Young Galaxy Candidates in the Hubble Frontier Fields. II. MACS J0416-2403

We searched for z ≳ 7 Lyman-break galaxies in the optical-to-mid-infrared Hubble Frontier Field and associated parallel field observations of the strong-lensing cluster MACS J0416−2403. We discovered 22 candidates, of which 6 lie at z ≳ 9 and 1 lies at z ≳ 10. Based on the Hubble and Spitzer photometry, all have secure photometric redshifts and a negligible probability of being at lower redshifts according to their peak-probability ratios, R. This substantial increase in the number of known high-redshift galaxies allows a solid determination of the luminosity function (LF) at z ≳ 8. The number of high-z candidates in the parallel field is considerably higher than that in the Abell 2744 parallel field. Our candidates have median stellar masses of log M_* ~ 8.44_(-0.31)^(+0.55) M_⊙, star formation rates (SFRs) of ~ 1.8_(-0.4)^(+0.5) M_⊙ yr^(−1), and SFR-weighted ages of ≾ 300_(-140)^(+70) Myr. Finally, we are able to put strong constraints on the z = 7, 8, 9, and 10 LFs. One of the objects in the cluster field is a z ≃ 10 candidate, with a magnification of μ ~ 20 ± 13. This object is likely the faintest z ~ 10 object known to date, allowing a first look into the extreme faint end (L ~ 0.04 L^*) of the z ~ 10 LF (It is named "Tayna" in the Aymara language).