The first Frontier Fields cluster: 4.5 μ m excess in a z ∼ 8 galaxy candidate in Abell 2744

Aims. We present in this letter the first analysis of a z ∼ 8 galaxy candidate found in the Hubble and Spitzer imaging data of Abell 2744 as part of the Hubble Frontier Fields legacy program. Methods. We applied the most commonly used methods to select exceptionally high-redshift galaxies by combining non-detection and color criteria using seven HST bands. We used GALFIT on IRAC images to fit and subtract contamination of bright nearby sources.The physical properties were inferred from spectral energy distribution-fitting using templates with and without nebular emission. Results. This letter is focused on the brightest candidate we found (m_F160W = 26.2) over the 4.9 arcmin² field of view covered by the WFC3. It is not detected in the ACS bands and at 3.6 μm, while it is clearly detected at 4.5 μm with rather similar depths. This break in the IRAC data might be explained by strong [OIII]+Hβ lines at z ∼ 8 that contribute to the 4.5 μm photometry. The best photo-z is found at z ∼ 8.0^+0.2_-0.5z8.0-0. 5+0.2, although solutions at low-redshift (z ∼ 1.9) cannot be completely excluded, but they are strongly disfavored by the SED-fitting. The amplification factor is relatively small at μ = 1.49 ± 0.02. The star formation rate in this object ranges from 8 to 60 M_⊙ yr^-1, the stellar mass is on the order of M^↠= (2.5-10) × 10⁹ M_⊙, and the size is r ≈ 0.35 ± 0.15 kpc. Conclusions. This object is one of the first z ∼ 8 Lyman break galaxy candidates showing a clear break between 3.6 μm and 4.5 μm, which is consistent with the IRAC properties of the first spectroscopically confirmed galaxy at a similar redshift. Due to its brightness, the redshift of this object could potentially be confirmed by near-infrared spectroscopy with current 8-10 m telescopes. The nature of this candidate will be revealed in the coming months with the arrival of new ACS and Spitzer data, increasing the depth at optical and near-infrared wavelengths.