arXiv subjects

Tohru Nagao

Publications and source records attributed to Tohru Nagao.

At least 235 records · Page 13Linked to original sources

arXiv2005

Narrow-Band Survey of the GOODS Fields: Search for Lyman-Alpha Emitters at z = 5.7

We present results from optical narrow-band lambda_c = 8150A ~ and Delta lambda = 120A) observations of the Great Observatories Origins Deep Survey (GOODS) fields, using Suprime-Cam on the Subaru Telescope. Using these narrow-band data, we then perform a survey of Lyman alpha Emitters (LAEs) at z~5.7. The LAE survey covers an area of approx 320 arcmin^2 and a co-moving volume of ~8.0 x 10^4 Mpc^3. We found a total of 10 (GOODS-N) and 4 (GOODS-S) LAE candidates at z~5.7. We perform a study of the spatial distribution, space density, and star formation properties of the LAEs at z~5.7.

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arXiv2005

The Evolution of the Broad-Line Region among SDSS Quasars

Based on 5344 quasar spectra taken from the SDSS Data Release 2, the dependences of various emission-line flux ratios on redshift and quasar luminosity are investigated in the ranges 2.0 < z < 4.5 and -24.5 > M_B > -29.5$. We show that the emission lines in the composite spectra are fitted better with power-law profiles than with double Gaussian or modified Lorentzian profiles, and in particular we show that the power-law profiles are more appropriate to measure broad emission-line fluxes than other methods. The composite spectra show that there are statistically significant correlations between quasar luminosity and various emission-line flux ratios, such as NV/CIV and NV/HeII, while there are only marginal correlations between quasar redshift and emission-line flux ratios. We obtain detailed photoionization models to interpret the observed line ratios. The correlation of line ratios with luminosity is interpreted in terms of higher gas metallicity in more luminous quasars. For a given quasar luminosity, there is no metallicity evolution for the redshift range 2.0 < z < 4.5. The typical metallicity of BLR gas clouds is estimated to be Z ~ 5 Z_sun, although the inferred metallicity depends on the assumed BLR cloud properties, such as their density distribution function and their radial distribution. The absence of a metallicity evolution up to z ~ 4.5 implies that the active star-formation epoch of quasar host galaxies occurred at z > 7.

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arXiv2005

Type I ULIRGs: Transition Stage from ULIRGs to QSOs

We examine whether the ultraluminous infrared galaxies that contain a type I Seyfert nucleus (a type I ULIRG) are in the transition stage from ULIRGs to quasi-stellar objects (QSOs). To inspect this issue, we compare the black hole (BH) mass, the bulge luminosity and the far infrared luminosity among type I ULIRGs, QSOs and elliptical galaxies. As a result, we find the following results; (1) The type I ULIRGs have systematically smaller BH masses in spite of the comparable bulge luminosity relative to QSOs and elliptical galaxies. (2) The far-infrared luminosity of most type I ULIRGs is larger than the Eddington luminosity. We show that above results do not change significantly for 3 type I ULIRGs that we can estimate the visual extinction from the column density. Also, for all 8 type I ULIRGs, we investigate the effect of uncertainties of BH mass measurments and our sample bias, so that it turns out that our results do not alter even if we consider above two effects. In addition, Anabuki (2004) revealed that their X-ray properties are similar to those of the narrow line Seyfert 1 galaxies. These would indicate that active galactic nuclei (AGNs) with a high mass accretion rate exist in the type I ULIRGs. Based on all of these findings, we conclude that it would be a natural interpretation that type I ULIRGs are the early phase of BH growth, namely the missing link between ULIRGs and QSOs. Moreover, by comparing our results with a theoretical model of a coevolution scenario of a QSO BH and a galactic bulge, we show clearly that this explanation would be valid.

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arXiv2005

Gas Metallicity in the Narrow-Line Regions of High-Redshift Active Galactic Nuclei

We analyze optical (UV rest-frame) spectra of X-ray selected narrow-line QSOs at redshift 1.5 < z < 3.7 found in the Chandra Deep Field South and of narrow-line radio galaxies at redshift 1.2 < z < 3.8 to investigate the gas metallicity of the narrow-line regions and their evolution in this redshift range. Such spectra are also compared with UV spectra of local Seyfert 2 galaxies. The observational data are inconsistent with the predictions of shock models, suggesting that the narrow-line regions are mainly photoionized. The photoionization models with dust grains predict line flux ratios which are also in disagreement with most of the observed values, suggesting that the high-ionization part of the narrow-line regions (which is sampled by the available spectra) is dust-free. The photoionization dust-free models provide two possible scenarios which are consistent with the observed data: low-density gas clouds (n < 10^3 cm^-3) with a sub-solar metallicity (0.2 < Z/Z_sun < 1.0), or high-density gas clouds (n ~ 10^5 cm^-3) with a wide range of gas metallicity (0.2 < Z/Z_sun < 5.0). Regardless of the specific interpretation, the observational data do not show any evidence for a significant evolution of the gas metallicity in the narrow-line regions within the redshift range 1.2 < z < 3.8. Instead, we find a trend for more luminous active galactic nuclei to have more metal-rich gas clouds (luminosity-metallicity relation), which is in agreement with the same finding in the studies of the broad-line regions. The lack of evolution for the gas metallicity of the narrow-line regions implies that the major epoch of star formation in the host galaxies of these active galactic nuclei is at z > 4.

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arXiv2005

An Observational Pursuit for Population III Stars in a Ly_alpha Emitter at z=6.33 through HeII Emission

We present a very deep near-infrared spectroscopic observation of a strong Ly_alpha emitter at z=6.33, SDF J132440.6+273607, which we used to search for HeII 1640. This emission line is expected if the target hosts a significant number of population III stars. Even after 42 ksec of integration with the Subaru/OHS spectrograph, no emission-line features are detected in the JH band, which confirms that SDF J132440.6+273607 is neither an active galactic nucleus nor a low-$z$ emission-line object. We obtained a 2sigma upper-limit of 9.06e-18 ergs/s/cm^2 on the HeII 1640 emission line flux, which corresponds to a luminosity of 4.11e42 ergs/s. This upper-limit on the HeII 1640 luminosity implies that the upper limit on population III star-formation rate is in the range 4.9--41.2 M_sun/yr if population III stars suffer no mass loss, and in the range 1.8--13.2 M_sun/yr if strong mass loss is present. The non-detection of HeII in SDF J132440.6+273607 at z=6.33 may thus disfavor weak feedback models for population III stars.

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arXiv2005

A survey of NB921 dropouts in the Subaru Deep Field

In order to search for high-redshift galaxies beyond $z = 6.6$ in the Subaru Deep Field, we have investigated NB921-dropout galaxies where NB921 is the narrowband filter centered at 919.6 nm with FWHM of 13.2 nm for the Suprime-Cam on the Subaru Telescope. There are no secure NB921-dropout candidates brighter than $z^\prime = 25.5$. Based on this result, we discuss the UV luminosity function of star-forming galaxies at $z > 6.6$.

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arXiv2005

New High-Redshift Galaxies at $z$ = 5.8 - 6.5 in the Subaru Deep Field

In order to search for high-redshift galaxies around $z \sim 6$ in the Subaru Deep Field, we have investigated NB816-dropout galaxies where NB816 is the narrowband filter centered at 815nm with FWHM of 12.5 nm for the Suprime-Cam on the Subaru Telescope. Since the NB816 imaging is so deep we can detect 10 well-defined NB816-dropout galaxies that we identify to lie at $z$ = 5.8 -- 6.5. We discuss their observational properties.

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arXiv2005

The Intermediate-band Dropout Method: A New Method to Search for High-Redshift Galaxies

We propose a new method to search for high-redshift galaxies that is based on an intermediate-band dropout technique rather than the usual broad-band dropout one. In this method, we use an intermediate-band filter whose central wavelength is longer than 7000 Å. This new method makes it possible to distinguish both very late-type stars such as L and T dwarfs and dusty galaxies at intermediate redshift from real high-$z$ Lyman break galaxies. The reason for this is that such interlopers do not show strong intermediate-band depression although they have very red broad-band colors that are indicative of Lyman break galaxies. Applying our new method to imaging data sets obtained with the Suprime-Cam on the Subaru Telescope, we find a new sample of Lyman break galaxies at $z \simeq 5$.

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arXiv2004

Subaru Spectropolarimetry of Mrk 573: The Hidden High-Ionization Nuclear Emission-Line Region inside the Dusty Torus

We report on the result of our high quality spectropolarimetric observation for the narrow-line region in a nearby Seyfert 2 galaxy, Mrk 573, by the Subaru Telescope. The polarized flux spectrum of Mrk 573 shows not only prominent scattered broad H$α$ emission but also various narrow forbidden emission lines. We find that the measured polarization degree of the observed forbidden emission lines is positively correlated with the ionization potential of the corresponding ions and the critical density of the corresponding transitions. We discuss some possible origins of these correlations, and then we point out that the correlations are caused due to the obscuration of the stratified narrow-line region in Mrk 573 by the optically and geometrically thick dusty torus, just similar to the previous study on NGC 4258.

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arXiv2004

A Subaru Search for Lyman-Alpha Emitters at z=5.8 with an Intermediate-Band Filter

We present the results of a survey for Ly-alpha emitters at z ~ 5.8 using a new intermediate-band filter centered at lambda_c = 8275 AA with Delta-lambda_FWHM = 340 AA (i.e., the spectroscopic resolution is R ~ 23) with a combination with a traditional narrow-band centered at lambda_c = 8150 AA with Delta-lambda_FWHM = 120 AA (R ~ 68). Our observations were made with use of the Subaru Prime Focus Camera, Suprime-Cam, on the 8.2 m Subaru telescope in a sky area surrounding the high redshift quasar, SDSSp J104433.04-012522.2 at z=5.74, covering an effective sky area with ~ 720 arcmin^2. In this survey, we have found four Ly-alpha-emitter candidates from the intermediate-band image (z \~ 5.8 with Delta z ~ 0.3). Combined with our previous results based on the NB816 imaging, we discuss the star formation activity in galaxies between z ~ 5.7 and z ~ 5.9.

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arXiv2004

Detection of Polarized Broad Emission in the Seyfert 2 Galaxy Mrk 573

We report the discovery of the scattered emission from a hidden broad-line region (BLR) in a Seyfert 2 galaxy, Mrk 573, based on our recent spectropolarimetric observation performed at the Subaru Telescope. This object has been regarded as a type 2 AGN without a hidden BLR by the previous observations. However, our high quality spectrum of the polarized flux of Mrk 573 shows prominent broad (~3000 km/s) H_alpha emission, broad weak H_beta emission, and subtle Fe II multiplet emission. Our new detection of these indications for the presence of the hidden BLR in the nucleus of Mrk 573 is thought to be owing to the high signal-to-noise ratio of our data, but the possibility of a time variation of the scattered BLR emission is also mentioned. Some diagnostic quantities such as the IRAS color, the radio power, and the line ratio of the emission from the narrow-line region of Mrk 573 are consistent with the distributions of such quantities of type 2 AGNs with a hidden BLR. Mrk 573 is thought to be an object whose level of the AGN activity is the weakest among the type 2 AGNs with a hidden BLR. In terms of the systematic differences between the type 2 AGNs with and without a hidden BLR, we briefly comment on an interesting Seyfert 2 galaxy, Mrk 266SW, which may possess a hidden BLR but has been treated as a type 2 AGNs without a hidden BLR.

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arXiv2003

Lyman$α$ Emitters beyond Redshift 5:The Dawn of Galaxy Formation

The 8m class telescopes in the ground-based optical astronomy together with help from the ultra-sharp eye of the Hubble Space Telescope have enabled us to observe forming galaxies beyond redshift $z=5$. In particular, more than twenty Ly$α$-emitting galaxies have already been found at $z > 5$. These findings provide us with useful hints to investigate how galaxies formed and then evolved in the early universe. Further, detailed analysis of Ly$α$ emission line profiles are useful in exploring the nature of the intergalactic medium because the trailing edge of cosmic reionization could be close to $z \sim 6$ -- 7, at which forming galaxies have been found recently. We also discuss the importance of superwinds from forming galaxies at high redshift, which has an intimate relationship between galaxies and the intergalactic medium. We then give a review of early cosmic star formation history based on recent progress in searching for Ly$α$-emitting young galaxies beyond redshift 5.

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arXiv2003

The Halpha Luminosity Function of the Galaxy Cluster Abell 521 at z = 0.25

We present an optical multicolor-imaging study of the galaxy cluster Abell 521 at $z = 0.25$, using Suprime-Cam on the Subaru Telescope, covering an area of $32 \times 20$ arcmin$^2$ ($9.4 \times 5.8 h_{50}^{-2}$ Mpc$^2$ at $z = 0.25$). Our imaging data taken with both a narrow-band filter, $NB816$ ($λ_0 = 8150$Åand $Δλ= 120$Å), and broad-band filters, $B,V,R_{\rm C}, i^\prime$, and $z^\prime$ allow us to find 165 H$α$ emitters. We obtain the H$α$ luminosity function (LF) for the cluster galaxies within 2 Mpc; the Schechter parameters are $α= -0.75 \pm 0.23$, $ϕ^\star = 10^{-0.25 \pm 0.20}$ Mpc$^{-3}$, and $L^\star = 10^{42.03 \pm 0.17}$ erg s$^{-1}$. Although the faint end slope, $α$, is consistent with that of the local cluster H$α$ LFs, the characteristic luminosity, $L^\star$, is about 6 times (or $\approx 2$ mag) brighter. This strong evolution implies that Abell 521 contains more active star-forming galaxies than the local clusters, being consistent with the observed Butcher-Oemler effect. However, the bright $L^\star$ of Abell 521 may be, at least in part, due to the dynamical condition of this cluster.

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arXiv2003

A Subaru Search for Lyman$α$ Emitters at Redshift 5.7

We present the results of a survey for Ly$α$ emitters at $z\approx 5.7$ based on optical narrow-band ($λ_{\rm c} = 8150$ Å~ and $Δλ= 120$ Å), and broad-band ($B$, $R_{\rm C}$, $I_{\rm C}$, and $z^\prime$) observations of the field surrounding the high redshift quasar, SDSSp J104433.04$-$012522.2, on the 8.2 m Subaru Telescope with the Subaru Prime Focus Camera, Suprime-Cam. This survey covers a sky area of $\approx 720$ arcmin$^2$ and a co-moving volume of $\simeq 2 \times 10^5 ~ h_{0.7}^{-3}$ Mpc$^3$. We have found 20 candidates of Ly$α$ emitters at $z \approx$ 5.7 with $Δz \approx 0.1$. Two of them have been confirmed star-forming galaxies at $z=5.655$ and $z=5.687$ from our follow-up optical spectroscopy. We discuss star-formation properties of the 20 objects from a statistical point of view. Our survey leads to a new estimate of the star formation rate density at $z \approx 5.7$, $\sim 1.2 \times 10^{-3} h_{0.7} M_\odot$ yr$^{-1}$ Mpc$^{-3}$.

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arXiv2003

Are Two z~6 Quasars Gravitationally Lensed ?

Several high-z (z > 5.7) quasars have been found in the course of Sloan Digital Sky Survey. The presence of such very high-z quasars is expected to give constraints on early structure formation. On one hand, it is suggested that these most luminous objects at high redshift are biased toward the highly magnified objects by gravitational lensing. To clarify the effect of gravitational lensing on the high-z quasars, we began the imaging survey of intervening lensing galaxies. Indeed our previous optical image showed that SDSSp J104433.04+012502.2 at z=5.74 is gravitationally magnified by a factor 2. In this paper, we report our new optical imaging of other two high-z quasars, SDSSp J103027.10+052455.0 at z=6.28 and SDSSp J130608.26+035626.3 at z=5.99. Since we find neither intervening galaxy nor counter image with i^{\prime} < 25.4-25.8 around each quasar, we conclude that they are not strongly magnified regardless that a lens galaxy is dusty.

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arXiv2003

Subaru High-Dispersion Spectroscopy of Narrow-Line Region in the Seyfert Galaxy NGC 4151

We report on a study of forbidden emission-line spectrum of nearby Seyfert 1.5 galaxy NGC 4151 based on the high-resolution (R ~ 45,000) optical spectrum obtained by using the High Dispersion Spectrograph boarded on the Subaru Telescope. The profile parameters such as the emission-line widths, the velocity shifts from the recession velocity of the host galaxy, and the asymmetry indices, for emission lines including very faint ones such as [Ar IV]4712,4740 and [Fe VI]5631,5677 are investigated. Statistically significant correlations between the measured profile parameters and the critical densities of transitions are found while there are no meaningful correlations between the profile parameters and the ionization potentials of ions. By comparing the results with photoionization model calculations, we remark that a simple power-law distribution of the gas density which is independent of the radius from the nucleus cannot explain the observed correlation between the emission-line widths and the critical densities of the transitions. Taking the additional dense gas component expected to exist at the innermost of the narrow-line region into account, the observed correlations between the emission-line width and the critical density of the transitions can be understood since high-critical-density emission lines can arise at such relatively inner regions even if their ionization potentials are low. The observed correlation between the blueshift amounts of emission lines and the critical densities of the ions is also explained if such dense gas clouds located closer to the nucleus have larger outflowing velocities.

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arXiv2002

Iron is not Depleted in High-Ionization Nuclear Emission-Line Regions of Active Galactic Nuclei

In order to examine whether or not high-ionization nuclear emission-line regions (HINERs) in narrow-line regions of active galactic nuclei are dusty, we focus on two high-ionization forbidden emission lines, [Fe VII]6087 and [Ne V]3426. We perform photoionization model calculations to investigate possible dependences of the flux ratio of [Fe VII]6087/[Ne V]3426 on various gas properties, in order to investigate how useful this flux ratio to explore the dust abundances in HINERs. Based on our photoionization model calculations, we show that the observed range of the flux ratio of [Fe VII]6087/[Ne V]3426 is consistent with the dust-free models while that is hard to be explained by the dusty models. This suggests that iron is not depleted at HINERs, which implies that the HINERs are not dusty. This results is consistent with the idea that the HINERs are located closer than the dust-sublimation radius (i.e., inner radius of dusty tori) and thus can be hidden by dusty tori when seen from a edge-on view toward the tori, which has been also suggested by the AGN-type dependence of the visibility of high-ionization emission lines.

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arXiv2002

SDSSp J104433.04$-$012502.2 at $z=5.74$ is Gravitationally Magnified by an Intervening Galaxy

During the course of our optical deep survey program on L$α$ emitters at $z \approx 5.7$ in the sky area surrounding the quasar SDSSp J104433.04$-$012502.2 at $z=5.74$, we found that a faint galaxy with $m_B$(AB) $\approx 25$ is located at \timeform{1".9} southwest of the quasar. Its broad-band color properties from $B$ to $z^\prime$ suggest that the galaxy is located at a redshift of $z \sim 1.5$ -- 2.5. This is consistent with no strong emission line in our optical spectroscopy. Since the counter image of the quasar cannot be seen in our deep optical images, the magnification factor seems not to be very high. Our modest estimate is that this quasar is gravitationally magnified by a factor of 2.

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