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.

💡 Research Summary

The paper presents a detailed investigation of spatially resolved infrared observations obtained during the performance‑verification (PV) phase of the AKARI All‑Sky Survey, focusing on a comparison with the legacy IRAS Point Source Catalogue (PSC). The authors first extracted reliable point sources from AKARI’s six photometric bands (9, 18, 65, 90, 140, and 160 µm) using stringent criteria: signal‑to‑noise ratio greater than five, positional uncertainty below 10 arcseconds, and removal of duplicate detections through cross‑matching across bands. A background‑modeling procedure, employing multi‑scale filtering especially for the 90 µm and 140 µm bands where Galactic cirrus is prominent, ensured that local background fluctuations were properly accounted for.

Next, the AKARI source list was cross‑matched with the IRAS PSC using a 30 arcsecond radius, reflecting the positional accuracy limits of IRAS. Approximately 78 % of AKARI sources found a counterpart in the IRAS catalogue, while a subset exhibited multiple AKARI counterparts for a single IRAS entry, highlighting the superior angular resolution of AKARI (≈30″) compared with IRAS (≈2′).

The core of the analysis involved a direct flux comparison in the overlapping wavelength regimes (IRAS 60 µm ↔ AKARI 65 µm, IRAS 100 µm ↔ AKARI 90 µm). The authors discovered systematic discrepancies: IRAS fluxes were on average 15 %–25 % higher than AKARI measurements, with the largest offsets occurring in regions of complex background such as the Galactic plane or bright star‑forming complexes. This bias is attributed to IRAS’s reliance on a global background subtraction, which fails to capture local variations, whereas AKARI’s processing incorporates a locally adaptive background estimate. Consequently, IRAS fluxes in crowded or high‑background environments are prone to over‑estimation.

A particularly valuable outcome of the AKARI data is the resolution of previously blended IRAS sources into multiple distinct point sources. For instance, the IRAS source designated 12233+1325 is resolved by AKARI into two separate objects (AKARI‑J122345+132501 and AKARI‑J122350+132540) with a flux ratio of roughly 1:0.6. Such de‑blending enables a more accurate assessment of source luminosities, star‑formation rates, and the physical association of nearby galaxies or interacting systems.

Beyond confirming and refining known IRAS entries, the authors identified 23 new AKARI point‑source candidates that lack IRAS counterparts. These detections exceed a 5σ significance threshold in at least one AKARI band and are predominantly located at high Galactic latitudes, where IRAS sensitivity was limited. Cross‑identification with radio (NVSS) and X‑ray (ROSAT) catalogues suggests that a subset of these new sources may be active galactic nuclei (AGN) or other extragalactic objects of interest.

The paper concludes that AKARI’s all‑sky observations provide a crucial benchmark for the calibration of historic IRAS measurements, demonstrating the necessity of local background corrections and the benefits of higher spatial resolution. By correcting systematic flux biases and revealing previously unresolved multiplicity, AKARI enhances the reliability of infrared point‑source catalogues, thereby supporting a wide range of astrophysical investigations—from studies of galaxy evolution and star‑formation histories to constraints on the cosmic infrared background. The authors anticipate that the full release of AKARI’s all‑sky data will enable comprehensive multi‑wavelength analyses when combined with Herschel, Spitzer, and future missions, allowing refined determinations of dust temperature, mass, and optical depth across diverse galactic environments.