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Register a new userThe First Source Counts at 18 microns from the AKARI NEP Survey
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We present the first galaxy counts at 18 microns using the Japanese AKARI satellites survey at the North Ecliptic Pole (NEP), produced from the images from the NEP-Deep and NEP-Wide surveys covering 0.6 and 5.8 square degrees respectively. We describe a procedure using a point source filtering algorithm to remove background structure and a minimum variance method for our source extraction and photometry that delivers the optimum signal to noise for our extracted sources, confirming this by comparison with standard photometry methods. The final source counts are complete and reliable over three orders of magnitude in flux density, resulting in sensitivities (80 percent completeness) of 0.15mJy and 0.3mJy for the NEP-Deep and NEP-Wide surveys respectively, a factor of 1.3 deeper than previous catalogues constructed from this field. The differential source counts exhibit a characteristic upturn from Euclidean expectations at around a milliJansky and a corresponding evolutionary bump between 0.2-0.4 mJy consistent with previous mid-infrared surveys with ISO and Spitzer at 15 and 24 microns. We compare our results with galaxy evolution models confirming the striking divergence from the non-evolving scenario. The models and observations are in broad agreement implying that the source counts are consistent with a strongly evolving population of luminous infrared galaxies at redshifts higher than unity. Integrating our source counts down to the limit of the NEP survey at the 150 microJy level we calculate that AKARI has resolved approximately 55 percent of the 18 micron cosmic infrared background relative to the predictions of contemporary source count models.
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We present galaxy counts at 15 microns using the Japanese AKARI satellites NEP-deep and NEP-wide legacy surveys at the North Ecliptic Pole. The total number of sources detected are approximately 6700 and 10,700 down to limiting fluxes of 117 and 250 microJy (5 sigma) for the NEP-deep and NEP-wide survey respectively. We construct the Euclidean normalized differential source counts for both data sets (assuming 80 percent completeness levels of 200 and 270 microJy respectively) to produce the widest and deepest contiguous survey at 15 microns to date covering the entire flux range from the deepest to shallowest surveys made with the Infrared Space Observatory (ISO) over areas sufficiently significant to overcome cosmic variance, detecting six times as many sources as the largest survey carried out with ISO.We compare the results from AKARI with the previous surveys with ISO at the same wavelength and the Spitzer observations at 16 microns using the peek-up camera on its IRS instrument. The AKARI source counts are consistent with other results to date reproducing the steep evolutionary rise at fluxes less than a milliJansky and super-Euclidean slopes. We find the the AKARI source counts show a slight excess at fluxes fainter than 200 microJanskys which is not predicted by previous source count models at 15 microns. However, we caution that at this level we may be suffering from the effects of source confusion in our data. At brighter fluxes greater than a milliJansky, the NEP-wide survey source counts agree with the Northern ISO-ELAIS field results, resolving the discrepancy of the bright end calibration in the ISO 15 micron source counts.
We present herein galaxy number counts of the nine bands in the 2-24 micron range on the basis of the AKARI North Ecliptic Pole (NEP) surveys. The number counts are derived from NEP-deep and NEP-wide surveys, which cover areas of 0.5 and 5.8 deg2, respectively. To produce reliable number counts, the sources were extracted from recently updated images. Completeness and difference between observed and intrinsic magnitudes were corrected by Monte Carlo simulation. Stellar counts were subtracted by using the stellar fraction estimated from optical data. The resultant source counts are given down to the 80% completeness limit; 0.18, 0.16, 0.10, 0.05, 0.06, 0.10, 0.15, 0.16, and 0.44 mJy in the 2.4, 3.2, 4.1, 7, 9, 11, 15, 18 and 24 um bands, respectively. On the bright side of all bands, the count distribution is flat, consistent with the Euclidean Universe, while on the faint side, the counts deviate, suggesting that the galaxy population of the distant universe is evolving. These results are generally consistent with previous galaxy counts in similar wavebands. We also compare our counts with evolutionary models and find them in good agreements. By integrating the models down to the 80% completeness limits, we calculate that the AKARI NEP-survey revolves 20%-50% of the cosmic infrared background, depending on the wavebands.
The extragalactic background suggests half the energy generated by stars reprocessed into the infrared (IR) by dust. At z$sim$1.3, 90% of star formation is obscured by dust. To fully understand the cosmic star formation history, it is critical to investigate infrared emission. AKARI has made deep mid-IR observation using its continuous 9-band filters in the NEP field (5.4 deg$^2$), using $sim$10% of the entire pointed observations available throughout its lifetime. However, there remain 11,000 AKARIs infrared sources undetected with the previous CFHT/Megacam imaging ($rsim$25.9ABmag). Redshift and IR luminosity of these sources are unknown. These sources may contribute significantly to the cosmic star-formation rate density (CSFRD). For example, if they all lie at 1$<z<$2, the CSFRD will be twice as high at the epoch. We are carrying out deep imaging of the NEP field in 5 broad bands ($g,r,i,z,$ and $y$) using Hyper Suprime-Camera (HSC), which has 1.5 deg field of view in diameter on Subaru 8m telescope. This will provide photometric redshift information, and thereby IR luminosity for the previously-undetected 11,000 faint AKARI IR sources. Combined with AKARIs mid-IR AGN/SF diagnosis, and accurate mid-IR luminosity measurement, this will allow a complete census of cosmic star-formation/AGN accretion history obscured by dust.
We present the Spitzer MIPS 24 micron source counts in the Extragalactic First Look Survey main, verification and ELAIS-N1 fields. Spitzers increased sensitivity and efficiency in large areal coverage over previous infrared telescopes, coupled with the enhanced sensitivity of the 24 micron band to sources at intermediate redshift, dramatically improve the quality and statistics of number counts in the mid-infrared. The First Look Survey observations cover areas of, respectively, 4.4, 0.26 and 0.015 sq.deg. and reach 3-sigma depths of 0.11, 0.08 and 0.03 mJy. The extragalactic counts derived for each survey agree remarkably well. The counts can be fitted by a super-Euclidean power law of index alpha=-2.9 from 0.2 to 0.9 mJy, with a flattening of the counts at fluxes fainter than 0.2 mJy. Comparison with infrared galaxy evolution models reveals a peaks displacement in the 24 micron counts. This is probably due to the detection of a new population of galaxies with redshift between 1 and 2, previously unseen in the 15 micron deep counts.
We have imaged a $sim$6 arcminute$^2$ region in the Bootes Deep Field using the 350 $mu$m-optimised second generation Submillimeter High Angular Resolution Camera (SHARC II), achieving a peak 1$sigma$ sensitivity of $sim$5 mJy. We detect three sources above 3$sigma$, and determine a spurious source detection rate of 1.09 in our maps. In the absence of $5sigma$ detections, we rely on deep 24 $mu$m and 20 cm imaging to deduce which sources are most likely to be genuine, giving two real sources. From this we derive an integral source count of 0.84$^{+1.39}_{-0.61}$ sources arcmin$^{-2}$ at $S>13$ mJy, which is consistent with 350 $mu$m source count models that have an IR-luminous galaxy population evolving with redshift. We use these constraints to consider the future for ground-based short-submillimetre surveys.
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