arXiv subjects

Yuichi Harikane

Publications and source records attributed to Yuichi Harikane.

180 records · Page 10Linked to original sources

arXiv2017

Minor Contribution of Quasars to Ionizing Photon Budget at z~6: Update on Quasar Luminosity Function at the Faint-end with Subaru/Suprime-Cam

We constrain the quasar contribution to cosmic reionization based on our deep optical survey of z~6 quasars down to z_R=24.15 using Subaru/Suprime-Cam in three UKIDSS-DXS fields covering 6.5 deg^2. In Kashikawa et al. (2015), we select 17 quasar candidates and report our initial discovery of two low-luminosity quasars (M_1450~ -23) from seven targets, one of which might be a Lyman alpha emitting galaxy. From an additional optical spectroscopy, none of the four candidates out of the remaining ten turn out to be genuine quasars. Moreover, the deeper optical photometry provided by the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP) shows that, unlike the two already-known quasars, the i-z and z-y colors of the last six candidates are consistent with M- or L-type brown dwarfs. Therefore, the quasar luminosity function (QLF) in the previous paper is confirmed. Compiling QLF measurements from the literature over a wide magnitude range, including an extremely faint AGN candidate from Parsa et al. (2017}, to fit them with a double power-law, we find that the best-fit faint-end slope is alpha=-2.04^+0.33_-0.18 (-1.98^+0.48_-0.21) and characteristic magnitude is M_1450^*=-25.8^+1.1_-1.9 (-25.7^+1.0_-1.8) in the case of two (one) quasar detection. Our result suggests that, if the QLF is integrated down to M_1450=-18, quasars produce ~1-12% of the ionizing photons required to ionize the whole universe at z~6 with 2sigma confidence level, assuming that the escape fraction is f_esc=1 and the IGM clumpy factor is C=3. Even when the systematic uncertainties are taken into account, our result supports the scenario that quasars are the minor contributors of reionization.

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arXiv2017

GOLDRUSH. III. A Systematic Search of Protoclusters at $z\sim4$ Based on the $>100\,\mathrm{deg^2}$ Area

We conduct a systematic search for galaxy protoclusters at $z\sim3.8$ based on the latest internal data release (S16A) of the Hyper SuprimeCam Subaru strategic program (HSC-SSP). In the Wide layer of the HSC-SSP, we investigate the large-scale projected sky distribution of $g$-dropout galaxies over an area of $121\,\mathrm{deg^2}$, and identify 216 large-scale overdense regions ($>4σ$ overdensity significance) that are good protocluster candidates. Of these, 37 are located within $8\,\mathrm{arcmin}$ ($3.4\,\mathrm{physicalMpc}$) from other protocluster candidates of higher overdensity, and are expected to merge into a single massive structure by $z=0$. Therefore, we find 179 unique protocluster candidates in our survey. A cosmological simulation that includes projection effects predicts that more than 76\% of these candidates will evolve into galaxy clusters with halo masses of at least $10^{14}\,M_{\odot}$ by $z=0$. The unprecedented size of our protocluster candidate catalog allowed us to perform, for the first time, an angular clustering analysis of the systematic sample of protocluster candidates. We find a correlation length of $35.0\,h^{-1}\,\mathrm{Mpc}$. The relation between correlation length and number density of $z\sim3.8$ protocluster candidates is consistent with the prediction of the $Λ$CDM model, and the correlation length is similar to that of rich clusters in the local universe. This result suggests that our protocluster candidates are tracing similar spatial structures as those expected of the progenitors of rich clusters and enhances the confidence that our method to identify protoclusters at high redshifts is robust. In the coming years, our protocluster search will be extended to the entire HSC-SSP Wide sky coverage of $\sim1400\,\mathrm{deg^2}$ to probe cluster formation over a wide redshift range of $z\sim2\mathrm{-}6$.

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arXiv2017

Great Optically Luminous Dropout Research Using Subaru HSC (GOLDRUSH). I. UV Luminosity Functions at $z \sim 4-7$ Derived with the Half-Million Dropouts on the 100 deg$^2$ Sky

We study the UV luminosity functions (LFs) at $z\sim 4$, $5$, $6,$ and $7$ based on the deep large-area optical images taken by the Hyper Suprime-Cam (HSC) Subaru strategic program (SSP). On the 100 deg$^2$ sky of the HSC SSP data available to date, we make enormous samples consisting of a total of 579,565 dropout candidates at $z\sim 4-7$ by the standard color selection technique, 358 out of which are spectroscopically confirmed by our follow-up spectroscopy and other studies. We obtain UV LFs at $z \sim 4-7$ that span a very wide UV luminosity range of $\sim 0.002 - 100 \, L_{\rm UV}^\ast$ ($-26 < M_{\rm UV} < -14$ mag) by combining LFs from our program and the ultra-deep Hubble Space Telescope legacy surveys. We derive three parameters of the best-fit Schechter function, $ϕ^\ast$, $M_{\rm UV}^\ast$, and $α$, of the UV LFs in the magnitude range where the AGN contribution is negligible, and find that $α$ and $ϕ^\ast$ decrease from $z\sim 4$ to $7$ with no significant evolution of $M_{\rm UV}^\ast$. Because our HSC SSP data bridge the LFs of galaxies and AGNs with great statistical accuracy, we carefully investigate the bright end of the galaxy UV LFs that are estimated by the subtraction of the AGN contribution either aided with spectroscopy or the best-fit AGN UV LFs. We find that the bright end of the galaxy UV LFs cannot be explained by the Schechter function fits at $> 2 σ$ significance, and require either double power-law functions or modified Schechter functions that consider a magnification bias due to gravitational lensing.

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arXiv2017

GOLDRUSH. II. Clustering of Galaxies at $z\sim 4-6$ Revealed with the Half-Million Dropouts Over the 100 deg$^2$ Area Corresponding to 1 Gpc$^3$

We present clustering properties from 579,492 Lyman break galaxies (LBGs) at z~4-6 over the 100 deg^2 sky (corresponding to a 1.4 Gpc^3 volume) identified in early data of the Hyper Suprime-Cam (HSC) Subaru strategic program survey. We derive angular correlation functions (ACFs) of the HSC LBGs with unprecedentedly high statistical accuracies at z~4-6, and compare them with the halo occupation distribution (HOD) models. We clearly identify significant ACF excesses in 10"<$θ$<90", the transition scale between 1- and 2-halo terms, suggestive of the existence of the non-linear halo bias effect. Combining the HOD models and previous clustering measurements of faint LBGs at z~4-7, we investigate dark-matter halo mass (Mh) of the z~4-7 LBGs and its correlation with various physical properties including the star-formation rate (SFR), the stellar-to-halo mass ratio (SHMR), and the dark matter accretion rate (dotMh) over a wide-mass range of Mh/M$_\odot$=4x10^10-4x10^12. We find that the SHMR increases from z~4 to 7 by a factor of ~4 at Mh~1x10^11 M$_\odot$, while the SHMR shows no strong evolution in the similar redshift range at Mh~1x10^12 M$_\odot$. Interestingly, we identify a tight relation of SFR/dotMh-Mh showing no significant evolution beyond 0.15 dex in this wide-mass range over z~4-7. This weak evolution suggests that the SFR/dotMh-Mh relation is a fundamental relation in high-redshift galaxy formation whose star formation activities are regulated by the dark matter mass assembly. Assuming this fundamental relation, we calculate the cosmic SFR densities (SFRDs) over z=0-10 (a.k.a. Madau-Lilly plot). The cosmic SFRD evolution based on the fundamental relation agrees with the one obtained by observations, suggesting that the cosmic SFRD increase from z~10 to 4-2 (decrease from z~4-2 to 0) is mainly driven by the increase of the halo abundance (the decrease of the accretion rate).

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arXiv2017

Enhancement of Galaxy Overdensity around Quasar Pairs at z<3.6 based on the Hyper Suprime-Cam Subaru Strategic Program Survey

We investigate the galaxy overdensity around proto-cluster scale quasar pairs at high (z>3) and low (z~1) redshift based on the unprecedentedly wide and deep optical survey of the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP). Using the first-year survey data covering effectively ~121 deg^2 with the 5sigma depth of i~26.4 and the SDSS DR12Q catalog, we find two luminous pairs at z~3.3 and 3.6 which reside in >5sigma overdense regions of g-dropout galaxies at i<25. The projected separations of the two pairs are R_perp=1.75 and 1.04 proper Mpc, and their velocity offsets are Delta V=692 and 1448 km s^{-1}, respectively. This result is in clear contrast to the average z~4 quasar environments as discussed in Uchiyama et al. (2017) and implies that the quasar activities of the pair members are triggered via major mergers in proto-clusters, unlike the vast majority of isolated quasars in general fields that may turn on via non-merger events such as bar and disk instabilities. At z~1, we find 37 pairs with R_perp<2 pMpc and Delta V<2300 km s^{-1} in the current HSC-Wide coverage, including four from Hennawi et al. (2006). The distribution of the peak overdensity significance within two arcminutes around the pairs has a long tail toward high density (>4sigma) regions. Thanks to the large sample size, we find a statistical evidence that this excess is unique to the pair environments when compared to single quasar and randomly selected galaxy environments at the same redshift range. Moreover, there are nine small-scale (R_perp<1 pMpc) pairs, two of which are found to reside in cluster fields. Our results demonstrate that <2 pMpc-scale quasar pairs at both redshift range tend to occur in massive haloes, although perhaps not the most massive ones, and that they are useful to search for rare density peaks.

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arXiv2017

First Data Release of the Hyper Suprime-Cam Subaru Strategic Program

The Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP) is a three-layered imaging survey aimed at addressing some of the most outstanding questions in astronomy today, including the nature of dark matter and dark energy. The survey has been awarded 300 nights of observing time at the Subaru Telescope and it started in March 2014. This paper presents the first public data release of HSC-SSP. This release includes data taken in the first 1.7 years of observations (61.5 nights) and each of the Wide, Deep, and UltraDeep layers covers about 108, 26, and 4 square degrees down to depths of i~26.4, ~26.5, and ~27.0 mag, respectively (5sigma for point sources). All the layers are observed in five broad bands (grizy), and the Deep and UltraDeep layers are observed in narrow bands as well. We achieve an impressive image quality of 0.6 arcsec in the i-band in the Wide layer. We show that we achieve 1-2 per cent PSF photometry (rms) both internally and externally (against Pan-STARRS1), and ~10 mas and 40 mas internal and external astrometric accuracy, respectively. Both the calibrated images and catalogs are made available to the community through dedicated user interfaces and database servers. In addition to the pipeline products, we also provide value-added products such as photometric redshifts and a collection of public spectroscopic redshifts. Detailed descriptions of all the data can be found online. The data release website is https://hsc-release.mtk.nao.ac.jp/.

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arXiv2017

Systematic Identification of LAEs for Visible Exploration and Reionization Research Using Subaru HSC (SILVERRUSH). I. Program Strategy and Clustering Properties of ~2,000 Lya Emitters at z=6-7 over the 0.3-0.5 Gpc$^2$ Survey Area

We present the SILVERRUSH program strategy and clustering properties investigated with $\sim 2,000$ Ly$α$ emitters at $z=5.7$ and $6.6$ found in the early data of the Hyper Suprime-Cam (HSC) Subaru Strategic Program survey exploiting the carefully designed narrowband filters. We derive angular correlation functions with the unprecedentedly large samples of LAEs at $z=6-7$ over the large total area of $14-21$ deg$^2$ corresponding to $0.3-0.5$ comoving Gpc$^2$. We obtain the average large-scale bias values of $b_{\rm avg}=4.1\pm 0.2$ ($4.5\pm 0.6$) at $z=5.7$ ($z=6.6$) for $\gtrsim L^*$ LAEs, indicating the weak evolution of LAE clustering from $z=5.7$ to $6.6$. We compare the LAE clustering results with two independent theoretical models that suggest an increase of an LAE clustering signal by the patchy ionized bubbles at the epoch of reionization (EoR), and estimate the neutral hydrogen fraction to be $x_{\rm HI}=0.15^{+0.15}_{-0.15}$ at $z=6.6$. Based on the halo occupation distribution models, we find that the $\gtrsim L^*$ LAEs are hosted by the dark-matter halos with the average mass of $\log (\left < M_{\rm h} \right >/M_\odot) =11.1^{+0.2}_{-0.4}$ ($10.8^{+0.3}_{-0.5}$) at $z=5.7$ ($6.6$) with a Ly$α$ duty cycle of 1 % or less, where the results of $z=6.6$ LAEs may be slightly biased, due to the increase of the clustering signal at the EoR. Our clustering analysis reveals the low-mass nature of $\gtrsim L^*$ LAEs at $z=6-7$, and that these LAEs probably evolve into massive super-$L^*$ galaxies in the present-day universe.

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arXiv2017

Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs). II. Discovery of 32 Quasars and Luminous Galaxies at 5.7 < z < 6.8

We present spectroscopic identification of 32 new quasars and luminous galaxies discovered at 5.7 < z < 6.8. This is the second in a series of papers presenting the results of the Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs) project, which exploits the deep multi-band imaging data produced by the Hyper Suprime-Cam (HSC) Subaru Strategic Program survey. The photometric candidates were selected by a Bayesian probabilistic algorithm, and then observed with spectrographs on the Gran Telescopio Canarias and the Subaru Telescope. Combined with the sample presented in the previous paper, we have now identified 64 HSC sources over about 430 deg2, which include 33 high-z quasars, 14 high-z luminous galaxies, 2 [O III] emitters at z ~ 0.8, and 15 Galactic brown dwarfs. The new quasars have considerably lower luminosity (M1450 ~ -25 to -22 mag) than most of the previously known high-z quasars. Several of these quasars have luminous (> 10^(43) erg/s) and narrow (< 500 km/s) Ly alpha lines, and also a possible mini broad absorption line system of N V 1240 in the composite spectrum, which clearly separate them from typical quasars. On the other hand, the high-z galaxies have extremely high luminosity (M1450 ~ -24 to -22 mag) compared to other galaxies found at similar redshift. With the discovery of these new classes of objects, we are opening up new parameter spaces in the high-z Universe. Further survey observations and follow-up studies of the identified objects, including the construction of the quasar luminosity function at z ~ 6, are ongoing.

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arXiv2017

Deep Optical Imaging of the COSMOS Field with Hyper Suprime-Cam Using Data from the Subaru Strategic Program and the University of Hawaii

We present the deepest optical images of the COSMOS field based on a joint dataset taken with Hyper Suprime-Cam (HSC) by the HSC Subaru Strategic Program (SSP) team and the University of Hawaii (UH). The COSMOS field is one of the key extragalactic fields with a wealth of deep, multi-wavelength data. However, the current optical data are not sufficiently deep to match with, e.g., the UltraVista data in the near-infrared. The SSP team and UH have joined forces to produce very deep optical images of the COSMOS field by combining data from both teams. The coadd images reach depths of g=27.8, r=27.7, i=27.6, z=26.8, and y=26.2 mag at 5 sigma for point sources based on flux uncertainties quoted by the pipeline and they cover essentially the entire COSMOS 2 square degree field. The seeing is between 0.6 and 0.9 arcsec on the coadds. We perform several quality checks and confirm that the data are of science quality; ~2% photometry and 30 mas astrometry. This accuracy is identical to the Public Data Release 1 from HSC-SSP. We make the joint dataset including fully calibrated catalogs of detected objects available to the community at https://hsc-release.mtk.nao.ac.jp/.

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arXiv2017

Clustering of quasars in a wide luminosity range at redshift 4 with Subaru Hyper Suprime-Cam wide field imaging

We examine the clustering of quasars over a wide luminosity range, by utilizing 901 quasars at $\overline{z}_{\rm phot}\sim3.8$ with $-24.73<M_{\rm 1450}<-22.23$ photometrically selected from the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP) S16A Wide2 date release and 342 more luminous quasars at $3.4<z_{\rm spec}<4.6$ having $-28.0<M_{\rm 1450}<-23.95$ from the Sloan Digital Sky Survey (SDSS) that fall in the HSC survey fields. We measure the bias factors of two quasar samples by evaluating the cross-correlation functions (CCFs) between the quasar samples and 25790 bright $z\sim4$ Lyman Break Galaxies (LBGs) in $M_{\rm 1450}<-21.25$ photometrically selected from the HSC dataset. Over an angular scale of \timeform{10.0"} to \timeform{1000.0"}, the bias factors are $5.93^{+1.34}_{-1.43}$ and $2.73^{+2.44}_{-2.55}$ for the low and high luminosity quasars, respectively, indicating no luminosity dependence of quasar clustering at $z\sim4$. It is noted that the bias factor of the luminous quasars estimated by the CCF is smaller than that estimated by the auto-correlation function (ACF) over a similar redshift range, especially on scales below \timeform{40.0"}. Moreover, the bias factor of the less-luminous quasars implies the minimal mass of their host dark matter halos (DMHs) is $0.3$-$2\times10^{12}h^{-1}M_{\odot}$, corresponding to a quasar duty cycle of $0.001$-$0.06$.

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arXiv2017

Evolution of N/O Abundance Ratios and Ionization Parameters from z~0 to 2 Investigated by the Direct Temperature Method

We present N/O abundance ratios, ionization parameters {$q_{\rm ion}$}, and oxygen abundance O/H for a total of 41 galaxies (11 individual galaxies and a 30-galaxy stack) including {Ly$α$} emitters and Lyman break galaxies at $z\sim 2$, and investigate galaxy evolution from $z\sim 0$ to $2$ in conjunction with 208,529 local SDSS galaxies and 9 green pea galaxies (GPs). In contrast with most of the recent studies, we obtain the N/O ratio, {$q_{\rm ion}$}, and O/H measurements by direct $T_{\rm e}$ methods with {[O\,\textsc{iii}]} $λ$4363 and {O\,\textsc{iii}]} $λ$1665 lines. Based on these reliable measurements, we find that there exist $z\sim 2$ galaxies with an excess of N/O falling beyond the local average of N/O--O/H relation, while the majority of the $z\sim 2$ galaxies have the N/O ratios nearly comparable with $z\sim 0$ galaxies in the N/O--stellar mass relation. Our galaxies place the upper limit of N/O ratio $\log ({\rm N/O})\le -1.26$ on average, suggesting that the N/O ratio evolves, if at all, by $<0.17$ dex. Exploiting our reliable measurements free from the N/O--{$q_{\rm ion}$}--O/H degeneracies, we identify, for the first time, that $z\sim 2$ galaxies with offsets in the BPT diagram indicate either 1) only an N/O excess, 2) only a {$q_{\rm ion}$} excess, or 3) both N/O and {$q_{\rm ion}$} excesses. We argue that the BPT offsets at $z\sim 2$ are not made by one of 1)--3) galaxy populations alone, but the composite of 1)--3) populations. We confirm that these 1)--3) populations also exist at $z\sim 0$, such as GPs and SDSS low-mass and high-SFR galaxies (LMHSs).

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arXiv2016

Clustering of infrared-bright dust-obscured galaxies revealed by the Hyper Suprime-Cam and WISE

We present measurements of the clustering properties of a sample of infrared (IR) bright dust-obscured galaxies (DOGs). Combining 125 deg$^2$ of wide and deep optical images obtained with the Hyper Suprime-Cam on the Subaru Telescope and all-sky mid-IR (MIR) images taken with Wide-Field Infrared Survey Explorer, we have discovered 4,367 IR-bright DOGs with $(i - [22])_{\rm AB}$ $>$ 7.0 and flux density at 22 $μ$m $>$ 1.0 mJy. We calculate the angular autocorrelation function (ACF) for a uniform subsample of 1411 DOGs with 3.0 mJy $<$ flux (22 $mu$m) $<$ 5.0 mJy and $i_{\rm AB}$ $<$ 24.0. The ACF of our DOG subsample is well-fit with a single power-law, $ω(θ)$ = (0.010 $\pm$ 0.003) $θ^{-0.9}$, where $θ$ in degrees. The correlation amplitude of IR-bright DOGs is larger than that of IR-faint DOGs, which reflects a flux-dependence of the DOG clustering, as suggested by Brodwin et al. (2008). We assume that the redshift distribution for our DOG sample is Gaussian, and consider 2 cases: (1) the redshift distribution is the same as IR-faint DOGs with flux at 22 $μ$m $<$ 1.0 mJy, mean and sigma $z$ = 1.99 $\pm$ 0.45, and (2) $z$ = 1.19 $\pm$ 0.30, as inferred from their photometric redshifts. The inferred correlation length of IR-bright DOGs is $r_0$ = 12.0 $\pm$ 2.0 and 10.3 $\pm$ 1.7 $h^{-1}$ Mpc, respectively. IR-bright DOGs reside in massive dark matter halos with a mass of $\log [\langle M_{\mathrm{h}} \rangle / (h^{-1} M_{\odot})]$ = 13.57$_{-0.55}^{+0.50}$ and 13.65$_{-0.52}^{+0.45}$ in the two cases, respectively.

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arXiv2016

Subaru high-z exploration of low-luminosity quasars (SHELLQs). I. Discovery of 15 quasars and bright galaxies at 5.7 < z < 6.9

We report the discovery of 15 quasars and bright galaxies at 5.7 < z < 6.9. This is the initial result from the Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs) project, which exploits the exquisite multiband imaging data produced by the Subaru Hyper Suprime-Cam (HSC) Strategic Program survey. The candidate selection is performed by combining several photometric approaches including a Bayesian probabilistic algorithm to reject stars and dwarfs. The spectroscopic identification was carried out with the Gran Telescopio Canarias and the Subaru Telescope for the first 80 deg2 of the survey footprint. The success rate of our photometric selection is quite high, approaching 100 % at the brighter magnitudes (zAB < 23.5 mag). Our selection also recovered all the known high-z quasars on the HSC images. Among the 15 discovered objects, six are likely quasars, while the other six with interstellar absorption lines and in some cases narrow emission lines are likely bright Lyman-break galaxies. The remaining three objects have weak continua and very strong and narrow Ly alpha lines, which may be excited by ultraviolet light from both young stars and quasars. These results indicate that we are starting to see the steep rise of the luminosity function of z > 6 galaxies, compared with that of quasars, at magnitudes fainter than M1450 ~ -22 mag or zAB ~24 mag. Follow-up studies of the discovered objects as well as further survey observations are ongoing.

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arXiv2016

Evolution of Stellar-to-Halo Mass Ratio at z=0-7 Identified by Clustering Analysis with the Hubble Legacy Imaging and Early Subaru/Hyper Suprime-Cam Survey Data

We present clustering analysis results from 10,381 Lyman break galaxies (LBGs) at z~ 4-7, identified in the Hubble legacy deep imaging and new complimentary large-area Subaru/Hyper Suprime-Cam data. We measure the angular correlation functions (ACFs) of these LBGs at z~4, 5, 6, and 7, and fit these measurements using halo occupation distribution (HOD) models that provide an estimate of halo masses, M_h~(1-20)x10^11 Msun. Our M_h estimates agree with those obtained by previous clustering studies in a UV-magnitude vs. M_h plane, and allow us to calculate stellar-to-halo mass ratios (SHMRs) of LBGs. By comparison with the z~0 SHMR, we identify evolution of the SHMR from z~0 to z~4, and z~4 to z~7 at the >98% confidence levels. The SHMR decreases by a factor of ~2 from z~0 to 4, and increases by a factor of ~4 from z~4 to 7. We compare our SHMRs with results of a hydrodynamic simulation and a semi-analytic model, and find that these theoretical studies do not predict the SHMR increase from z~4 to 7. We obtain the baryon conversion efficiency (BCE) of LBGs at z~4, and find that the BCE increases with increasing dark matter halo mass. Finally, we compare our clustering+HOD estimates with results from abundance matching techniques, and conclude that the M_h estimates of the clustering+HOD analyses agree with those of the simple abundance matching within a factor of 3, and that the agreement improves when using more sophisticated abundance matching techniques that include subhalos, incompleteness, and/or evolution in the star formation and stellar mass functions.

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arXiv2016

Very Compact Dense Galaxy Overdensity with δ ~ 130 Identified at z ~ 8: Implications for Early Protocluster and Cluster-Core Formation

We report the first identification of a compact dense galaxy overdensity at $z\sim8$ dubbed A2744z8OD. A2744z8OD consists of eight $Y$-dropout galaxies behind Abell 2744 that is originally pinpointed by Hubble Frontier Fields studies. However, no studies have, so far, derived basic physical quantities of structure formation or made comparisons with theoretical models. We obtain a homogeneous sample of dropout galaxies at $z\sim8$ from eight field data of Hubble legacy images that are as deep as the A2744z8OD data. Using the sample, we find that a galaxy surface overdensity value of A2744z8OD is very high $δ\simeq130$, where $δ$ is defined by an overdensity in a small circle of $6"$ ($\simeq30$ physical kpc) radius. Because there is no such a large $δ$ value reported for high-$z$ overdensities to date, A2744z8OD is a system clearly different from those found in previous high-$z$ overdensity studies. In the galaxy+structure formation models of Henriques et al. (2015), there exist a very similar overdensity, Modelz8OD, that is made of eight model dropout galaxies at $z\sim8$ in a $6"$-radius circle. Modelz8OD is a progenitor of a today's $10^{14}M_\odot$ cluster, and more than a half of the seven Modelz8OD galaxies are merged into the brightest cluster galaxy (BCG) of the cluster. If Modelz8OD is a counterpart of A2744z8OD, the models suggest that A2744z8OD would be a part of a forming cluster core of a today's $10^{14}M_\odot$ cluster that started star formation at $z>12$.

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arXiv2016

Morphologies of ~190,000 Galaxies at z=0-10 Revealed with HST Legacy Data II. Evolution of Clumpy Galaxies

We investigate evolution of clumpy galaxies with the Hubble Space Telescope (HST) samples of ~17,000 photo-z and Lyman break galaxies at z~0-8. We detect clumpy galaxies with off-center clumps in a self-consistent algorithm that is well tested with previous study results, and measure the number fraction of clumpy galaxies at the rest-frame UV, f_clumpy^UV. We identify an evolutionary trend of f_clumpy^UV over z~0-8 for the first time: f_clumpy^UV increases from z~8 to z~1-3 and subsequently decreases from z~1 to z~0, which follows the trend of Madau-Lilly plot. A low average Sersic index of n~1 is found in the underlining components of our clumpy galaxies at z~0-2, indicating that typical clumpy galaxies have disk-like surface brightness profiles. Our f_clumpy^UV values correlate with physical quantities related to star formation activities for star-forming galaxies at z~0-7. We find that clump colors tend to be red at a small galactocentric distance for massive galaxies with log(M_*/M_sun)>~11. All of these results are consistent with a picture that a majority of clumps form in the violent disk instability and migrate into the galactic centers.

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arXiv2015

Morphologies of ~190,000 Galaxies at z=0-10 Revealed with HST Legacy Data I. Size Evolution

We present redshift evolution of galaxy effective radius r_e obtained from the HST samples of ~190,000 galaxies at z=0-10. Our HST samples consist of 176,152 photo-z galaxies at z=0-6 from the 3D-HST+CANDELS catalogue and 10,454 LBGs at z=4-10 identified in CANDELS, HUDF09/12, and HFF parallel fields, providing the largest data set to date for galaxy size evolution studies. We derive r_e with the same technique over the wide-redshift range of z=0-10, evaluating the optical-to-UV morphological K-correction and the selection bias of photo-z galaxies+LBGs as well as the cosmological surface brightness dimming effect. We find that r_e values at a given luminosity significantly decrease towards high-z, regardless of statistics choices. For star-forming galaxies, there is no evolution of the power-law slope of the size-luminosity relation and the median Sersic index (n~1.5). Moreover, the r_e-distribution is well represented by log-normal functions whose standard deviation σ_{\ln{r_e}} does not show significant evolution within the range of σ_{\ln{r_e}}~0.45-0.75. We calculate the stellar-to-halo size ratio from our r_e measurements and the dark-matter halo masses estimated from the abundance matching study, and obtain a nearly constant value of r_e/r_vir=1.0-3.5% at z=0-8. The combination of the r_e-distribution shape+standard deviation, the constant r_e/r_vir, and n~1.5 suggests a picture that typical high-z star-forming galaxies have disk-like stellar components in a sense of dynamics and morphology over cosmic time of z~0-6. If high-z star-forming galaxies are truly dominated by disks, the r_e/r_vir value and the disk formation model indicate that the specific angular momentum of the disk normalized by the host halo is j_d/m_d=0.5-1. These are statistical results for galaxies' major stellar components, and the detailed study of clumpy sub-components is presented in the paper II.

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arXiv2014

MOSFIRE and LDSS3 Spectroscopy for an [OII] Blob at z=1.18: Gas Outflow and Energy Source

We report our Keck/MOSFIRE and Magellan/LDSS3 spectroscopy for an [OII] Blob, OIIB10, that is a high-$z$ galaxy with spatially extended [OII]$λ\lambda3726,3729$ emission over 30 kpc recently identified by a Subaru large-area narrowband survey. The systemic redshift of OIIB10 is $z=1.18$ securely determined with [OIII]$λ\lambda4959,5007$ and H$β$ emission lines. We identify FeII$λ$2587 and MgII$λλ$2796,2804 absorption lines blueshifted from the systemic redshift by $80\pm50$ and $260\pm40$ km s$^{-1}$, respectively, which indicate gas outflow from OIIB10 with the velocity of $\sim 80-260$ km s$^{-1}$. This outflow velocity is comparable with the escape velocity, $250\pm140$ km s$^{-1}$, estimated under the assumption of a singular isothermal halo potential profile. Some fraction of the outflowing gas could escape from the halo of OIIB10, suppressing OIIB10's star-formation activity. We estimate a mass loading factor, $η$, that is a ratio of mass outflow rate to star-formation rate, and obtain $η>0.8\pm 0.1$ which is relatively high compared with low-$z$ starbursts including U/LIRGs and AGNs. The major energy source of the outflow is unclear with the available data. Although no signature of AGN is found in the X-ray data, OIIB10 falls in the AGN/star-forming composite region in the line diagnostic diagrams. It is possible that the outflow is powered by star formation and a type-2 AGN with narrow FWHM emission line widths of $70-130$ km s$^{-1}$. This is the first detailed spectroscopic study of oxygen-line blobs, which includes the analyses of the escape velocity, the mass loading factor, and the presence of an AGN, and a significant step to understanding the nature of oxygen-line blobs and the relation with gas outflow and star-formation quenching at high redshift.

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