[Paper Review] The ROSAT Deep Survey: VI. X-ray sources and Optical identifications of the Ultra Deep Survey

I. Lehmann, G. Hasinger+11 more|ArXiv.org|Mar 22, 2001

Astrophysical Phenomena and ObservationsPhysics and Astronomy43 references111 citations

TL;DR

This paper presents the ROSAT Ultra Deep Survey (UDS), a deep X-ray survey in the Lockman Hole reaching a flux limit of 1.2×10⁻¹⁵ erg cm⁻² s⁻¹ in the 0.5–2.0 keV band, 4.6 times fainter than the ROSAT Deep Survey. Using Keck spectroscopy, 90% of 94 X-ray sources were spectroscopically identified: 57 are type I AGNs, 13 are type II AGNs (likely obscured), 10 are groups/clusters of galaxies, and five are stars; three unclassified very red sources (R−K′ > 5.0) show photometric redshifts of 1.2–2.7, suggesting they may be high-luminosity, obscured type II QSOs.

ABSTRACT

We describe in this paper the ROSAT Ultra Deep Survey (UDS), an extension of the ROSAT Deep Survey (RDS) in the Lockman Hole. The UDS reaches a flux level of 1.2 x 10E-15 erg/cm2/s in 0.5-2.0 keV energy band, a level ~4.6 times fainter than the RDS. We present nearly complete spectroscopic identifications (90%) of the sample of 94 X-ray sources based on low-resolution Keck spectra. The majority of the sources (57) are broad emission line AGNs (type I), whereas a further 13 AGNs show only narrow emission lines or broad Balmer emission lines with a large Balmer decrement (type II AGNs) indicating significant optical absorption. The second most abundant class of objects (10) are groups and clusters of galaxies (~11%). Further we found five galactic stars and one ''normal'' emission line galaxy. Eight X-ray sources remain spectroscopically unidentified. The photometric redshift determination indicates in three out of the eight sources the presence of an obscured AGN in the range of 1.2 < z < 2.7. These objects could belong to the long-sought population of type 2 QSOs, which are predicted by the AGN synthesis models of the X-ray background. Finally, we discuss the optical and soft X-ray properties of the type I AGN, type II AGN, and groups and clusters of galaxies, and the implications to the X-ray backround.

Motivation & Objective

• To extend the ROSAT Deep Survey to fainter flux levels to resolve more of the soft X-ray background.
• To identify the nature of X-ray sources in the ultra-deep X-ray survey using optical and spectroscopic follow-up.
• To test whether the population of X-ray sources changes at fainter fluxes, particularly regarding obscured AGNs.
• To search for high-redshift, obscured type II QSOs predicted by AGN unification models.

Proposed method

• ROSAT PSPC and HRI observations were used to detect X-ray sources in the Lockman Hole down to 1.2×10⁻¹⁵ erg cm⁻² s⁻¹ in the 0.5–2.0 keV band.
• Optical imaging and photometry were performed using the Keck telescope, with follow-up low-resolution spectroscopy via the LRIS instrument.
• Photometric redshifts were derived for very red sources (R−K′ > 5.0) assuming stellar SEDs in the optical/near-infrared.
• Spectral energy distribution (SED) modeling was applied to identify obscured AGNs among spectroscopically unidentified sources.
• X-ray luminosities were calculated assuming standard cosmology (H₀ = 50 km s⁻¹ Mpc⁻¹, q₀ = 0.5, Λ = 0).
• Statistical comparison of X-ray and optical properties was used to classify sources into AGNs, clusters, stars, and emission-line galaxies.

Experimental results

Research questions

• RQ1 Are the X-ray source populations in the ultra-deep survey consistent with those in the deeper ROSAT Deep Survey?
• RQ2 Do the most highly obscured AGNs, such as high-luminosity type II QSOs, appear at these faint flux levels?
• RQ3 Can photometric redshifts of very red, spectroscopically unidentified sources reveal high-redshift obscured AGNs?
• RQ4 What is the contribution of groups and clusters of galaxies to the soft X-ray background at these flux levels?
• RQ5 Is there evidence for a change in the AGN luminosity function or obscuration fraction at the faintest flux levels?

Key findings

• Ninety percent of the 94 X-ray sources in the Ultra Deep Survey were spectroscopically identified, with 57 classified as type I AGNs.
• Thirteen sources are type II AGNs, showing narrow or broad Balmer lines with large Balmer decrements, indicating significant optical absorption.
• Ten sources are groups or clusters of galaxies, with X-ray luminosities spanning 41.5 < log L_X < 43.5 and redshifts from 0.20 to 1.26, including the most distant X-ray-selected cluster known at z = 1.263.
• Five sources are identified as galactic stars, and one is a normal emission-line galaxy.
• Three spectroscopically unidentified, very red sources (R−K′ > 5.0) have photometric redshifts of 1.22 to 2.71, suggesting they may be high-luminosity, obscured type II QSOs with L_X ≈ 10⁴⁴ erg s⁻¹.
• No significant change in source population was observed between the ROSAT Deep Survey and the Ultra Deep Survey, and the number of high-luminosity obscured AGNs remains lower than predicted by simple unification models.