Unveiling the weak radio quasar population at $z\ge 4$

Abstract

We applied image stacking on empty-field Faint Images of the Radio Sky at Twenty-Centimeters (FIRST) survey maps centred on optically identified high-redshift quasars at z ≥ 4 to uncover the hidden µJy radio emission in these active galactic nuclei (AGN). The median stacking procedure for the full sample of 2229 optically identified AGN uncovered an unresolved point source with an integrated flux density of 52 µJy, with a signal-to-noise ratio ∼ 10. We co-added the individual image centre pixels to estimate the characteristic monochromatic radio power at 1.4 GHz considering various values for the radio spectral index, revealing a radio population with P 1.4GHz ∼ 10 24 W Hz −1 . Assuming that the entire radio emission originates from star-forming (SF) activity in the nuclear region of the host galaxy, we obtained an upper limit on the characteristic star formation rate, ∼ 4200 M ⊙ yr −1 . The angular resolution of FIRST images is insufficient to distinguish between the SF and AGN origin of radio emission at these redshifts. However, a comparison with properties of individual sources from the literature indicates that a mixed nature is likely. Future very long baseline interferometry radio observations and ultra-deep Square Kilometre Array surveys are expected to be sensitive enough to detect and resolve the central 1−10 kpc region in the host galaxies, and thus discriminate between SF and AGN related emission.

Radio emission from dust-obscured galaxies

“…Thus, these values indeed characterize the random noise distribution of FIRST cutout images. Similar results, SNRs between 4 − 5, were achieved by Perger et al (2019) when median stacking of FIRST images around fake positions were performed.…”

“…Radio emission is clearly detected in the stacked images of the otherwise individually undetected PL DOG positions. We can derive the characteristic 1.4-GHz radio power (𝑃 1.4 GHz ) corresponding to this detection following the procedure described in Perger et al (2019). The cumulative flux density can be given as…”

“…calculating the median intensity for each pixel in the stack of images, has the advantage to exclude the outlier pixel values. Median stacking of FIRST cutout images were used to look for the faint radio emission of e.g., optically identified quasars (White et al 2007), low-luminosity AGN (de Vries et al 2007), radio-quiet quasars (Hodge et al 2008;Hwang et al 2018), special class of quiescent galaxies with ionized gas emission features (Roy et al 2018), and high-redshift AGN (Perger et al 2019).…”

What is the origin of the stacked radio emission in radio-undetected quasars?: Insights from a radio-infrared image stacking analysis

“…White et al (2015) using a stacking analysis of photometrically and spectroscopically selected quasars with 0 < z < 3.1 in the VISTA Deep Extragalactic Observations (VIDEO, Jarvis et al 2013) survey found that the radio emission of the quasars in their sample is likely caused by accretion activity. Finally, Perger et al (2019) found a 1.4 GHz flux density of 52 µJy for 2229 RQQs with 4.0 < z < 7.5 using FIRST radio maps.…”

A method for checking high-redshift identification of radio AGNs

“…It would solve the conflict between the anomalously fast apparent jet component motion and the high redshift in a straightforward way. Further supporting pieces of evidence against the high-redshift scenario are the γ-ray emitting nature of J2346+0705, its optical magnitudes that are brighter by at least 3 m than those of typical z ∼ 5 quasars (see the catalogue 5 of z > 4 AGNs compiled by Perger et al 2019), and the g − r = 0.71 optical colour from SDSS DR16 data which is incompatible with the high redshift (cf. Alexandroff et al 2013).…”