Exploring the radio-loudness of SDSS quasars with spectral stacking

M. I. Arnaudova, D. J. B. Smith, M. J. Hardcastle, Soumyadeep Das, A. Drake, Kenneth Duncan, G. Gürkan, M. Magliocchetti, L. K. Morabito, J. W. Petley, S. Shenoy, C. Tasse

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We use new 144 MHz observations over 5634 deg 2 from the LOFAR (Low Frequency Array) Two-metre Sky Survey (LoTSS) to compile the largest sample of uniformly selected, spectroscopically confirmed quasars from the 14th data release of the Sloan Digital Sky Survey (SDSS-DR14). Using the classical definition of radio loudness, R = log (L 1.4GHz/L i), we identify 3697 radio-loud (RL) and 111 132 radio-quiet (RQ) sources at 0.6 < z < 3.4. To study their properties, we develop a new rest-frame spectral stacking algorithm, designed with forthcoming massively multiplexed spectroscopic surveys in mind, and use it to create high signal-to-noise composite spectra of each class, matched in redshift and absolute i-band magnitude. We show that RL quasars have redder continuum and enhanced [O II] emission than their RQ counterparts. These results persist when additionally matching in black hole mass, suggesting that this parameter is not the defining factor in making a quasi-stellar object (QSO) RL. We find that these features are not gradually varying as a function of radio loudness, but are maintained even when probing deeper into the RQ population, indicating that a clear-cut division in radio loudness is not apparent. Upon examining the star formation rates (SFRs) inferred from the [O II] emission line, with the contribution from active galactic nucleus removed using the [Ne V] line, we find that RL quasars have a significant excess of star formation relative to RQ quasars out to z = 1.9 at least. Given our findings, we suggest that RL sources either preferably reside in gas-rich systems with rapidly spinning black holes, or represent an earlier obscured phase of QSO evolution.

Original languageEnglish
Article numberstae233
Pages (from-to)4547–4567
Number of pages21
JournalMonthly Notices of the Royal Astronomical Society
Issue number3
Early online date22 Jan 2024
Publication statusE-pub ahead of print - 22 Jan 2024


  • astro-ph.GA
  • quasars: general
  • galaxies: active
  • techniques: spectroscopic
  • radio continuum: galaxies


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