The evolution of the luminosity functions in the FORS Deep Field from low to high redshift I. The blue bands

Abstract

We use the very deep and homogeneous I-band selected dataset of the FORS Deep Field (FDF) to trace the evolution of the luminosity function over the redshift range 0.5 < z < 5.0. We show that the FDF I-band selection down to IAB = 26.8 misses of the order of 10% of the galaxies that would be detected in a K-band selected survey with magnitude limit KAB = 26.3 (like FIRES). Photometric redshifts for 5558 galaxies are estimated based on the photometry in 9 filters (U, B, Gunn g, R, I, SDSS z, J, K and a special filter centered at 834 nm). A comparison with 362 spectroscopic redshifts shows that the achieved accuracy of the photometric redshifts is Δz/(z spec + 1) ≤ 0.03 with only ∼1% outliers. This allows us to derive luminosity functions with a reliability similar to spectroscopic surveys. In addition, the luminosity functions can be traced to objects of lower luminosity which generally are not accessible to spectroscopy. We investigate the evolution of the luminosity functions evaluated in the restframe UV (1500 Å and 2800 Å), u′, B, and g′ bands. Comparison with results from the literature shows the reliability of the derived luminosity functions. Out to redshifts of z ∼ 2.5 the data are consistent with a slope of the luminosity function approximately constant with redshift, at a value of -1.07 ± 0.04 in the UV (1500 Å, 2800 Å) as well as u′, and -1.25 ± 0.03 in the blue (g′, B). We do not see evidence for a very steep slope (α ≤ -1.6) in the UV at 〈z〉 ∼ 3.0 and 〈z〉 ∼ 4.0 favoured by other authors. There may be a tendency for the faint-end slope to become shallower with increasing redshift but the effect is marginal. We find a brightening of M* and a decrease of φ* with redshift for all analyzed wavelengths. The effect is systematic and much stronger than what can be expected to be caused by cosmic variance seen in the FDF. The evolution of M* and φ* from z = 0 to z = 5 is well described by the simple approximations M*(z) = M*0 + a ln (1 + z) and φ*(z) = φ*0(1 + z)b for M* and φ*. The evolution is very pronounced at shorter wavelengths (a = -2.19, and b = -1.76 for 1500 Å rest wavelength) and decreases systematically with increasing wavelength, but is also clearly visible at the longest wavelength investigated here (a = -1.08, and b = -1.29 for g′). Finally we show a comparison with semi-analytical galaxy formation models.