arXiv subjects

Satoshi Yamanaka

Publications and source records attributed to Satoshi Yamanaka.

24 records · Page 2Linked to original sources

arXiv2021

Big Three Dragons: A [N II] 122 $μ$m Constraint and New Dust-continuum Detection of A $z = 7.15$ Bright Lyman Break Galaxy with ALMA

We present new Atacama Large Millimeter/submillimeter Array Band 7 observational results of a Lyman break galaxy at $ z=7.15 $, B14-65666 ("Big Three Dragons"), which is an object detected in [OIII] 88 $\rm{μm}$, [CII] 158 $\rm{μm}$, and dust-continuum emission during the epoch of reionization. Our targets are the [NII] 122 $\rm{μm}$ fine-structure emission line and underlying 120 $\rm{μm}$ dust continuum. The dust continuum is detected with a $ \sim $19$ σ$ significance. From far-infrared spectral energy distribution sampled at 90, 120, and 160 $\rm{μm}$, we obtaine a best-fit dust temperature of $ 40 $ K ($ 79 $ K) and an infrared luminosity of $ \log_{10}(L_{\rm IR}/{\rm L}_\odot)=11.6$ ($12.1$) at the emissivity index $ β= 2.0 $ (1.0). The [NII] 122 $\rm{μm}$ line is not detected. The 3$ σ$ upper limit of the [NII] luminosity is $ 8.1 \times 10^7\ {\rm L}_\odot$. From the [NII], [OIII], and [CII] line luminosities, we use the Cloudy photoionization code to estimate nebular parameters as functions of metallicity. If the metallicity of the galaxy is high ($ Z > 0.4\ {\rm Z}_\odot$), the ionization parameter and hydrogen density are $ \log_{10} U \simeq -2.7\pm0.1$ and $ n_\text{H} \simeq 50$-$250\ {\rm cm}^{-3}$, respectively, which are comparable to those measured in low-redshift galaxies. The nitrogen-to-oxygen abundance ratio, $\rm{N/O}$, is constrained to be sub-solar. At $ Z < 0.4\ {\rm Z}_\odot$, the allowed $ U $ drastically increases as the assumed metallicity decreases. For high ionization parameters, the $\rm{N/O}$ constraint becomes weak. Finally, our Cloudy models predict the location of B14-65666 on the BPT diagram, thereby allowing a comparison with low-redshift galaxies.

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arXiv2021

SILVERRUSH X: Machine Learning-Aided Selection of $9318$ LAEs at $z=2.2$, $3.3$, $4.9$, $5.7$, $6.6$, and $7.0$ from the HSC SSP and CHORUS Survey Data

We present a new catalog of $9318$ Ly$α$ emitter (LAE) candidates at $z = 2.2$, $3.3$, $4.9$, $5.7$, $6.6$, and $7.0$ that are photometrically selected by the SILVERRUSH program with a machine learning technique from large area (up to $25.0$ deg$^2$) imaging data with six narrowband filters taken by the Subaru Strategic Program with Hyper Suprime-Cam (HSC SSP) and a Subaru intensive program, Cosmic HydrOgen Reionization Unveiled with Subaru (CHORUS). We construct a convolutional neural network that distinguishes between real LAEs and contaminants with a completeness of $94$% and a contamination rate of $1$%, enabling us to efficiently remove contaminants from the photometrically selected LAE candidates. We confirm that our LAE catalogs include $177$ LAEs that have been spectroscopically identified in our SILVERRUSH programs and previous studies, ensuring the validity of our machine learning selection. In addition, we find that the object-matching rates between our LAE catalogs and our previous results are $\simeq 80$-$100$% at bright NB magnitudes of $\lesssim 24$ mag. We also confirm that the surface number densities of our LAE candidates are consistent with previous results. Our LAE catalogs will be made public on our project webpage.

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arXiv2020

CHORUS. I. Cosmic HydrOgen Reionization Unveiled with Subaru: Overview

To determine the dominant sources for cosmic reionization, the evolution history of the global ionizing fraction, and the topology of the ionized regions, we have conducted a deep imaging survey using four narrow-band (NB) and one intermediate-band (IB) filters on the Subaru/Hyper Suprime-Cam (HSC), called Cosmic HydrOgen Reionization Unveiled with Subaru (CHORUS). The central wavelengths and full-widths-at-half-maximum of the CHORUS filters are, respectively, 386.2 nm and 5.5 nm for NB387, 526.0 nm and 7.9 nm for NB527, 717.1 nm and 11.1 nm for NB718, 946.2 nm and 33.0 nm for IB945, and 971.2 nm and 11.2 nm for NB973. This combination, including NB921 (921.5 nm and 13.5 nm) from the Subaru Strategic Program with HSC (HSC SSP), are carefully designed, as if they were playing a chorus, to observe multiple spectral features simultaneously, such as Lyman continuum, Ly$α$, C~{\sc iv}, and He~{\sc ii} for $z=2$--$7$. The observing field is the same as that of the deepest footprint of the HSC SSP in the COSMOS field and its effective area is about 1.6 deg$^2$. Here, we present an overview of the CHORUS project, which includes descriptions of the filter design philosophy, observations and data reduction, multiband photometric catalogs, assessments of the imaging quality, measurements of the number counts, and example use cases of the data. All the imaging data, photometric catalogs, masked pixel images, data of limiting magnitudes and point spread functions, results of completeness simulations, and source number counts are publicly available through the HSC SSP database.

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arXiv2020

Testing an indirect method for identifying galaxies with high levels of Lyman continuum leakage

Using a sample of galaxies at $z\approx 3$ with detected Lyman Continuum (LyC) leakage in the SSA22 field, we attempt to verify a proposed indirect method for identifying cases with high LyC escape fraction $f_\mathrm{esc}$ based on measurements of the H$β$ equivalent width (EW) and the $β$ slope of the UV continuum. To this end, we present Keck/MOSFIRE H$β$ emission line flux measurements of LyC galaxies (LCGs) at spectroscopic redshifts $z_{\mathrm{spec}} \sim 3.3$, Lyman break galaxies (LBGs) at photometric redshifts $z_{\mathrm{phot}} = 2.7$--$3.7$, and Ly$α$ emitters at $z_{\mathrm{phot}} = 3.1$. We also reconfirm the spectroscopic redshifts and measure the H$β$ emission line fluxes from 2 LCGs and 6 LBGs. For the LCG in our sample with the most extreme $f_\mathrm{esc}$, as revealed by the direct detection of LyC photons, we find that the EW(H$β$)--$β$ method gives a broadly consistent estimate for $f_\mathrm{esc}$, although the error bars remain very large. We also discuss how a combination of $f_\mathrm{esc}$ measurements based on direct and indirect methods can shed light on the LyC escape mechanism and the anisotropy of the leakage.

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arXiv2019

Balmer Break Galaxy Candidates at $z \sim 6$: a Potential View on the Star-Formation Activity at $z \gtrsim 14$

We search for galaxies with a strong Balmer break (Balmer Break Galaxies; BBGs) at $z \sim 6$ over a 0.41 deg$^2$ effective area in the COSMOS field. Based on rich imaging data, including data obtained with the Atacama Large Millimeter/submillimeter Array (ALMA), three candidates are identified by their extremely red $K - [3.6]$ colors as well as by non-detection in X-ray, optical, far-infrared (FIR), and radio bands. The non-detection in the deep ALMA observations suggests that they are not dusty galaxies but BBGs at $z \sim 6$, although contamination from Active Galactic Nuclei (AGNs) at $z \sim 0$ cannot be completely ruled out for the moment. Our spectral energy distribution (SED) analyses reveal that the BBG candidates at $z \sim 6$ have stellar masses of $\approx 5 \times 10^{10} M_{\odot}$ dominated by old stellar populations with ages of $\gtrsim 700$ Myr. Assuming that all the three candidates are real BBGs at $z \sim 6$, we estimate the stellar mass density (SMD) to be $2.4^{+2.3}_{-1.3} \times 10^{4} M_{\odot}$ Mpc$^{-3}$. This is consistent with an extrapolation from the lower redshift measurements. The onset of star formation in the three BBG candidates is expected to be several hundred million years before the observed epoch of $z \sim 6$. We estimate the star-formation rate density (SFRD) contributed by progenitors of the BBGs to be 2.4 -- 12 $\times 10^{-5} M_{\odot}$ yr$^{-1} $Mpc$^{-3}$ at $z > 14$ (99.7\% confidence range). Our result suggests a smooth evolution of the SFRD beyond $z = 8$.

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arXiv2019

The UV spectral slope beta and stellar population of most active star-forming galaxies at z~4

We investigate the stellar population of star-forming galaxies at $z \sim 4$ by focusing on their slope of rest-frame ultraviolet (UV) continuum, $β$ where $f_λ \propto λ^β$. We investigate the sample of bright Lyman Break Galaxies (LBGs) with $i' \leq 26.0$ in the Subaru/XMM-Newton Deep Survey field by using the SED fitting analysis. We find that the apparently redder ($β_{\mathrm{obs}} > -1.73$) LBGs tend to be dusty ($\mathrm{Av} > 1.0$), have the young stellar population ($β_{\mathrm{int}} < -2.42$), and the intrinsically active star-forming galaxies (SFR $\gtrsim$ a few $\times 10^{2}$ $\mathrm{M_{\odot}} \mathrm{yr^{-1}}$). It means that a significant fraction of the UV-selected LBGs at $z \sim 4$ is on-going active and dust obscured star-forming galaxies. We compare the IR to UV luminosity ratio assuming the dust attenuation laws with the sub-millimeter observations from previous works. The result suggests that the Calzetti-like dust attenuation law is preferable for the active and dusty star-forming LBGs at $z = 4$. We also find that an extrapolation of the $β_{\mathrm{int}}$-$M_{\mathrm{UV,int}}$ relation toward the fainter magnitude range below our sample magnitude limit intersects the $β_{\mathrm{obs}}$-$M_{\mathrm{UV,obs}}$ relation previously obtained in the deeper narrow-area observations at $M_{\mathrm{UV}} = -18.9$ and $β= -1.94$, which coincides with the break point of $β_{\mathrm{obs}}$-$M_{\mathrm{UV,obs}}$ relation observed so far. The coincidence suggest that we see the almost dust-free population at $M_{\mathrm{UV,obs}} \gtrsim -18.9$.

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