arXiv subjects

Tetsuo Nishimura

Publications and source records attributed to Tetsuo Nishimura.

At least 37 records · Page 2Linked to original sources

arXiv2015

Near-IR High-Resolution Imaging Polarimetry of the SU Aur Disk: Clues for Tidal Tails?

We present new high-resolution ($\sim$0\farcs09) $H$-band imaging observations of the circumstellar disk around the T Tauri star SU Aur. Our observations with Subaru-HiCIAO have revealed the presence of scattered light as close as 0\farcs15 ($\sim$20 AU) to the star. Within our image, we identify bright emission associated with a disk with a minimum radius of $\sim$90 AU, an inclination of $\sim$35\degr from the plane of the sky, and an approximate P.A. of 15\degr for the major axis. We find a brightness asymmetry between the northern and southern sides of the disk due to a non-axisymmetric disk structure. We also identify a pair of asymmetric tail structures extending east and west from the disk. The western tail extends at least 2\farcs5 (350 AU) from the star, and is probably associated with a reflection nebula previously observed at optical and near-IR wavelengths. The eastern tail extends at least 1\arcsec (140 AU) at the present signal-to-noise. These tails are likely due to an encounter with an unseen brown dwarf, but our results do not exclude the explanation that these tails are outflow cavities or jets.

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arXiv2014

SEEDS Adaptive Optics Imaging of the Asymmetric Transition Disk Oph IRS 48 in Scattered Light

We present the first resolved near infrared imagery of the transition disk Oph IRS 48 (WLY 2-48), which was recently observed with ALMA to have a strongly asymmetric sub-millimeter flux distribution. H-band polarized intensity images show a $\sim$60AU radius scattered light cavity with two pronounced arcs of emission, one from Northeast to Southeast and one smaller, fainter and more distant arc in the Northwest. K-band scattered light imagery reveals a similar morphology, but with a clear third arc along the Southwestern rim of the disk cavity. This arc meets the Northwestern arc at nearly a right angle, revealing the presence of a spiral arm or local surface brightness deficit in the disk, and explaining the East-West brightness asymmetry in the H-band data. We also present 0.8-5.4$μ$m IRTF SpeX spectra of this object, which allow us to constrain the spectral class to A0$\pm$1 and measure a low mass accretion rate of 10$^{-8.5}$M$_{\odot}$/yr, both consistent with previous estimates. We investigate a variety of reddening laws in order to fit the mutliwavelength SED of Oph IRS 48 and find a best fit consistent with a younger, higher luminosity star than previous estimates.

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arXiv2014

Indications of M-dwarf Deficits in the Halo and Thick Disk of the Galaxy

We compared the number of faint stars detected in deep survey fields with the current stellar distribution model of the Galaxy and found that the detected number in the H band is significantly smaller than the predicted number. This indicates that M-dwarfs, the major component, are fewer in the halo and the thick disk. We used archived data of several surveys in both the north and south field of GOODS (Great Observatories Origins Deep Survey), MODS in GOODS-N, and ERS and CANDELS in GOODS-S. The number density of M-dwarfs in the halo has to be 20+/-13% relative to that in the solar vicinity, in order for the detected number of stars fainter than 20.5 mag in the H band to match with the predicted value from the model. In the thick disk, the number density of M-dwarfs must be reduced (52+/-13%) or the scale height must be decreased (~600 pc). Alternatively, overall fractions of the halo and thick disks can be significantly reduced to achieve the same effect, because our sample mainly consists of faint M-dwarfs. Our results imply that the M-dwarf population in regions distant from the Galactic plane is significantly smaller than previously thought. We then discussed the implications this has on the suitability of the model predictions for the prediction of non-companion faint stars in direct imaging extrasolar planet surveys by using the best-fit number densities.

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arXiv2014

Surface Geometry of Protoplanetary Disks Inferred From Near-Infrared Imaging Polarimetry

We present a new method of analysis for determining the surface geometry of five protoplanetary disks observed with near-infrared imaging polarimetry using Subaru-HiCIAO. Using as inputs the observed distribution of polarized intensity (PI), disk inclination, assumed properties for dust scattering, and other reasonable approximations, we calculate a differential equation to derive the surface geometry. This equation is numerically integrated along the distance from the star at a given position angle. We show that, using these approximations, the local maxima in the PI distribution of spiral arms (SAO 206462, MWC 758) and rings (2MASS J16042165-2130284, PDS 70) are associated with local concave-up structures on the disk surface. We also show that the observed presence of an inner gap in scattered light still allows the possibility of a disk surface that is parallel to the light path from the star, or a disk that is shadowed by structures in the inner radii. Our analysis for rings does not show the presence of a vertical inner wall as often assumed in studies of disks with an inner gap. Finally, we summarize the implications of spiral and ring structures as potential signatures of ongoing planet formation.

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arXiv2013

The Formation of the massive galaxies in the SSA22 z=3.1 protocluster

The properties of K-band selected galaxies (K_AB<24) in the z = 3.09 SSA22 protocluster field are studied. 430 galaxies at 2.6 < z_phot < 3.6 are selected as potential protocluster members in a 112 arcmin^2 area based on their photometric redshifts. We find that \approx 20% of the massive galaxies with stellar masses >10^11 M_sun at z_phot \sim 3.1 have colors consistent with those of quiescent galaxies with ages > 0.5 Gyr. This fraction increases to \approx 50% after correcting for unrelated foreground/background objects. We also find that 30% of the massive galaxies are heavily reddened dusty star-forming galaxies. Few such quiescent galaxies at similar redshifts are seen in typical survey fields. An excess surface density of 24μm sources at z_phot \sim 3.1 is also observed, implying the presence of dusty star-formation activity in the protocluster. Cross-correlation with the X-ray data indicates that the fraction of K-band selected protocluster galaxies hosting active galactic nuclei (AGN) is also high compared with the field. The sky distribution of the quiescent galaxies, the 24μm sources, and the X-ray AGNs show clustering around a density peak of z=3.1 Lyαemitters (LAEs). A significant fraction of the massive galaxies have already become quiescent, while the dusty star-formation is still active in the SSA22 protocluster. These findings indicate that we are witnessing the formation epoch of massive early-type galaxies at the center of predecessors to present-day rich galaxy clusters.

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arXiv2013

A Discovery of a Candidate Companion to a Transiting System KOI-94: A Direct Imaging Study for a Possibility of a False Positive

We report a discovery of a companion candidate around one of {\it Kepler} Objects of Interest (KOIs), KOI-94, and results of our quantitative investigation of the possibility that planetary candidates around KOI-94 are false positives. KOI-94 has a planetary system in which four planetary detections have been reported by {\it Kepler}, suggesting that this system is intriguing to study the dynamical evolutions of planets. However, while two of those detections (KOI-94.01 and 03) have been made robust by previous observations, the others (KOI-94.02 and 04) are marginal detections, for which future confirmations with various techniques are required. We have conducted high-contrast direct imaging observations with Subaru/HiCIAO in $H$ band and detected a faint object located at a separation of $\sim0.6''$ from KOI-94. The object has a contrast of $\sim 1\times 10^{-3}$ in $H$ band, and corresponds to an M type star on the assumption that the object is at the same distance of KOI-94. Based on our analysis, KOI-94.02 is likely to be a real planet because of its transit depth, while KOI-94.04 can be a false positive due to the companion candidate. The success in detecting the companion candidate suggests that high-contrast direct imaging observations are important keys to examine false positives of KOIs. On the other hand, our transit light curve reanalyses lead to a better period estimate of KOI-94.04 than that on the KOI catalogue and show that the planetary candidate has the same limb darkening parameter value as the other planetary candidates in the KOI-94 system, suggesting that KOI-94.04 is also a real planet in the system.

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arXiv2013

High-Contrast Near-Infrared Imaging Polarimetry of the Protoplanetary Disk around RY Tau

We present near-infrared coronagraphic imaging polarimetry of RY Tau. The scattered light in the circumstellar environment was imaged at H-band at a high resolution (~0".05) for the first time, using Subaru-HiCIAO. The observed polarized intensity (PI) distribution shows a butterfly-like distribution of bright emission with an angular scale similar to the disk observed at millimeter wavelengths. This distribution is offset toward the blueshifted jet, indicating the presence of a geometrically thick disk or a remnant envelope, and therefore the earliest stage of the Class II evolutionary phase. We perform comparisons between the observed PI distribution and disk models with: (1) full radiative transfer code, using the spectral energy distribution (SED) to constrain the disk parameters; and (2) monochromatic simulations of scattered light which explore a wide range of parameters space to constrain the disk and dust parameters. We show that these models cannot consistently explain the observed PI distribution, SED, and the viewing angle inferred by millimeter interferometry. We suggest that the scattered light in the near-infrared is associated with an optically thin and geometrically thick layer above the disk surface, with the surface responsible for the infrared SED. Half of the scattered light and thermal radiation in this layer illuminates the disk surface, and this process may significantly affect the thermal structure of the disk.

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arXiv2013

Direct Imaging Search for Extrasolar Planets in the Pleiades

We carried out an imaging survey for extrasolar planets around stars in the Pleiades (125 Myr, 135 pc) in the $H$ and $K_{S}$ bands using HiCIAO combined with the adaptive optics, AO188, on the Subaru telescope. We found 13 companion candidates fainter than 14.5 mag in the $H$ band around 9 stars. Five of these 13 were confirmed to be background stars by measurement of their proper motion. One was not found in the second epoch observation, and thus was not a background or companion object. One had multi-epoch image, but the precision of its proper motion was not sufficient to conclude whether it was background object. Four other candidates are waiting for second epoch observations to determine their proper motion. Finally, the remaining 2 were confirmed to be 60 $M_{J}$ brown dwarf companions orbiting around HD 23514 (G0) and HII 1348 (K5) respectively, as had been reported in previous studies. In our observations, the average detection limit for a point source was 20.3 mag in the $H$ band beyond 1''.5 from the central star. On the basis of this detection limit, we calculated the detection efficiency to be 90% for a planet with 6 to 12 Jovian masses and a semi-major axis of 50--1000 AU. For this we extrapolated the distribution of planet mass and semi-major axis derived from RV observations and adopted the planet evolution model of Baraffe et al. (2003). As there was no detection of a planet, we estimated the frequency of such planets to be less than 17.9% ($2σ$) around one star of the Pleiades cluster.

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arXiv2013

Mapping H-band Scattered Light Emission in the Mysterious SR21 Transitional Disk

We present the first near infrared (NIR) spatially resolved images of the circumstellar transitional disk around SR21. These images were obtained with the Subaru HiCIAO camera, adaptive optics and the polarized differential imaging (PDI) technique. We resolve the disk in scattered light at H-band for stellocentric 0.1"<r<0.6" (12<r<75AU). We compare our results with previously published spatially-resolved 880 micron continuum Submillimeter Array (SMA) images that show an inner r<36AU cavity in SR21. Radiative transfer models reveal that the large disk depletion factor invoked to explain SR21's sub-mm cavity cannot be "universal" for all grain sizes. Even significantly more moderate depletions (delta=0.1, 0.01 relative to an undepleted disk) than those that reproduce the sub-mm cavity (delta~10^-6) are inconsistent with our H-band images when they are assumed to carry over to small grains, suggesting that surface grains scattering in the NIR either survive or are generated by whatever mechanism is clearing the disk midplane. In fact, the radial polarized intensity profile of our H-band observations is smooth and steeply inwardly-increasing (r^-3), with no evidence of a break at the 36AU sub-mm cavity wall. We hypothesize that this profile is dominated by an optically thin disk envelope or atmosphere component. We also discuss the compatibility of our data with the previously postulated existence of a sub-stellar companion to SR21 at r~10-20AU, and find that we can neither exclude nor verify this scenario. This study demonstrates the power of multiwavelength imaging of transitional disks to inform modeling efforts, including the debate over precisely what physical mechanism is responsible for clearing these disks of their large midplane grains.

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arXiv2012

A Common Proper Motion Stellar Companion to HAT-P-7

We report that HAT-P-7 has a common proper motion stellar companion. The companion is located at $\sim3.9$ arcsec to the east and estimated as an M5.5V dwarf based on its colors. We also confirm the presence of the third companion, which was first reported by Winn et al. (2009), based on long-term radial velocity measurements. We revisit the migration mechanism of HAT-P-7b given the presence of those companions, and propose sequential Kozai migration as a likely scenario in this system. This scenario may explain the reason for an outlier in the discussion of the spin-orbit alignment timescale for HAT-P-7b by Albrecht et al. (2012).

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arXiv2012

Assembly of Massive Galaxies in a High-z Protocluster

We present the results of wide-field deep JHK imaging of the SSA22 field using MOIRCS instrument equipped with Subaru telescope. The observed field is 112 arcmin^2 in area, which covers the z=3.1 protocluster characterized by the overdensities of Ly Alpha emitters (LAEs) and Ly Alpha Blobs (LABs). The 5 sigma limiting magnitude is K_{AB} = 24.3. We extract the potential protocluster members from the K-selected sample by using the multi-band photometric-redshift selection as well as the simple color cut for distant red galaxies (DRGs; J-K_{AB}>1.4). The surface number density of DRGs in our observed fields shows clear excess compared with those in the blank fields, and the location of the densest area whose projected overdensity is twice the average coincides with the large-scale density peak of LAEs. We also found that K-band counterparts with z_{phot} = 3.1 are detected for 75% (15/20) of the LABs within their Ly Alpha halo, and the 40 % (8/20) of LABs have multiple components, which gives a direct evidence of the hierarchical multiple merging in galaxy formation. The stellar mass ofLABs correlates with their luminosity, isophotal area, and the Ly Alpha velocity widths, implying that the physical scale and the dynamical motion of Ly Alpha emission are closely related to their previous star-formation activities. Highly dust-obscured galaxies such as hyper extremely red objects (HEROs; J-K_{AB}>2.1) and plausible K-band counterparts of submillimeter sources are also populated in the high density region.

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arXiv2010

MOIRCS Deep Survey. IX. Deep Near-Infrared Imaging Data and Source Catalog

We present deep J-, H-, and Ks-band imaging data of the MOIRCS Deep Survey (MODS), which was carried out with Multi-Object Infrared Camera and Spectrograph (MOIRCS) mounted on the Subaru telescope in the GOODS-North region. The data reach 5sigma total limiting magnitudes for point sources of J=23.9, H=22.8, and Ks=22.8 (Vega magnitude) over 103 arcmin^2 (wide field). In 28 arcmin^2 of the survey area, which is ultra deep field of the MODS (deep field), the data reach the 5sigma depths of J=24.8, H=23.4, and Ks=23.8. The spatial resolutions of the combined images are FWHM ~ 0.6 arcsec and ~ 0.5 arcsec for the wide and deep fields in all bands, respectively. Combining the MODS data with the multi-wavelength public data taken with the HST, Spitzer, and other ground-based telescopes in the GOODS field, we construct a multi-wavelength photometric catalog of Ks-selected sources. Using the catalog, we present Ks-band number counts and near-infrared color distribution of the detected objects, and demonstrate some selection techniques with the NIR colors for high redshift galaxies. These data and catalog are publicly available via internet.

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arXiv2010

MOIRCS Deep Survey. VII: NIR Morphologies of Star-forming Galaxies at Redshift z~1

We investigate rest-frame near-infrared (NIR) morphologies of a sample of 139 galaxies with M_{s} >= 1 x 10^{10} M_{sun} at z=0.8-1.2 in the GOODS-North field using our deep NIR imaging data (MOIRCS Deep Survey, MODS). We focus on Luminous Infrared Galaxies (LIRGs), which dominate high star formation rate (SFR) density at z~1, in the sample identified by cross-correlating with the Spitzer/MIPS 24um source catalog. We perform two-dimensional light profile fitting of the z~1 galaxies in the Ks-band (rest-frame J-band) with a single component Sersic model. We find that at z~1, ~90% of LIRGs have low Sersic indices (n<2.5, similar to disk-like galaxies) in the Ks-band, and those disk-like LIRGs consist of ~60% of the whole disk-like sample above M_{s} >= 3 x 10^{10} M_{sun}. The z~1 disk-like LIRGs are comparable or ~20% small at a maximum in size compared to local disk-like galaxies in the same stellar mass range. If we examine rest-frame UV-optical morphologies using the HST/ACS images, the rest-frame B-band sizes of the z~1 disk-like galaxies are comparable to those of the local disk-like galaxies as reported by previous studies on size evolution of disk-like galaxies in the rest-frame optical band. Measuring color gradients (galaxy sizes as a function of wavelength) of the z~1 and local disk-like galaxies, we find that the z~1 disk-like galaxies have 3-5 times steeper color gradient than the local ones. Our results indicate that (i) more than a half of relatively massive disk-like galaxies at z~1 are in violent star formation epochs observed as LIRGs, and also (ii) most of those LIRGs are constructing their fundamental disk structure vigorously. The high SFR density in the universe at z~1 may be dominated by such star formation in disk region in massive galaxies.

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arXiv2010

MOIRCS Deep Survey. VI. Near-Infrared Spectroscopy of K-Selected Star-Forming Galaxies at z~2

We present near-infrared (NIR) spectroscopy for 37 BzK-color-selected star-forming galaxies with MOIRCS on Subaru. The sample is drawn from the Ks-band selected catalog of the MOIRCS Deep Survey (MODS) in the GOODS-N region. About half of our samples are selected from the MIPS 24um-source catalog. Ha emission lines are detected from 23 galaxies, of which the median redshift is 2.12. We derived the star formation rates (SFRs) from extinction-corrected Ha luminosities. The extinction correction is estimated from the SED fitting of multi-band photometric data covering UV to NIR wavelengths. The Balmer decrement of the stacked emission lines shows that the amount of extinction for the ionized gas is larger than that for the stellar continuum. From a comparison of the extinction corrected Ha luminosity and other SFR indicators we found that the relation between the dust properties of stellar continuum and ionized gas is different depending on the intrinsic SFR. The comparison between the Ha SFR and stellar mass estimated from SED fitting shows no correlation between them. Some galaxies with stellar mass smaller than ~10^10 Msun show SFRs higher than ~100 Msun/yr. The specific SFRs (SSFRs) of these galaxies are remarkably high. The average stellar-population age of these high-SSFR galaxies is younger than 100 Myr, which is consistent with the implied high SSFR. The large SFR implies the possibility that the high SSFR galaxies significantly contribute to the cosmic SFR density of the universe at z~2. The total SFR density estimated from the Ha emission line galaxies is 0.089-0.136 Msun/yr/Mpc^3, which is consistent with the total SFR densities in the literature. The metallicity of the high-SSFR galaxies is larger than that expected from the mass-metallicity relation of UV-selected galaxies at z~2 by Erb et al. (2006a). [Abridged]

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arXiv2010

Search for Outer Massive Bodies around Transiting Planetary Systems: Candidates of Faint Stellar Companions around HAT-P-7

We present results of direct imaging observations for HAT-P-7 taken with the Subaru HiCIAO and the Calar Alto AstraLux. Since the close-in transiting planet HAT-P-7b was reported to have a highly tilted orbit, massive bodies such as giant planets, brown dwarfs, or a binary star are expected to exist in the outer region of this system. We show that there are indeed two candidates for distant faint stellar companions around HAT-P-7. We discuss possible roles played by such companions on the orbital evolution of HAT-P-7b. We conclude that as there is a third body in the system as reported by Winn et al. (2009, ApJL, 763, L99), the Kozai migration is less likely while planet-planet scattering is possible.

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arXiv2008

Subaru/MOIRCS Near-Infrared Imaging in the Proto-Cluster Region at z=3.1

We present the results of deep near-infrared imaging observations of the z=3.1 proto-cluster region in the SSA22a field taken by MOIRCS mounted on the Subaru Telescope. We observed a 21.7 arcmin^2 field to the depths of J=24.5, H=24.3, and K=23.9 (5 sigma). We examine the distribution of the K-selected galaxies at z~3 by using the simple color cut for distant red galaxies (DRGs) as well as the photometric-redshift selection technique. The marginal density excess of DRGs and the photo-z selected objects are found around the two most luminous Ly alpha blobs (LABs). We investigate the correlation between the K-selected objects and the LABs, and find that several galaxies with stellar mass M_* = 10^9-10^11 M_solar exist in vicinity of LABs, especially around the two most luminous ones. We also find that 7 of the 8 LABs in the field have plausible K_s-band counterparts and the sum of the stellar mass possibly associated with LABs correlates with the luminosity and surface brightness of them, which implies that the origin of Ly alpha emission may be closely correlated with their stellar mass or their previous star formation phenomena.

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arXiv2007

MOIRCS Deep Survey. II. Clustering Properties of K-band Selected Galaxies in GOODS-North Region

We present the first measurement of clustering properties of low mass galaxies with a stellar mass down to M_*~10^9 Msun at 1 1.3 show a large bias of b~7.2+-1.3 on scales up to θ~100" or 3.1 comoving Mpc, while blue galaxies with 0.5 ~10^{10} Msun, we estimate a correlation length and bias to be r_0~4.5 h^{-1} Mpc and b=1.9-3.5, which are much larger than those of low mass (M_*~10^9-10^{10} Msun) galaxies. The comparison of our measurements with analytic CDM models constrains the properties of hosting dark halos, and indicates that the low mass galaxies would be progenitors of galaxies with a typical luminosity of L<~L_* in the local Universe. The blue galaxies in low mass samples are more strongly clustered in more massive halos with higher occupation numbers than low mass red galaxies. This fact suggests an environment effect due to the halo mass on star formation activity at high-z.

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arXiv2006

MOIRCS Deep Survey. I: DRG Number Counts

We use very deep near-infrared imaging data taken with Multi-Object InfraRed Camera and Spectrograph (MOIRCS) on the Subaru Telescope to investigate the number counts of Distant Red Galaxies (DRGs). We have observed a 4x7 arcmin^2 field in the Great Observatories Origins Deep Survey North (GOODS-N), and our data reach J=24.6 and K=23.2 (5sigma, Vega magnitude). The surface density of DRGs selected by J-K>2.3 is 2.35+-0.31 arcmin^-2 at K<22 and 3.54+-0.38 arcmin^-2 at K<23, respectively. These values are consistent with those in the GOODS-South and FIRES. Our deep and wide data suggest that the number counts of DRGs turn over at K~22, and the surface density of the faint DRGs with K>22 is smaller than that expected from the number counts at the brighter magnitude. The result indicates that while there are many bright galaxies at 2 22 suggest that the mass-dependent color distribution, where most of low-mass galaxies are blue while more massive galaxies tend to have redder colors, had already been established at that epoch.

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