中川 貴雄

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 and Far Infrared Surveyor, all of which are cooled down to cryogenic temperature to achieve superior sensitivity. AKARI has a unique cryogenic system, which consists of cryogen (liquid helium) and mechanical coolers (2-stage Stirling coolers). With the help of 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 a pre-flight test, and the boil-off gas flow rate is as small as 0.32 mg s . We observed an increase in the major axis of the AKARI orbit, which can be explained by thrust due to the thermal pressure of vented helium gas. © 2007. Astronomical Society of Japan. -1

We present spatially resolved observations of IRAS sources from the Japanese infrared astronomy satellite AKARI All-Sky Survey during the performance verification phase of the mission. We extracted reliable point sources matched with the 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. © 2007. Astronomical Society of Japan.

We report on the detection of an Hα 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 AKARI. This is the first spectroscopic detection of an Ha 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; also, the flux ratio of F(Lyα)/F(Hα) is consistent with those of previous reports for lower-redshift quasars. © 2007. Astronomical Society of Japan.

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 has been detected in all four observed IRC bands, covering 2.6-15 μm. 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 an 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 H 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 SNRIC 443. Our results highlight the potential of AKARIIRC observations in studying SNRs, especially for diagnosing SNR shocks. © 2007. Astronomical Society of Japan. 2

We report initial results of far-infrared observations of the Lockman Hole with Far-Infrared Surveyor onboard the AKARI infrared satellite. On the basis of slow-scan observations of a 0.°6 × l.°2 contiguous area, we obtained source number counts at 65, 90, and 140 μm down to 77, 26, and 194 mJy (3σ), respectively. The counts at 65 and 140 μm show good agreement with the Spitzer results. However, our 90 μm counts are clearly lower than the predicted counts by recent evolutionary models that fit the Spitzer counts in all the MIPS bands. Our 90 μm 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. © 2007. Astronomical Society of Japan.

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. © 2006 RAS.

We have made one-dimensional raster scan observations of the ρ Oph and σ Sco star-forming regions with two spectrometers (SWS and LWS) on board the ISO. In the ρ Oph region, [Si II] 35 μm, [O I] 63 μm, 146 μm, [C II] 158 μm, and the H pure rotational transition lines S(0) to S(3) are detected, and the photodissociation region (PDR) properties are derived as the radiation field scaled by the solar neighborhood value G ∼ 30-500, the gas density n ∼ 250-2500 cm , and the surface temperature T ∼ 100-400 K. The ratio of [Si II] 35 μm to [O I] 146 μm indicates that silicon of 10%-20% of the solar abundance must be in the gaseous form in the PDR, suggesting that efficient dust destruction is ongoing even in the PDR and that a fraction of the silicon atoms may be contained in volatile forms in dust grains. The [O I] 63 μm and [C II] 158 μm emissions are too weak relative to [O I] 146 μm 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 [O I] 63 μm and [C II] 158 μm emissions, and show that the proposed model reproduces the observed line intensities fairly well. In the σ Sco region, we have detected three fine-structure lines, [O I] 63 μm, [N II] 122 μm, and [C II] 158 μm, and derived that 30%-80% of the [C II] emission comes from the ionized gas. The upper limit of the [Si II] 35 μm is compatible with the solar abundance relative to nitrogen, and no useful constraint on the gaseous Si is obtained for the σ Sco region. © 2006. The American Astronomical Society. All rights reserved. 2 0 -3

Half the energy produced since decoupling emerges in the far-IR, hence a complete understanding of the history of stellar nucleosynthesis and galaxy evolution requires a systematic study of dust-enshrouded energy release at all epochs. Sensitive far-IR spectroscopy is the natural tool for this, and we present the first opportunity for cosmological far-IR spectroscopy with new cryogenic telescopes and instruments. We are studying a US-built spectrograph BLISS for the Japanese SPICA mission to launch in 2013, and NASA's SAFIR mission, envisioned for 2025. With sensitive detectors and the very low backgrounds provided by the cold telescope, we anticipate 5-8 orders of magnitude speed improvement relative to preceeding platforms, enabling measurements throughout the epoch of peak activity in galaxies, to z ∼ 5. © 2005 Elsevier B.V. All rights reserved.

We examine whether the ultraluminous infrared galaxies that contain a type 1 Seyfert nucleus (a type I ULIRG) are in the transition stage from ULIRGs to quasi-stellar objects (QSOs). To investigate 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 the above results do not change significantly for three type I ULIRGs for which we can estimate the visual extinction from the column density. Also, for all eight type I ULIRGs, we investigate the effect of uncertainties of BH mass measurements and our sample bias to make sure that our results are not altered even if we consider the above two effects. In addition, Anabuki recently 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 type I ULIRGs. On the basis of 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 could be valid. © 2006. The American Astronomical Society. All rights reserved.

The advanced two-stage stirling cooler has been developed for the cryogenic system of next space missions, SPICA, NeXT, ASTRO-G promoted by JAXA. In order to meet the cooling requirements of these missions, improvement of cooling performance and mechanical reliability is required based on the existing two-stage stirling cooler with cooling capacity 0.2W at 20K by power input 90W, which is operated on the infrared astronomical satellite AKARI (ASTRO-F) launched in 2006/2/22. This paper describes the cooling capacity evaluation test by simulating contamination of the working gas and mechanical attrition reduction by flat-spring-supported displacer for development of the advanced two-stage stirling cooler.

Fluctuations in the brightness of the background radiation can lead to confusion with real point sources. This type of confusion with background emission is relevant when making infrared (IR) observations with relatively large beam sizes, since the amount of fluctuation tends to increase with the angular scale. To quantitively assess the effect of the background emission on the detection of point sources for current and future far-IR observations by space-borne missions such as Spitzer, ASTRO-F, Herschel and Space Infrared Telescope for Cosmology and Astrophysics (SPICA), we have extended the Galactic emission map to a higher level of angular resolution than that of the currently available data. Using this high-resolution map, we estimate the sky confusion noise owing 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 results from realistic simulations. Although sky confusion noise becomes dominant in long wavelength bands (< 100 μm) with 60-90 cm aperture missions, it is expected to be two orders of magnitude lower for the next generation of space missions (with larger aperture sizes) such as Herschel and SPICA.