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Infrared-faint radio sources (IFRS) form a new class of galaxiescharacterised by radio flux densities between tenths and tens of mJy and faintor absent infrared counterparts. It has been suggested that these objects areradio-loud active galactic nuclei (AGNs) at significant redshifts (z >~ 2).Whereas the high redshifts of IFRS have been recently confirmed based onspectroscopic data, the evidence for the presence of AGNs in IFRS is mainlyindirect. So far, only two AGNs have been unquestionably confirmed in IFRSbased on very long baseline interferometry (VLBI) observations. In this work,we test the hypothesis that IFRS contain AGNs in a large sample of sourcesusing VLBI. We observed 57 IFRS with the Very Long Baseline Array (VLBA) downto a detection sensitivity in the sub-mJy regime and detected compact cores in35 sources. Our VLBA detections increase the number of VLBI-detected IFRS from2 to 37 and provide strong evidence that most - if not all - IFRS contain AGNs.We find that IFRS have a marginally higher VLBI detection fraction thanrandomly selected sources with mJy flux densities at arcsec-scales. Moreover,our data provide a positive correlation between compactness - defined as theratio of milliarcsec- to arcsec-scale flux density - and redshift for IFRS, butsuggest a decreasing mean compactness with increasing arcsec-scale radio fluxdensity. Based on these findings, we suggest that IFRS tend to contain youngAGNs whose jets have not formed yet or have not expanded, equivalent to verycompact objects. We found two IFRS that are resolved into two components. Thetwo components are spatially separated by a few hundred milliarcseconds in bothcases. They might be components of one AGN, a binary black hole, or the resultof gravitational lensing.

Active galactic nuclei cores in infrared-faint radio sources: Very long baseline interferometry observations using the Very Long Baseline Array