A spectroscopic survey of X-ray-selected AGNs in the northern XMM-XXL field

作者：Menzel M L^*; Merloni A^*; Georgakakis A; Salvato M; Aubourg E; Brandt W N; Brusa M; Buchner J; Dwelly T; Nandra K; Paris I; Petitjean P; Schwope A

来源：Monthly Notices of the Royal Astronomical Society, 2016, 457(1): 110-132.

DOI：10.1093/mnras/stv2749

• techniques: spectroscopic
• galaxies: active
• quasars: emission lines
• galaxies: starburst
• infrared: galaxies
• X-rays: galaxies

This paper presents a survey of X-ray-selected active galactic nuclei (AGNs) with optical spectroscopic follow-up in a similar to 18 deg(2) area of the equatorial XMM-XXL north field. A sample of 8445 point-like X-ray sources detected by XMM-Newton above a limiting flux of F0.5-10 (keV) > 10(-15) erg cm(-2) s(-1) was matched to optical (Sloan Digital Sky Survey, SDSS) and infrared (IR; WISE) counterparts. We followed up 3042 sources brighter than r = 22.5 mag with the SDSS Baryon Oscillation Spectroscopic Survey (BOSS) spectrograph. The spectra yielded a reliable redshift measurement for 2578 AGNs in the redshift range z = 0.02-5.0, with 0.5-2 keV luminosities ranging from 10(39)-10(46) erg s(-1). This is currently the largest published spectroscopic sample of X-ray-selected AGNs in a contiguous area. The BOSS spectra of AGN candidates show a distribution of optical line widths which is clearly bimodal, allowing an efficient separation between broad-and narrow-emission line AGNs. The former dominate our sample (70 per cent) due to the relatively bright X-ray flux limit and the optical BOSS magnitude limit. We classify the narrow-emission line objects (22 per cent of the full sample) using standard optical emission line diagnostics: the majority have line ratios indicating the dominant source of ionization is the AGN. A small number (8 per cent of the full sample) exhibit the typical narrow line ratios of star-forming galaxies, or only have absorption lines in their spectra. We term the latter two classes 'elusive' AGN, which would not be easy to identify correctly without their X-ray emission. We also compare X-ray (XMM-Newton), optical colour (SDSS) and and IR (WISE) AGN selections in this field. X-ray observations reveal, by far, the largest number of AGN. The overlap between the selections, which is a strong function of the imaging depth in a given band, is also remarkably small. We show using spectral stacking that a large fraction of the X-ray AGNs would not be selectable via optical or IR colours due to host galaxy contamination. A substantial fraction of AGN may therefore be missed by these longer wavelength selection methods.

• 出版日期2016-3-21