High-redshift galaxies in J-PLUS
Identifying and studying high redshift galaxies is crucial for our understanding of the early epochs of galaxy evolution. At the beginning of the nineties, the implementation of the so called dropout technique opened the era for detections of copious numbers of these early galaxies. They are identified based on their broadband colours, i.e. by measuring the drop in brightness due to the Lyman break at rest frame 912 Å and/ or the Lyman forest between 912 Å and 1216 Å. For high redshift galaxies (z ≳ 2) these features are shifted to optical or infrared wavelengths and permit the detection of these so-called Lyman-break galaxies (LBGs) from the ground. While the dropout technique is efficient at selecting high redshift galaxies, it is also affected by significant incompleteness and contamination, losing some fraction of the population at the selected redshift, or allowing galaxies at other redshifts to enter the sample. While the latter can be dealt with by obtaining spectroscopic redshifts, the former remains a serious difficulty. We are not yet at the point of spectroscopic blind surveys, hence, a step forward towards less biased candidate selection is offered by multifilter surveys, such as J-PLUS. They combine the efficiency and unbiased nature of photometric surveys with very low resolution spectral information, permitting us to derive more information on the surveyed objects such as their accurate photometric redshifts.
In Figure above a composite spectrum of 811 LBGs at redshift ~ 3 (Shapley et al. 2003) is shown together with the J-PLUS photometry obtained by convolving the same spectrum with J-PLUS filters. At redshifts z ~ 2 – 3, J-PLUS filters sample both the Lyman forest and the Lyman-α break, enabling a reliable determination of the photometric redshift. In Figures 1 and 2 high-redshift galaxy number counts derived from Advanced Large, Homogeneous Area Medium Band Redshift Astronomical (ALHAMBRA) survey are shown. Due to the limited depth of J-PLUS narrow-band filters (mAB ~ 21.5), we can only expect to sample the very brightest end of these counts with J-PLUS data. However, thanks to the large area (~ 8500 deg2) J-PLUS will cover, we will have a very good sampling of these brightest high-redshift galaxies. From the counts shown in Figures 1 and 2 we can derive an expected number of > 10 000 high-redshift galaxies brighter than mAB ~ 21.5 detectable by J-PLUS. With this data we will populate the brightest end of the LBG luminosity function at z = 2-3 with unprecedented detail.