2019A&A...629A..68B

We present the discovery and properties of DES J014132.4-542749.9 (DES0141-54), a new powerful radio-loud active galactic nucleus (AGN) in the early Universe (z=5.0). It was discovered by cross-matching the first data release of the Dark Energy Survey (DES DR1) with the Sidney University Molonglo Survey (SUMSS) radio catalog at 0.843GHz. This object is the first radio-loud AGN at high redshift discovered in the DES. The radio properties of DES0141-54, namely its very large radio-loudness (R>10⁴), the high radio luminosity (L_0.8GHz=1.73x10²⁸W/Hz), and the flatness of the radio spectrum (α=0.35) up to very high frequencies (120GHz in the source's rest frame), classify this object as a blazar, meaning, a radio-loud AGN observed along the relativistic jet axis. However, the X-ray luminosity of DES0141-54 is much lower compared to those of the high redshift (z≥4.5) blazars discovered so far. Moreover its X-ray-to-radio luminosity ratio (log(L_{[0.5–10]keV}/L_1.4GHz)=9.96±0.30Hz) is small also when compared to lower redshift blazars: only 2% of the low-z population has a similar ratio. By modeling the spectral energy distribution we found that this peculiar X-ray weakness and the powerful radio emission could be related to a particularly high value of the magnetic field. Finally, the mass of the central black hole is relatively small (M_BH=3-8x10⁸M_☉) compared to other confirmed blazars at similar redshift, making DES0141-54 the radio-loud AGN that host the smallest supermassive black hole ever discovered at z≥5.