arXiv subjects

Michitoshi Yoshida

Publications and source records attributed to Michitoshi Yoshida.

At least 109 records · Page 6Linked to original sources

arXiv2008

Planetary Companions to Evolved Intermediate-Mass Stars: 14 Andromedae, 81 Ceti, 6 Lyncis, and HD 167042

We report on the detection of four extrasolar planets orbiting evolved intermediate-mass stars from a precise Doppler survey of G and K giants at Okayama Astrophysical Observatory. All of the host stars are considered to be formerly early F-type or A-type dwarfs when they were on the main sequence. 14 And (K0 III) is a clump giant with a mass of 2.2 M_solar and has a planet of minimum mass m_2sin i=4.8 M_Jup in a nearly circular orbit with a 186 day period. This is one of the innermost planets around evolved intermediate-mass stars and such planets have only been discovered in clump giants. 81 Cet (G5 III) is a clump giant with 2.4 M_solar hosting a planet of m_2sin i=5.3 M_Jup in a 953 day orbit with an eccentricity of e=0.21. 6 Lyn (K0 IV) is a less evolved subgiant with 1.7 M_solar and has a planet of m_2sin i=2.4 M_Jup in a 899 day orbit with e=0.13. HD 167042 (K1 IV) is also a less evolved star with 1.5 M_solar hosting a planet of m_2sin i=1.6 M_Jup in a 418 day orbit with e=0.10. This planet was independently announced by Johnson et al. (2008, ApJ, 675, 784). All of the host stars have solar or sub-solar metallicity, which supports the lack of metal-rich tendency in planet-harboring giants in contrast to the case of dwarfs.

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arXiv2008

The Subaru/XMM-Newton Deep Survey (SXDS). IV. Evolution of Lya Emitters from z=3.1 to 5.7 in the 1 deg^2 Field: Luminosity Functions and AGN

We present luminosity functions (LFs) and various properties of Lya emitters (LAEs) at z=3.1, 3.7, and 5.7, in a 1 deg^2 sky of the Subaru/XMM-Newton Deep Survey (SXDS) Field. We obtain a photometric sample of 858 LAE candidates based on deep Subaru/Suprime-Cam imaging data, and a spectroscopic sample of 84 confirmed LAEs from Subaru/FOCAS and VLT/VIMOS spectroscopy in a survey volume of ~10^6 Mpc^3 with a limiting Lya luminosity of ~3x10^42 erg/s. We derive the LFs of Lya and UV-continuum (~1500 Å) for each redshift, taking into account the statistical error and the field-to-field variation. We find that the apparent Lya LF shows no significant evolution between z=3.1 and 5.7 within factors of 1.8 and 2.7 in L* and phi*, respectively. On the other hand, the UV LF of LAEs increases from z=3.1 to 5.7, indicating that galaxies with Lya emission are more common at earlier epochs. We identify six LAEs with AGN activities from our spectra combined with VLA, Spitzer, and XMM-Newton data. Among the photometrically selected LAEs at z=3.1 and 3.7, only ~1 % show AGN activities, while the brightest LAEs with logL(Lya) >~ 43.4-43.6 erg/s appear to always host AGNs. Our LAEs are bluer in UV-continuum color than dropout galaxies, suggesting lower extinction and/or younger stellar populations. Our stacking analyses provide upper limits to the radio luminosity and the f(HeII)/f(Lya) line fraction, and constrain the hidden star formation (+low-luminosity AGN) and the primordial population in LAEs.

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arXiv2008

The 2006 November outburst of EG Aquarii: the SU UMa nature revealed

We report time-resolved CCD photometry of the cataclysmic variable EG Aquarii during the 2006 November outburst During the outburst, superhumps were unambiguously detected with a mean period of 0.078828(6) days, firstly classifying the object as an SU UMa-type dwarf nova. It also turned out that the outburst contained a precursor. At the end of the precursor, immature profiles of humps were observed. By a phase analysis of these humps, we interpreted the features as superhumps. This is the second example that the superhumps were shown during a precursor. Near the maximum stage of the outburst, we discovered an abrupt shift of the superhump period by ${\sim}$ 0.002 days. After the supermaximum, the superhump period decreased at the rate of $\dot{P}/P$=$-8.2{\times}10^{-5}$, which is typical for SU UMa-type dwarf novae. Although the outburst light curve was characteristic of SU UMa-type dwarf novae, long-term monitoring of the variable shows no outbursts over the past decade. We note on the basic properties of long period and inactive SU UMa-type dwarf novae.

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arXiv2008

Evidence for a companion to BM Gem, a silicate carbon star

Balmer and Paschen continuum emission as well as Balmer series lines of P Cygni-type profile from H_gamma through H_23 are revealed in the violet spectra of BM Gem, a carbon star associated with an oxygen-rich circumstellar shell (`silicate carbon star') observed with the high dispersion spectrograph (HDS) on the Subaru telescope. The blue-shifted absorption in the Balmer lines indicates the presence of an outflow, the line of sight velocity of which is at least 400 km s^-1, which is the highest outflow velocity observed to date in a carbon star. We argue that the observed unusual features in BM Gem are strong evidence for the presence of a companion, which should form an accretion disk that gives rise to both an ionized gas region and a high velocity, variable outflow. The estimated luminosity of ~0.2 (0.03-0.6) L_sun for the ionized gas can be maintained by a mass accretion rate to a dwarf companion of ~10^-8 M_sun yr^-1, while ~10^-10 M_sun yr^-1 is sufficient for accretion to a white dwarf companion. These accretion rates are feasible for some detached binary configurations on the basis of the Bond-Hoyle type accretion process. We concluded that the carbon star BM Gem is in a detached binary system with a companion of low mass and low luminosity. However, we are unable to determine whether this companion object is a dwarf or a white dwarf. The upper limits for binary separation are 210 AU and 930 AU for a dwarf and a white dwarf, respectively. We also note that the observed features of BM Gem mimic those of Mira (omi Cet), which may suggest actual similarities in their binary configurations and circumstellar structures.

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arXiv2008

Planetary Companions around Three Intermediate-Mass G and K Giants: 18 Del, xi Aql, and HD 81688

We report the detection of 3 new extrasolar planets from the precise Doppler survey of G and K giants at Okayama Astrophysical Observatory. The host stars, namely, 18 Del (G6 III), xi Aql (K0 III) and HD 81688 (K0 III-IV), are located at the clump region on the HR diagram with estimated masses of 2.1-2.3 M_solar. 18 Del b has a minimum mass of 10.3 M_Jup and resides in a nearly circular orbit with period of 993 days, which is the longest one ever discovered around evolved stars. xi Aql b and HD 81688 b have minimum masses of 2.8 and 2.7 M_Jup, and reside in nearly circular orbits with periods of 137 and 184 days, respectively, which are the shortest ones among planets around evolved stars. All of the substellar companions ever discovered around possible intermediate-mass (1.7-3.9 M_solar) clump giants have semimajor axes larger than 0.68 AU, suggesting the lack of short-period planets. Our numerical calculations suggest that Jupiter-mass planets within about 0.5 AU (even up to 1 AU depending on the metallicity and adopted models) around 2-3 M_solar stars could be engulfed by the central stars at the tip of RGB due to tidal torque from the central stars. Assuming that most of the clump giants are post-RGB stars, we can not distinguish whether the lack of short-period planets is primordial or due to engulfment by central stars. Deriving reliable mass and evolutionary status for evolved stars is highly required for further investigation of formation and evolution of planetary systems around intermediate-mass stars.

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arXiv2007

A Substellar Companion to the Intermediate-Mass Giant 11 Com

We report the detection of a substellar companion orbiting the intermediate-mass giant star 11 Com (G8 III). Precise Doppler measurements of the star from Xinglong station and Okayama Astrophysical Observatory (OAO) revealed Keplerian velocity variations with an orbital period of 326.03 +/- 0.32 days, a semiamplitude of 302.8 +/- 2.6 m/s, and an eccentricity of 0.231 +/- 0.005. Adopting a stellar mass of 2.7 +/- 0.3 M_solar, the minimum mass of the companion is 19.4 +/- 1.5 M_Jup, well above the deuterium burning limit, and the semimajor axis is 1.29 +/- 0.05 AU. This is the first result from the joint planet search program between China and Japan aiming at revealing statistics of substellar companions around intermediate-mass giants. 11 Com b emerged from 300 targets of the planet search program at OAO. The current detection rate of a brown dwarf candidate seems to be comparable to that around solar-type stars within orbital separations of $\sim$3 AU.

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arXiv2007

Deep Spectroscopy of Systematically Surveyed Extended Lyman-alpha Sources at z~3-5

Spatially extended Ly-alpha sources that are faint and/or compact in coninuum are candidates for extremely young (~< 10^7 yrs) galaxies at high redshifts. We present medium-resolution (R~2000) spectroscopy of such extended Ly-alpha sources found in our previous study at z~3-5, using VLT/VIMOS. The deep spectroscopy showed that all 18 objects we observed have large equivalent widths (EWs) exceeding 100 A. For about 30% of our sample (five objects), we identified conspicuous asymmetry on the profiles of the Ly-alpha line. They show broad wing emission components on the red side, and sharp cut-off on the blue side of the Ly-alpha line. Such asymmetry is often seen in superwind galaxies known to date, and also consistent with a theoretical prediction of superwind activity. There are eight objects (8/18 ~ 40%) that have large EWs exceeding 200 A, and no clear signature of superwind activities. Such large EWs cannot be explained in terms of photo-ionization by a moderately old (>10^7 yrs) stellar population, even with a top-heavy IMF or an extremely low metallicity. These eight objects clearly show a positive correlation between the Ly-alpha luminosity and the velocity width. This suggests that these eight objects are good candidates for forming-galaxies in a gas-cooling phase.

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arXiv2007

The Remarkable 60 x 2 kpc Optical Filament Associated with a Poststarburst Galaxy in Coma Cluster

In the deep narrow band image of the Coma Cluster taken with Suprime-Cam of the Subaru telescope, we found an extremely long and narrow (~ 60 kpc x 2 kpc) Halpha emitting region associated with a poststarburst galaxy (D100). Follow up spectroscopy shows that the region has the same redshift as D100. The surface brightness of the region is typically 25 mag(AB) arcsec^-2 in Halpha, which corresponds to 0.5--4x10^-17 erg s^-1 cm^-2 arcsec^-2. We set two possible explanations for the origin of the region; gas stripped off from a merged dwarf, or gas stripped off from D100 by ram pressure. Either scenario has difficulty to fully explain all the observed characteristics of the region.

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arXiv2006

Systematic Survey of Extended Lyman-alpha Sources over z~3-5

Spatially extended Ly-alpha sources which are faint and compact in coninuum are candidates for extremely young galaxies (age of ~<10e+7 yrs) at high redshifts. We carried out a systematic survey for extended Ly-alpha sources, using deep intermediate-band imaging data taken with the Suprime-Cam on the Subaru Telescope. Our survey covers a field of view of 33' * 25' and a redshift range of 3.24<z<4.95, down to a Ly-alpha flux of ~1e-17 ergs/cm^2/s. We identified 41 extended Ly-alpha sources. The redshift distribution of these sources shows that this kind of objects are common in the early universe over the surveyed redshift range. The identified objects have typical sizes of \~10-15 kpc and luminosities of ~10^42 ergs/s. Follow-up spectroscopy made for 7 of the 41 objects showed that our sample suffers from little contamination. All 7 objects have large equivalent widths of Ly-alpha emission line, all but one exceeding 240A in the rest frame. The large equivalent widths suggest that their extended Ly-alpha emissions are unlikely to be due to normal starbursts, but possibly originated from very young galaxies. All 41 objects in our sample have much smaller Ly-alpha luminosities than the two Ly-alpha Blobs (LABs) found by Steidel et al. (2000) in spite of our much larger survey volume. This suggests that large and luminous extended Ly-alpha objects like the two LABs are very rare and are clustered in overdense regions.

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arXiv2006

Low- and Medium-Dispersion Spectropolarimetry of Nova V475 Sct (Nova Scuti 2003): Discovery of an Asymmetric High-Velocity Wind in a Moderately Fast Nova

We present low-resolution ($R\sim 90$) and medium-resolution ($R\sim 2500$) spectropolarimetry of Nova V475 Sct with the HBS instrument, mounted on the 0.91-m telescope at the Okayama Astrophysical Observatory, and with FOCAS, mounted on the 8.2-m Subaru telescope. We estimated the interstellar polarization toward the nova from the steady continuum polarization components and H$α$ line emission components. After subtracting the interstellar polarization component from the observations, we found that the H$α$ emission seen on 2003 October 7 was clearly polarized. In the polarized flux spectrum, the H$α$ emission had a distinct red wing extending to $\sim +4900$ km s$^{-1}$ and a shoulder around $+3500$ km s$^{-1}$, showing a constant position angle of linear polarization $θ_{\rm *}\simeq 155\arcdeg\pm 15\arcdeg$. This suggests that the nova had an asymmetric outflow with a velocity of $v_{\rm wind}\simeq 3500$ km s$^{-1}$ or more, which is six times higher than the expansion velocity of the ionized shell at the same epoch. Such a high-velocity component has not previously been reported for a nova in the `moderately fast' speed class. Our observations suggest the occurrence of violent mass-loss activity in the nova binary system even during the common-envelope phase. The position angle of the polarization in the H$α$ wing is in good agreement with that of the continuum polarization found on 2003 September 26 ($p_{\rm *}\simeq 0.4$--0.6 %), which disappeared within the following 2 d. The uniformity of the PA between the continuum polarization and the wing polarization on October 7 suggests that the axis of the circumstellar asymmetry remained nearly constant during the period of our observations.

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arXiv2005

The Number Density of Old Passively-Evolving Galaxies at z=1 in the Subaru/XMM-Newton Deep Survey Field

We obtained the number counts and the rest-frame B-band luminosity function of the color-selected old passively-evolving galaxies (OPEGs) at z=1 with very high statistical accuracy using a large and homogeneous sample of about 4000 such objects with z' <25 detected in the area of 1.03 deg^2 in the Subaru/XMM-Newton Deep Survey (SXDS) field. Our selection criteria are defined on the i'-z' and R-z' color-magnitude plane so that OPEGs at z=0.9-1.1 with formation redshift z_f=2-10 are properly sampled. The limiting magnitude corresponds to the luminosity of galaxies with M_*+3 at z=0. We made a pilot redshift observations for 99 OPEG candidates with 19 < z' < 22 and found that at least 78% (73/93) of the entire sample, or 95% (73/77) of these whose redshifts were obtained are indeed lie between z=0.87 and 1.12 and the most of their spectra show the continuum break and strong Ca H and K lines, indicating that these objects are indeed dominated by the old stellar populations. We then compare our results with the luminosity functions of the color- or the morphologically-selected early type galaxies at z=0 taking the evolutionary factor into account and found that the number density of old passive galaxies with sim M_* magnitude at z~1 averaged over the SXDS area is 40-60% of the equivalently red galaxies and 60-85% of the morphologically-selected E/S0 galaxies at z=0 depending on their luminosity evolution. It is revealed that more than half, but not all, of the present-day early-type galaxies had already been formed into quiescent passive galaxies at z=1.

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arXiv2005

The Color Selection of Quasars from Redshifts 5 to 10: Cloning and Discovery

We present simulations of quasar colors, magnitudes, and numbers at redshifts 5 6 epoch taking advantage of the new UKIDSS near-infrared filter set, and we examine the redshift intervals of maximum productivity, discussing color selection and survey depth issues. On the basis of the SDSS sample, we find that the surface density of z>4.7 quasars increases by a factor of 3X by extending 0.7 i-magnitudes deeper than the SDSS spectroscopic survey limit of i=20.2 -- correspondingly we predict a total of ~400 faint quasars in the SDSS main area that have redshift z>4.7 and magnitudes $<20.9.

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arXiv2003

Dusty ERO Search behind Two Massive Clusters

We performed deep K'-band imaging observations of 2 massive clusters, MS 0451.6-0305 at z = 0.55 and MS 0440.5+0204 at z = 0.19, for searching counterparts of the faint sub-mm sources behind these clusters, which would provide one of the deepest extremely red object(ERO) samples. Comparing our near-infrared images with optical images taken by the Hubble Space Telescope and by the Subaru Telescope, we identified 13 EROs in these fields. The sky distributions of EROs are consistent with the previous results, that there is a sign of strong clustering among detected EROs. Also, the surface density with corrected lensing amplification factors in both clusters are in good agreement with that derived from previous surveys. We found 7 EROs and 3 additional very red objects in a small area (\sim 0.6 arcmin^2) of the MS 0451.6-0305 field around an extended SCUBA source. Many of their optical and near-infrared colors are consistent with dusty star-forming galaxies at high redshifts(z \sim 1.0-4.0), and they may be constituting a cluster of dusty starburst galaxies and/or lensed star-forming galaxies at high redshift. Their red J-K' colors and faint optical magnitudes suggest they are relatively old massive stellar systems with ages(>300 Mega years) suffering from dust obscuration. We also found a surface-density enhancement of EROs around the SCUBA source in the MS 0440.5+0204 field.

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arXiv2003

Superwind-driven Intense H$_2$ Emission in NGC 6240 II: Detailed Comparison of Kinematical and Morphological Structures of the Warm and Cold Molecular Gas

We report on our new analysis of the spatial and kinematical distribution of warm and cold molecular gas in NGC 6240, which was undertaken to explore the origin of its unusually luminous H$_2$ emission. By comparing three-dimensional emission-line data (in space and velocity) of CO (J=2-1) in the radio and H$_2$ in the near infrared, we are able to study the H$_2$ emitting efficiency, defined in terms of the intensity ratio of H$_2$ to CO [$I$(H$_2$)/$I$(CO)], as a function of velocity. The integrated H$_2$ emitting efficiency is calculated by integrating the velocity profile of H$_2$ emitting efficiency in blue, red, and total (blue + red) velocity regions of the profile. We find that (1) both the total H$_2$ emitting efficiency and the blue-to-red ratio of the efficiency are larger in regions surrounding the CO and H$_2$ intensity peaks, and (2) the H$_2$ emitting efficiency and the kinematical conditions in the warm molecular gas are closely related to each other. A collision between the molecular gas concentration and the external superwind outflow from the southern nucleus seems plausible to explain these characteristics, since it can reproduce the enhanced emitting efficiency of blueshifted H$_2$ around the molecular gas concentration, if we assume that the superwind blows from the southern nucleus toward us, hitting the entire gas concentration from behind. In this model, internal cloud-cloud collisions within the molecular gas concentration are enhanced by the interaction with the superwind outflow, and efficient and intense shock-excited H$_2$ emission is expected as a result of the cloud-crushing mechanism.

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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

Decomposition of the Superwind in M82

We present new optical images ($B$, $V$, and H$α$) of the archetypical starburst/superwind galaxy M82 obtained with the 8.2 m Subaru Telescope to reveal new detailed structures of the superwind-driven nebula and the high-latitude dark lanes. The emission-line nebula is decomposed into (1) a ridge-dominated component comprising numerous filament/loop sub-structures whose overall morphology appears as a pair of narrow cylinders, and (2) a diffuse component extended over much wider opening angle from the nucleus. We suggest that these two components have different origins. The ridge-dominated component appears as a pair of cylinders rather than a pair of cones. Since this morphological property is similar to that of hot plasma probed by soft X-ray, this component seems to surround the hot plasma. On the other hand, the diffuse component may arise from dust grains which scatter stellar light from the galaxy. Since inner region of this component is seen over the prominent ^^ ^^ X"-shaped dark lanes streaming out from the nuclear region and they can be reproduced as a conical distribution of dust grains, there seems to be a dusty cold outflow as well as the hot one probed by soft X-ray and shock-excited optical emission lines. If this is the case, the presence of such high-latitude dust grains implies that neutral gaseous matter is also blown out during the course of the superwind activity.

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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

A Shock-Induced Pair of Superbubbles in the High-Redshift Powerful Radio Galaxy MRC 0406-244

We present new optical spectroscopy of the high-redshift powerful radio galaxy MRC 0406$-$244 at redshift of 2.429. We find that the two extensions toward NW and SE probed in the rest-frame ultraviolet image are heated mainly by the nonthermal continuum of the active galactic nucleus. However, each extension shows a shell-like morphology, suggesting that they are a pair of superbubbles induced by the superwind activity rather than by the interaction between the radio jet and the ambient gas clouds. If this is the case, the intense starburst responsible for the formation of superbubbles could occur $\sim 1 \times 10^9$ yr ago. On the other hand, the age of the radio jets may be of the order of $\sim 10^6$ yr, being much shorter than the starburst age. Therefore, the two events, i.e., the starburst and the radio-jet activities, are independent phenomena. However, their directions of the expanding motions could be governed by the rotational motion of the gaseous component in the host galaxy. This idea appears to explain the alignment effect of MRC 0406$-$244.

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