Speaker
Description
The Square Kilometre Array (SKA) will open a new observational window on the radio-faint sky. The large sky coverage and the high sensitivity of this facility will enable us to reconstruct, for the first time, the distribution of the sub-µJy radio sources. The milliarcsecond-scale spatial resolution delivered by the SKA-VLBI project (SKA - phase2) will also allow to detect compact radio cores at brightness temperatures of $T_b > 10^5 K$. This measurement will serve as a reliable AGN diagnostic in the sub-µJy range.
The contribution of faint, previously hidden sub-µJy AGN has been suggested as a major source of contamination in key star-formation–regulated relations, such as the star formation rate (SFR)–radio luminosity relation. For instance, the Infrared - Radio Correlation (IRRC), which arises from the relationship between radio luminosity and obscured SFR (e.g., SFR measured in the infrared), is well-known to flatten at high ($\approx 10^{10} M_\odot$) stellar masses. Contamination to the total radio luminosity from AGN jets buried in the highly star forming galactic nuclei was hypothesized to explain this flattening. Understanding the role of such contaminants is crucial, as uncertainties in these relations bias our interpretation of star-forming processes and, ultimately, our understanding of galaxy growth and evolution.
Recent pioneering VLBA surveys have already attempted to address this challenge, though these studies were limited by the narrow field of view and the relatively lower sensitivity of the VLBA with respect to SKA. The AGN-sCAN survey, for instance, targeted 500 galaxies at 1.4GHz in the COSMOS field, reaching a sensitivity limit of 25 μJy/beam. These galaxies were selected to lie around the IRRC, therefore classified as pure star forming galaxies by previous low-resolution ($\sim$ arcsec scale) VLA observations. Their high stellar masses (>10$^{10} M_\odot$) made them promising candidate to host hidden AGNs. Thanks to the high-spatial resolution (~ mas scale) of the AGN-sCAN - VLBA observations, we were able to disentangle the emission from AGN and stars in the galactic nuclei, and identify the faintest AGN ever detected in massive star-forming galaxies. In the deepest regions we detect 4 VLBA sources, which translates into an effective AGN detection rate of 9%, in good agreement with the predicted AGN number counts at these flux densities. Interestingly, we find that the IRRC is completely unaffected by this correction, even though the AGN flux contamination (∼30%) in our individual VLBA detections is non-negligible. However, we could not rule out the incidence of radio-silent AGN at sub-μJy levels, below the VLBA sensitivity limit.
The next decisive step will come with SKA. Crucially, SKA will make it possible to extend this analysis into the sub-µJy regime and over much larger areas of sky. This will allow us to rigorously test predictions of AGN number counts in this faint regime and definitively assess the contribution of non radio-excess AGN on the observed relations between star formation and radio emission.
| Topics | Galaxy Evolution & AGN |
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