SIMBAD references

A complete demographic of active galactic nuclei (AGNs) is essential to understand the evolution of the Universe. Optical surveys estimate the population of AGNs in the local Universe to be of ∼ 4 per cent. However, these results could be biased towards bright sources, not affected by the host galaxy attenuation. An alternative method for detecting these objects is through the X-ray emission. In this work, we aim to complement the AGN population of the optical CALIFA survey (941 sources), by using X-ray data from Chandra, which provides the best spatial resolution to date, essential to isolate the nuclear emission from the host galaxy. We study a total of 138 sources with available data. We find 34 new bona fide AGNs and 23 AGN candidates, which could increase the AGN population to 7-10 per cent among the CALIFA survey. X-rays are particularly useful for low-luminosity AGNs since they are excluded by the criterion of large equivalent width of the $\rm {H\alpha }$ emission line when applied to optical selections. Indeed, placing such a restrictive criteria might cause a loss of up to 70 per cent of AGN sources. X-ray detected sources are preferentially located in the right side of the [${\mathrm{O}}\, {\small {\rm III}}$ ]/Hβ versus [${\mathrm{N}}\, {\small {\rm II}}$ ]/Hα diagram, suggesting that this diagram might be the most reliable at classifying AGN sources. Our results support the idea that multiwavelength studies are the best way to obtain a complete AGN population.