The deepest image of the universe at a wavelength of 15 microns

Hopwood, R.; Serjeant, S.; Negrello, M.; Pearson, C.; Egami, E.; Im, M.; Kneib, J.-P.; Ko, J. and Smail, I. (2009). The deepest image of the universe at a wavelength of 15 microns. In: Onaka, Takashi; White, Glenn; Nakagawa, Takao and Yamamura, Issei eds. AKARI, a Light to Illuminate the Misty Universe. Astronomical Society of the Pacific Conference Series, 418. San Francisco, USA: Astronomical Society of the Pacific, pp. 499–502.



We present photometry, photometric redshifts and extra galactic number counts for ultra deep 15 micron mapping of the gravitational lensing cluster Abell 2218 (A2218), which is the deepest image taken by any facility at this wavelength. This data resolves the cosmic infrared background (CIRB) beyond the 80% that blank field AKARI surveys aim to achieve. To gain an understanding of galaxy formation and evolution over the age of the Universe a necessary step is to fully resolve the CIRB, which represents the dust-shrouded cosmic star formation history. Observing through A2218 gives magnifications of up to a factor of 10, thus allowing the sampling of a more representative spread of high redshift galaxies, which comprise the bulk of the CIRB. 19 pointed observations were taken by AKARI’s IRC MIR-L channel, and a final combined image with an area of 122.3 square arcminutes and effective integration time of 8460 seconds was achieved. The 5σ sensitivity limit is estimated at 41.7 μJy. An initial 5σ catalogue of 565 sources was extracted giving 39 beams per source, which shows the image is confusion limited. Our 15 micron number counts show strong evolution consistent with galaxy evolution models that incorporate downsizing in star formation.

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