arXiv subjects

Sadanori Okamura

Publications and source records attributed to Sadanori Okamura.

71 records · Page 4Linked to original sources

arXiv2003

ERO R1 in CL0939+4713 field - Evidence for an S0-like galaxy at z \sim 1.5

We present further observations of the extremely red object ERO J094258+4659.2, identified by \citet{iye00} as ERO R1 in their deep images of the cluster A851. We estimate its redshift independently by eight-band photometric redshift determination and cross-correlation of a new H-band spectrum with the optical spectra of local E/S0 galaxies, and conclude that it lies at $z \sim 1.5$. Although its colors are consistent both with an elliptical galaxy and an S0 galaxy at that redshift, its elongated shape and exponential luminosity profile suggest the presence of an evolved stellar disk component. We rule out the possibility that these properties are strongly influenced by gravitational lensing by the foreground cluster, and therefore conclude that this object is more likely to be an S0-like galaxy, rather than a lensed elliptical. The H-band spectrum does not show strong H$α$ emission and the star formation rate therefore appears to be very modest. The presence of such a galaxy with an apparently relaxed disk of stars at this high redshift provides a new and strong constraint on theoretical models which aim to explain the formation and evolution of galaxies.

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arXiv2003

Hdelta-Selected Galaxies in the Sloan Digital Sky Survey I: The Catalog

[Abridged] We present here a new and homogeneous sample of 3340 galaxies selected from the Sloan Digital Sky Survey (SDSS) based solely on the observed strength of their Hdelta absorption line. These galaxies are commonly known as ``post-starburst'' or ``E+A'' galaxies, and the study of these galaxies has been severely hampered by the lack of a large, statistical sample of such galaxies. In this paper, we rectify this problem by selecting a sample of galaxies which possess an absorption Hdelta equivalent width of EW(Hdelta_max) - Delta EW(Hdelta_max) > 4A from 106682 galaxies in the SDSS. We have performed extensive tests on our catalog including comparing different methodologies of measuring the Hdelta absorption and studying the effects of stellar absorption, dust extinction, emission-filling and measurement error. The measured abundance of our Hdelta-selected (HDS) galaxies is 2.6 +/- 0.1% of all galaxies within a volume-limited sample of 0.05<z<0.1 and M(r*)<-20.5, which is consistent with previous studies of such galaxies in the literature. We find that only 25 of our HDS galaxies in this volume-limited sample (3.5+/-0.7%) show no evidence for OII and Halpha emission, thus indicating that true E+A (or k+a) galaxies are extremely rare objects at low redshift, i.e., only 0.09+/-0.02% of all galaxies in this volume-limited sample are true E+A galaxies. In contrast, 89+/-5% of our HDS galaxies in the volume-limited sample have significant detections of the OII and Halpha emission lines. We find 27 galaxies in our volume-limited HDS sample that possess no detectable OII emission, but do however possess detectable Halpha emission. These galaxies may be dusty star-forming galaxies. We provide the community with this new catalog of Hdelta-selected galaxies to aid in the understanding of these galaxies.

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arXiv2002

Isolated Star Formation: A Compact HII Region in the Virgo Cluster

We report on the discovery of an isolated, compact HII region in the Virgo cluster. The object is located in the diffuse outer halo of NGC 4388, or could possibly be in intracluster space. Star formation can thus take place far outside the main star forming regions of galaxies. This object is powered by a small starburst with an estimated mass of $\sim 400\msun$ and age of $\sim 3\myr$. From a total sample of 17 HII region candidates, the present rate of isolated star formation estimated in our Virgo field is small, $\sim 10^{-6} Msun arcmin}^{-2} yr^{-1}$. However, this mode of star formation might have been more important at higher redshifts and be responsible for a fraction of the observed intracluster stars and total cluster metal production. This object is relevant also for distance determinations with the planetary nebula luminosity function from emission line surveys, for high-velocity clouds and the in situ origin of B stars in the Galactic halo, and for local enrichment of the intracluster gas by Type II supernovae.

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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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arXiv2002

Subaru Prime Focus Camera -- Suprime-Cam --

We have built an 80-mega pixels (10240 X 8192) mosaic CCD camera, called Suprime-Cam, for the wide-field prime focus of the 8.2 m Subaru telescope. Suprime-Cam covers a field of view 34' X 27', a unique facility among the the 8-10 m class telescopes, with a resolution of 0.''202 per pixel. The focal plane consists of ten high-resistivity 2k X 4k CCDs developed by MIT Lincoln Laboratory, which are cooled by a large stirling-cycle cooler. The CCD readout electronics was designed to be scalable, which allows the multiple read-out of tens of CCDs. It takes 50 seconds to readout entire arrays. We designed a filter-exchange mechanism of the jukebox type that can hold up to ten large filters (205 X 170 X 15 mm^3). The wide-field corrector is basically a three-lens Wynne-type, but has a new type of atmospheric dispersion corrector. The corrector provides a flat focal plane and an un-vignetted field of view of 30' in diameter. The achieved co-planarity of the focal array mosaic is smaller than 30 micron peak-to-peak, which realizes mostly the seeing limited image over the entire field. The median seeing in the Ic-band, measured over one year and a half, is 0.''61. The PSF anisotropy in Suprime-Cam images, estimated by stellar ellipticities, is about 2% under this median seeing condition. At the time of commissioning, Suprime-Cam had the largest survey speed, which is defined as the field of view multiplied by the primary mirror area of the telescope, among those cameras built for sub-arcsecond imaging.

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arXiv2002

Subaru Deep Survey III. Evolution of Rest-Frame Luminosity Functions Based on the Photometric Redshifts for A K'-band Selected Galaxy Sample

We have constructed a very deep K'-selected multicolor BVRIz'JK' sample of 439 field galaxies. Photometric redshifts for sample galaxies were estimated. The overall redshift distribution N(z) for the K'<21.0 sample is consistent with previous observations, and for the first time we derive the N(z) down to K'=24.0. After taking account of the dust extinction and selection effects of the sample, the observed N(z) distribution is well described with the PLE model, while the hierarchical galaxy formation model shows an apparent deficiency of galaxies especially at z>2. The photometric redshift and the best-fit SED model evaluations allow the derivation of the rest-frame K', B, and UV(2000A) luminosity functions (LFs) and their evolutions. The rest-K' LF shows almost no evolution up to z=3, while the rest B LF shows mild luminosity evolution, and the rest UV LF shows strong luminosity evolution. This trend corresponds to the evolution of the rest UV-K' color, which gets bluer with increasing redshift. We also found that more massive galaxies are redder in this rest-frame color in every epoch. The rest-UV LF of our K'-selected galaxies shows a much shallower faint end slope at z=3 than that of previous estimations for rest-UV selected Lyman break galaxies. As a consequence, the contribution to the global star formation rate of our K'-selected galaxies is about 42% of that derived from the integration of LF of Lyman break galaxies at z=3. This result suggests that a large fraction of the star formation rate density at z>1.5 comes from the contribution from the faint (M_2000A>-20) blue galaxy population at high redshift universe that have not yet obviously been identified. (abridged)

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arXiv2002

The Galaxy Luminosity Function and Luminosity Density at Redshift z=0.1

Using a catalog of 147,986 galaxy redshifts and fluxes from the Sloan Digital Sky Survey (SDSS) we measure the galaxy luminosity density at z=0.1 in five optical bandpasses corresponding to the SDSS bandpasses shifted to match their restframe shape at z=0.1. We denote the bands {0.1}{u}, {0.1}{g}, {0.1}{r}, {0.1}{i}, {0.1}{z}, with λ_{eff} = [3216, 4240, 5595, 6792, 8111] Angstroms respectively. We use a maximum likelihood method which allows for a general form for the shape of the luminosity function, simple luminosity and number evolution, incorporates flux uncertainties, and accounts for the flux limits of the survey. We find luminosity densities at z=0.1 in absolute AB magnitudes in a Mpc^3 of [-14.10 \pm 0.15, -15.18 \pm 0.03, -15.90 \pm 0.03, -16.24 \pm 0.03, -16.56 \pm 0.02] in [{0.1}{u}, {0.1}{g}, {0.1}{r}, {0.1}{i}, {0.1}{z}], respectively, using Ω_0 =0.3, Ω_Λ=0.7, and h=1, and using Petrosian magnitudes. Similar results are obtained using Sersic model magnitudes, suggesting that flux from outside the Petrosian apertures is not a major correction. In the {0.1}{r} band, the best fit Schechter function to our results has ϕ_\ast = (1.49 \pm 0.04) \times 10^{-2} h^3 Mpc^{-3}, M_\ast - 5\log_{10} h = -20.44 \pm 0.01, and α= -1.05\pm 0.01. In solar luminosities, the luminosity density in {0.1}{r} is (1.84 \pm 0.04) h 10^8 L_{{0.1}{r},\odot} Mpc^{-3}. Our results are consistent with other estimates of the luminosity density, from the Two-degree Field Galaxy Redshift Survey and the Millenium Galaxy Catalog. They represent a substantial change (\sim 0.5 mag) from earlier SDSS luminosity density results based on commissioning data, almost entirely because of the inclusion of evolution in the luminosity function model.

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arXiv2002

A Search for Lyman alpha Emitters at Redshift 3.7

We present the results of a survey for emission-line objects based on optical intermediate-band ($λ_{\rm c}$ = 5736 Å~ and $Δλ$ = 280 Å) and broad-band ($B$, $V$, $R$, and $i^\prime$) observations of the Subaru/XMM-Newton Deep Field on the 8.2 m Subaru telescope with the Subaru Prime Focus Camera, Suprime-Cam. All the data were obtained during the guaranteed time observations of the Suprime-Cam instrument. The intermediate-band image covered a sky area with 10\minpoint62 $\times 12\minpoint40 \approx 132$ arcmin$^2$ in the Subaru/XMM-Newton Deep Field (Ouchi et al.). Using this image, we have found 23 emission-line sources whose observed emission-line equivalent widths are greater than 250 Å. Their optical multicolor properties indicate that six emission-line sources are Ly$α$ emitters at $z \approx$ 3.7 ($Δz \approx 0.22$). They are either intense starburst galaxies or active galactic nuclei like quasars at $z \approx$ 3.7. Two more emission-line sources may also be Ly$α$ emitters at $z \approx$ 3.7 although their multicolor properties are marginal. Among the remaining 15 emission-line objects, eight objects appear strong emission-line galaxies at lower redshift; e.g., [O {\sc ii}] $λ$3727 emitters at $z \approx 0.54$, H$β$ at $z \approx 0.18$, or [O {\sc iii}]$λ$5007 emitters at $z \approx 0.15$. The remaining seven objects are unclassified because they are too faint to be detected in broad-band images. We discuss observational properties of these strong emission-line sources. In particular, our data allow us to estimate the star formation density at $z \approx 3.7$ for the first time.

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arXiv2001

Discovery of Very Extended Emission-Line Region around the Seyfert 2 Galaxy NGC 4388

We found a very large, 35 kpc, emission-line region around the Seyfert type 2 galaxy NGC 4388, using deep narrow-band imaging with the prime focus camera (Suprime-Cam) of the Subaru telescope. This region consists of many faint gas clouds or filaments, and extends northeastwards from the galaxy. The typical H-alpha luminosity L(H-alpha) of the clouds is 10^37 erg s^-1, and the total L(H-alpha) of the region within 10 kpc from the nucleus is 2 x 10^38 erg s^-1, which corresponds to an ionized gas mass of 10^5 M_solar. The map of the emission-line intensity ratio I([OIII])/I(H-alpha) indicates that the inner (r < 12 kpc) region of the VEELR may be excited by nuclear ionizing radiation. The excitation mechanism of the outer (r > 12 kpc) region is unclear, but it is likely that the nuclear radiation is also a dominant source of its ionization. We discuss the origin of the ionized gas. Two plausible origins of the gas in the VEELR are (i) the tidal debris resulting from a past interaction with a gas-rich dwarf galaxy, i.e., a minor merger, or (ii) the interstellar medium of NGC 4388, stripped by the ram pressure of the hot intracluster medium of the Virgo cluster.

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arXiv2001

Galaxy Population in a Cluster of Galaxies around the Radio Galaxy 3C 324 at z=1.2

We discuss the properties of galaxies around the radio galaxy 3C 324 at z=1.2 based on BVRIK' multi-band imaging data. We have applied a photometric-redshift technique to objects in the 3C 324 field, and identified 35 objects as plausible cluster members. We have found that red and luminous members are concentrated in a small region enclosed by a circle of 40" radius (0.33 Mpc at z=1.2 for Ω_0=0.3, λ_0=0.7, H_0=70km/s/Mpc cosmology) from the 3C 324 galaxy. The 3C 324 cluster is probably much more compact in size compared with the local clusters. We constructed a K'-band luminosity function of the cluster members and fit a Schechter function, and found the characteristic magnitude to be K*_{AB}=20.2+-0.6. This value is consistent with the extrapolation of the pure passive evolution seen for z<1 clusters. We have identified eight bright galaxies which form a red color-magnitude sequence. The slope of the sequence is consistent with the passive evolution model down to K'_{AB}<22; we also found that there is no clear age variation in these bright red galaxies. However, seven out of these eight galaxies exhibit a significant excess in the rest UV light with respect to the passive evolution model. This may suggest that the massive early-type galaxies in this high-redshift cluster are still forming stars to some extent. We have confirmed a truncation of the color-magnitude sequence at K'_{AB}\sim22; faint passively-evolving galaxies may not yet be present in this cluster at z\sim1.2. The overall color distribution of the cluster members, selected by the photometric redshift technique, is found to be very broad. We derived the fraction of blue galaxies in this cluster following a

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arXiv2001

Statistical Properties of Lyman Break Galaxies at z~4

We study the luminosity function and the correlation function of about 1200 z~4 Lyman break galaxies (LBGs) with i'<26 that are photometrically selected from deep BRi' imaging data of a 618 arcmin^2 area in the Subaru/XMM-Newton Deep Field taken with Subaru Prime Focus Camera. The contamination and completeness of our LBG sample are evaluated, on the basis of the Hubble Deep Field-North (HDF-N) objects, to be 17% and 45%, respectively. We derive the UV (rest 1700A) luminosity functions (LFs) and find a large population of UV-luminous galaxies at z~4. The LFs of the red and blue subsamples imply that the bright LBGs are redder in the UV continuum than the average color of the LBGs. Then we calculate the correlation function over theta = 2''-1000'' and find that it is fitted fairly well by a power law, omega(theta)=A_omega theta^(-0.8), with A_omega=0.71 +/- 0.26. We estimate the correlation length r_0 (in comoving units) of the two-point spatial correlation function xi(r) = (r/r_0)^(-1.8) to be r_0=2.7 +0.5/-0.6 h^(-1) Mpc (Omega_m=0.3 and Omega_Lambda=0.7). The correlation function shows an excess of omega (theta) on small scales (theta < 5''), departing from the power-law fit at > 3 sigma significance level. Interpreting this as being due to galaxy mergers, we evaluate the fraction of galaxies undergoing mergers to be 3.0 +/- 0.9%, which is significantly smaller than those of galaxies at intermediate redshifts.

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arXiv2001

Clustering Properties of Galaxies at z~4 in the Subaru/XMM Deep Survey Field

We study the clustering properties of about 1200 z~4 Lyman Break Galaxy (LBG) candidates with i'<26 which are selected by color from deep BRi' imaging data of a 618 arcmin^2 area in the Subaru/XMM-Newton Deep Field taken with Subaru Prime Focus Camera. The contamination and completeness of our LBG sample are evaluated, on the basis of the Hubble Deep Field North (HDFN) objects, to be 17% and 45%, respectively. We derive the angular correlation function over theta = 2''-1000'', and find that it is fitted fairly well by a power law, omega(theta)=A_omega theta^{-0.8}, with A_omega = 0.71 +/- 0.26. We then calculate the correlation length r0 (in comoving units) of the two-point spatial correlation function xi(r) = (r/r0)^{-1.8} from A_omega using the redshift distribution of LBGs derived from the HDFN, and find r0=2.7 (+0.5/-0.6) h^{-1} Mpc in a Lambda-dominated universe (Omega_m=0.3 and Omega_Lambda=0.7). This is twice larger than the correlation length of the dark matter at z~4 predicted from an analytic model by Peacock & Dodds but about twice smaller than that of bright galaxies predicted by a semi-analytic model of Baugh et al. We find an excess of omega(theta) on small scales (theta < 5'') departing from the power law fit over 3 sigma significance levels. Interpreting this as due to galaxy mergers, we estimate the fraction of galaxies undergoing mergers in our LBG sample to be 3.0 +/- 0.9%, which is significantly smaller than those of galaxies at intermediate redshifts.

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arXiv1999

New Improved Photometric Redshifts of Galaxies in the HDF

We report new improved photometric redshifts of 1048 galaxies in the Hubble Deep Field (HDF). A standard chi^2 minimizing method is applied to seven-color UBVIJHK photometry by Fernandez-Soto, Lanzetta, & Yahil (1999). We use 187 template SEDs representing a wide variety of morphology and age of observed galaxies based on a population synthesis model by Kodama & Arimoto (1997). We introduce two new recipes. First, the amount of the internal absorption is changed as a free parameter in the range of E(B-V)=0.0 to 0.5 with an interval of 0.1. Second, the absorption due to intergalactic HI clouds is also changed by a factor of 0.5, 1.0, and 1.5 around the opacity given by Madau (1995). The total number of template SEDs is thus 187x6x3=3,366, except for the redshift grid. The dispersion sigma_z of our photometric redshifts with respect to spectroscopic redshifts is sigma_z=0.08 and 0.24 for z<2 and z>2, respectively, which are smaller than the corresponding values (sigma_z=0.09 and 0.45) by Fernandez-Soto et al. Improvement is significant, especially in z>2. This is due to smaller systematic errors which are largely reduced mainly by including three opacities due to intergalactic HI clouds. We discuss redshift distribution N(z) and cosmic star formation rate based on our new photometric redshifts.

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arXiv1999

Probing the evolution of early-type galaxies using multi-colour number counts and redshift distributions

We investigate pure luminosity evolution models for early-type (elliptical and S0) galaxies (i.e., no number density change or morphology transition), and examine whether these models are consistent with observed number counts in the B, I and K bands and redshift distributions of two samples of faint galaxies selected in the I and K bands. The models are characterized by the star formation time scale $τ_{SF}$ and the time $t_{gw}$ when galactic wind blows in addition to several other conventional parameters. We find the single-burst model ($τ_{SF}$=0.1 Gyr and $t_{gw}$=0.353 Gyr), which is known to reproduce the photometric properties of early-type galaxies in clusters, is inconsistent with redshift distributions of early-type galaxies in the field environment due to overpredictions of galaxies at $z\gsim1.4$ even with strong extinction which is at work until $t_{gw}$. In order for dust extinction to be more effective, we change $τ_{SF}$ and $t_{gw}$ as free parameters, and find that models with $τ_{SF}\gsim0.5$ Gyr and $t_{gw}>1.0$ Gyr can be made consistent with both the observed redshift distributions and number counts, if we introduce strong extinction $(E(B-V)\geq1$ as a peak value). These results suggest that early-type galaxies in the field environment do not have the same evolutionary history as described by the single-burst model.

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arXiv1998

Retrieving Bulge and Disk Parameters and Asymptotic Magnitudes from the Growth Curves of Galaxies

We show that the growth curves of galaxies can be used to determine their bulge and disk parameters and bulge-to-total luminosity ratios, in addition to their conventional asymptotic magnitudes, provided that the point spread function is accurately known and signal-to-noise ratio is modest (S/N$\gtrsim30$). The growth curve is a fundamental quantity that most future large galaxy imaging surveys will measure. Bulge and disk parameters retrieved from the growth curve will enable us to perform statistical studies of luminosity structure for a large number of galaxies.

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arXiv1998

An Objective and Automatic Cluster Finder: An Improvement of the Matched-Filter Method

We describe an objective and automated method for detecting clusters of galaxies from optical imaging data. This method is a variant of the so-called `matched-filter' technique pioneered by Postman et al. (1996). With simultaneous use of positions and apparent magnitudes of galaxies, this method can, not only find cluster candidates, but also estimate their redshifts and richnesses as byproducts of detection. We examine errors in the estimation of cluster's position, redshift, and richness with a number of Monte Carlo simulations. No systematic discrepancies between the true and estimated values are seen for either redshift or richness. Spurious detection rate of the method is about less than 10% of those of conventional ones which use only surface density of galaxies. A cluster survey in the North Galactic Pole is executed to verify the performance characteristics of the method with real data. Two known real clusters are successfully detected. We expect these methods based on `matched-filter' technique to be essential tools for compiling large and homogeneous optically-selected cluster catalogs.

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arXiv1998

An Unbiased Estimate of the Global Hubble Constant in the Region of Pisces-Perseus

We obtain an unbiased estimate of the global Hubble constant H0 in the volume of cz<12000km/s in the region of Pisces-Perseus. The Tully-Fisher (TF) relation is applied to a magnitude limited sample of 441 spirals selected from the Arecibo 21cm catalog. The photometry data were calibrated with CCD observations and we achieve 0.13mag for the photometric internal error. We use a maximum likelihood method for the TF analysis. Monte-Carlo simulations demonstrate that our method reproduces a given H0 at the 95% confidence level. By applying the method to our sample galaxies, we obtain the unbiased global Hubble constant H0=65+-2(+20,-14) km/s/Mpc; the first and the second terms represent the internal random error and the external errors, respectively. We also find a good agreement for our H0 with those recently obtained via Cepheid observation, the TF relation and supernovae. Hubble velocities of the spirals inferred from our H0 show no significant systematic difference from those given in the Mark III catalog. The same analysis for H0 is carried out using r-band photometry data of the Pisces-Perseus region given by Willick et al.(1997). We obtain a global H0 which is consistent with that obtained from our B-band analysis. A bulk motion in the Pisces-Perseus region is briefly discussed, based on our calibration of H0. Our r-band TF analysis supports the notion of a coherent streaming motion of the Pisces-Perseus ridge with -200km/s with respect to the CMB, in agreement with most modern studies.

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