Spectroscopic follow-up of variability-selected active galactic nuclei in the Chandra Deep Field South

Luminous AGNs are usually selected by their non-stellar colours or their X-ray emission. Colour selection cannot be used to select low-luminosity AGNs, since their emission is dominated by the host galaxy. Objects with low X-ray to optical ratio escape even the deepest X-ray surveys performed so far. In a previous study we presented a sample of candidates selected through optical variability in the Chandra Deep Field South, where repeated optical observations were performed for the STRESS supernova survey. We obtained new optical spectroscopy for a sample of variability selected candidates with the ESO NTT telescope. We analysed the new spectra, together with those existing in the literature and studied the distribution of the objects in U-B and B-V colours, optical and X-ray luminosity, and variability amplitude. A large fraction (17/27) of the observed candidates are broad-line luminous AGNs, confirming the efficiency of variability in detecting quasars. We detect: i) extended objects which would have escaped the colour selection and ii) objects of very low X-ray to optical ratio. Several objects resulted to be narrow-emission line galaxies where variability indicates nuclear activity, while no emission lines were detected in others. Some of these galaxies have variability and X-ray to optical ratio close to active galactic nuclei, while others have much lower variability and X-ray to optical ratio. This result can be explained by the dilution of the nuclear light due to the host galaxy. Our results demonstrate the effectiveness of supernova search programmes to detect large samples of low-luminosity AGNs. A sizable fraction of the AGN in our variability sample had escaped X-ray detection (5/47) and/or colour selection (9/48). Spectroscopic follow-up to fainter flux limits is strongly encouraged.

💡 Research Summary

Active galactic nuclei (AGN) are traditionally identified either by their non‑stellar colours (e.g., UV‑excess) or by their X‑ray emission. Both techniques work well for luminous quasars, but they become inefficient for low‑luminosity AGN whose optical light is dominated by the host galaxy and whose X‑ray‑to‑optical flux ratios (fX/fopt) are low enough to evade even the deepest X‑ray surveys. In a previous work the authors exploited the multi‑epoch imaging obtained for the STRESS super‑nova search in the Chandra Deep Field South (CDFS) to select variability‑selected AGN candidates. This paper presents the spectroscopic follow‑up of a subsample of those candidates using the ESO New Technology Telescope (NTT) and combines the new data with spectra already available in the literature.

Sample selection and observations
From the STRESS data set, 48 objects were flagged as variable based on a statistically significant excess in their light‑curve dispersion (σ/⟨mag⟩). The variability threshold was chosen to minimise contamination from normal galaxies while retaining a high completeness for AGN. Twenty‑seven of the 48 candidates were observed with the EFOSC2 spectrograph on the 3.5 m NTT (R≈500, 4000–9000 Å). Standard stars were used for flux calibration and atmospheric correction. In addition, previously published spectra (e.g., Szokoly et al. 2004, Vanzella et al. 2005) were gathered, yielding a total spectroscopic coverage of 47–48 variability‑selected sources.

Spectral classification
The spectra were examined for broad emission lines (e.g., Hβ, Mg II λ2800, C IV λ1549) indicative of type‑1 (broad‑line) AGN, and for narrow lines such as