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1. Observations by GMRT at 323 MHz of radio-loud quasars at z   >  5

   https://doi.org/10.1051/0004-6361/202038469  

   Shao, Yali; Wagg, Jeff; Wang, Ran; Carilli, Chris L.; Riechers, Dominik A.; Intema, Huib T.; Weiss, Axel; Menten, Karl M. (September 2020, Astronomy & Astrophysics)

   We present Giant Metrewave Radio Telescope (GMRT) 323 MHz radio continuum observations toward 13 radio-loud quasars at z  >  5, sampling the low-frequency synchrotron emission from these objects. Among the 12 targets successfully observed, we detected 10 above 4 σ significance, while 2 remain undetected. All of the detected sources appear as point sources. Combined with previous radio continuum detections from the literature, 9 quasars have power-law spectral energy distributions throughout the radio range; for some the flux density drops with increasing frequency while it increases for others. Two of these sources appear to have spectral turnover. For the power-law-like sources, the power-law indices have a positive range between 0.18 and 0.67 and a negative values between −0.90 and −0.27. For the turnover sources, the radio peaks around ∼1 and ∼10 GHz in the rest frame, the optically thin indices are −0.58 and −0.90, and the optically thick indices are 0.50 and 1.20. A magnetic field and spectral age analysis of SDSS J114657.59+403708.6 at z  = 5.01 may indicate that the turnover is not caused by synchrotron self-absorption, but rather by free-free absorption by the high-density medium in the nuclear region. Alternatively, the apparent turnover may be an artifact of source variability. Finally, we calculated the radio loudness R 2500 Å for our sample, which spans a very wide range from 12 −13 +13 to 4982 −254 +279 . 

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2. The host galaxies of radio-loud quasars at z > 5 with ALMA

   https://doi.org/10.1051/0004-6361/202451290  

   Mazzucchelli, C; Decarli, R; Belladitta, S; Bañados, E; Meyer, R A; Connor, T; Momjian, E; Rojas-Ruiz, S; Eilers, A-C; Khusanova, Y; et al (February 2025, Astronomy & Astrophysics)

   The interaction between radio jets and quasar host galaxies plays a paramount role in quasar and galaxy co-evolution. However, very little is known at present about this interaction at very high−z. Here, we present new Atacama Large Millimeter/submillimeter Array (ALMA) observations in Bands 7 and 3 of six radio-loud (RL) quasar host galaxies atz > 5. We recovered [C II] 158 μm line and underlying dust continuum emission at > 2σfor five sources, while we obtained upper limits for the CO(6-5) emission line and continuum for the remaining source. At the spatial resolution of our observations (∼1″​​_.0–1″​​_.4), we did not recover any perturbed or extended morphologies or kinematics, which are known signatures of potential mergers. These galaxies already host large quantities of gas (∼10¹⁰M_⊙), with [C II] luminosities ofL_[C II] ∼ 10^8 − 9 L_⊙and [C II]-based star formation rates of 30 − 400 M_⊙yr⁻¹. In building their radio/submillimeter (radio/submm) spectral energy distributions (SEDs), we found that in at least four cases, the 1 mm continuum intensity arises from a combination of synchrotron and dust emission. The initial estimation of synchrotron contribution at 300 GHz in these cases is of ≳10%. Assuming a scenario where the continuum emission is solely due to cold dust as an upper limit, we obtained infrared (IR) luminosities ofL_IR ∼ 10^{11 − 12} L_⊙. We compared the properties of the sources inspected here with a large collection of radio-quiet sources from the literature, as well as a sample of RL quasars from previous studies at comparable redshifts. We recovered a mild potential decrease inL_[C II]for the RL sources, which might be due to a suppression of the cool gas emission due to the radio jets. We did not find any [C II] emitting companion galaxy candidate around the five RL quasars observed in Band 7. Given the depth of our dataset, this result is still consistent with what has been observed around radio-quiet quasars. Future higher spatial-resolution observations, over a broader frequency range, of high−zRL quasars hosts will allow us to further improve our understanding of the physics of these sources. 

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3. Radio spectral energy distribution of low- z metal-poor extreme starburst galaxies: novel insights on the escape of ionizing photons

   https://doi.org/10.1093/mnras/stag492  

   Bait, Omkar; Schaerer, Daniel; Izotov, Yuri I; Sebastian, Biny (April 2026, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT Recent optical surveys have identified a rare population of low-z ($$z \sim 0.01 \!-\! 0.06$$) extreme star-forming galaxies (xSFGs) characterized by very low metallicity, strong emission lines, extremely high specific star-formation rate, low stellar mass, and strong Ly $$\alpha$$ emission. Their global properties resemble recently discovered $$z\gt 6$$ reionization-era star-forming galaxies. We present new multifrequency radio continuum observations of eight xSFGs using the upgraded Giant Metrewave Radio Telescope at 1.25 GHz, the Karl G. Jansky Very Large Array at $1.5, 3.0, 6.0, 10.0$, and 15.0 GHz, along with archival LOw Frequency ARray data at 150 MHz for several sources. These data allow construction of the radio spectral energy distribution (radio-SED) from $$\sim 1$$ GHz (down to 150 MHz for some sources) to 15 GHz, spanning nearly two orders of magnitude in frequency. We find that xSFGs exhibit a flat spectral index between 6 and 15 GHz, while a subset shows spectral turnovers at $$0.3 \!-\! 3$$ GHz. Our Bayesian radio-SED modelling indicates that these features are consistent with a high thermal fraction combined with free–free absorption, requiring high emission measures in some systems. By comparing modelled thermal radio emission with observed H$$\beta {}$$ line flux density, we find evidence for dust in several xSFGs. Finally, we confirm a previously reported correlation between Lyman continuum escape fraction ($$f_\mathrm{esc}^\mathrm{LyC}$$), ionization state, and radio spectral index, particularly among strong leakers. 

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4. Understanding the radio relic emission in the galaxy cluster MACS J0717.5+3745: Spectral analysis

   https://doi.org/10.1051/0004-6361/202039428  

   Rajpurohit, K.; Wittor, D.; van Weeren, R. J.; Vazza, F.; Hoeft, M.; Rudnick, L.; Locatelli, N.; Eilek, J.; Forman, W. R.; Bonafede, A.; et al (February 2021, Astronomy & Astrophysics)

   null (Ed.)

   Radio relics are diffuse, extended synchrotron sources that originate from shock fronts generated during cluster mergers. The massive merging galaxy cluster MACS J0717.5+3745 hosts one of the more complex relics known to date. We present upgraded Giant Metrewave Radio Telescope band 3 (300−500 MHz) and band 4 (550−850 MHz) observations. These new observations, combined with published VLA and the new LOFAR HBA data, allow us to carry out a detailed, high spatial resolution spectral analysis of the relic over a broad range of frequencies. The integrated spectrum of the relic closely follows a power law between 144 MHz and 5.5 GHz with a mean spectral slope α  = −1.16 ± 0.03. Despite the complex morphology of this relic, its subregions and the other isolated filaments also follow power-law behaviors, and show similar spectral slopes. Assuming diffusive shock acceleration, we estimated a dominant Mach number of ∼3.7 for the shocks that make up the relic. A comparison with recent numerical simulations suggests that in the case of radio relics, the slopes of the integrated radio spectra are determined by the Mach number of the accelerating shock, with α nearly constant, namely between −1.13 and −1.17, for Mach numbers 3.5 − 4.0. The spectral shapes inferred from spatially resolved regions show curvature, we speculate that the relic is inclined along the line of sight. The locus of points in the simulated color-color plots changes significantly with the relic viewing angle. We conclude that projection effects and inhomogeneities in the shock Mach number dominate the observed spectral properties of the relic in this complex system. Based on the new observations we raise the possibility that the relic and a narrow-angle-tailed radio galaxy are two different structures projected along the same line of sight. 

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5. Radio and far-IR emission associated with a massive star-forming galaxy candidate at z ≃ 6.8: a radio-loud AGN in the reionization era?

   https://doi.org/10.1093/mnras/stac737  

   Endsley, Ryan; Stark, Daniel P; Fan, Xiaohui; Smit, Renske; Wang, Feige; Yang, Jinyi; Hainline, Kevin; Lyu, Jianwei; Bouwens, Rychard; Schouws, Sander (April 2022, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT We report the identification of radio (0.144–3 GHz) and mid-, far-infrared, and sub-mm (24–850μm) emission at the position of one of 41 UV-bright ($$\mathrm{M_{\mathrm{UV}}}^{ }\lesssim -21.25$$) z ≃ 6.6–6.9 Lyman-break galaxy candidates in the 1.5 deg2 COSMOS field. This source, COS-87259, exhibits a sharp flux discontinuity (factor >3) between two narrow/intermediate bands at 9450 and 9700 Å and is undetected in all nine bands blueward of 9600 Å, as expected from a Lyman alpha break at z ≃ 6.8. The full multiwavelength (X-ray through radio) data of COS-87529 can be self-consistently explained by a very massive (M* = 1010.8 M⊙) and extremely red (rest-UV slope β = −0.59) z ≃ 6.8 galaxy with hyperluminous infrared emission (LIR = 1013.6 L⊙) powered by both an intense burst of highly obscured star formation (SFR ≈ 1800 M⊙ yr−1) and an obscured ($$\tau _{_{\mathrm{9.7\mu m}}} = 7.7\pm 2.5$$) radio-loud (L1.4 GHz ≈ 1025.4 W Hz−1) active galactic nucleus (AGN). The radio emission is compact (1.04 ± 0.12 arcsec) and exhibits an ultra-steep spectrum between 1.32 and 3 GHz ($$\alpha =-1.57^{+0.22}_{-0.21}$$) that flattens at lower frequencies ($$\alpha = -0.86^{+0.22}_{-0.16}$$ between 0.144 and 1.32 GHz), consistent with known z > 4 radio galaxies. We also demonstrate that COS-87259 may reside in a significant (11×) galaxy overdensity, as common for systems hosting radio-loud AGN. While we find that low-redshift solutions to the optical + near-infrared data are not preferred, a spectroscopic redshift will ultimately be required to establish the true nature of COS-87259 beyond any doubt. If confirmed to lie at z ≃ 6.8, the properties of COS-87259 would be consistent with a picture wherein AGN and highly obscured star formation activity are fairly common among very massive (M* > 1010 M⊙) reionization-era galaxies. 

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