arXiv subjects

Tohru Nagao

Publications and source records attributed to Tohru Nagao.

At least 163 records · Page 9Linked to original sources

arXiv2017

A 16 deg$^2$ survey of emission-line galaxies at z<1.5 in HSC-SSP PDR1

We present initial results from the Subaru Strategic Program (SSP) with Hyper Suprime-Cam (HSC) on a comprehensive survey of emission-line galaxies at z<1.5 based on narrowband (NB) imaging. The first Public Data Release (PDR1) provides us with data from two NB filters, specifically NB816 and NB921 over 5.7 deg$^2$ and 16.2 deg$^2$ respectively. The $5 σ$ limiting magnitudes are 25.2 (UDeep layer, 1.4 deg$^2$) and 24.8 (Deep layer, 4.3 deg$^2$) mag in NB816, and 25.1 (UDeep, 2.9 deg$^2$) and 24.6--24.8 (Deep, 13.3 deg$^2$) mag in NB921. The wide-field imaging allows us to construct unprecedentedly large samples of 8,054 H$α$ emitters at z ~ 0.25 and 0.40, 8,656 [OIII] emitters at z ~ 0.63 and 0.84, and 16,877 [OII] emitters at z ~ 1.19 and 1.47. We map the cosmic web on scales out to about 50 comoving Mpc that includes galaxy clusters, identified by red sequence galaxies, located at the intersection of filamentary structures of star-forming galaxies. The luminosity functions of emission-line galaxies are measured with precision and consistent with published studies. The wide field coverage of the data enables us to measure the luminosity functions up to brighter luminosities than previous studies. The comparison of the luminosity functions between the different HSC-SSP fields suggests that a survey volume of $>5\times10^5$ Mpc$^3$ is essential to overcome cosmic variance. Since the current data have not reached the full depth expected for the HSC-SSP, the color cut in i-NB816 or z-NB921 induces a bias towards star-forming galaxies with large equivalent widths, primarily seen in the stellar mass functions for the H$α$ emitters at z ~ 0.25--0.40. Even so, the emission-line galaxies clearly cover a wide range of luminosity, stellar mass, and environment, thus demonstrating the usefulness of the NB data from the HSC-SSP to investigate star-forming galaxies at z<1.5.

astro-ph.GA↗

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/.

astro-ph.IM↗

arXiv2017

Active Galactic Nuclei with a Low-Metallicity Narrow-Line Region

Low-metallicity active galactic nuclei (AGNs) are interesting to study the early phase of the AGN evolution. However most AGNs are chemically matured and accordingly low-metallicity AGNs are extremely rare. One approach to search for low-metallicity AGNs systematically is utilizing the so-called BPT diagram that consists of the [O III]$λ$5007/H$β$$\lambda4861$ and [N II]$\lambda6584$/H$α$$\lambda6563$ flux ratios. Specifically, photoionization models predict that low-metallicity AGNs show a high [O III]$λ$5007/H$β$$λ$4861 ratio and a relatively low [N II]$λ$6584/H$α$$λ$6563 ratio, that corresponds to the location between the sequence of star-forming galaxies and that of usual AGNs on the BPT diagram (hereafter "the BPT valley"). However, other populations of galaxies such as star-forming galaxies and AGNs with a high electron density or a high ionization parameter could be also located in the BPT valley, not only low-metallicity AGNs. In this paper, we examine whether most of emission-line galaxies at the BPT valley are low-metallicity AGNs or not. We select 70 BPT-valley objects from 212,866 emission line galaxies obtained by the Sloan Digital Sky Survey. Among the 70 BPT-valley objects, 43 objects show firm evidence of the AGN activity; i.e., the He II$λ$4686 emission and/or weak but significant broad H$α$ emission. Our analysis shows that those 43 BPT-valley AGNs are not characterized by a very high gas density nor ionization parameter, inferring that at least 43 among 70 BPT-valley objects (i.e., $>60$ %) are low-metallicity AGNs. This suggests that the BPT diagram is an efficient tool to search for low-metallicity AGNs.

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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.

astro-ph.GA↗

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.

astro-ph.GA↗

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 $0.001$-$0.06$.

astro-ph.GA↗

arXiv2017

The Quasar Luminosity Function at Redshift 4 with Hyper Suprime-Cam Wide Survey

We present the luminosity function of z=4 quasars based on the Hyper Suprime-Cam Subaru Strategic Program Wide layer imaging data in the g, r, i, z, and y bands covering 339.8 deg^2. From stellar objects, 1666 z~4 quasar candidates are selected by the g-dropout selection down to i=24.0 mag. Their photometric redshifts cover the redshift range between 3.6 and 4.3 with an average of 3.9. In combination with the quasar sample from the Sloan Digital Sky Survey in the same redshift range, the quasar luminosity function covering the wide luminosity range of M1450=-22 to -29 mag is constructed. It is well described by a double power-law model with a knee at M1450=-25.36+-0.13 mag and a flat faint-end slope with a power-law index of -1.30+-0.05. The knee and faint-end slope show no clear evidence of redshift evolution from those at z~2. The flat slope implies that the UV luminosity density of the quasar population is dominated by the quasars around the knee, and does not support the steeper faint-end slope at higher redshifts reported at z>5. If we convert the M1450 luminosity function to the hard X-ray 2-10keV luminosity function using the relation between UV and X-ray luminosity of quasars and its scatter, the number density of UV-selected quasars matches well with that of the X-ray-selected AGNs above the knee of the luminosity function. Below the knee, the UV-selected quasars show a deficiency compared to the hard X-ray luminosity function. The deficiency can be explained by the lack of obscured AGNs among the UV-selected quasars.

astro-ph.GA↗

arXiv2017

Far-infrared properties of infrared bright dust-obscured galaxies selected with IRAS and AKARI far-infrared all-sky survey

We investigate the star forming activity of a sample of infrared (IR)-bright dust-obscured galaxies (DOGs) that show an extreme red color in the optical and IR regime, $(i - [22])_{\rm AB} > 7.0$. Combining an IR-bright DOG sample with the flux at 22 $μ$m $>$ 3.8 mJy discovered by Toba & Nagao (2016) with IRAS faint source catalog version 2 and AKARI far-IR (FIR) all-sky survey bright source catalog version 2, we selected 109 DOGs with FIR data. For a subsample of 7 IR-bright DOGs with spectroscopic redshift ($0.07 < z < 1.0$) that was obtained from literature, we estimated their IR luminosity, star formation rate (SFR), and stellar mass based on the spectral energy distribution fitting. We found that (i) WISE 22 $μ$m luminosity at observed frame is a good indicator of IR luminosity for IR-bright DOGs and (ii) the contribution of active galactic nucleus (AGN) to IR luminosity increases with IR luminosity. By comparing the stellar mass and SFR relation for our DOG sample and literature, we found that most of IR-bright DOGs lie significantly above the main sequence of star-forming galaxies at similar redshift, indicating that the majority of IRAS- and/or AKARI-detected IR-bright DOGs are starburst galaxies.

astro-ph.GA↗

arXiv2017

Molecular gas on large circumgalactic scales at z=3.47

We report ALMA observations of the most massive (star forming) galaxy in the redshift range 3<z<4 within the whole GOODS-S field. We detect a large elongated structure of molecular gas around the massive primeval galaxy, traced by the CO(4-3) emission, and extended over 40 kpc. We infer a mass of the large gaseous structure of Mgas~2-6x10^11 Msun. About 60% of this mass is not directly associated with either the central galaxy or its two lower mass satellites. The CO extended structure is also detected in continuum thermal emission. The kinematics of the molecular gas shows the presence of different components, which cannot be ascribed to simple rotation. Furthermore, on even larger scales, we detect nine additional CO systems within a radius of 250 kpc from the massive galaxy and mostly distributed in the same direction as the CO elongated structure found in the central 40 kpc. The stacked images of these CO systems show detections in the thermal continuum and in the X-rays, suggesting that these systems are forming stars at a rate of 30-120 Msun/yr. We suggest that the extended gas structure, combined with its kinematic properties, and the gas rich star forming systems detected on larger scales, are tracing the inner and densest regions of large scale accreting streams, feeding the central massive galaxy. These results corroborate models of galaxy formation, in which accreting streams are clumpy and undergo some star formation (hence enriching the streams with metals) even before accreting onto the central galaxy.

astro-ph.GA↗

arXiv2017

A search for Ly-alpha emitters around a concentrated-region of strong Ly-alpha absorbers at z=2.3

In order to investigate the physical relationship between strong Ly-alpha absorbers (logN_HI>20.0 cm^-2) such as damped Ly-alpha absorption systems (DLAs) and young star-forming galaxies at high redshift, we have conducted narrow-band observations of Ly-alpha emitters (LAEs) in a concentrated region of strong Ly-alpha absorbers at z= 2.3, the J1230+34 field. Using a catalog of Ly-alpha absorbers with logN_HI>20.0 cm^-2 based on the baryon oscillation spectroscopic survey (BOSS), we found 6 fields where 3 or more absorbers are concentrated within a (50 Mpc)^3 cubic box in the comoving scale. Among them, we focus on the J1230+34 field, where 2 DLAs and 2 sub-DLAs present. Our narrow-band imaging observations with Subaru/Suprime-Cam using a custom-made filter, NB400 (lambda_c=4003 A and FWHM=92 A) yield a sample of 149 LAEs in this field. In the large scale (~50 Mpc), we have found no differences between the obtained Ly-alpha luminosity function and those in the blank fields at similar redshifts. We also compare the frequency distribution of the Ly-alpha rest-frame equivalent width (EW_0) in the target field and other fields including both overdensity region and blank field, but find no differences. On the other hand, in the small scale (~10 Mpc), we have found a possible overdensity of LAEs around a DLA with the highest HI column density (N_HI=21.08 cm^-2) in the target field while there are no density excess around the other absorbers with a lower N_HI.

astro-ph.GA↗

arXiv2017

A new quadruple gravitational lens from the Hyper Suprime-Cam Survey: the puzzle of HSC~J115252+004733

We report the serendipitous discovery of a quadruply lensed source at $z_{\rm s}=3.76$, HSC~J115252+004733, from the Hyper Suprime-Cam (HSC) Survey. The source is lensed by an early-type galaxy at $z_{\rm l}=0.466$ and a satellite galaxy. Here, we investigate the properties of the source by studying its size and luminosity from the imaging and the luminosity and velocity width of the Ly-$α$ line from the spectrum. Our analyses suggest that the source is most probably a low-luminosity active galactic nucleus (LLAGN) but the possibility of it being a compact bright galaxy (e.g., a Lyman-$α$ emitter or Lyman Break Galaxy) cannot be excluded. The brighter pair of lensed images appears point-like except in the HSC $i$-band (with a seeing $\sim0.5"$). The extended emission in the $i$-band image could be due to the host galaxy underneath the AGN, or alternatively, due to a highly compact lensed galaxy (without AGN) which appears point-like in all bands except in $i$-band. We also find that the flux ratio of the brighter pair of images is different in the Ks-band compared to optical wavelengths. Phenomena such as differential extinction and intrinsic variability cannot explain this chromatic variation. While microlensing from stars in the foreground galaxy is less likely to be the cause, it cannot be ruled out completely. If the galaxy hosts an AGN, then this represents the highest redshift quadruply imaged AGN known to date, enabling study of a distant LLAGN. Discovery of this unusually compact and faint source demonstrates the potential of the HSC survey.

astro-ph.CO↗

arXiv2016

ALMA reveals Strong [CII] emission in a galaxy embedded in a giant Lyman-alpha blob at z=3.1

We report the result from observations conducted with the Atacama Large Millimeter/submillimeter Array (ALMA) to detect [CII] 158 um fine structure line emission from galaxies embedded in one of the most spectacular Lyman-alpha blobs (LABs) at z=3.1, SSA22-LAB1. Of three dusty star-forming galaxies previously discovered by ALMA 860 um dust continuum survey toward SSA22-LAB1, we detected the [CII] line from one, LAB1-ALMA3 at z=3.0993+/-0.0004. No line emission was detected, associated with the other ALMA continuum sources or from three rest-frame UV/optical selected z_spec~3.1 galaxies within the field of view. For LAB1-ALMA3, we find relatively bright [CII] emission compared to the infrared luminosity (L_[CII]/L_[CII]) and an extremely high [CII] 158 um and [NII] 205 um emission line ratio (L_[CII]/L_[NII]>55). The relatively strong [CII] emission may be caused by abundant photodissociation regions and sub-solar metallicity, or by shock heating. The origin of the unusually strong [CII] emission could be causally related to the location within the giant LAB, although the relationship between extended Lyman-alpha emission and ISM conditions of associated galaxies is yet to be understand.

astro-ph.GA↗

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.

astro-ph.GA↗

arXiv2016

Near-infrared spectroscopy of nearby Seyfert galaxies: Is there evidence for the shock excitation in narrow-line regions?

One of the unsettled, important problems in active galactic nuclei (AGNs) is the major ionization mechanism of gas clouds in their narrow-line regions (NLRs). In order to investigate this issue, we present our $J$-band spectroscopic observations for a sample of 26 nearby Seyfert galaxies. In our study, we use the flux ratio of the following two forbidden emission lines, [Fe II]1.257$μ$m and [P II]1.188$μ$m because it is known that this ratio is sensitive to the ionization mechanism. We obtain the [Fe II]/[P II] flux ratio or its lower limit for 19 objects. In addition to our data, we compile this flux ratio (or its lower limit) of 23 nearby Seyfert galaxies from the literature. Based on the collected data, we find that three Seyfert galaxies show very large lower limits of [Fe II]/[P II] flux ratios ($\gtrsim$10); NGC 2782, 5005, and Mrk 463. It is thus suggested that the contribution of the fast shock in the gas excitation is significantly large for them. However, more than half of Seyfert galaxies in our sample show moderate [Fe II]/[P II] flux ratios ($\sim$2), that is consistent to the pure photoionization by power-law ionizing continuum emission. We also find that the [Fe II]/[P II] flux ratio shows no clear correlation with the radio loudness, suggesting that the radio jet is not the primary origin of shocks in NLRs of Seyfert galaxies.

astro-ph.GA↗

arXiv2016

Revisiting the Completeness and the Luminosity Function in High-Redshift Low-Luminosity Quasar Surveys

Recent studies have derived quasar luminosity functions (QLFs) at various redshifts. However, the faint side of the QLF at high redshifts is still too uncertain. An accurate estimate of the survey completeness is essential to derive an accurate QLF for use in studying the luminosity-dependent density evolution of the quasar population. Here we investigate how the luminosity dependence of quasar spectra (the Baldwin effect) and the attenuation model for the inter-galactic medium (IGM) affect the completeness estimates. For this purpose, we revisit the completeness of quasar surveys specifically at $z\sim4-5$, using the COSMOS images observed with Subaru/Suprime-Cam. As the result, we find that the completeness estimates are sensitive to the luminosity dependence of the quasar spectrum and difference in the IGM attenuation models. At $z\sim4$, the number density of quasars when we adopt the latest IGM model and take the luminosity dependence of spectra into account are $(3.49\pm1.62)\times10^{-7}$ Mpc$^{-3}$ mag$^{-1}$ for $-24.09<M_{1450}<-23.09$ and $(5.24\pm2.13)\times10^{-7}$ Mpc$^{-3}$ mag$^{-1}$ for $-23.09<M_{1450}<-22.09$, respectively, which are $\sim$$24$$\%$ lower than that estimated by the conventional method. On the other hand, at $z\sim5$, a $1σ$ confidence upper limit of the number density at $-24.5<M_{1450}<-22.5$ in our new estimates is $\sim$$43$$\%$ higher than that estimated previously. The results suggest that the luminosity dependence of the quasar spectrum and the IGM model are important for deriving accurate number density of high-$z$ quasars. Even taking these effects into account, the inferred luminosity-dependent density evolution of quasars is consistent with the AGN down-sizing evolutionary picture.

astro-ph.GA↗

arXiv2016

The Metal Abundances across Cosmic Time ($\mathcal{MACT}$) Survey. II. Evolution of the Mass-Metallicity Relation over 8 Billion Years, using [OIII]$λ$4363Å-based Metallicities

We present the first results from MMT and Keck spectroscopy for a large sample of $0.1\leq z\leq1$ emission-line galaxies selected from our narrow-band imaging in the Subaru Deep Field. We measured the weak [OIII]$λ$4363 emission line for 164 galaxies (66 with at least 3$σ$ detections, and 98 with significant upper limits). The strength of this line is set by the electron temperature for the ionized gas. Because the gas temperature is regulated by the metal content, the gas-phase oxygen abundance is inversely correlated with [OIII]$λ$4363 line strength. Our temperature-based metallicity study is the first to span $\approx$8 Gyr of cosmic time and $\approx$3 dex in stellar mass for low-mass galaxies, $\log{\left(M_{\rm star}/M_{\rm sun}\right)}\approx6.0-9.0$. Using extensive multi-wavelength photometry, we measure the evolution of the stellar mass--gas metallicity relation and its dependence on dust-corrected star formation rate (SFR). The latter is obtained from high signal-to-noise Balmer emission-line measurements. Our mass-metallicity relation is consistent with Andrews & Martini at $z\leq0.3$, and evolves toward lower abundances at a given stellar mass, $\log{({\rm O/H})}\propto(1+z)^{-2.32^{+0.52}_{-0.26}}$. We find that galaxies with lower metallicities have higher SFRs at a given stellar mass and redshift, although the scatter is large ($\approx$0.3 dex), and the trend is weaker than seen in local studies. We also compare our mass--metallicity relation against predictions from high-resolution galaxy formation simulations, and find good agreement with models that adopt energy- and momentum-driven stellar feedback. We have identified 16 extremely metal-poor galaxies with abundances less than a tenth of solar; our most metal-poor galaxy at $z\approx0.84$ is similar to I Zw 18.

astro-ph.GA↗

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.

astro-ph.GA↗

arXiv2016

An Effective Selection Method for Low-Mass Active Black Holes and First Spectroscopic Identification

We present a new method to effectively select objects which may be low-mass active black holes (BHs) at galaxy centers using high-cadence optical imaging data, and our first spectroscopic identification of an active 2.7x10^6 Msun BH at z=0.164. This active BH was originally selected due to its rapid optical variability, from a few hours to a day, based on Subaru Hyper Suprime-Cam~(HSC) g-band imaging data taken with 1-hour cadence. Broad and narrow H-alpha and many other emission lines are detected in our optical spectra taken with Subaru FOCAS, and the BH mass is measured via the broad H-alpha emission line width (1,880 km s^{-1}) and luminosity (4.2x10^{40} erg s^{-1}) after careful correction for the atmospheric absorption around 7,580-7,720A. We measure the Eddington ratio to be as low as 0.05, considerably smaller than those in a previous SDSS sample with similar BH mass and redshift, which indicates one of the strong potentials of our Subaru survey. The g-r color and morphology of the extended component indicate that the host galaxy is a star-forming galaxy. We also show effectiveness of our variability selection for low-mass active BHs.

astro-ph.GA↗