Youichi Ohyama

The Infrared Astronomical Mission AKARI

AKARI, the first Japanese satellite dedicated to infrared astronomy, was launched on 2006 February 21, and started observations in May of the same year. AKARI has a 68.5 cm cooled telescope, together with two focal-plane instruments, which survey the sky in six wavelength bands from mid- to far-infrared. The instruments also have a capability for imaging and spectroscopy in the wavelength range 2-180 mu m in the pointed observation mode, occasionally inserted into a continuous survey operation. The in-orbit cryogen lifetime is expected to be one and a half years. The All-Sky Survey will cover more than 90% of the whole sky with a higher spatial resolution and a wider wavelength coverage than that of the previous IRAS all-sky survey. Point-source catalogues of the All-Sky Survey will be released to the astronomical community. Pointed observations will be used for deep surveys of selected sky areas and systematic observations of important astronomical targets. These will become an additional future heritage of this mission.

The Infrared Camera (IRC) for AKARI–Design and Imaging Performance

The Infrared Camera (IRC) is one of two focal-plane instruments on the AKARI satellite. It is designed for wide-field deep imaging and low-resolution spectroscopy in the nearto mid-infrared (1.8–26.5 m) in the pointed observation mode of AKARI. The IRC is also operated in the survey mode to make an All-Sky Survey at 9 and 18 m. It comprises three channels. The NIR channel (1.8–5.5 m) employs a 512 412 InSb array, whereas both the MIR-S (4.6–13.4 m) and MIR-L (12.6–26.5 m) channels use 256 256 Si:As impurity band conduction arrays. Each of the three channels has a field-of-view of about 100 100, and they are operated simultaneously. The NIR and MIR-S share the same field-of-view by virtue of a beam splitter. The MIR-L observes the sky about 250 away from the NIR/MIR-S field-of-view. The IRC gives us deep insights into the formation and evolution of galaxies, the evolution of planetary disks, the process of star-formation, the properties of interstellar matter under various physical conditions, and the nature and evolution of solar system objects. The in-flight performance of the IRC has been confirmed to be in agreement with the pre-flight expectation. This paper summarizes the design and the in-flight operation and imaging performance of the IRC.

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.

An Asymmetric Energetic Type Ic Supernova Viewed Off-Axis, and a Link to Gamma Ray Bursts

Type Ic supernovae, the explosions after the core collapse of massive stars that have previously lost their hydrogen and helium envelopes, are particularly interesting because of their link with long-duration gamma ray bursts. Although indications exist that these explosions are aspherical, direct evidence has been missing. Late-time observations of supernova SN 2003jd, a luminous type Ic supernova, provide such evidence. Recent Subaru and Keck spectra reveal double-peaked profiles in the nebular lines of neutral oxygen and magnesium. These profiles are different from those of known type Ic supernovae, with or without a gamma ray burst, and they can be understood if SN 2003jd was an aspherical axisymmetric explosion viewed from near the equatorial plane. If SN 2003jd was associated with a gamma ray burst, we missed the burst because it was pointing away from us.

Errata : The SUBARU Deep Field Project: Lyman α Emitters at a Redshift of 6.6

We present new results of a deep optical imaging survey using a narrow band filter (NB921) centered at λ = 9196 u A together with B, V, R, i � ,a ndzbroadband filters in the sky area of the Subaru Deep Field, which has been promoted as one of legacy programs of the 8.2m Subaru Telescope. We obtained a photometric sample of 58 Ly α emitter candidates at z ≈ 6.5-6.6 among ∼ 180 strong NB921-excess (z � −NB921 > 1.0) objects together with a color criterion of i � −z � > 1.3. We then obtained optical spectra of 20 objects in our NB921-excess sample, and identified at least nine Ly α emitters at z ∼ 6.5-6.6, including the two emitters reported by Kodaira et al. (2003, PASJ, 55, L17). Since our Ly α-emitter candidates are free from strong amplification of gravitational lensing, we are able to discuss their observational properties from a statistical point of view. Based on these new results, we obtained a lower limit of the star-formation rate density of ρSFR � 5.7 ×10 −4 h0.7 Myr −1 Mpc −3 at z ≈ 6.6, being

The broad-lined Type Ic supernova 2003jd

The results of a world-wide coordinated observational campaign on the broad-lined Type Ic SN 2003jd are presented. In total, 74 photometric data points and 26 spectra were collected using 11 different telescopes. SN 2003jd is one of the most luminous SN Ic ever observed. A comparison with other Type Ic supernovae (SNe Ic) confirms that SN 2003jd represents an intermediate case between broad-line events (2002ap, 2006aj), and highly energetic SNe (1997ef, 1998bw, 2003dh, 2003lw), with an ejected mass of M_{ej} = 3.0 +/- 1 Mo and a kinetic energy of E_{k}(tot) = 7_{-2}^{+3} 10^{51} erg. SN 2003jd is similar to SN 1998bw in terms of overall luminosity, but it is closer to SNe 2006aj and 2002ap in terms of light-curve shape and spectral evolution. The comparison with other SNe Ic, suggests that the V-band light curves of SNe Ic can be partially homogenized by introducing a time stretch factor. Finally, due to the similarity of SN 2003jd to the SN 2006aj/XRF 060218 event, we discuss the possible connection of SN 2003jd with a GRB.

Discovery of a Very Extended Emission-Line Region around the Seyfert 2 Galaxy NGC 4388

We found a very large, D35 kpc, emission-line region around the Seyfert type 2 galaxy NGC 4388, using deep narrowband imaging with the prime focus camera (Suprime-Cam) of the Subaru Telescope. This region consists of many faint gas clouds, or —laments, and extends northeastward from the galaxy. The typical Ha luminosity L (Ha) of the clouds is D1037 ergs s~1, and the total L (Ha) of the region within 10 kpc of the nucleus is D2 ] 1038 ergs s~1, which corresponds to an ionized gas mass of D105 The map of the emission-line intensity ratio I([O III])/I(Ha) indicates that the inner (r \ 12 kpc) M _ . region of the very extended emission-line region (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 (1) the tidal debris resulting from a past interaction with a gas-rich dwarf galaxy, i.e., a minor merger, and (2) the interstellar medium of NGC 4388, stripped by the ram pressure of the hot intracluster medium of the Virgo Cluster.

Evolution of infrared luminosity functions of galaxies in the AKARI NEP-deep field - Revealing the cosmic star formation history hidden by dust

Aims. Dust-obscured star-formation increases with increasing intensity and increasing redshift. We aim to reveal the cosmic starformation history obscured by dust using deep infrared observation with AKARI. Methods. We constructed restframe 8 μm, 12 μm, and total infrared (TIR) luminosity functions (LFs) at 0.15 < z < 2.2 using 4128 infrared sources in the AKARI NEP-deep field. A continuous filter coverage in the mid-IR wavelength (2.4, 3.2, 4.1, 7, 9, 11, 15, 18, and 24 μm) by the AKARI satellite allowed us to estimate restframe 8 μm and 12 μm luminosities without using a large extrapolation based on an SED fit, which was the largest uncertainty in previous work. Results. We find that all 8 μm (0.38 < z < 2.2), 12 μm (0.15 < z < 1.16), and TIR LFs (0.2 < z < 1.6) show continuous and strong evolution toward higher redshift. Our direct estimate of 8 μm LFs is useful since previous work often had to use a large extrapolation from the Spitzer 24 μm to 8 μm, where SED modeling is more difficult because of the PAH emissions. In terms of cosmic infrared luminosity density (Ω_(IR)), which was obtained by integrating analytic fits to the LFs, we find good agreement with previous work at z < 1.2. We find the ΩIR evolves as ∝(1 + z)^(4.4±1.0). When we separate contributions to Ω_(IR) by LIRGs and ULIRGs, we found more IR luminous sources are increasingly more important at higher redshift. We find that the ULIRG (LIRG) contribution increases by a factor of 10 (1.8) from z = 0.35 to z = 1.4.

Optical Spectropolarimetry of SN 2002ap: A High-Velocity Asymmetric Explosion*

We present spectropolarimetry of the Type Ic supernova SN 2002ap and give a preliminary analysis: the data were taken at two epochs, close to and 1 month later than the visual maximum (2002 February 8). In addition, we present June 9 spectropolarimetry without analysis. The data show the development of linear polarization. Distinct polarization profiles were seen only in the O I λ7773 multiplet/Ca II IR triplet absorption trough at maximum light and in the O I λ7773 multiplet and Ca II IR triplet absorption troughs a month later, with the latter showing a peak polarization as high as ~2%. The intrinsic polarization shows three clear position angles: 80° for the February continuum, 120° for the February line feature, and 150° for the March data. We conclude that there are multiple asymmetric components in the ejecta. We suggest that the supernova has a bulk asymmetry with an axial ratio projected on the sky that is different from 1 by an order of 10%. Furthermore, we suggest very speculatively that a high-velocity ejecta component moving faster than ~0.115c (e.g., a jet) contributes to polarization in the February epoch.

On the Spectrum and Spectropolarimetry of Type Ic Hypernova SN 2003dh/GRB 030329

Spectroscopic and spectropolarimetric observations of SN 2003dh/GRB 030329 obtained in 2003 May using the Subaru 8.2 m Telescope are presented. The properties of the supernova (SN) are investigated through a comparison with spectra of the Type Ic hypernovae SN 1997ef and SN 1998bw (hypernovae being a tentatively defined class of SNe with very broad absorption features: these features suggest a large velocity of the ejected material and possibly a large explosion kinetic energy). Comparison with spectra of other hypernovae shows that the spectrum of SN 2003dh obtained on 2003 May 8 and 9, i.e., 34-35 rest-frame days after the gamma-ray burst (GRB; for z = 0.1685), are similar to those of SN 1997ef obtained ~34-42 days after the fiducial time of explosion of that SN. The match with SN 1998bw spectra is not as good (at rest 7300-8000 A), but again spectra obtained ~33-43 days after GRB 980425 are preferred. This indicates that the SN may have intermediate properties between SNe 1997ef and 1998bw. On the basis of the analogy with the other hypernovae, the time of explosion of SN 2003dh is then constrained to be between -8 and +2 days of the GRB. The Si and O P Cygni lines of SN 2003dh seem comparable to those of SN 1997ef, which suggests that the ejected mass in SN 2003dh may match that in SN 1997ef. Polarization was marginally detected at optical wavelengths. This is consistent with measurements of the late afterglow, implying that it originated mostly in the interstellar medium of the host galaxy.