arXiv subjects

Takao Nakagawa

Publications and source records attributed to Takao Nakagawa.

At least 109 records · Page 6Linked to original sources

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 Far-Infrared Surveyor (FIS) for AKARI

The Far-Infrared Surveyor (FIS) is one of two focal plane instruments on the AKARI satellite. FIS has four photometric bands at 65, 90, 140, and 160 um, and uses two kinds of array detectors. The FIS arrays and optics are designed to sweep the sky with high spatial resolution and redundancy. The actual scan width is more than eight arcmin, and the pixel pitch is matches the diffraction limit of the telescope. Derived point spread functions (PSFs) from observations of asteroids are similar to the optical model. Significant excesses, however, are clearly seen around tails of the PSFs, whose contributions are about 30% of the total power. All FIS functions are operating well in orbit, and its performance meets the laboratory characterizations, except for the two longer wavelength bands, which are not performing as well as characterized. Furthermore, the FIS has a spectroscopic capability using a Fourier transform spectrometer (FTS). Because the FTS takes advantage of the optics and detectors of the photometer, it can simultaneously make a spectral map. This paper summarizes the in-flight technical and operational performance of the FIS.

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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

Flight Performance of the AKARI Cryogenic System

We describe the flight performance of the cryogenic system of the infrared astronomical satellite AKARI, which was successfully launched on 2006 February 21 (UT). AKARI carries a 68.5 cm telescope together with two focal plane instruments, Infrared Cameras (IRC) and Far Infrared Surveyor (FIS), all of which are cooled down to cryogenic temperature to achieve superior sensitivity. The AKARI cryogenic system is a unique hybrid system, which consists of cryogen (liquid helium) and mechanical coolers (2-stage Stirling coolers). With the help of the mechanical coolers, 179 L (26.0 kg) of super-fluid liquid helium can keep the instruments cryogenically cooled for more than 500 days. The on-orbit performance of the AKARI cryogenics is consistent with the design and pre-flight test, and the boil-off gas flow rate is as small as 0.32 mg/s. We observed the increase of the major axis of the AKARI orbit, which can be explained by the thrust due to thermal pressure of vented helium gas.

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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

The Far-Infrared Properties of Spatially Resolved AKARI Observations

We present the spatially resolved observations of IRAS sources from the Japanese infrared astronomy satellite AKARI All-Sky Survey during the performance verification (PV) phase of the mission. We extracted reliable point sources matched with IRAS point source catalogue. By comparing IRAS and AKARI fluxes, we found that the flux measurements of some IRAS sources could have been over or underestimated and affected by the local background rather than the global background. We also found possible candidates for new AKARI sources and confirmed that AKARI observations resolved IRAS sources into multiple sources. All-Sky Survey observations are expected to verify the accuracies of IRAS flux measurements and to find new extragalactic point sources.

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arXiv2007

Microscopic surface structure of C/SiC composite mirrors for space cryogenic telescopes

We report on the microscopic surface structure of carbon-fiber-reinforced silicon carbide (C/SiC) composite mirrors that have been improved for the Space Infrared Telescope for Cosmology and Astrophysics (SPICA) and other cooled telescopes. The C/SiC composite consists of carbon fiber, silicon carbide, and residual silicon. Specific microscopic structures are found on the surface of the bare C/SiC mirrors after polishing. These structures are considered to be caused by the different hardness of those materials. The roughness obtained for the bare mirrors is 20 nm rms for flat surfaces and 100 nm rms for curved surfaces. It was confirmed that a SiSiC slurry coating is effective in reducing the roughness to 2 nm rms. The scattering properties of the mirrors were measured at room temperature and also at 95 K. No significant change was found in the scattering properties through cooling, which suggests that the microscopic surface structure is stable with changes in temperature down to cryogenic values. The C/SiC mirror with the SiSiC slurry coating is a promising candidate for the SPICA telescope.

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arXiv2007

AKARI Far-Infrared Source Counts in the Lockman Hole

We report initial results of far-infrared observations of the Lockman hole with Far-Infrared Surveyor (FIS) onboard the AKARI infrared satellite. On the basis of slow scan observations of a 0.6 deg x 1.2 deg contiguous area, we obtained source number counts at 65, 90 and 140 um down to 77, 26 and 194 mJy (3 sigma), respectively. The counts at 65 and 140 um show good agreement with the Spitzer results. However, our 90 um counts are clearly lower than the predicted counts by recent evolutionary models that fit the Spitzer counts in all the MIPS bands. Our 90 um counts above 26 mJy account for about 7% of the cosmic background. These results provide strong constraints on the evolutionary scenario and suggest that the current models may require modifications.

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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

Detection of CFIRB with AKARI/FIS Deep Observations

The Cosmic Far-Infrared Background (CFIRB) contains information about the number and distribution of contributing sources and thus gives us an important key to understand the evolution of galaxies. Using a confusion study to set a fundamental limit to the observations, we investigate the potential to explore the CFIRB with AKARI/FIS deep observations. The Far-Infrared Surveyor (FIS) is one of the focal-plane instruments on the AKARI (formerly known as ASTRO-F) satellite, which was launched in early 2006. Based upon source distribution models assuming three different cosmological evolutionary scenarios (no evolution, weak evolution, and strong evolution), an extensive model for diffuse emission from infrared cirrus, and instrumental noise estimates, we present a comprehensive analysis for the determination of the confusion levels for deep far-infrared observations. We use our derived sensitivities to suggest the best observational strategy for the AKARI/FIS mission to detect the CFIRB fluctuations. If the source distribution follows the evolutionary models, observations will be mostly limited by source confusion. We find that we will be able to detect the CFIRB fluctuations and that these will in turn provide information to discriminate between the evolutionary scenarios of galaxies in most low-to-medium cirrus regions.

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arXiv2007

An Optical Source Catalog of the North Ecliptic Pole Region

We present a five (u*,g',r',i',z') band optical photometry catalog of the sources in the North Ecliptic Pole (NEP) region based on deep observations made with MegaCam at CFHT. The source catalog covers about 2 square degree area centered at the NEP and reaches depths of about 26 mag for u*, g', r' bands, about 25 mag for i' band, and about 24 mag for z' band (4 sigma detection over an 1 arcsec aperture). The total number of cataloged sources brighter than r'= 23 mag is about 56,000 including both point sources and extended sources. From the investigation of photometric properties using the color-magnitude diagrams and color-color diagrams, we have found that the colors of extended sources are mostly (u*-r') < 3.0 and (g'-z') > 0.5. This can be used to separate the extended sources from the point sources reliably, even for the faint source domain where typical morphological classification schemes hardly work efficiently. We have derived an empirical color-redshift relation of the red sequence galaxies using the Sloan Digital Sky Survey data. By applying this relation to our photometry catalog and searching for any spatial overdensities, we have found two galaxy clusters and one nearby galaxy group.

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arXiv2007

AKARI Detection of the Infrared-Bright Supernova Remnant B0104-72.3 in the Small Magellanic Cloud

We present a serendipitous detection of the infrared-bright supernova remnant (SNR) B0104-72.3 in the Small Magellanic Cloud by the Infrared Camera (IRC) onboard AKARI. An elongated, partially complete shell is detected in all four observed IRC bands covering 2.6-15 um. The infrared shell surrounds radio, optical, and X-ray emission associated with the SNR and is probably a radiative SNR shell. This is the first detection of a SNR shell in this near/mid-infrared waveband in the Small Magellanic Cloud. The IRC color indicates that the infrared emission might be from shocked H2 molecules with some possible contributions from ionic lines. We conclude that B0104-72.3 is a middle-aged SNR interacting with molecular clouds, similar to the Galactic SNR IC 443. Our results highlight the potential of AKARI IRC observations in studying SNRs, especially for diagnosing SNR shocks.

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arXiv2007

A Spitzer IRS Low-Resolution Spectroscopic Search for Buried AGNs in Nearby Ultraluminous Infrared Galaxies - A Constraint on Geometry between Energy Sources and Dust -

We present the results of Spitzer IRS low-resolution infrared 5-35 micron spectroscopy of nearby ultraluminous infrared galaxies (ULIRGs) at z < 0.15. We focus on the search for the signatures of buried active galactic nuclei (AGNs) in the complete sample of ULIRGs classified optically as non-Seyferts (LINERs or HII-regions). In addition to polycyclic aromatic hydrocarbon (PAH) emission features at 6.2 micron, 7.7 micron, and 11.3 micron, the conventional tool of starburst-AGN separation, we use the optical depths of the 9.7 micron and 18 micron silicate dust absorption features to infer the geometry of energy sources and dust at the nuclei of these ULIRGs, namely, whether the energy sources are spatially well mixed with dust(a normal starburst) or are more centrally concentrated than the dust (a buried AGN). Infrared spectra of at least 30%, and possibly 50%, of the observed optical non-Seyfert ULIRGs are naturally explained by emission consisting of (1) energetically insignificant, modestly obscured (Av < 20-30 mag) PAH-emitting normal starbursts, and (2) energetically dominant, highly dust-obscured, centrally concentrated energy sources with no PAH emission. We interpret the latter component as a buried AGN. The fraction of ULIRGs showing some buried AGN signatures is higher in LINER ULIRGs than in HII-region ULIRGs. Most of the luminous buried AGN candidates are found in ULIRGs with cool far-infrared colors. Where the absorption-corrected intrinsic AGN luminosities are derivable with little uncertainty, they are found to be of the order of 10^12Lsun, accounting for the bulk of the ULIRGs' luminosities. The 5-35 micron spectroscopic starburst/AGN classifications are generally consistent with our previous classifications based on 3-4 micron spectra for the same sample.

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arXiv2006

Deep Extragalactic Surveys around the Ecliptic Poles with AKARI (ASTRO-F)

AKARI (formerly ASTRO-F) is an infrared space telescope designed for an all-sky survey at 10-180 (mu)m, and deep pointed surveys of selected areas at 2-180 (mu)m. The deep pointed surveys with AKARI will significantly advance our understanding of galaxy evolution, the structure formation of the Universe, the nature of the buried AGNs, and the cosmic infrared background. Here we describe the important characteristics of the AKARI mission: the orbit, and the attitude control system, and investigate the optimum survey area based on the updated pre-flight sensitivities of AKARI, taking into account the cirrus confusion noise as well as the surface density of bright stars. The North Ecliptic Pole (NEP) is concluded to be the best area for 2-26 (mu)m deep surveys, while the low-cirrus noise regions around the South Ecliptic Pole (SEP) are worth considering for 50-180 (mu)m pointed surveys to high sensitivities limited by the galaxy confusion noise. Current observational plans of these pointed surveys are described in detail. Comparing these surveys with the deep surveys with the Spitzer Space Telescope, the AKARI deep surveys are particularly unique in respect of their continuous wavelength coverage over the 2-26 (mu)m range in broad-band deep imaging, and their slitless spectroscopy mode over the same wavelength range.

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arXiv2006

Far-IR Detection Limits II: Probing Confusion including Source Confusion

We present a comprehensive analysis for the determination of the confusion levels for the current and the next generation of far-infrared surveys assuming three different cosmological evolutionary scenarios. We include an extensive model for diffuse emission from infrared cirrus in order to derive absolute sensitivity levels taking into account the source confusion noise due to point sources, the sky confusion noise due to the diffuse emission, and instrumental noise. We use our derived sensitivities to suggest best survey strategies for the current and the future far-infrared space missions Spitzer, AKARI (ASTRO-F), Herschel, and SPICA. We discuss whether the theoretical estimates are realistic and the competing necessities of reliability and completeness. We find the best estimator for the representation of the source confusion and produce predictions for the source confusion using far-infrared source count models. From these confusion limits considering both source and sky confusions, we obtain the optimal, confusion limited redshift distribution for each mission. Finally, we predict the Cosmic Far-Infrared Background (CFIRB) which includes information about the number and distribution of the contributing sources.

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arXiv2005

Large Silicon Abundance in Photodissociation Regions

We have made one-dimensional raster-scan observations of the rho Oph and sigma Sco star-forming regions with two spectrometers (SWS and LWS) on board the ISO. In the rho Oph region, [SiII] 35um, [OI] 63um, 146um, [CII] 158um, and the H2 pure rotational transition lines S(0) to S(3) are detected, and the PDR properties are derived as the radiation field scaled by the solar neighborhood value G_0~30-500, the gas density n~250--2500 /cc, and the surface temperature T~100-400 K. The ratio of [SiII] 35um to [OI] 146um indicates that silicon of 10--20% of the solar abundance must be in the gaseous form in the photodissociation region (PDR), suggesting that efficient dust destruction is undergoing even in the PDR and that part of silicon atoms may be contained in volatile forms in dust grains. The [OI] 63um and [CII] 158um emissions are too weak relative to [OI] 146um to be accounted for by standard PDR models. We propose a simple model, in which overlapping PDR clouds along the line of sight absorb the [OI] 63um and [CII] 158um emissions, and show that the proposed model reproduces the observed line intensities fairly well. In the sigma Sco region, we have detected 3 fine-structure lines, [OI] 63um, [NII] 122um, and [CII] 158um, and derived that 30-80% of the [CII] emission comes from the ionized gas. The upper limit of the [SiII] 35um is compatible with the solar abundance relative to nitrogen and no useful constraint on the gaseous Si is obtained for the sigma Sco region.

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arXiv2005

Type I ULIRGs: Transition Stage from ULIRGs to QSOs

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

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arXiv2004

Far-IR Detection Limits I: Sky Confusion Due to Galactic Cirrus

Fluctuations in the brightness of the background radiation can lead to confusion with real point sources. Such background emission confusion will be important for infrared observations with relatively large beam sizes since the amount of fluctuation tends to increase with angular scale. In order to quantitively assess the effect of the background emission on the detection of point sources for current and future far-infrared observations by space-borne missions such as Spitzer, ASTRO-F, Herschel and SPICA, we have extended the Galactic emission map to higher angular resolution than the currently available data. Using this high resolution map, we estimate the sky confusion noise due to the emission from interstellar dust clouds or cirrus, based on fluctuation analysis and detailed photometry over realistically simulated images. We find that the confusion noise derived by simple fluctuation analysis agrees well with the result from realistic simulations. Although the sky confusion noise becomes dominant in long wavelength bands (> 100 um) with 60 - 90cm aperture missions, it is expected to be two order of magnitude smaller for the next generation space missions with larger aperture sizes such as Herschel and SPICA.

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