The GLEAMing of the first supermassive black holes

Abstract

We present the results of a new selection technique to identify powerful ( $L_{\rm 500\,MHz} \gt 10^{27}\,\text{WHz}^{-1}$ ) radio galaxies towards the end of the Epoch of Reionisation. Our method is based on the selection of bright radio sources showing radio spectral curvature at the lowest frequency ( ${\sim}100\,\text{MHz}$ ) combined with the traditional faintness in K-band for high-redshift galaxies. This technique is only possible, thanks to the Galactic and Extra-galactic All-sky Murchison Wide-field Array survey which provides us with 20 flux measurements across the 70– $230\,\text{MHz}$ range. For this pilot project, we focus on the GAMA 09 field to demonstrate our technique. We present the results of our follow-up campaign with the Very Large Telescope, Australian Telescope Compact Array, and the Atacama Large Millimetre Array to locate the host galaxy and to determine its redshift. Of our four candidate high-redshift sources, we find two powerful radio galaxies in the $1<z<3$ range, confirm one at $z=5.55$ , and present a very tentative $z=10.15$ candidate. Their near-infrared and radio properties show that we are preferentially selecting some of the most radio luminous objects, hosted by massive galaxies very similar to powerful radio galaxies at $1<z<5$ . Our new selection and follow-up technique for finding powerful radio galaxies at $z>5.5$ has a high 25–50% success rate.

Radio detection of VIK J2318−3113, the most distant radio-loud quasar (z = 6.44)

“…Nevertheless, we expect that upcoming and ongoing wide-area surveys such as RACS and the development of dedicated selection techniques in the radio band (e.g. Drouart et al 2020) will find many more radio-powerful sources at z > 6 (e.g. Amarantidis et al 2019).…”

Radio and Far-IR Emission Associated with a Massive Star-forming Galaxy Candidate at z$\simeq$6.8: A Radio-Loud AGN in the Reionization Era?

“…3; Drouart et al 2020). While the observed 1.4 GHz flux density of COS-87259 is ≈1000× fainter than that of most currently known 𝑧 > 4 radio galaxies (Saxena et al 2018;Drouart et al 2020;Yamashita et al 2020), this can be explained by the fact that 𝑧 > 4 radio galaxies are often identified from all-sky surveys that probe much higher radio luminosities relative to the extremely deep VLA-COSMOS data. There are many radio galaxies at lower redshifts (𝑧 < 4) with rest-frame 1.4 GHz luminosities comparable to that of COS-87259 (L 1.4GHz ∼ 10 25.5 W Hz −1 ; e.g.…”

“…Ker et al 2012;Saxena et al 2018;Yamashita et al 2020). As an example, we show the radio SED of GLEAM 0856 at 𝑧 = 5.55 in the inset panel which shows an ultra-steep (𝛼 = −1.51) radio slope at 1.4 GHz yet a relatively flat slope (𝛼 = −0.78) at 1 GHz (Drouart et al 2020).…”

“…For example, GLEAM 0856 at 𝑧 = 5.55 exhibits an ultra-steep (𝛼 = −1.51) radio slope at 1.4 GHz yet a relatively flat slope (𝛼 = −0.78) at 1 GHz (Fig. 3; Drouart et al 2020). While the observed 1.4 GHz flux density of COS-87259 is ≈1000× fainter than that of most currently known 𝑧 > 4 radio galaxies (Saxena et al 2018;Drouart et al 2020;Yamashita et al 2020), this can be explained by the fact that 𝑧 > 4 radio galaxies are often identified from all-sky surveys that probe much higher radio luminosities relative to the extremely deep VLA-COSMOS data.…”

“…We discuss the possibility of lowredshift solutions and compare the properties of COS-87259 to the sample of known high-redshift radio galaxies (e.g. Saxena et al 2018;Drouart et al 2020;Yamashita et al 2020) as well as hot dust-obscured galaxies (e.g. Stern et al 2014;Assef et al 2015;Fan et al 2016).…”