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

M. L. Menzel, A. Merloni, A. Georgakakis, M. Salvato, E. Aubourg, W. N. Brandt, M. Brusa, J. Buchner, T. Dwelly, K. Nandra, I. Pâris, P. Petitjean, A. Schwope

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Abstract

This paper presents a survey of X-ray-selected active galactic nuclei (AGNs) with optical spectroscopic follow-up in a ~18 deg2 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=22.5mag 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 1039-1046 erg s-1. This is currently the largest published spectroscopic sample of X-ray-selectedAGNs in a contiguous area. The BOSS spectra ofAGN 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.

Original languageEnglish (US)
Pages (from-to)110-132
Number of pages23
JournalMonthly Notices of the Royal Astronomical Society
Volume457
Issue number1
DOIs
StatePublished - Jan 11 2016

Fingerprint

XMM-Newton telescope
active galactic nuclei
x rays
Wide-field Infrared Survey Explorer
baryons
oscillation
erg
newton
oscillations
galaxies
color
stacking
ionization
spectrographs
light emission
contamination
wavelength
luminosity
stars
wavelengths

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science

Cite this

Menzel, M. L. ; Merloni, A. ; Georgakakis, A. ; Salvato, M. ; Aubourg, E. ; Brandt, W. N. ; Brusa, M. ; Buchner, J. ; Dwelly, T. ; Nandra, K. ; Pâris, I. ; Petitjean, P. ; Schwope, A. / A spectroscopic survey of X-ray-selected AGNs in the northern XMM-XXL field. In: Monthly Notices of the Royal Astronomical Society. 2016 ; Vol. 457, No. 1. pp. 110-132.
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abstract = "This paper presents a survey of X-ray-selected active galactic nuclei (AGNs) with optical spectroscopic follow-up in a ~18 deg2 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=22.5mag 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 1039-1046 erg s-1. This is currently the largest published spectroscopic sample of X-ray-selectedAGNs in a contiguous area. The BOSS spectra ofAGN 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.",
author = "Menzel, {M. L.} and A. Merloni and A. Georgakakis and M. Salvato and E. Aubourg and Brandt, {W. N.} and M. Brusa and J. Buchner and T. Dwelly and K. Nandra and I. P{\^a}ris and P. Petitjean and A. Schwope",
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Menzel, ML, Merloni, A, Georgakakis, A, Salvato, M, Aubourg, E, Brandt, WN, Brusa, M, Buchner, J, Dwelly, T, Nandra, K, Pâris, I, Petitjean, P & Schwope, A 2016, 'A spectroscopic survey of X-ray-selected AGNs in the northern XMM-XXL field', Monthly Notices of the Royal Astronomical Society, vol. 457, no. 1, pp. 110-132. https://doi.org/10.1093/mnras/stv2749

A spectroscopic survey of X-ray-selected AGNs in the northern XMM-XXL field. / Menzel, M. L.; Merloni, A.; Georgakakis, A.; Salvato, M.; Aubourg, E.; Brandt, W. N.; Brusa, M.; Buchner, J.; Dwelly, T.; Nandra, K.; Pâris, I.; Petitjean, P.; Schwope, A.

In: Monthly Notices of the Royal Astronomical Society, Vol. 457, No. 1, 11.01.2016, p. 110-132.

Research output: Contribution to journalArticle

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T1 - A spectroscopic survey of X-ray-selected AGNs in the northern XMM-XXL field

AU - Menzel, M. L.

AU - Merloni, A.

AU - Georgakakis, A.

AU - Salvato, M.

AU - Aubourg, E.

AU - Brandt, W. N.

AU - Brusa, M.

AU - Buchner, J.

AU - Dwelly, T.

AU - Nandra, K.

AU - Pâris, I.

AU - Petitjean, P.

AU - Schwope, A.

PY - 2016/1/11

Y1 - 2016/1/11

N2 - This paper presents a survey of X-ray-selected active galactic nuclei (AGNs) with optical spectroscopic follow-up in a ~18 deg2 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=22.5mag 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 1039-1046 erg s-1. This is currently the largest published spectroscopic sample of X-ray-selectedAGNs in a contiguous area. The BOSS spectra ofAGN 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.

AB - This paper presents a survey of X-ray-selected active galactic nuclei (AGNs) with optical spectroscopic follow-up in a ~18 deg2 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=22.5mag 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 1039-1046 erg s-1. This is currently the largest published spectroscopic sample of X-ray-selectedAGNs in a contiguous area. The BOSS spectra ofAGN 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.

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