arXiv subjects

Youichi Ohyama

Publications and source records attributed to Youichi Ohyama.

At least 73 records · Page 4Linked to original sources

arXiv2010

Detection of parent H2O and CO2 molecules in the 2.5--5 micron spectrum of comet C/2007 N3 (Lulin) observed with AKARI

Comet C/2007 N3 (Lulin) was observed with the Japanese infrared satellite AKARI in the near-infrared at a post-perihelion heliocentric distance of 1.7 AU. Observations were performed with the spectroscopic (2.5--5.0 micron) and imaging (2.4, 3.2, and 4.1 micron) modes on 2009 March 30 and 31 UT, respectively. AKARI images of the comet exhibit a sunward crescent-like shape coma and a dust tail extended toward the anti-solar direction. The 4.1 micron image (CO/CO2 and dust grains) shows a distribution different from the 2.4 and 3.2 micron images (H2O and dust grains). The observed spectrum shows distinct bands at 2.66 and 4.26 micron, attributed to H2O and CO2, respectively. This is the fifth comet in which CO2 has been directly detected in the near-infrared spectrum. In addition, CO at 4.67 micron and a broad 3.2--3.6 micron emission band from C-H bearing molecules were detected in the AKARI spectrum. The relative abundance ratios CO2/H2O and CO/H2O derived from the molecular production rates are \sim 4%--5% and < 2%, respectively. Comet Lulin belongs to the group that has relatively low abundances of CO and CO2 among the comets observed ever.

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arXiv2010

The AKARI/IRC Mid-Infrared All-Sky Survey

Context : AKARI is the first Japanese astronomical satellite dedicated to infrar ed astronomy. One of the main purposes of AKARI is the all-sky survey performed with six infrared bands between 9 and 200um during the period from 2006 May 6 to 2007 August 28. In this paper, we present the mid-infrared part (9um and 18um b ands) of the survey carried out with one of the on-board instruments, the Infrar ed Camera (IRC). Aims : We present unprecedented observational results of the 9 and 18um AKARI al l-sky survey and detail the operation and data processing leading to the point s ource detection and measurements. Methods : The raw data are processed to produce small images for every scan and point sources candidates, above the 5-sigma noise level per single scan, are der ived. The celestial coordinates and fluxes of the events are determined statisti cally and the reliability of their detections is secured through multiple detect ions of the same source within milli-seconds, hours, and months from each other. Results : The sky coverage is more than 90% for both bands. A total of 877,091 s ources (851,189 for 9um, 195,893 for 18um) are confirmed and included in the cur rent release of the point source catalogue. The detection limit for point source s is 50mJy and 90mJy for the 9um and 18um bands, respectively. The position accu racy is estimated to be better than 2". Uncertainties in the in-flight absolute flux calibration are estimated to be 3% for the 9um band and 4% for the 18um ban d. The coordinates and fluxes of detected sources in this survey are also compar ed with those of the IRAS survey and found to be statistically consistent.

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arXiv2009

A Multiwavelength Study of a Young, Z-shaped, FR I Radio Galaxy NGC 3801

We present preliminary results from a multi-wavelength study of a merger candidate, NGC3801, hosting a young FR I radio galaxy, with a Z-shaped structure. Analysing archival data from the VLA, we find two HI emission blobs on either side of the host galaxy, suggesting a 30 kpc sized rotating gas disk aligned with stellar rotation, but rotating significantly faster than the stars. Broad, faint, blue-shifted absorption wing and an HI absorption clump associated with the shocked shell around the eastern lobe are also seen, possibly due to an jet-driven outflow. While 8.0 um dust and PAH emission, from Spitzer and near and far UV emission from GALEX is seen on a large scale in an S-shape, partially coinciding with the HI emission blobs, it reveals a ~2 kpc radius ring-like, dusty, starforming structure in the nuclear region, orthogonal to the radio jet axis. Its similarities with Kinematically Decoupled Core galaxies and other evidences have been argued for a merger origin of this young, bent jet radio galaxy.

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arXiv2008

Galaxy Clusters at 0.9<z<1.7 in the AKARI NEP deep field

There is a huge gap between properties of red-sequence selected massive galaxy clusters at z<1 and Lyman-break selected proto-clusters at z>3. It is important to understand when and how the z>3 proto-clusters evolve into passive clusters at z<1. We aim to fill this cluster desert by using the space-based N4(4um) imaging with the AKARI. The z'-N4 color is a powerful separator of cluster galaxies at z>1, taking advantage of the 4000A break and the 1.6um bump. We carefully selected 16 promising cluster candidates at 0.9<z<1.7, which all show obvious over-density of galaxies and a prominent red-sequence. At this redshift range, the mid-infrared S15um/S9um flux ratio is an extinction-free indicator of galaxy star formation activity due to the redshifted PAH emission lines (6.2,7.7 and 8.6um). We show statistically that the cluster galaxies have a lower S15um/S9um flux ratio than field galaxies, i.e., cluster galaxies already have lower star-formation activity at 0.9<z<1.7, pushing the formation epoch of these galaxy clusters to a higher redshift.

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arXiv2008

Asphericity in Supernova Explosions from Late-Time Spectroscopy

Core-collapse supernovae (CC-SNe) are the explosions that announce the death of massive stars. Some CC-SNe are linked to long-duration gamma-ray bursts (GRBs) and are highly aspherical. One important question is to what extent asphericity is common to all CC-SNe. Here we present late-time spectra for a number of CC-SNe from stripped-envelope stars, and use them to explore any asphericity generated in the inner part of the exploding star, near the site of collapse. A range of oxygen emission-line profiles is observed, including a high incidence of double-peaked profiles, a distinct signature of an aspherical explosion. Our results suggest that all CC-SNe from stripped-envelope stars are aspherical explosions and that SNe accompanied by GRBs exhibit the highest degree of asphericity.

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arXiv2007

Near-infrared and Mid-infrared Spectroscopy with the Infrared Camera (IRC) for AKARI

The Infrared Camera (IRC) is one of the two instruments on board the AKARI satellite. In addition to deep imaging from 1.8-26.5um for the pointed observation mode of the AKARI, it has a spectroscopic capability in its spectral range. By replacing the imaging filters by transmission-type dispersers on the filter wheels, it provides low-resolution (lambda/d_lambda ~ 20-120) spectroscopy with slits or in a wide imaging field-of-view (approximately 10'X10'). The IRC spectroscopic mode is unique in space infrared missions in that it has the capability to perform sensitive wide-field spectroscopic surveys in the near- and mid-infrared wavelength ranges. This paper describes specifications of the IRC spectrograph and its in-orbit performance.

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arXiv2007

Optical Identification of 15 Micron Sources in the AKARI Performance Verification Field toward the North Ecliptic Pole

We present the results of optical identifications for 257 mid-infrared sources detected with a deep 15um survey over approximately 80 arcmin^2 area in the AKARI performance verification field near the North Ecliptic Pole. The 15um fluxes of the sources range from 1 mJy down to 40 uJy, approximately a half of which are below 100 uJy. Optical counterparts were searched for within a 2-3 arcsec radius in both the BVRi'z' catalog generated by using the deep Subaru/Suprime-cam field which covers one-third of the performance verification field, and the g'r'i'z' catalog based on observations made with MegaCam at CFHT. We found B-R and R-z' colours of sources with successful optical identifications are systematically redder than that of the entire optical sample in the same field. Moreover, approximately 40% of the 15um sources show colours R-L15>5, which cannot be explained by the spectral energy distribution (SED) of normal quiescent spiral galaxies, but are consistent with SEDs of redshifted (z>1) starburst or ultraluminous infrared galaxies. This result indicates that the fraction of the ultraluminous infrared galaxies in our faint 15um sample is much larger than that in our brighter 15um sources, which is consistent with the evolving mid-infrared luminosity function derived by recent studies based on the Spitzer 24um deep surveys. Based on an SED fitting technique, the nature of the faint 15um sources is further discussed for a selected number of sources with available K_s-band data.

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arXiv2007

The Infrared Camera (IRC) deep survey in the performance verification phase

We report the first results of a near- and mid- infrared deep survey with the Infrared Camera (IRC) onboard AKARI in the performance verification phase. Simultaneous observations by the NIR, MIR-S and MIR-L channels of the IRC with effective integration times of 4529, 4908, and 4417 seconds at 3, 7, and 15 micron, covering 86.0, 70.3, and 77.3 arcmin^2 area, detected 955, 298 and 277 sources, respectively. The 5 sigma detection limits of the survey are 6.0, 31.5 and 71.2 micro Jy and the 50% completeness limit are 24.0, 47.5, and 88.1 micro Jy at 3, 7, and 15 micron, respectively. The observation is limited by source confusion at 3 micron. We have confirmed the turnover in the 15 micron differential source counts around 400 micro Jy, previously detected by surveys with the Infrared Space Observatory. The faint end of 15 micron raw source counts agree with the results from the deep surveys in the GOODS fields carried out with the Spitzer IRS peak up imager and the predictions of current galaxy evolution models. These results indicate that deep surveys with comprehensive wavelength coverage at mid-infrared wavelength are very important to investigate the evolution of infrared galaxies at high redshifts.

astro-ph↗

arXiv2007

AKARI infrared imaging of reflection nebulae IC4954 and IC4955

We present the observations of the reflection nebulae IC4954 and IC4955 region with the Infrared Camera (IRC) and the Far-Infrared Surveyor (FIS) on board the infrared astronomical satellite AKARI during its performance verification phase. We obtained 7 band images from 7 to 160um with higher spatial resolution and higher sensitivities than previous observations. The mid-infrared color of the S9W (9um) and L18W (18um) bands shows a systematic variation around the exciting sources. The spatial variation in the mid-infrared color suggests that the star-formation in IC4954/4955 is progressing from south-west to north-east. The FIS data also clearly resolve two nebulae for the first time in the far-infrared. The FIS 4-band data from 65um to 160um allow us to correctly estimate the total infrared luminosity from the region, which is about one sixth of the energy emitted from the existing stellar sources. Five candidates for young stellar objects have been detected as point sources for the first time in the 11um image. They are located in the red S9W to L18W color regions, suggesting that current star-formation has been triggered by previous star-formation activities. A wide area map of the size of about 1 x 1 (deg^2) around the IC4954/4955 region was created from the AKARI mid-infrared all-sky survey data. Together with the HI 21cm data, it suggests a large hollow structure of a degree scale, on whose edge the IC4954/4955 region has been created, indicating star formation over three generations in largely different spatial scales.

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arXiv2007

Nature of Infrared Sources in 11 micron Selected Sample from Early Data of the AKARI North Ecliptic Pole Deep Survey

We present the properties of 11 $μ$m selected sources detected in the early data of the North Ecliptic Pole Deep (NEP-Deep) Survey of AKARI. The data set covers 6 wavelength bands from 2.5 to 11 $μ$m, with the exposure time of 10 ~ 20 minutes. This field lies within the CFHT survey with four filter bands ($g', r', i',z'), enabling us to establish nearly continuous spectral energy distributions (SEDs) for wavelengths ranging from 0.4 to 11 $μ$m. The main sample studied here consists of 71 sources whose 11 $μ$m AB magnitudes are equal to or brighter than 18.5 (251 $μ$Jy), which is complete to more than 90%. The 11 $μ$m band has an advantage of sampling star forming galaxies with low to medium redshifts since the prominent PAH feature shifts into this band. As expected, we find that the majority (~68%) of 11 $μ$m bright sources are star forming galaxies at 0.2 < z < 0.7 with $L_{IR} ~ 10^{10}$ -- $10^{12} L_{\odot}$ based on the detailed modelling of SEDs. We also find four AGNs lying at various redshifts in the main sample. In addition, we discuss a few sources which have non-typical SEDs of the main sample, including a brown dwarf candidate, a steep power-law source, flat spectrum object, and an early-type galaxy at moderate redshift.

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arXiv2007

Detection of an H-alpha Emission Line on a Quasar, RX J1759.4+6638, at z=4.3 with AKARI

We report the detection of an H-alpha emission line in the low resolution spectrum of a quasar, RX J1759.4+6638, at a redshift of 4.3 with the InfraRed Camera (IRC) onboard the AKARI. This is the first spectroscopic detection of an H-alpha emission line in a quasar beyond z=4. The overall spectral energy distribution (SED) of RX J1759.4+6638 in the near- and mid-infrared wavelengths agrees with a median SED of the nearby quasars and the flux ratio of F(Ly-alpha)/F(H-alpha) is consistent with those of previous reports for lower-redshift quasars.

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arXiv2007

AKARI observations of circumstellar dust in the globular clusters NGC104 and NGC362

We report preliminary results of AKARI observations of two globular clusters, NGC104 and NGC362. Imaging data covering areas of about 10x10 arcmin^2 centered on the two clusters have been obtained with InfraRed Camera (IRC) at 2.4, 3.2, 4.1, 7.0, 9.0, 11.0, 15.0, 18.0 and 24.0 mu. We used F11/F2 and F24/F7 flux ratios as diagnostics of circumstellar dust emission. Dust emissions are mainly detected from variable stars obviously on the asymptotic giant branch, but some variable stars that reside below the tip of the first-ascending giant branch also show dust emissions. We found eight red sources with F24/F7 ratio greater than unity in NGC362. Six out of the eight have no 2MASS counterparts. However, we found no such source in NGC104.

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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

Near-Infrared search for C IV absorption counterparts along the line-of-sights to pair quasars

We carried out a Subaru and UKIRT near infrared imaging survey for H-alpha emitting galaxies around two pair quasar systems (Q0301-005/Q0302-003 and Q2343+125/Q2344+125), and a triple quasar system (KP76/KP77/KP78). Narrow band near infrared filters covering the H-alpha emission expected for galaxies at the confirmed C IV absorption redshift toward the quasar systems were used for this survey. These quasar pairs or triplet are separated at most by 17 arcmins (~5 h^-1 Mpc in proper distance) from each other on the sky, and have common C IV absorption lines at almost identical redshifts at z=2.24-2.43, which suggests there could be a Mpc-scale absorbing systems such as a cluster, or a group, of galaxies that cover all the line-of-sights to the pair/triple quasars. Using narrow-band deep images, we detected five candidates for H-alpha emitting galaxies around two of the six fields, Q2343+125 and Q2344+125, whose apparent star formation rates are, extremely high, 20-466 M_solar/year. However, all or most of them are not likely to be galaxies at the absorption redshift but galaxies at lower redshift, because of their extreme brightness. In the fields of the other quasars, we detected no star-forming galaxies, nor did we find any number excess of galaxy counts around them. This no-detection results could be because the luminosities and star formation rates of galaxies are lower than the detection limits of our observations (K' > 21 and SFR < 1.8-240 h^-2 M_solar/year). They could be located outside of the observed fields by chance. Otherwise, most C IV absorption lines could be ascribed not to cluster of galaxies, but to isolated star forming pockets far from bright galaxies and could be analogous objects to weak Mg II absorbers.

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arXiv2005

Dark and stellar matter in strong lensing galaxies from a joint lensing and stellar dynamics

We present Subaru spectroscopy of the early-type lensing galaxies in the lens systems HST14113+5211 (z=0.464) and B2045+265 (z=0.868), being aimed at measuring the velocity dispersion of the lensing galaxies as an important component to their mass distributions and internal dynamics. For HST14113+5211 we have obtained 174+/-20km/s (1-sigma) inside an aperture of 0.9", and for B2045+265 we have obtained 213+/-23km/s inside an aperture of 1.2". To extract the significance of these information on the mass distributions and stellar dynamics of the lensing galaxies, we construct two alternative models for both the mass and velocity distributions. It is found that the mass-to-light ratios as derived from a joint lensing and stellar dynamics analysis are virtually consistent with those obtained from the use of the Fundamental Plane. Also, for HST14113+5211, we find that the total mass distribution is well reproduced by a power-law density profile with an index of 2, thereby suggesting that a singular isothermal model is a good fit to the lensing galaxy. In contrast, the corresponding slope for B2045+265 is flatter than isothermal, suggesting that additional contributions to lensing mass from surrounding structures (possibly the group of galaxies) provide the observed angular separations of multiple images.

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arXiv2004

Subaru Deep Spectroscopy of a Star-forming Companion Galaxy of BR 1202-0725 at $z=4.7$

We present deep spatially-resolved optical spectroscopy of the NW companion galaxy of the quasar BR 1202-0725 at $z=4.7$. Its rest-frame UV spectrum shows star-forming activity in the nuclear region. The Ly$α$ emission profile is symmetric with wavelength while its line width is unusually wide (FWHM $\simeq 1100$ km s$^{-1}$) for such a high-$z$ star-forming galaxy. Spectrum taken along the Ly$α$ nebula elongation, which is almost along the minor axis of the companion host galaxy, reveals that off-nuclear Ly$α$ nebulae have either flat-topped or multi-peaked profiles along the extension. All these properties can be understood in terms of superwind activity in the companion galaxy. We also find a diffuse continuum component around the companion, which shows similar morphology to that of Ly$α$ nebula, and is most likely due to scattering of the quasar light at dusty halo around the companion. We argue that the superwind could expel dusty material out to the halo region, making a dusty halo for scattering.

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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.

astro-ph↗

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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