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Register a new userCalibration of the AKARI Far-infrared All Sky Survey Maps
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We present an initial analysis of the properties of the all-sky image obtained by the Far-Infrared Surveyor (FIS) onboard the AKARI satellite, at 65~$mu$m (N60), 90~$mu$m (WIDE-S), 140~$mu$m (WIDE-L),and 160~$mu$m (N160). Absolute flux calibration was determined by comparing the data with the COBE/DIRBE data sets, and the intensity range was as wide as from a few MJy~sr$^{-1}$ to $>$1~GJy~sr$^{-1}$. The uncertainties are considered to be the standard deviations with respect to the DIRBE data, and they are less than 10% for intensities above 10, 3, 25, and 26~MJy~sr$^{-1}$ at the N60, WIDE-S, WIDE-L, and N160 bands, respectively. The characteristics of point sources in the image were also determined by stacking maps centred on photometric standard stars. The full width at half maxima of the point spread functions (PSFs) were 63$$, 78$$, and 88$$ at the N60, WIDE-S, and WIDE-L bands, respectively. The PSF at the N160 band was not obtained due to the sensitivity, but it is thought to be the same as that of the WIDE-L one.
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Far-infrared observations provide crucial data for the investigation and characterisation of the properties of dusty material in the Interstellar Medium (ISM), since most of its energy is emitted between ~100 and 200 um. We present the first all-sky image from a sensitive all-sky survey using the Japanese AKARI satellite, in the wavelength range 50 -- 180 um. Covering >99% of the sky in four photometric bands with four filters centred at 65 um, 90 um, 140 um, and 160 um wavelengths, this achieved spatial resolutions from 1 to 2 arcmin and a detection limit of <10 MJy sr-1, with absolute and relative photometric accuracies of <20%. All-sky images of the Galactic dust continuum emission enable astronomers to map the large-scale distribution of the diffuse ISM cirrus, to study its thermal dust temperature, emissivity and column density, and to measure the interaction of the Galactic radiation field and embedded objects with the surrounding ISM. In addition to the point source population of stars, protostars, star-forming regions, and galaxies, the high Galactic latitude sky is shown to be covered with a diffuse filamentary-web of dusty emission that traces the potential sites of high latitude star formation. We show that the temperature of dust particles in thermal equilibrium with the ambient interstellar radiation field can be estimated by using 90 um, 140 um, and 160 um data. The FIR AKARI full-sky maps provide a rich new data set within which astronomers can investigate the distribution of interstellar matter throughout our Galaxy, and beyond.
We present a far-infrared all-sky atlas from a sensitive all-sky survey using the Japanese $AKARI$ satellite. The survey covers $> 99$% of the sky in four photometric bands centred at 65 $mu$m, 90 $mu$m, 140 $mu$m, and 160 $mu$m with spatial resolutions ranging from 1 to 1.5 arcmin. These data provide crucial information for the investigation and characterisation of the properties of dusty material in the Interstellar Medium (ISM), since significant portion of its energy is emitted between $sim$50 and 200 $mu$m. The large-scale distribution of interstellar clouds, their thermal dust temperatures and column densities, can be investigated with the improved spatial resolution compared to earlier all-sky survey observations. In addition to the point source distribution, the large-scale distribution of ISM cirrus emission, and its filamentary structure, are well traced. We have made the first public release of the full-sky data to provide a legacy data set for use by the astronomical community.
We demonstrate the capability of AKARI for mapping diffuse far-infrared emission and achieved reliability of all-sky diffuse map. We have conducted an all-sky survey for more than 94 % of the whole sky during cold phase of AKARI observation in 2006 Feb. -- 2007 Aug. The survey in far-infrared waveband covers 50 um -- 180 um with four bands centered at 65 um, 90 um, 140 um, and 160 um and spatial resolution of 3000 -- 4000 (FWHM).This survey has allowed us to make a revolutionary improvement compared to the IRAS survey that was conducted in 1983 in both spatial resolution and sensitivity after more than a quarter of a century. Additionally, it will provide us the first all-sky survey data with high-spatial resolution beyond 100 um. Considering its extreme importance of the AKARI far-infrared diffuse emission map, we are now investigating carefully the quality of the data for possible release of the archival data. Critical subjects in making image of diffuse emission from detected signal are the transient response and long-term stability of the far-infrared detectors. Quantitative evaluation of these characteristics is the key to achieve sensitivity comparable to or better than that for point sources (< 20 -- 95 [MJy/sr]). We describe current activities and progress that are focused on making high quality all-sky survey images of the diffuse far-infrared emission.
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.
The AKARI Infrared Astronomical Satellite produced the all-sky survey (AFASS) maps in the far-IR at roughly arc-minute spatial resolution, enabling us to investigate the whole sky in the far-IR for objects having surface brightnesses greater than a few to a couple of dozen MJy/sr. While the AFASS maps are absolutely calibrated against large-scale diffuse emission, it was uncertain whether or not an additional flux correction for point sources was necessary. Here, we verify that calibration for point-source photometry in the AFASS maps is proper. With the aperture correction method based on the empirical point-spread-function templates derived directly from the AFASS maps, fluxes in the AKARI bright source catalogue (BSC) are reproduced. The AKARI BSC fluxes are also satisfactorily recovered with the 1 sigma aperture, which is the empirical equivalent of an infinite aperture. These results confirm that in the AFASS maps far-IR photometry can be properly performed by using the aperture correction method for point sources and by summing all pixel values within an appropriately defined aperture of the intended target (i.e., the aperture photometry method) for extended sources.
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