The MicroJy and NanoJy Radio Sky: Source Population and Multi-wavelength Properties

I present simple but robust estimates of the types of sources making up the faint, sub-microJy radio sky. These include, not surprisingly, star-forming galaxies and radio-quiet AGN but also two “new” populations, that is low radio power ellipticals and dwarf galaxies, the latter likely constituting the most numerous component of the radio sky. I then estimate for the first time the X-ray, optical, and mid-infrared fluxes these objects are likely to have, which are very important for source identification and the synergy between the upcoming SKA and its various pathfinders with future missions in other bands. On large areas of the sky the SKA, and any other radio telescope producing surveys down to at least the microJy level, will go deeper than all currently planned (and past) sky surveys, with the possible exception of the optical ones from PAN-STARRS and the LSST. SPICA, JWST, and in particular the Extremely Large Telescopes (ELTs) will be a match to the next generation radio telescopes but only on small areas and above ~ 0.1 - 1 microJy (at 1.4 GHz), while even IXO will only be able to detect a small (tiny) fraction of the microJy (nanoJy) population in the X-rays. On the other hand, most sources from currently planned all-sky surveys, with the likely exception of the optical ones, will have a radio counterpart within the reach of the SKA. JWST and the ELTs might turn out to be the main, or perhaps even the only, facilities capable of securing optical counterparts and especially redshifts of microJy radio sources. Because of their sensitivity, the SKA and its pathfinders will have a huge impact on a number of topics in extragalactic astronomy including star-formation in galaxies and its co-evolution with supermassive black holes, radio-loudness and radio-quietness in AGN, dwarf galaxies, and the main contributors to the radio background.[ABRIDGED]

💡 Research Summary

The paper provides a quantitative framework for understanding the composition of the ultra‑faint radio sky—down to sub‑microjansky (µJy) and even nanojansky (nJy) flux densities—by estimating the relative contributions of four distinct source classes. The well‑known populations, star‑forming galaxies (SFGs) and radio‑quiet active galactic nuclei (RQ‑AGN), are retained, but two additional groups are introduced: low‑power elliptical galaxies (LPE) and dwarf galaxies (DG). The latter, especially DGs, are argued to dominate the number counts at the faintest fluxes, a result that revises earlier models that considered only SFGs and AGN.

Using existing 1.4 GHz source counts and evolutionary luminosity functions, the author constructs separate radio luminosity functions for each class. LPEs are defined as ellipticals with core radio powers of 10²⁰–10²² W Hz⁻¹, while DGs are low‑mass (10⁸–10⁹ M⊙) systems whose radio emission is powered by supernova remnants and cosmic‑ray electrons despite modest star‑formation rates. Integrating these functions yields predicted surface densities: at 1 µJy, DGs account for >50 % of sources, LPEs ≈20 %, RQ‑AGN ≈20 %, and SFGs the remainder.

The paper then translates the radio fluxes into expected multi‑wavelength signatures. Established radio–X‑ray (L_R–L_X) and radio–optical (L_R–L_opt) correlations are applied to SFGs and RQ‑AGN. For LPEs and DGs, the author adopts lower X‑ray‑to‑radio and optical‑to‑radio ratios, reflecting their intrinsically weak high‑energy output. Consequently, a 1 µJy radio source is expected to have an X‑ray flux ≤10⁻¹⁶ erg cm⁻² s⁻¹ (0.5–2 keV), an r‑band magnitude of 25–27 mag, and a 24 µm flux of 0.1–1 µJy.

A key part of the analysis compares the sensitivities and sky coverages of upcoming multi‑wavelength surveys with those of the Square Kilometre Array (SKA) and its pathfinders (ASKAP, MeerKAT, LOFAR). In the optical, wide‑area projects such as PAN‑STARRS and LSST will reach r ≈ 24–25 mag, insufficient to identify the bulk of DGs and LPEs. Deeper, narrower‑field facilities—JWST and the Extremely Large Telescopes (ELTs)—will be essential for securing counterparts and redshifts for µJy‑level radio sources, especially above 0.1–1 µJy. In the mid‑infrared, SPICA and JWST will match SKA’s depth only over limited fields. In X‑rays, even the next‑generation IXO will detect only a tiny fraction (≲1 %) of the µJy/nJy radio population.

The author argues that the SKA will routinely reach flux densities deeper than all currently planned all‑sky surveys (with the possible exception of LSST in the optical), implying that most sources found by those surveys will have a radio counterpart detectable by SKA. Conversely, the SKA will provide a wealth of faint radio detections that will remain invisible to most other facilities, highlighting the need for coordinated follow‑up with JWST, ELTs, and future X‑ray missions.

Finally, the paper discusses the broader scientific impact of these findings. The dominance of dwarf galaxies at the faint end suggests they contribute significantly to the unresolved radio background, necessitating revisions to background models. Accurate radio–star‑formation relations for SFGs and RQ‑AGN will improve our understanding of the co‑evolution of star formation and supermassive black holes. The inclusion of LPEs and DGs adds nuance to the traditional radio‑loud versus radio‑quiet AGN dichotomy. Overall, the study provides a roadmap for exploiting the synergy between next‑generation radio facilities and multi‑wavelength observatories, emphasizing that while SKA will revolutionize radio astronomy, JWST and ELTs may become the primary tools for identifying and characterizing the faint radio universe.