter mass $m_{u}$ and $f_u$, the fraction of ultra-light dark matter with respect to the total matter, which can be probed by the 21cm forest. We find that 21cm forest can potentially put the dark matter mass lower bound $m_u \gtrsim 10^{-18}$ eV for $f_u=1$, which is 3 orders of magnitude bigger mass scale than those probed by the current Lyman-$α$ forest observations.While the effects of the ultra-light particles on the structure formation become smaller when the dominant component of dark matter is composed of the conventional cold dark matter, we find that the 21cm forest is still powerful enough to probe the sub-component ultra-light dark matter mass up to the order of $10^{-19}$ eV. The Fisher matrix analysis shows that $(m_u,f_u)\sim (10^{-20}\mathrm{eV}, 0.3)$ is the most optimal parameter set which the 21cm forest can probe with the minimal errors for a sub-component ultra-light dark matter scenario.

astro-ph.CO↗

arXiv2019

Density Fluctuation Reconstruction using KS test and D'Agostino's K-squared test

We propose a new non-parametric method to constrain the cosmological model through the growth factor of large-scale structure. To constrain the cosmological model from observations such as cosmic microwave background or large-scale structure, the power spectrum or correlation function have been used since they are well described by the linear perturbation theory or little correction is needed for the non-linearity; however, it is always important to find an alternative route to constrain cosmological models in order to avoid systematic effects due to the estimator and make the constraint more robust.Our density fluctuation reconstruction method constrains the cosmological model directly from time evolution of density fluctuation. We propose to apply Kolmogorov-Smirnov test and D'Agostino's K-squared test to the distribution function of density fluctuations, which are observed on a light cone. The density fluctuation recovers the Gaussian distribution only if we reconstruct the density fluctuation with correct linear growth factors. The method is verified by the use of Gaussian random field and N-body simulations. This method will open a new window both to separate the initial three dimensional density fluctuation and the growth factor of large scale structure.

astro-ph.CO↗

arXiv2019

Cross-correlation between 21-cm radiation and CMB B modes from the cosmic birefringence in the presence of a light scalar field

We study the cross-correlation between the 21cm and CMB B mode fluctuations which are induced by the cosmic birefringence when the non-constant scalar field couples to the electromagnetic field strength. Such multi-wavelength signals can potentially probe the reionization history of the Universe and also explore the nature of fundamental physics such as the parity violation and the scalar field dynamics in the early Universe. We illustrate the feasibility to detect such 21cm-B mode cross-correlations through commonly discussed scalar field models, the quintessence-like scalar field which is responsible for the current dark energy and the axion-like scalar field which is responsible for the current dark matter density.

astro-ph.CO↗

arXiv2019

Simultaneous determination of the cosmic birefringence and miscalibrated polarisation angles from CMB experiments

We show that the cosmic birefringence and miscalibrated polarisation angles can be determined simultaneously by cosmic microwave background (CMB) experiments using the cross-correlation between $E$- and $B$-mode polarisation data. This is possible because polarisation angles of the CMB are rotated by both the cosmic birefringence and miscalibration effects,whereas those of the Galactic foreground emission only by the latter. Our method does not require prior knowledge of the $E$- and $B$-mode power spectra of the foreground emission, but uses only the knowledge of the CMB polarisation spectra. Specifically, we relate the observed $EB$ correlation to the difference between the $\mathit{observed}$ $E$- and $B$-mode spectra in the sky, and use different multipole dependence of the CMB (given by theory) and foreground spectra (with no assumption) to derive the likelihood for the miscalibration angle $α$ and the birefringence angle $β$. We show that a future satellite mission similar to LiteBIRD can determine $β$ with a precision of ten arcminutes.

astro-ph.CO↗

arXiv2019

Delta-map method to remove CMB foregrounds with spatially varying spectra

We extend the internal template foreground removal method by accounting for spatially varying spectral parameters such as the spectral indices of synchrotron and dust emission and the dust temperature. As the previous algorithm had to assume that the spectral parameters are uniform over the full sky (or some significant fraction of the sky), it resulted in a bias in the tensor-to-scalar ratio parameter $r$ estimated from foreground-cleaned polarization maps of the cosmic microwave background (CMB). The new algorithm, "Delta-map method", accounts for spatially varying spectra to first order in perturbation. The only free parameters are the cosmological parameters such as $r$ and the sky-averaged foreground parameters. We show that a cleaned CMB map is the maximum likelihood solution to first order in perturbation, and derive the posterior distribution of $r$ and the sky-averaged foreground parameters using Bayesian statistics. Applying this to realistic simulated sky maps including polarized CMB, synchrotron and thermal dust emission, we find that the new algorithm removes the bias in $r$ down to undetected level in our noiseless simulation ($r\lesssim 4\times 10^{-4}$). We show that the frequency decorrelation of polarized foregrounds due to averaging of spatially varying spectra can be accounted for to first order in perturbation by using slightly different spectral parameters for the Stokes $Q$ and $U$ maps. Finally, we show that the effect of polarized anomalous microwave emission on foreground removal can be absorbed into the curvature parameter of the synchrotron spectrum.

astro-ph.CO↗

arXiv2018

Thermal Sunyaev-Zel'dovich effect in the intergalactic medium with primordial magnetic fields

The presence of ubiquitous magnetic fields in the universe is suggested from observations of radiation and cosmic ray from galaxies or the intergalactic medium (IGM). One possible origin of cosmic magnetic fields is the magnetogenesis in the primordial universe. Such magnetic fields are called primordial magnetic fields (PMFs), and are considered to affect the evolution of matter density fluctuations and the thermal history of the IGM gas. Hence the information of PMFs is expected to be imprinted on the anisotropies of the cosmic microwave background (CMB) through the thermal Sunyaev-Zel'dovich (tSZ) effect in the IGM. In this study, given an initial power spectrum of PMFs as $P(k)\propto B_{\rm 1Mpc}^2 k^{n_{B}}$, we calculate dynamical and thermal evolutions of the IGM under the influence of PMFs, and compute the resultant angular power spectrum of the Compton $y$-parameter on the sky. As a result, we find that two physical processes driven by PMFs dominantly determine the power spectrum of the Compton $y$-parameter; (i) the heating due to the ambipolar diffusion effectively works to increase the temperature and the ionization fraction, and (ii) the Lorentz force drastically enhances the density contrast just after the recombination epoch. These facts result in making the tSZ angular power spectrum induced by the PMFs more remarkable at $\ell >10^4$ than that by galaxy clusters even with $B_{\rm 1Mpc}=0.1$ nG and $n_{B}=-1.0$ because the contribution from galaxy clusters decreases with increasing $\ell$. The measurement of the tSZ angular power spectrum on high $\ell$ modes can provide the stringent constraint on PMFs.

astro-ph.CO↗

arXiv2018

Thermal Sunyaev-Zel'dovich Effect in the IGM due to Primordial Magnetic Fields

In the present universe, magnetic fields exist with various strengths and on various scales. One possible origin of these cosmic magnetic fields is the primordial magnetic fields (PMFs) generated in the early universe. PMFs are considered to contribute to matter density evolution via Lorentz force and the thermal history of intergalactic medium (IGM) gas due to ambipolar diffusion. Therefore, information about PMFs should be included in the temperature anisotropy of the Cosmic Microwave Background through the thermal Sunyaev-Zel'dovich (tSZ) effect in IGM. In this article, given an initial power spectrum of PMFs, we show the spatial fluctuation of mass density and temperature of the IGM and tSZ angular power spectrum created by the PMFs. Finally, we find that the tSZ angular power spectrum induced by PMFs becomes significant on small scales, even with PMFs below the observational upper limit. Therefore, we conclude that the measurement of tSZ anisotropy on small scales will provide the most stringent constraint on PMFs.

astro-ph.CO↗