Radio spectra of bright compact sources at z>4.5

Rocco Coppejans, Sjoert van Velzen, Huib T. Intema, Cornelia Müller, Sándor Frey, Deanne L. Coppejans, Dávid Cseh, Wendy Williams, Heino Falcke, Elmar G. Körding, Emanuela Orrú, Zsolt Paragi, Krisztina É. Gabányi

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    High-redshift quasars are important to study galaxy and active galactic nuclei (AGN) evolution, test cosmological models, and study supermassive black hole growth. Optical searches for high-redshift sources have been very successful, but radio searches are not hampered by dust obscuration and should be more effective at finding sources at even higher redshifts. Identifying high-redshift sources based on radio data is, however, not trivial. Here we report on new multi-frequency Giant Metrewave Radio Telescope (GMRT) observations of eight z>4.5 sources previously studied at high angular resolution with very long baseline interferometry (VLBI). Combining these observations with those from the literature, we construct broad-band radio spectra of all 30 z>4.5 sources that have been observed with VLBI. In the sample we found flat, steep and peaked spectra in approximately equal proportions. Despite several selection effects, we conclude that the z>4.5 VLBI (and likely also non-VLBI) sources have diverse spectra and that only about a quarter of the sources in the sample have flat spectra. Previously, the majority of high-redshift radio sources were identified based on their ultra-steep spectra (USS). Recently a new method has been proposed to identify these objects based on their megahertz-peaked spectra (MPS). Neither method would have identified more than 18% of the high-redshift sources in this sample. More effective methods are necessary to reliably identify complete samples of high-redshift sources based on radio data.
    Original languageEnglish
    Pages (from-to)2039-2060
    Number of pages62
    JournalMonthly Notices of the Royal Astronomical Society
    Issue number2
    Early online date25 Jan 2017
    Publication statusPublished - 1 May 2017


    • galaxies: active
    • galaxies: high-redshift
    • radio continuum: galaxies


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