An extremely X-ray weak blazar at z = 5

Abstract

We present the discovery and properties of DESJ014132.4-542749.9 (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 0.843 GHz. 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>10 4 ), the high radio luminosity (L 0.8GHz =1.73×10 28 W Hz −1 ), and the flatness of the radio spectrum (α=0.35) up to very high frequencies (120 GHz 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 DESJ0141-54 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±0.30 Hz) 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 = 3-8 ×10 8 M ) 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. 1 The criteria adopted to classify a source as a blazar are not unique and they can be based on both radio and X-ray properties. Here the six sources considered as blazars at z≥5.0 are: J0906+6930 (z=5.47, Romani et al.

X-ray properties of z > 4 blazars

“…All the sources analysed in this work have been selected from the CLASS survey (Myers et al 2003;Browne et al 2003), a density flux limited survey (S 5GH z >30 mJy) of flat spectrum radio sources, which covers most of the northern sky (16300 deg 2 ) and that contains more than 11000 objects (see Fig 1). It was built by combining the NRAO VLA Sky Survey (NVSS) at 1.4 GHz (Condon et al 1998) with the Green-Bank Survey (GB6) at 5 GHz (Gregory et al 1996) and by selecting only those objects with a flat (-0.5 <α <0.5) spectrum between 1.4 and 5 GHz.…”

What do observations tell us about the highest-redshift supermassive black holes?

“…The highest-z quasars ing extremely powerful radio emission, thought to originate from a relativistic jet (e.g., Bañados et al 2018c, Belladitta et al 2019. If (some of) these systems are interpreted as beamed AGN (i.e., blazars; e.g., Sbarrato et al 2012, Ghisellini et al 2015, they may potentially allow us to further constrain the accretion efficiencies and space densities of all (actively) accreting SMBHs at z 5 -including those which are too faint to be picked up by current optical/NIR surveys (Volonteri et al 2011, Ghisellini et al 2013).…”

What do observations tell us about the highest-redshift supermassive black holes?

“…(e.g., Bañados et al 2018c;Belladitta et al 2019). If (some of) these systems are interpreted as beamed AGN (i.e., blazars; e.g., Sbarrato et al 2012;Ghisellini et al 2015), they may potentially allow us to further constrain the accretion efficiencies and space densities of all (actively) accreting SMBHs at z ∼ > 5 -including those which are too faint to be picked up by current optical/NIR surveys (Volonteri et al 2011;Ghisellini et al 2013).…”