Frontier Fields: Combining HST, VLT, andSpitzerdata to explore thez~ 8 Universe behind the lensing cluster MACSJ0416.1−2403

Abstract

Context. The Hubble Space Telescope (HST) Frontier Fields (HFFs) project started at the end of 2013 with the aim of providing extremely deep images of six massive galaxy clusters. One of the main goals of this program is to push several telescopes to their limits to provide the best current view of the earliest stages of the Universe. The analysis of the initial data has already demonstrated the huge capabilities of the program. Aims. We present a detailed analysis of z ∼ 8 objects behind the HFFs lensing cluster, MACSJ0416.1-2403, combining 0.3−1.6 µm imaging from HST, ground-based K s imaging from VLT HAWK-I, and 3.6 µm and 4.5 µm Spitzer Space Telescope. The images probe to 5σ depths of ≈29 AB for HST, 25.6 AB for HAWK-I, and ≈0.310 and 0.391 µJy at 3.6 and 4.5 µm, respectively. With these datasets, we assess the photometric properties of z ∼ 8 galaxies in this field, as well as their distribution in luminosity, to unprecedented sensitivity. Methods. We applied the classical Lyman break (LB) technique, which combines non detection criteria in bands blueward of the Lyman break at z ∼ 8 and color-selection in bands redward of the break. To avoid contamination by mid-z interlopers, we required a strong break between optical and near-infrared data. We determined the photometric properties of z ∼ 8 selected candidates using spectral energy distribution (SED)-fitting with standard library templates. The luminosity function at z ∼ 8 is computed using a Monte-Carlo method taking advantage of the SED-fitting results. A piece of cautionary information is gleaned from new deep optical photometry of a previously identified z ∼ 8 galaxy in this cluster, which is now firmly detected as a mid-z interloper with a strong ≈1.5 mag Balmer break (between F606W and F125W). Using the SED of this interloper, we estimated the contamination rate of our MACSJ0416.1−2403 sample, and that of previous samples in Abell 2744 that were based on HFF data, we highlight the dangers of pushing the LB technique too close to the photometry limits. Results. Our selection reliably recovers four objects with m F160W ranging from 26.0 to 27.9 AB that are located in modestamplification regions (µ < 2.4). Two of the objects display a secondary break between the IRAC 3.6 µm and 4.5 µm bands, which could be associated to the Balmer break or emission lines at z ∼ 8. The SED-fitting analysis suggests that all of these objects favor high-z solutions with no reliable secondary solutions. The candidates generally have star formation rates around ∼10 M /yr and sizes ranging from 0.2 to 0.5 kpc, which agrees well with previous observations and expectations for objects in the early Universe. The sample size and luminosity distribution are consistent with previous findings.

The ASTRODEEP Frontier Fields catalogues

“…4, which indicates a good consistency with respect to previous estimates. A similar result is found by making a comparison with the much smaller LBG samples from the M0416 fields (Laporte et al 2015;Coe et al 2015;McLeod et al 2015) and from the A2744 parallel field (Ishigaki et al 2015;McLeod et al 2015). We inspected the 16 LBGs missing from our A2744 cluster catalogue and found that in three cases they are undetected, while 13 are very close to bright galaxies and are not deblended from them.…”

The Frontier Fields: Survey Design and Initial Results

“…The full Spitzer Frontier Fields observations are complete and were publicly released in early 2016. The first Frontier Fields observations have already probed galaxies during the epoch of reionization to intrinsic luminosities fainter than any previously seen (e.g., Zitrin et al 2014;Atek et al 2015;Laporte 2015;Livermore et al 2017) and improved our statistical accounting of z>8 galaxies (e.g., Kawamata et al 2015;McLeod et al 2015McLeod et al , 2016Ishigaki et al 2016). The full data set will place strong statistical constraints on the faint end of the luminosity function during this era (Robertson et al 2015).…”

The ASTRODEEP Frontier Fields catalogues

“…Moreover, the program offers a crucial test for our current capabilities of data analysis, from the perspective of the new, large amount of data that will come from future facilities like the James Webb Space Telescope. Published works on FF data include McLeod et al (2015), Wang et al (2015), Oesch et al (2015), Laporte et al (2015), Atek et al (2015).…”