Astronomers Discovered Most Distant Radio-loud Quasar (Astronomy)

Radio jets from active galactic nuclei (AGNs) are thought to play a key role in the coevolution of supermassive black holes and their host galaxies, as well as in the early growth of massive black holes. Yet, strong radio emission seems to be a rare or at least short-lived phenomenon. Only about 10% of all quasars are strong radio emitters, almost independent of their redshifts up to z ∼ 6. Now, Banados and colleagues presented the discovery of the most distant radioloud source to date, the quasar P172+18 with an Mg II-based redshift of z = 6.823 and is so distant that light from it has travelled for about 13 billion years to reach us: we see it as it was when the Universe was just around 780 million years old.. Their study recently appeared in ArXiv on dated 4 Mar 2021.

The quasar has a black hole mass of ∼ 2.9×108 M and an Eddington ratio of ∼2.2. It is known that there are large uncertainties on the estimates of black hole mass and Eddington ratio associated with the scaling relations used. Therefore, they compared the properties of P172+18 to other quasars using the same scaling relation and bolometric correction. With this in mind, P172+18 is among the fastest accreting quasars at both low and high redshift (Featured image).

The quasar shows a strong Lyα line that can be modeled with a narrow Gaussian and a broad one (shown in fig 1). The large measured near-zone size, RNZ,corr ∼ 6 pMpc, suggests an ionized intergalactic medium (IGM) around the quasar and implies that P172+18’s lifetime exceeds the average lifetime of the z ⪰ 6 quasar population.

Figure 1: Zoom-in on the main broad emission lines from near-infrared spectroscopy © Banados et al.

The quasar’s radio emission is unresolved (with size smaller than 1.”90 × 0.”87) and showed a steep radio spectrum (α = −1.31 ± 0.08) between 1.5 and 3.0 GHz (∼11–23 GHz in the rest frame). Extrapolating the spectrum to 5 GHz rest frame, the quasar has a radio-loudness of R2500 = 91 ± 9 (shown in fig. 2 below).

Figure 2. Radio spectral energy distribution of P172+18, including data from their VLA follow-up observations (red diamond), the VLBI measurements (green circles) from Momjian et al. (2021), and a 3σ upper limit from the TGSS (purple square). The power-law index, α, is shown between the measurements as well as the radio-loudness by extrapolating the radio emission to rest-frame 5GHz. The dotted line with α = 1.24 represents the turnover required for P172+18 to be classified as radio-quiet (i.e., R2500 < 10). © Banados et al.

The follow-up L-band radio data are a factor ∼ 2 fainter than what is expected from the FIRST observations taken two decades previously. This fact, together with the long lifetime implied by the size of P172+18’s near-zone, could indicate that they are witnessing the quasar phase turning off.

Figure 3. VLA L- and S-band observations (blue contours) centered on the position of P172+18 over optical and Near-infrared imaging as labeled in the figure; north is up and east is left © Banados et al.

In addition to detecting the quasar, the follow-up VLA radio observations revealed a second radio source 23.’’1 from P172+18 at a position angle of 128.25° (see Figure 3). This source was not detected in the FIRST survey and has no counterpart in their current optical/near-infrared images.

Featured image: Black hole mass vs. bolometric luminosity. The gray points and contours showed the distribution of SDSS DR7 quasars at 0.35 < z < 2.25. Red points and contours highlight the SDSS DR7 radio-loud quasar subsample. They showed z > 6 radio-quiet and radio-loud quasars from a collection of studies in the literature with blue circles and magenta squares, respectively. P172+18 (orange star) is consistent with accreting matter at super-Eddington rate. The dominant systematic uncertainty on black hole mass estimates from scaling relations (∼0.55 dex) is shown in the bottom right corner. All black hole masses shown here are estimated using the same scaling relation, and the same bolometric correction was applied for all bolometric luminosities © Banados et al.

Reference: Eduardo Banados, Chiara Mazzucchelli, Emmanuel Momjian, Anna-Christina Eilers, Feige Wang, Jan-Torge Schindler, Thomas Connor, Irham Taufik Andika, Aaron J. Barth, Chris Carilli, Frederick B. Davies, Roberto Decarli, Xiaohui Fan, Emanuele Paolo Farina, Joseph F. Hennawi, Antonio Pensabene, Daniel Stern, Bram P. Venemans, Lukas Wenzl, Jinyi Yang, “The discovery of a highly accreting, radio-loud quasar at z=6.82”, Astrophysical Journal, 2021.

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