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

Abstract

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.

arXiv comment14 pages, 11 figures, to appear in A&A

References in corpus (37)

• The Host Galaxies and Classification of Active Galactic Nuclei
• Cosmic-Ray Rejection by Laplacian Edge Detection
• The Chandra Deep Field-North Survey. XIII. 2 Ms Point-Source Catalogs
• The Chandra Deep Field South: the 1 Million Second
• The Chandra Deep Field South: Optical Spectroscopy I
• A catalogue of the Chandra Deep Field South with multi-colour classification and photometric redshifts from COMBO-17
• The FIRST Bright Quasar Survey. II. 60 Nights and 1200 Spectra Later
• The GOODS-MUSIC sample: a multicolour catalog of near-IR selected galaxies in the GOODS-South field
• The VIMOS VLT Deep Survey: Public release of 1599 redshifts to IAB<=24 across the Chandra Deep Field South
• The Extended Chandra Deep Field-South Survey. Chandra Point-Source Catalogs
• The Chandra Deep Field-South Survey: 2 Ms Source Catalogs
• The evolution of faint AGN between z~1 and z~5 from the COMBO-17 survey
• Deep ATLAS radio observations of the CDFS-SWIRE field
• The Murmur of the Sleeping Black Hole: Detection of Nuclear Ultraviolet Variability in LINER Galaxies
• The HELLAS2XMM survey: II. Multiwavelength observations of P3: an X-ray bright, optically inactive galaxy
• ESO Imaging Survey. Deep Public Survey: Multi-Color Optical Data for the Chandra Deep Field South
• Supernova rates from the Southern inTermediate Redshift ESO Supernova Search (STRESS)
• 531 new spectroscopic redshifts from the CDFS and a test on the cosmological relevance of the GOODS-South field
• Are Optically-Selected Quasars Universally X-Ray Luminous? X-Ray/UV Relations in Sloan Digital Sky Survey Quasars
• Variability-Selected Quasars in MACHO Project Magellanic Cloud Fields
• The VLA Survey of the Chandra Deep Field South: I. Overview of the Radio Data
• The Ensemble Variability Properties of Faint QSOs
• Spectroscopic identification of ten faint hard X-ray sources discovered by Chandra
• Death rate of massive stars at redshift ~0.3
• Chandra-SDSS Normal and Star-Forming Galaxies I: X-ray Source Properties of Galaxies Detected by Chandra in SDSS DR2
• ESO Imaging Survey. The Stellar Catalogue in the Chandra Deep Field South
• Variability-selected active galactic nuclei from supernova search in the Chandra deep field south
• A V-Band Survey for Variable Galactic Nuclei in the Hubble Deep Field
• Variable Extended Objects in SA 57
• The DEEP Groth Strip Survey VI. Spectroscopic, Variability, and X-ray Detection of AGN
• The Properties and Redshift Evolution of Intermediate-Luminosity Off-Nuclear X-ray Sources in the Chandra Deep Fields
• The Subaru/XMM-Newton Deep Survey (SXDS) - V. Optically Faint Variable Object Survey
• Optical Variability of Infrared Power Law-Selected Galaxies & X-ray Sources in the GOODS-South Field
• The Subaru/XMM-Newton Deep Survey (SXDS) - VI. Properties of Active Galactic Nuclei Selected by Optical Variability
• Redshifts in the Southern Abell Redshift Survey Clusters. I. The Data
• An X-ray Survey in SA 57 with XMM-Newton
• Optical Spectroscopy of Active Galactic Nuclei in SA57

Cited by in corpus (14)

• Cosmic X-ray Surveys of Distant Active Galaxies: The Demographics, Physics, and Ecology of Growing Supermassive Black Holes
• An ALMA survey of submillimetre galaxies in the Extended Chandra Deep Field South: Spectroscopic redshifts
• Quasar Selection Based on Photometric Variability
• Variability Selected Low-Luminosity Active Galactic Nuclei in the 4 Ms Chandra Deep Field-South
• The Chandra Deep Fields: Lifting the Veil on Distant Active Galactic Nuclei and X-Ray Emitting Galaxies
• Variability-selected Active Galactic Nuclei in the VST-SUDARE/VOICE Survey of the COSMOS Field
• Variability-based AGN selection using image subtraction in the SDSS and LSST era
• Optically variable active galactic nuclei in the 3 yr VST survey of the COSMOS field
• SUDARE-VOICE variability-selection of Active Galaxies in the Chandra Deep Field South and the SERVS/SWIRE region
• On the UV/optical variation in NGC 5548: new evidence against the reprocessing diagram
• The Cosmic History of Hot Gas Cooling and Radio AGN Activity in Massive Early-Type Galaxies
• Continuum optical-UV and X-ray variability of AGN: current results and future challenges
• The XMM Deep Survey in the CDF-S VII. UV catalogue of the Optical Monitor observations
• The relationship of SMBHs and host galaxies at $z<4$ in the deep optical variability-selected AGN sample in the COSMOS field