arXiv subjects

Ryota Kawamata

Publications and source records attributed to Ryota Kawamata.

8 recordsLinked to original sources

arXiv2021

Calibration Method of the Monocular Omnidirectional Stereo Camera

Compact and low-cost devices are needed for autonomous driving to image and measure distances to objects 360-degree around. We have been developing an omnidirectional stereo camera exploiting two hyperbolic mirrors and a single set of a lens and sensor, which makes this camera compact and cost efficient. We establish a new calibration method for this camera considering higher-order radial distortion, detailed tangential distortion, an image sensor tilt, and a lens-mirror offset. Our method reduces the calibration error by 6.0 and 4.3 times for the upper- and lower-view images, respectively. The random error of the distance measurement is 4.9% and the systematic error is 5.7% up to objects 14 meters apart, which is improved almost nine times compared to the conventional method. The remaining distance errors is due to a degraded optical resolution of the prototype, which we plan to make further improvements as future work.

eess.IV↗

arXiv2018

Extreme magnification of a star at redshift 1.5 by a galaxy-cluster lens

Galaxy-cluster gravitational lenses can magnify background galaxies by a total factor of up to ~50. Here we report an image of an individual star at redshift z=1.49 (dubbed "MACS J1149 Lensed Star 1 (LS1)") magnified by >2000. A separate image, detected briefly 0.26 arcseconds from LS1, is likely a counterimage of the first star demagnified for multiple years by a >~3 solar-mass object in the cluster. For reasonable assumptions about the lensing system, microlensing fluctuations in the stars' light curves can yield evidence about the mass function of intracluster stars and compact objects, including binary fractions and specific stellar evolution and supernova models. Dark-matter subhalos or massive compact objects may help to account for the two images' long-term brightness ratio.

astro-ph.GA↗

arXiv2018

Size--luminosity relations and UV luminosity functions at $z=6-9$ simultaneously derived from the complete Hubble Frontier Fields data

We construct $z\sim6-7$, 8, and 9 faint Lyman break galaxy samples (334, 61, and 37 galaxies, respectively) with accurate size measurements with the software $\texttt{glafic}$ from the complete Hubble Frontier Fields (FF) cluster and parallel fields data. These are the largest samples hitherto and reach down to the faint ends of recently obtained deep luminosity functions. At faint magnitudes, however, these samples are highly incomplete for galaxies with large sizes, implying that derivation of the luminosity function (LF) sensitively depends on the intrinsic size--luminosity (RL) relation. We thus conduct simultaneous maximum-likelihood estimation of LF and RL relation parameters from the observed distribution of galaxies on the RL plane with help of a completeness map as a function of size and luminosity. At $z\sim6-7$, we find that the intrinsic RL relation expressed as $r_\textrm{e} \propto L^β$ has a notably steeper slope of $β=0.46^{+0.08}_{-0.09}$ than those at lower redshifts, which in turn implies that the LF has a relatively shallow faint-end slope of $α=-1.86^{+0.17}_{-0.18}$. This steep $β$ can be reproduced by a simple analytical model in which smaller galaxies have lower specific angular momenta. The $β$ and $α$ values for the $z\sim8$ and 9 samples are consistent with those for $z\sim6-7$ but with larger errors. For all three samples there is a large, positive covariance between $β$ and $α$, implying that the simultaneous determination of these two parameters is important. We also provide new strong lens mass models of Abell S1063 and Abell 370 as well as updated mass models of Abell 2744 and MACS J0416.1$-$2403.

astro-ph.GA↗

arXiv2018

Full-Data Results of Hubble Frontier Fields: UV Luminosity Functions at $z\sim6-10$ and a Consistent Picture of Cosmic Reionization

We present UV luminosity functions of dropout galaxies at $z\sim6-10$ with the complete Hubble Frontier Fields data. We obtain a catalog of $\sim450$ dropout-galaxy candidates (350, 66, and 40 at $z\sim6-7$, 8, and 9, respectively), whose UV absolute magnitudes reach $\sim-14$ mag, $\sim2$ mag deeper than the Hubble Ultra Deep Field detection limits. We carefully evaluate number densities of the dropout galaxies by Monte-Carlo simulations, including all lensing effects such as magnification, distortion, and multiplication of images as well as detection completeness and contamination effects in a self-consistent manner. We find that UV luminosity functions at $z\sim6-8$ have steep faint-end slopes, $α\sim-2$, and likely steeper slopes, $α\lesssim-2$ at $z\sim9-10$. We also find that the evolution of UV luminosity densities shows a non-accelerated decline beyond $z\sim8$ in the case of $M_\mathrm{trunc}=-15$, while an accelerated in the case of $M_\mathrm{trunc}=-17$. We examine whether our results are consistent with the Thomson scattering optical depth from the Planck satellite and the ionized hydrogen fraction $Q_\mathrm{HII}$ at $z\lesssim7$ based on the standard analytic reionization model. We find that there exist reionizaiton scenarios that consistently explain all the observational measurements with the allowed parameters of $f_{\rm esc}=0.17^{+0.07}_{-0.03}$ and $M_\mathrm{trunc}>-14.0$ for $\logξ_\mathrm{ion}/[\mathrm{erg}^{-1}\ \mathrm{Hz}]=25.34$, where $f_{\rm esc}$ is the escape fraction, $M_\mathrm{trunc}$ is the faint limit of the UV luminosity function, and $ξ_\mathrm{ion}$ is the conversion factor of the UV luminosity to the ionizing photon emission rate. The length of the reionization period is estimated to be $Δz=3.9^{+2.0}_{-1.6}$ (for $0.1<Q_\mathrm{HII}<0.99$), consistent with the recent estimate from Planck.

astro-ph.GA↗

arXiv2017

Angular momentum evolution of stellar disks at high redshifts

The stellar disk size of a galaxy depends on the ratio of the disk stellar mass to the halo mass, $m_\star \equiv M_\star/M_{\rm dh}$, and the fraction of the dark halo angular momentum transferred to the stellar disk, $j_\star \equiv J_\star/J_{\rm dh}$. Since $m_{\star}$ and $j_{\star}$ are determined by many star-formation related processes, measuring $j_\star$ and $m_\star$ at various redshifts is essential to understand the formation history of disk galaxies. We use the 3D-HST GOODS-S, COSMOS, and AEGIS imaging data and photo-$z$ catalog to examine $j_\star$ and $m_\star$ for star-forming galaxies at $z \sim$ 2, 3, and 4, when disks are actively forming. We find that the $j_\star/m_\star$ ratio is $\simeq 0.77\pm 0.06$ for all three redshifts over the entire mass range examined, $8\times 10^{10} < M_{\rm dh}/h^{-1} M_\odot < 2\times 10^{12}$, with a possible ($<30\%$) decrease with mass. This high ratio is close to those of local disk galaxies, descendants of our galaxies in terms of $M_{\rm dh}$ growth, implying a nearly constant $j_\star/m_\star$ over past 12 Gyr. These results are remarkable because mechanisms controlling angular momentum transfer to disks such as inflows and feedbacks depend on both cosmic time and halo mass and indeed theoretical studies tend to predict $j_\star/m_\star$ changing with redshift and mass. It is found that recent theoretical galaxy formation simulations predict smaller $j_{\star}/m_{\star}$ than our values. We also find that a significant fraction of our galaxies appears to be unstable against bar formation.

astro-ph.GA↗

arXiv2016

Precise Strong Lensing Mass Modeling of Four Hubble Frontier Fields Clusters and a Sample of Magnified High-Redshift Galaxies

We conduct precise strong lensing mass modeling of four ${\it Hubble}$ Frontier Fields (HFF) clusters, Abell$~$2744, MACS$~$J0416.1$-$2403, MACS$~$J0717.5$+$3745, and MACS$~$J1149.6$+$2223, for which HFF imaging observations are completed. We construct a refined sample of more than 100 multiple images for each cluster by taking advantage of the full depth HFF images, and conduct mass modeling using the ${\small \rm GLAFIC}$ software, which assumes simply parametrized mass distributions. Our mass modeling also exploits a magnification constraint from the lensed Type Ia supernova HFF14Tom for Abell$~$2744 and positional constraints from the multiple images S1$-$S4 of the lensed supernova SN Refsdal for MACS$~$J1149.6$+$2223. We find that our best-fitting mass models reproduce the observed image positions with RMS errors of $\sim 0.4$ arcsec, which are smaller than RMS errors in previous mass modeling that adopted similar numbers of multiple images. Our model predicts a new image of SN Refsdal with a relative time delay and magnification that are fully consistent with a recent detection of reappearance. We then construct catalogs of $z\sim 6-9$ dropout galaxies behind the four clusters and estimate magnification factors for these dropout galaxies with our best-fitting mass models. The dropout sample from the four cluster fields contains $\sim 120$ galaxies at $z\gtrsim 6$, about 20 of which are predicted to be magnified by a factor of more than 10. Some of the high-redshift galaxies detected in the HFF have lensing-corrected magnitudes of $M_{\rm UV}\sim -15$ to $-14$. Our analysis demonstrates that the HFF data indeed offer an ideal opportunity to study faint high-redshift galaxies. All lensing maps produced from our mass modeling will be made available on the STScI website.

astro-ph.GA↗

arXiv2015

The sizes of $z\sim6-8$ lensed galaxies from the Hubble Frontier Fields Abell 2744 data

We investigate sizes of $z\sim6-8$ dropout galaxies using the complete data of the Abell 2744 cluster and parallel fields in the Hubble Frontier Fields program. By directly fitting light profiles of observed galaxies with lensing-distorted Sérsic profiles on the image plane with the \texttt{glafic} software, we accurately measure intrinsic sizes of 31 $z\sim6-7$ and eight $z\sim8$ galaxies, including those as faint as $M_{\mathrm{UV}}\simeq-16.6$. We find that half-light radii $r_\mathrm{e}$ positively correlates with UV luminosity at each redshift, although the correlation is not very tight. Largest ($r_\mathrm{e}>0.8$ kpc) galaxies are mostly red in UV color while smallest ($r_\mathrm{e} < 0.08$ kpc) ones tend to be blue. We also find that galaxies with multiple cores tend to be brighter. Combined with previous results at $2.5\lesssim z\lesssim12$, our result confirms that the average $r_{\mathrm{e}}$ of bright ($(0.3-1)L^\star_{z=3}$) galaxies scales as $r_{\mathrm{e}}\propto(1+z)^{-m}$ with $m=1.24\pm0.1$. We find that the ratio of $r_\mathrm{e}$ to virial radius is virtually constant at $3.3\pm0.1\%$ over a wide redshift range, where the virial radii of hosting dark matter halos are derived based on the abundance matching. This constant ratio is consistent with the disk formation model by Mo et al. (1998) with $j_\mathrm{d}\sim m_\mathrm{d}$, where $j_\mathrm{d}$ and $m_\mathrm{d}$ are the fractions of the angular momentum and mass within halos confined in the disks. A comparison with various types of local galaxies indicates that our galaxies are most similar to circumnuclear star-forming regions of barred galaxies in the sense that a sizable amount of stars are forming in a very small area.

astro-ph.GA↗

arXiv2014

Hubble Frontier Fields First Complete Cluster Data: Faint Galaxies at $z\sim 5-10$ for UV Luminosity Functions and Cosmic Reionization

We present the comprehensive analyses of faint dropout galaxies up to $z\sim10$ with the first full-depth data set of Abell 2744 lensing cluster and parallel fields observed by the Hubble Frontier Fields (HFF) program. We identify $54$ dropouts at $z\sim5-10$ in the HFF fields, and enlarge the size of $z\sim9$ galaxy sample obtained to date. Although the number of highly magnified ($μ\sim10$) galaxies is small due to the tiny survey volume of strong lensing, our study reaches the galaxies' intrinsic luminosities comparable to the deepest-field HUDF studies. We derive UV luminosity functions with these faint dropouts, carefully evaluating the combination of observational incompleteness and lensing effects in the image plane by intensive simulations including magnification, distortion, and multiplication of images, with the evaluations of mass model dependences. Our results confirm that the faint-end slope, $α$, is as steep as $-2$ at $z\sim6-8$, and strengthen the evidence of the rapid decrease of UV luminosity densities, $ρ_\mathrm{UV}$, at $z>8$ from the large $z\sim9$ sample. We examine whether the rapid $ρ_\mathrm{UV}$ decrease trend can reconcile with the large Thomson scattering optical depth, $τ_\mathrm{e}$, measured by CMB experiments allowing a large space of free parameters such as average ionizing photon escape fraction and stellar-population dependent conversion factor. No parameter set can reproduce both the rapid $ρ_\mathrm{UV}$ decrease and the large $τ_\mathrm{e}$. It is possible that the $ρ_\mathrm{UV}$ decrease moderates at $z\gtrsim11$, that the free parameters significantly evolve towards high-$z$, or that there exist additional sources of reionization such as X-ray binaries and faint AGNs.

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