Toshinobu Takagi

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.

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 mu m, 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 arcminutes, and the pixel pitch matches the diffraction limit of the telescope. Derived point-spread functions (PSFs) from observations of asteroids are similar to those given by 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 the 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.

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.

The SCUBA Half-Degree Extragalactic Survey (SHADES) – VIII. The nature of faint submillimetre galaxies in SHADES, SWIRE and SXDF surveys

We present the optical-to-submillimetre spectral energy distributions (SEDs) for 33 radio and mid-infrared (mid-IR) identified submillimetre galaxies discovered via the SHADES 850-μm SCUBA imaging in the Subaru-XMM Deep Field (SXDF). Optical data for the sources come from the SXDF and mid- and far-IR fluxes from SWIRE. We obtain photometric redshift estimates for our sources using optical and IRAC 3.6- and 4.5-μm fluxes. We then fit SED templates to the longer wavelength data to determine the nature of the far-IR emission that dominates the bolometric luminosity of these sources. The IR template fits are also used to resolve ambiguous identifications and cases of redshift aliasing. The redshift distribution obtained broadly matches previous results for submillimetre sources and on the SHADES SXDF field. Our template fitting finds that active galactic nuclei, while present in about 10 per cent of our sources, do not contribute significantly to their bolometric luminosity. Dust heating by starbursts, with either Arp220 or M82 type SEDs, appears to be responsible for the luminosity in most sources (23/33 are fitted by Arp220 templates, 2/33 by the warmer M82 templates). 8/33 sources, in contrast, are fitted by a cooler cirrus dust template, suggesting that cold dust has a role in some of these highly luminous objects. Three of our sources appear to have multiple identifications or components at the same redshift, but we find no statistical evidence that close associations are common among our SHADES sources. Examination of rest-frame K-band luminosity suggests that ‘downsizing’ is underway in the submillimetre galaxy population, with lower redshift systems lying in lower mass host galaxies. Of our 33 identifications six are found to be of lower reliability but their exclusion would not significantly alter our conclusions.

Mapping dusty star formation in and around a cluster at z= 0.81 by wide-field imaging with AKARI

We present environmental dependence of dusty star-forming activity in and around the cluster RXJ1716.4+6708 at z= 0.81 based on wide-field and multiwavelength observations with the Prime Focus Camera on the Subaru Telescope (Suprime-Cam) and the Infrared Camera onboard the AKARI satellite. Our optical data show that the optical colour distribution of galaxies starts to dramatically change from blue to red at the medium-density environment such as cluster outskirts, groups and filaments. By combining with the AKARI infrared data, we find that 15-μm-detected galaxies tend to have optical colours between the red sequence and the blue cloud with a tail into the red sequence, consistent with being dusty star-forming galaxies. The spatial distribution of the 15-μm-detected galaxies over ∼200 arcmin2 around the cluster reveals that few 15-μm galaxies are detected in the cluster central region. This is probably due to the low star-forming activity in the cluster core. However, interestingly, the fraction of 15-μm-detected galaxies in the medium-density environments is as high as in the low-density field, despite the fact that the optical colours start to change in the medium-density environments. Furthermore, we find that 15-μm-detected galaxies which have optically red colours (candidates for dusty red galaxies) and galaxies with high specific star formation rates are also concentrated in the medium-density environment. These results imply that the star-forming activity in galaxies in groups and filaments is enhanced due to some environmental effects specific to the medium-density environment (e.g. galaxy–galaxy interaction), and such a phenomenon is probably directly connected to the truncation of star-forming activity in galaxies seen as the dramatic change in optical colours in such environment.

Luminosity functions of local infrared galaxies with AKARI: implications for the cosmic star formation history and AGN evolution

Infrared (IR) luminosity is fundamental to understanding the cosmic star formation history and active galactic nuclei (AGN) evolution, since their most intense stages are often obscured by dust. However, local IR luminosity function estimates today are still based on the IRAS survey in the 1980s, with wavelength coverage only up to 100 μm. The AKARI IR space telescope performed an all-sky survey in six IR bands (9, 18, 65, 90, 140 and 160 μm) with 3–10 times better sensitivity, covering the crucial far-IR wavelengths across the peak of the dust emission. Combined with a better spatial resolution, AKARI can much more precisely measure the total infrared luminosity (L_(TIR)) of individual galaxies, and thus, the total infrared luminosity density in the local Universe. By fitting modern IR spectral energy distribution (SED) models, we have remeasured L_(TIR) of the IRAS Revised Bright Galaxy Sample, which is a complete sample of local galaxies with S_(60μm) > 5.24 Jy. We present mid-IR monochromatic luminosity (νL_ν) to L_(TIR) correlations for Spitzer 8 μm, AKARI 9 μm, IRAS 12 μm, WISE 12 μm, ISO 15 μm, AKARI 18 μm, WISE 22 μm and Spitzer 24 μm filters. These measures of L_(MIR) are well correlated with L_(TIR), with scatter in the range 13–44 per cent. The best-fitting L_(MIR)-to-L_(TIR) conversions provide us with estimates of L_(TIR) using only a single MIR band, in which several deep all-sky surveys are becoming available such as AKARI MIR and WISE. Although we have found some overestimates of L_(TIR) by IRAS due to contaminating cirrus/ sources, the resulting AKARI IR luminosity function (LF) agrees well with that from IRAS. We integrate the LF weighted by L_(TIR) to obtain a cosmic IR luminosity density of Ω_(TIR) = (8.5^(+1.5)_(−2.3)) × 10^7 L_⊙ Mpc^(−3), of which 7 ± 1 per cent is produced by luminous infrared galaxies (LIRGs) (L_(TIR) > 10^(11) L_⊙), and only 0.4 ± 0.1 per cent is from ultraluminous infrared galaxies (ULIRGs) (L_(TIR) > 10^(12) L_⊙) in the local Universe, in stark contrast to high-redshift results. We separate the contributions from AGN and star-forming galaxies (SFGs). The SFG IR LF shows a steep decline at the bright end. Combined with high-redshift results from the AKARI NEP deep survey, these data show a strong evolution of Ω^(SF)_(TIR) ∝ (1 + z)^(4.0 ± 0.5) and Ω^(AGN)_(TIR) ∝ (1 + z)^(4.4 ± 0.4). For Ω^(AGN)_(TIR), the ULIRG contribution exceeds that from LIRGs already by z ~ 1. A rapid evolution in both Ω^(AGN)_(TIR) and Ω^(SFG)_(TIR) suggests the correlation between star formation and black hole accretion rate continues up to higher redshifts. We compare the evolution of Ω^(AGN)_(TIR) to that of X-ray luminosity density. The Ω^(AGN)_(TIR)/Ω^(AGN)_(X-ray) ratio shows a possible increase at z > 1, suggesting an increase of obscured AGN at z > 1.

Polycyclic aromatic hydrocarbon (PAH) luminous galaxies at z 1

Aims. The NEP-deep survey, an extragalactic AKARI survey towards the north ecliptic pole (NEP), provides a comprehensive wavelength coverage from 2 to 24 μm using all 9 photometric bands of the infrared camera (IRC). It allows us to photometrically identify galaxies whose mid-IR emission is clearly dominated by PAHs. Methods. We propose a single-colour selection method to identify such galaxies, using two mid-IR flux ratios at 11-to-7 μ ma nd 15-to-9 μm (PAH-to-continuum flux ratio in the rest frame), which are useful for identifying starburst galaxies at z ∼ 0.5 and 1, respectively. We perform a fitting of the spectral energy distributions (SEDs) from optical to mid-IR wavelengths, using an evolutionary starburst model with a proper treatment of radiative transfer (SBURT), in order to investigate their nature. Results. The SBURT model reproduces observed optical-to-mid-IR SEDs of more than a half of the PAH-selected galaxies. Based on the 8 μm luminosity, we find ultra luminous infrared galaxies (ULIRGs) among PAH-selected galaxies. Their PAH luminosity is higher than local ULIRGs with a similar luminosity, and the PAH-to-total IR luminosity ratio is consistent with that of less luminous starburst galaxies. They are a unique galaxy population at high redshifts, and we call these PAH-selected ULIRGs “PAH-luminous”Aims. Using an AKARI multi-wavelength mid-infrared (IR) survey, we identify luminous starburst galaxies at z >∼ 0.5 based on the PAH luminosity, and investigate the nature of these PAH-sel ect d starbursts. Methods. An extragalactic survey with AKARI towards the north eclipt ic pole (NEP), the NEP-Deep survey, is unique in terms of a comprehensive wavelength coverage from 2 to 24 μm using all 9 photometric bands of the InfraRed Camera (IRC). This survey allows us to photometrically identify galaxies whose mid-IR emiss ion is clearly dominated by PAHs. We propose a single colour s election method to identify such galaxies, using two mid-IR flux ratio s at 11-to-7μm and 15-to-9μm (PAH-to-continuum flux ratio in the rest-frame), which are useful to identify starburst galaxi es atz ∼ 0.5 and 1, respectively. We perform a fitting of the spectral ene rgy distributions (SEDs) from optical to mid-IR wavelengths, u ing an evolutionary starburst model with a proper treatmen of radiative transfer (SBURT), in order to investigate their nature. Results. The SBURT model reproduces observed optical-to-mid-IR SED s of more than a half of PAH-selected galaxies. Based on the 8μm luminosity, we find ultra luminous infrared galaxies (ULIR Gs) among PAH-selected galaxies. Their PAH luminosity is hi gher than local ULIRGs with a similar luminosity, and the PAH-tototal IR luminosity ratio is consistent with that of less lum inous starburst galaxies. They are a unique galaxy population at high redshi fts and we call these PAH-selected ULIRGs “PAH-luminous” ga laxies. Although they are not as massive as submillimetre galaxies a t z ∼ 2, they have the stellar mass of > 3 × 1010 M⊙ and therefore moderately massive.

The AKARI NEP-Deep survey: a mid-infrared source catalogue

We present a new catalogue of mid-IR sources using the AKARI NEP-Deep survey. The InfraRed Camera (IRC) onboard AKARI has a comprehensive mid-IR wavelength coverage with 9 photometric bands at 2–24 μm. We utilized all of these bands to cover a nearly circular area adjacent to the north ecliptic pole (NEP). We designed the catalogue to include most of sources detected in 7, 9, 11, 15 and 18 μm bands, and found 7284 sources in a 0.67 deg 2 area. From our simulations, we estimate that the catalogue is ~80 per cent complete to 200 μJy at 15–18 μm, and ~10 per cent of sources are missed, owing to source blending. Star-galaxy separation is conducted using only AKARI photometry, as a result of which 10 per cent of catalogued sources are found to be stars. The number counts at 11, 15, 18, and 24 μm are presented for both stars and galaxies. A drastic increase in the source density is found in between 11 and 15 μm at the flux level of ~ 300 μJy. This is likely due to the redshifted PAH emission at 8 μm, given our rough estimate of redshifts from an AKARI colour–colour plot. Along with the mid-IR source catalogue, we present optical-NIR photometry for sources falling inside a Subaru/Sprime-cam image covering part of the AKARI NEP-Deep field, which is deep enough to detect most of AKARI mid-IR sources, and useful to study optical characteristics of a complete mid-IR source sample.

A Tale of Two Feedbacks: Star Formation in the Host Galaxies of Radio AGNs

Several lines of argument support the existence of a link between activity at the nuclei of galaxies, in the form of an accreting supermassive black hole, and star formation activity in these galaxies. Radio jets have long been argued to be an ideal mechanism that allows active galactic nuclei (AGNs) to interact with their host galaxies and affect star formation. We use a sample of radio sources in the North Ecliptic Pole (NEP) field to study the nature of this putative link, by means of spectral energy distribution (SED) fitting. We employ the excellent spectral coverage of the AKARI infrared space telescope and the rich ancillary data available in the NEP to build SEDs extending from UV to far-IR wavelengths. We find a significant AGN component in our sample of relatively faint radio sources (<mJy). A positive correlation is found between the luminosity of the AGN component and that of star formation in the host galaxy, independent of the radio luminosity. In contrast, for narrow redshift and AGN luminosity ranges, we find that increasing radio luminosity leads to a decrease in the specific star formation rate. The most radio-loud AGNs are found to lie on the main sequence of star formation for their respective redshifts. For the first time, we potentially see such a two-sided feedback process in the same sample. We discuss the possible suppression of star formation, but not total quenching, in systems with strong radio jets, that supports the maintenance nature of feedback from radio AGN jets.