arXiv subjects

Masayuki Akiyama

Publications and source records attributed to Masayuki Akiyama.

At least 91 records · Page 5Linked to original sources

arXiv2016

Constraint on the inflow/outflow rates in star-forming galaxies at z~1.4 from molecular gas observations

We constrain the rate of gas inflow into and outflow from a main-sequence star-forming galaxy at z~1.4 by fitting a simple analytic model for the chemical evolution in a galaxy to the observational data of the stellar mass, metallicity, and molecular gas mass fraction. The molecular gas mass is derived from CO observations with a metallicity-dependent CO-to-H2 conversion factor, and the gas metallicity is derived from the Hα and [NII]λ 6584 emission line ratio. Using a stacking analysis of CO integrated intensity maps and the emission lines of Hα and [NII], the relation between stellar mass, metallicity, and gas mass fraction is derived. We constrain the inflow and outflow rates with least-chi-square fitting of a simple analytic chemical evolution model to the observational data. The best-fit inflow and outflow rates are ~1.7 and ~0.4 in units of star-formation rate, respectively. The inflow rate is roughly comparable to the sum of the star-formation rate and outflow rate, which supports the equilibrium model for galaxy evolution; i.e., all inflow gas is consumed by star formation and outflow.

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arXiv2016

A New Milky Way Satellite Discovered In The Subaru/Hyper Suprime-Cam Survey

We report the discovery of a new ultra-faint dwarf satellite companion of the Milky Way based on the early survey data from the Hyper Suprime-Cam Subaru Strategic Program. This new satellite, Virgo I, which is located in the constellation of Virgo, has been identified as a statistically significant (5.5 sigma) spatial overdensity of star-like objects with a well-defined main sequence and red giant branch in their color-magnitude diagram. The significance of this overdensity increases to 10.8 sigma when the relevant isochrone filter is adopted for the search. Based on the distribution of the stars around the likely main sequence turn-off at r ~ 24 mag, the distance to Virgo I is estimated as 87 kpc, and its most likely absolute magnitude calculated from a Monte Carlo analysis is M_V = -0.8 +/- 0.9 mag. This stellar system has an extended spatial distribution with a half-light radius of 38 +12/-11 pc, which clearly distinguishes it from a globular cluster with comparable luminosity. Thus, Virgo I is one of the faintest dwarf satellites known and is located beyond the reach of the Sloan Digital Sky Survey. This demonstrates the power of this survey program to identify very faint dwarf satellites. This discovery of VirgoI is based only on about 100 square degrees of data, thus a large number of faint dwarf satellites are likely to exist in the outer halo of the Milky Way.

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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

Multi time-step wave-front reconstruction for tomographic Adaptive-Optics systems

In tomographic adaptive-optics (AO) systems, errors due to tomographic wave-front reconstruction limit the performance and angular size of the scientific field of view (FoV), where AO correction is effective. We propose a multi time-step tomographic wave-front reconstruction method to reduce the tomographic error by using the measurements from both the current and the previous time-steps simultaneously. We further outline the method to feed the reconstructor with both wind speed and direction of each turbulence layer. An end-to-end numerical simulation, assuming a multi-object AO (MOAO) system on a 30 m aperture telescope, shows that the multi time-step reconstruction increases the Strehl ratio (SR) over a scientific FoV of 10 arcminutes in diameter by a factor of 1.5--1.8 when compared to the classical tomographic reconstructor, depending on the guide star asterism and with perfect knowledge of wind speeds and directions. We also evaluate the multi time-step reconstruction method and the wind estimation method on the RAVEN demonstrator under laboratory setting conditions. The wind speeds and directions at multiple atmospheric layers are measured successfully in the laboratory experiment by our wind estimation method with errors below 2 \ms. With these wind estimates, the multi time-step reconstructor increases the SR value by a factor of 1.2--1.5, which is consistent with a prediction from end-to-end numerical simulation.

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arXiv2016

The Subaru FMOS Galaxy Redshift Survey (FastSound). II. The Emission Line Catalog and Properties of Emission Line Galaxies

We present basic properties of $\sim$3,300 emission line galaxies detected by the FastSound survey, which are mostly H$α$ emitters at $z \sim$ 1.2-1.5 in the total area of about 20 deg$^2$, with the H$α$ flux sensitivity limit of $\sim 1.6 \times 10^{-16} \rm erg \ cm^{-2} s^{-1}$ at 4.5 sigma. This paper presents the catalogs of the FastSound emission lines and galaxies, which will be open to the public in the near future. We also present basic properties of typical FastSound H$α$ emitters, which have H$α$ luminosities of $10^{41.8}$-$10^{43.3}$ erg/s, SFRs of 20--500 $M_\odot$/yr, and stellar masses of $10^{10.0}$--$10^{11.3}$ $M_\odot$. The 3D distribution maps for the four fields of CFHTLS W1--4 are presented, clearly showing large scale clustering of galaxies at the scale of $\sim$ 100--600 comoving Mpc. Based on 1,105 galaxies with detections of multiple emission lines, we estimate that contamination of non-H$α$ lines is about 4% in the single-line emission galaxies, which are mostly [OIII]$λ$5007. This contamination fraction is also confirmed by the stacked spectrum of all the FastSound spectra, in which H$α$, [NII]$λλ$6548,6583, [SII]$λλ$6717, 6731, and [OI]$λλ$6300,6364 are seen.

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arXiv2016

The Subaru FMOS galaxy redshift survey (FastSound). IV. New constraint on gravity theory from redshift space distortions at $z\sim 1.4$

We measure the redshift-space correlation function from a spectroscopic sample of 2783 emission line galaxies from the FastSound survey. The survey, which uses the Subaru Telescope and covers the redshift ranges of $1.19<z<1.55$, is the first cosmological study at such high redshifts. We detect clear anisotropy due to redshift-space distortions (RSD) both in the correlation function as a function of separations parallel and perpendicular to the line of sight and its quadrupole moment. RSD has been extensively used to test general relativity on cosmological scales at $z<1$. Adopting a LCDM cosmology with the fixed expansion history and no velocity dispersion $σ_{\rm v}=0$, and using the RSD measurements on scales above 8Mpc/h, we obtain the first constraint on the growth rate at the redshift, $f(z)σ_8(z)=0.482\pm 0.116$ at $z\sim 1.4$ after marginalizing over the galaxy bias parameter $b(z)σ_8(z)$. This corresponds to $4.2σ$ detection of RSD. Our constraint is consistent with the prediction of general relativity $fσ_8\sim 0.392$ within the $1-σ$ confidence level. When we allow $σ_{\rm v}$ to vary and marginalize it over, the growth rate constraint becomes $fσ_8=0.494^{+0.126}_{-0.120}$. We also demonstrate that by combining with the low-z constraints on $fσ_8$, high-z galaxy surveys like the FastSound can be useful to distinguish modified gravity models without relying on CMB anisotropy experiments.

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arXiv2016

Properties of the Interstellar Medium in Star-Forming Galaxies at z~1.4 revealed with ALMA

We conducted observations of 12CO(J=5-4) and dust thermal continuum emission toward twenty star-forming galaxies on the main sequence at z~1.4 using ALMA to investigate the properties of the interstellar medium. The sample galaxies are chosen to trace the distributions of star-forming galaxies in diagrams of stellar mass-star formation rate and stellar mass-metallicity. We detected CO emission lines from eleven galaxies. The molecular gas mass is derived by adopting a metallicity-dependent CO-to-H2 conversion factor and assuming a CO(5-4)/CO(1-0) luminosity ratio of 0.23. Molecular gas masses and its fractions (molecular gas mass/(molecular gas mass + stellar mass)) for the detected galaxies are in the ranges of (3.9-12) x 10^{10} Msun and 0.25-0.94, respectively; these values are significantly larger than those in local spiral galaxies. The molecular gas mass fraction decreases with increasing stellar mass; the relation holds for four times lower stellar mass than that covered in previous studies, and that the molecular gas mass fraction decreases with increasing metallicity. Stacking analyses also show the same trends. The dust thermal emissions were clearly detected from two galaxies and marginally detected from five galaxies. Dust masses of the detected galaxies are (3.9-38) x 10^{7} Msun. We derived gas-to-dust ratios and found they are 3-4 times larger than those in local galaxies. The depletion times of molecular gas for the detected galaxies are (1.4-36) x 10^{8} yr while the results of the stacking analysis show ~3 x 10^{8} yr. The depletion time tends to decrease with increasing stellar mass and metallicity though the trend is not so significant, which contrasts with the trends in local galaxies.

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arXiv2015

The Subaru FMOS Galaxy Redshift Survey (FastSound). III. The mass-metallicity relation and the fundamental metallicity relation at $z\sim1.4$

We present the results from a large near-infrared spectroscopic survey with Subaru/FMOS (\textit{FastSound}) consisting of $\sim$ 4,000 galaxies at $z\sim1.4$ with significant H$α$ detection. We measure the gas-phase metallicity from the [N~{\sc ii}]$λ$6583/H$α$ emission line ratio of the composite spectra in various stellar mass and star-formation rate bins. The resulting mass-metallicity relation generally agrees with previous studies obtained in a similar redshift range to that of our sample. No clear dependence of the mass-metallicity relation with star-formation rate is found. Our result at $z\sim1.4$ is roughly in agreement with the fundamental metallicity relation at $z\sim0.1$ with fiber aperture corrected star-formation rate. We detect significant [S~{\sc ii}]$λλ$6716,6731 emission lines from the composite spectra. The electron density estimated from the [S~{\sc ii}]$λλ$6716,6731 line ratio ranges from 10 -- 500 cm$^{-3}$, which generally agrees with that of local galaxies. On the other hand, the distribution of our sample on [N~{\sc ii}]$λ$6583/H$α$ vs. [S~{\sc ii}]$λλ$6716,6731/H$α$ is different from that found locally. We estimate the nitrogen-to-oxygen abundance ratio (N/O) from the N2S2 index, and find that the N/O in galaxies at $z\sim1.4$ is significantly higher than the local values at a fixed metallicity and stellar mass. The metallicity at $z\sim1.4$ recalculated with this N/O enhancement taken into account decreases by 0.1 -- 0.2 dex. The resulting metallicity is lower than the local fundamental metallicity relation.

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arXiv2015

Optical-Infrared Properties of Faint 1.3 mm Sources Detected with ALMA

We report optical-infrared (IR) properties of faint 1.3 mm sources (S_1.3mm = 0.2-1.0 mJy) detected with the Atacama Large Millimeter/submillimeter Array (ALMA) in the Subaru/XMM-Newton Deep Survey (SXDS) field. We searched for optical/IR counterparts of 8 ALMA-detected sources (>=4.0 sigma, the sum of the probability of spurious source contamination is ~1) in a K-band source catalog. Four ALMA sources have K-band counterpart candidates within a 0.4" radius. Comparison between ALMA-detected and undetected K-band sources in the same observing fields shows that ALMA-detected sources tend to be brighter, more massive, and more actively forming stars. While many of the ALMA-identified submillimeter-bright galaxies (SMGs) in previous studies lie above the sequence of star-forming galaxies in stellar mass--star-formation rate plane, our ALMA sources are located in the sequence, suggesting that the ALMA-detected faint sources are more like `normal' star-forming galaxies rather than `classical' SMGs. We found a region where multiple ALMA sources and K-band sources reside in a narrow photometric redshift range (z ~ 1.3-1.6) within a radius of 5" (42 kpc if we assume z = 1.45). This is possibly a pre-merging system and we may be witnessing the early phase of formation of a massive elliptical galaxy.

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arXiv2015

The Subaru-XMM-Newton Deep Survey (SXDS) VIII.: Multi-wavelength Identification, Optical/NIR Spectroscopic Properties, and Photometric Redshifts of X-ray Sources

We report the multi-wavelength identification of the X-ray sources found in the Subaru-XMM-Newton Deep Survey (SXDS) using deep imaging data covering the wavelength range between the far-UV to the mid-IR. We select a primary counterpart of each X-ray source by applying the likelihood ratio method to R-band, 3.6micron, near-UV, and 24micron source catalogs as well as matching catalogs of AGN candidates selected in 1.4GHz radio and i'-band variability surveys. Once candidates of Galactic stars, ultra-luminous X-ray sources in a nearby galaxy, and clusters of galaxies are removed there are 896 AGN candidates in the sample. We conduct spectroscopic observations of the primary counterparts with multi-object spectrographs in the optical and NIR; 65\% of the X-ray AGN candidates are spectroscopically-identified. For the remaining X-ray AGN candidates, we evaluate their photometric redshift with photometric data in 15 bands. Utilising the multi-wavelength photometric data of the large sample of X-ray selected AGNs, we evaluate the stellar masses, M*, of the host galaxies of the narrow-line AGNs. The distribution of the stellar mass is remarkably constant from z=0.1 to 4.0. The relation between M* and 2--10 keV luminosity can be explained with strong cosmological evolution of the relationship between the black hole mass and M*. We also evaluate the scatter of the UV-MIR spectral energy distribution (SED) of the X-ray AGNs as a function of X-ray luminosity and absorption to the nucleus. The scatter is compared with galaxies which have redshift and stellar mass distribution matched with the X-ray AGN. The UV-NIR SEDs of obscured X-ray AGNs are similar to those of the galaxies in the matched sample. In the NIR-MIR range, the median SEDs of X-ray AGNs are redder, but the scatter of the SEDs of the X-ray AGN broadly overlaps that of the galaxies in the matched sample.

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arXiv2015

The Subaru FMOS Galaxy Redshift Survey (FastSound). I. Overview of the Survey Targeting on H$α$ Emitters at $z \sim 1.4$

FastSound is a galaxy redshift survey using the near-infrared Fiber Multi-Object Spectrograph (FMOS) mounted on the Subaru Telescope, targeting H$α$ emitters at $z \sim 1.18$--$1.54$ down to the sensitivity limit of H$α$ flux $\sim 2 \times 10^{-16} \ \rm erg \ cm^{-2} s^{-1}$. The primary goal of the survey is to detect redshift space distortions (RSD), to test General Relativity by measuring the growth rate of large scale structure and to constrain modified gravity models for the origin of the accelerated expansion of the universe. The target galaxies were selected based on photometric redshifts and H$α$ flux estimates calculated by fitting spectral energy distribution (SED) models to the five optical magnitudes of the Canada France Hawaii Telescope Legacy Survey (CFHTLS) Wide catalog. The survey started in March 2012, and all the observations were completed in July 2014. In total, we achieved $121$ pointings of FMOS (each pointing has a $30$ arcmin diameter circular footprint) covering $20.6$ deg$^2$ by tiling the four fields of the CFHTLS Wide in a hexagonal pattern. Emission lines were detected from $\sim 4,000$ star forming galaxies by an automatic line detection algorithm applied to 2D spectral images. This is the first in a series of papers based on FastSound data, and we describe the details of the survey design, target selection, observations, data reduction, and emission line detections.

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arXiv2015

From Diversity to Dichotomy, and Quenching: Milky-Way-Like and Massive-Galaxy Progenitors at 0.5<z<3.0

Using the HST/WFC3 and ACS multi-band imaging data taken in CANDELS and 3D-HST, we study the general properties and the diversity of the progenitors of the Milky Way (MWs) and local massive galaxy (MGs) at 0.5 < z < 3.0, based on a constant cumulative number density analysis. After careful data reduction and stacking analysis, we conduct a radially resolved pixel SED fitting to obtain the radial distributions of the stellar mass and rest-frame colors. The stellar mass of MWs increases in self-similar way, irrespective of the radial distance, while that of MGs grows in inside-out way where they obtain ~ 75% of the total mass at outer (> 2.5 kpc) radius since z ~ 2. Although the radial mass profiles evolve in distinct ways, the formation and quenching of the central dense region (or bulge) ahead of the outer disk formation are found to be common for both systems. The sudden reddening of bulge at z ~ 1.6 and z ~ 2.4 for MWs and MGs, respectively, suggests the formation of bulge and would give a clue to the different gas accretion histories and quenching. A new approach to evaluate the morphological diversity is conducted by using the average surface density profile and its dispersion. The variety of the radial mass profiles for MGs peaks at higher redshift (z > 2.8), and then rapidly converges to more uniform shape at z < 1.5, while that for MWs remains in the outer region over the redshift. Compared with the observed star formation rates and color profiles, the evolution of variety is consistently explained by the star formation activities.

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arXiv2014

The gas inflow and outflow rate in star-forming galaxies at $z\sim1.4$

We try to constrain the gas inflow and outflow rate of star-forming galaxies at $z\sim1.4$ by employing a simple analytic model for the chemical evolution of galaxies. The sample is constructed based on a large near-infrared (NIR) spectroscopic sample observed with Subaru/FMOS. The gas-phase metallicity is measured from the [\ion{N}{2}]$λ$6584/H$α$ emission line ratio and the gas mass is derived from the extinction corrected H$α$ luminosity by assuming the Kennicutt-Schmidt law. We constrain the inflow and outflow rate from the least-$χ^{2}$ fittings of the observed gas mass fraction, stellar mass, and metallicity with the analytic model. The joint $χ^{2}$ fitting shows the best-fit inflow rate is $\sim1.8$ and the outflow rate is $\sim0.6$ in unit of star-formation rate (SFR). By applying the same analysis to the previous studies at $z\sim0$ and $z\sim2.2$, it is shown that the both inflow rate and outflow rate decrease with decreasing redshift, which implies the higher activity of gas flow process at higher redshift. The decreasing trend of the inflow rate from $z\sim2.2$ to $z\sim0$ agrees with that seen in the previous observational works with different methods, though the absolute value is generally larger than the previous works. The outflow rate and its evolution from $z\sim2.2$ to $z\sim0$ obtained in this work agree well with the independent estimations in the previous observational works.

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arXiv2014

FIELD: An automated emission-line detection software for Subaru/FMOS near-infrared spectroscopy

We describe the development of automated emission line detection software for the Fiber Multi-Object Spectrograph (FMOS), which is a near-infrared spectrograph fed by $400$ fibers from the $0.2$ deg$^2$ prime focus field of view of the Subaru Telescope. The software, FIELD (FMOS software for Image-based Emission Line Detection), is developed and tested mainly for the FastSound survey, which is targeting H$α$ emitting galaxies at $z \sim 1.3$ to measure the redshift space distortion as a test of general relativity beyond $z \sim 1$. The basic algorithm is to calculate the line signal-to-noise ratio ($S/N$) along the wavelength direction, given by a 2-D convolution of the spectral image and a detection kernel representing a typical emission line profile. A unique feature of FMOS is its use of OH airglow suppression masks, requiring the use of flat-field images to suppress noise around the mask regions. Bad pixels on the detectors and pixels affected by cosmic-rays are efficiently removed by using the information obtained from the FMOS analysis pipeline. We limit the range of acceptable line-shape parameters for the detected candidates to further improve the reliability of line detection. The final performance of line detection is tested using a subset of the FastSound data; the false detection rate of spurious objects is examined by using inverted frames obtained by exchanging object and sky frames. The false detection rate is $< 1$\% at $S/N > 5$, allowing an efficient and objective emission line search for FMOS data at the line flux level of $\gtrsim 1.0 \times 10^{-16}$[erg/cm$^2$/s].

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arXiv2014

Toward the Standard Population Synthesis Model of the X-Ray Background: Evolution of X-Ray Luminosity and Absorption Functions of Active Galactic Nuclei Including Compton-Thick Populations

We present the most up-to-date X-ray luminosity function (XLF) and absorption function of Active Galactic Nuclei (AGNs) over the redshift range from 0 to 5, utilizing the largest, highly complete sample ever available obtained from surveys performed with Swift/BAT, MAXI, ASCA, XMM-Newton, Chandra, and ROSAT. The combined sample, including that of the Subaru/XMM-Newton Deep Survey, consists of 4039 detections in the soft (0.5--2 keV) and/or hard ($>2$ keV) band. We utilize a maximum likelihood method to reproduce the count-rate versus redshift distribution for each survey, by taking into account the evolution of the absorbed fraction, the contribution from Compton-thick (CTK) AGNs, and broad band spectra of AGNs including reflection components from tori based on the luminosity and redshift dependent unified scheme. We find that the shape of the XLF at $z \sim 1-3$ is significantly different from that in the local universe, for which the luminosity dependent density evolution model gives much better description than the luminosity and density evolution model. These results establish the standard population synthesis model of the X-Ray Background (XRB), which well reproduces the source counts, the observed fractions of CTK AGNs, and the spectrum of the hard XRB. The number ratio of CTK AGNs to the absorbed Compton-thin (CTN) AGNs is constrained to be $\approx$0.5--1.6 to produce the 20--50 keV XRB intensity within present uncertainties, by assuming that they follow the same evolution as CTN AGNs. The growth history of supermassive black holes is discussed based on the new AGN bolometric luminosity function.

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arXiv2013

A Study of Selection Methods for H alpha Emitting Galaxies at z~1.3 for the Subaru/FMOS Galaxy Redshift Survey for Cosmology (FastSound)

The efficient selection of high-redshift emission galaxies is important for future large galaxy redshift surveys for cosmology. Here we describe the target selection methods for the FastSound project, a redshift survey for H alpha emitting galaxies at z=1.2-1.5 using Subaru/FMOS to measure the linear growth rate fσ8 via Redshift Space Distortion (RSD) and constrain the theory of gravity. To select ~400 target galaxies in the 0.2 deg^2 FMOS field-of-view from photometric data of CFHTLS-Wide (u*g'r'i'z'), we test several different methods based on color-color diagrams or photometric redshift estimates from spectral energy distribution (SED) fitting. We also test the improvement in selection efficiency that can be achieved by adding near-infrared data from the UKIDSS DXS (J). The success rates of H alpha detection with FMOS averaged over two observed fields using these methods are 11.3% (color-color, optical), 13.6% (color-color, optical+NIR), 17.3% (photo-z, optical), and 15.1% (photo-z, optical+NIR). Selection from photometric redshifts tends to give a better efficiency than color-based methods, although there is no significant improvement by adding J band data within the statistical scatter. We also investigate the main limiting factors for the success rate, by using the sample of the HiZELS H alpha emitters that were selected by narrow-band imaging. Although the number density of total H alpha emitters having higher H alpha fluxes than the FMOS sensitivity is comparable with the FMOS fiber density, the limited accuracy of photometric redshift and H alpha flux estimations have comparable effects on the success rate of <~20% obtained from SED fitting.

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arXiv2013

First Systematic Search for Oxygen-Line Blobs at High Redshift: Uncovering AGN Feedback and Star-Formation Quenching

We present the first systematic search for extended metal-line [OII]λλ3726,3729 nebulae, or [OII] blobs (OIIBs), at z=1.2 using deep narrowband imaging with a survey volume of 1.9x10^5 Mpc^3 on the 0.62 deg^2 sky of Subaru-XMM Deep Survey (SXDS) field. We discover a giant OIIB, dubbed 'OIIB 1', with a spatial extent over ~75 kpc at a spectroscopic redshift of z=1.18, and also identify a total of twelve OIIBs with a size of >30 kpc. Our optical spectrum of OIIB 1 presents [NeV]λ3426 line at the 6σ level, indicating that this object harbors an obscured type-2 AGN. The presence of gas outflows in this object is suggested by two marginal detections of FeIIλ2587 absorption and FeII*λ2613 emission lines both of which are blueshifted at as large as 500-600 km/s, indicating that the heating source of OIIB 1 is AGN or associated shock excitation rather than supernovae produced by starbursts. The number density of OIIB 1-type giant blobs is estimated to be ~5x10^{-6} Mpc^{-3} at z~1.2, which is comparable with that of AGNs driving outflow at a similar redshift, suggesting that giant OIIBs are produced only by AGN activity. On the other hand, the number density of small OIIBs, 6x10^{-5} Mpc^{-3}, compared to that of z~1 galaxies in the blue cloud in the same M_B range, may imply that 3% of star-forming galaxies at z~1 are quenching star formation through outflows involving extended [OII] emission.

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arXiv2013

The mass-metallicity relation at z~1.4 revealed with Subaru/FMOS

We present a stellar mass-metallicity relation at z~1.4 with an unprecedentedly large sample of ~340 star-forming galaxies obtained with FMOS on the Subaru Telescope. We observed K-band selected galaxies at 1.2 < z_{ph} < 1.6 in the SXDS/UDS fields with M_{*} > 10^{9.5} M_{\sun}, and expected F(Hα) > 5 \times 10^{-17} erg s^{-1} cm^{-2}. Among the observed ~1200 targets, 343 objects show significant Hαemission lines. The gas-phase metallicity is obtained from [NII]λ6584/Hαline ratio, after excluding possible active galactic nuclei (AGNs). Due to the faintness of the [NII]λ6584 lines, we apply the stacking analysis and derive the mass-metallicity relation at z~1.4. Our results are compared to past results at different redshifts in the literature. The mass-metallicity relation at z~1.4 is located between those at z~0.8 and z~2.2; it is found that the metallicity increases with decreasing redshift from z~3 to z~0 at fixed stellar mass. Thanks to the large size of the sample, we can study the dependence of the mass-metallicity relation on various galaxy physical properties. The average metallicity from the stacked spectra is close to the local FMR in the higher metallicity part but >0.1 dex higher in metallicity than the FMR in the lower metallicity part. We find that galaxies with larger E(B-V), B-R, and R-H colours tend to show higher metallicity by ~0.05 dex at fixed stellar mass. We also find relatively clearer size dependence that objects with smaller half light radius tend to show higher metallicity by ~0.1 dex at fixed stellar mass, especially in the low mass part.

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