Deep 15 μ m AKARI observations in the CDFS : estimating dust luminosities for a MIR - selected sample and for Lyman Break Galaxies

Deep observations of the Chandra Deep Field South have been secured at 15μm with AKARI/IRC infrared space telescope. From these observations, we define a sample of mid infrared-selected galaxies at 15μm and we also obtain 15μm flux densities for a sample of Lyman Break Galaxies at z ∼ 1 already observed at 24μm with Spitzer/MIPS. Number counts for the mid infrared-selected sample show a bump around a 15μm flux density of 0.2mJy that can be attributed to galaxies at z > 0.4 and at z > 0.8 for the fainter part of the bump. This bump seems to be shifted as compared to other works and a possible origin can be the Cosmic variance. Thanks to this dataset, we have tested, on the two above samples z ∼ 1, the validity of the conversions from monochromatic luminosities νfν at a rest-frame wavelength of 8μm by a comparison with total dust luminosities estimated from Spitzer rest-frame 12μm data that we use as a reference. We find that the 8μm dust luminosities are not all consistent and that some of them are better when compared to Ldust evaluated from longer wavelength luminosities. We also find that the rest-frame 8μm luminosities provide globally good estimates of Ldust. By comparing our data for the two samples to several libraries of spectral energy distributions, we find that models can explain the diversity of the observed f24/f15 ratio quite reasonably for the MIR-selected sample and better for the LBG sample which are less dispersed than the MIR selection. However, when we analyse the luminosity dependence of this ratio, we find important discrepancies. Finally, we revisit the evolution of Ldust/LUV ratio with the redshift z by re-calibrating previous Ldust at z ∼ 2 based on our results and added new data points at higher redshifts. The decreasing trend is amplified as compared to the previous estimate.