Lyman Break Galaxies at z ∼ 5: Luminosity Function

We present the results of a search for Lyman break galaxies (LBGs) atz = ∼ 5 in a 618 square-arcmin field including the Hubble Deep Field-North (HDF-N) taken by the Subaru Prime Focus Camera. Utilizing the published redshift data of the HDF-N and its flanking fields, the color selection criteria were chosen so that LBGs could be picked out most efficiently and least contaminated by foreground objects.The numbers of detected LBG candidates were 96 in 23.0 ≡ I_c (mag) ≡ 24.5 and 310 in 23.0 ≡ I_c ≡ 25.5. There is a hint of a deficiency of bright blue galaxies, although it is not as clear as has been suggested for LBGs at z ∼ 3 to 4. With 305 LBG candidates in a 575 square-arcmin field, the rest-frame UV luminosity function of LBGs at4.4 <∼ z <∼ 5.3 was derived statistically by considering both the contamination by objects at the intermediate redshift and the incompleteness of the survey. The fraction of the contamination was estimated to be ∼ 50% in the faintest magnitude range.The completeness of the survey was ∼ 80% at the bright part of the sample, and ∼ 20% in the faintest magnitude range(25.0 < I_c ≤ 25.5).The luminosity function of LBG candidates at z ∼ 5 did not show a significant difference from those of LBGs at z ∼ 3 and 4, though there might be a slight decrease in the fainter part.The UV luminosity density within the observational limit was 0.56-0.69 times smaller than that obtained for LBGs at z ∼ 3, depending on the adopted cosmology and the integration range of the luminosity function. This decrease in the UV luminosity at z ∼ 5 compared to that at z ∼ 3 is due to the smaller number density of faint galaxies at z ∼ 5. The similarity of the luminosity functions at redshifts 5 to 3 implies that most of the LBGs at z ∼ 5 should have faded outat z ∼ 3, and that the LBGs at z ∼ 5 are different galaxies from those seen at z ∼ 3, if we take the face values for the ages of the LBGs at z ∼ 3 obtained by a SED fitting in which a continuous star formation in an individual galaxy was assumed. However, if the star formation in LBGs is sporadic, the similarity of the luminosity function at z ∼ 3 and 5 would be explained. Such sporadic star formation has been suggested by hydrodynamical simulations and semi-analytic models with collisional starbursts, and the trend of the cosmic star formation history predicted by these studies resembles that estimated from an integration of the UV luminosity density within the observational limit.