Revealing X-ray obscured quasars in SWIRE sources with extreme mid-IR/optical flux ratios

Abstract

Recent works have suggested that selection criteria based on mid-IR properties, i.e. extreme colors and bright flux levels, can be used to reveal a population of dust-enshrouded, extremely-luminous quasars at z ∼ 1-2. However, the X-ray spectral properties of these intriguing objects still remain largely unexplored. We have performed an X-ray study of a large sample of bright mid-IR (F 24 μm > 1.3 mJy) galaxies showing an extreme MIR/Optical flux ratio (F 24 μm /F R > 2000) in order to confirm the presence of a luminous active nucleus in these very red objects. Sampling of a large area is required to pick up objects at the highest luminosities given their low surface density. Accordingly, we have applied our selection criteria to an area of ∼6 deg 2 covered by XMM-Newton/Chandra observations within the ∼50 deg 2 SWIRE survey, resulting in a final sample of 44 objects. The vast majority of the source redshifts, both spectroscopic and photometric, are in the range 0.7 < ∼ z < ∼ 2.5. The X-ray coverage of the sample is highly inhomogeneous (from snap-shot 5 ks Chandra observations to medium-deep XMM-Newton exposures of 70 ks) and, consequently, a sizable fraction of them (≈43%) remains undetected in the 0.5-10 keV band. Using spectral or hardness information we were able to estimate the value of the absorbing column density in 23 sources. 95% of them are consistent with being obscured by neutral gas with an intrinsic column density of N H ≥ 10 22 cm −2 . Remarkably, we also find that ∼55% of these sources can be classified as type 2 quasars on the basis of their absorption properties and X-ray luminosity. Moreover, most of the X-ray undetected sources show extreme mid-IR colors, consistent with being luminous AGN-powered objects, suggesting they might host heavily obscured (possibly Compton-thick) quasars in X-rays. This demonstrates that our selection criteria applied to a wide area survey is very efficient in finding a large number of type 2 quasars at z > ∼ 1. The existence of this class of very powerful, obscured quasars at high z could have important implications in the context of the formation and cosmological evolution of accreting supermassive black holes and their host galaxies.

X-ray observations of highly obscuredτ9.7μm > 1 sources: an efficient method for selecting Compton-thick AGN?

“…However, because of the lack of X-ray spectroscopy the fraction of Compton-thick sources cannot be readily quantified. Based on brighter sources with X-ray spectroscopy in the GOODS (Georgantopoulos et al 2011), AEGIS (Georgakakis et al 2010) and SWIRE fields (Lanzuisi et al 2009), the fractions of Compton-thick sources found among IR-excess AGN range between 0% and 50%. Another widely applied mid-IR method for the selection of Compton-thick AGN ) relies on low X-ray to IR ratios.…”

X-ray detected infrared excess AGN in theChandradeep fields: a moderate fraction ofCompton-thick sources

“…The columns are: (1) X-ray ID as in Table 1; (2) HST ID from the catalogue of Giavalisco et al (2004) and Grazian et al (2006) for the CDF-N and CDF-S respectively; (3) HST coordinates (J2000); (4) z-band AB magnitude (F850LP) (5) comment on morphology. Lanzuisi et al (2009) have recently performed an X-ray study of luminous DOGs (F 24 μm /F R > 2000 and F 24 μm > 1.3 mJy) in the Spitzer Wide-Area Infrared Extragalactic (SWIRE) survey. An area of 6deg 2 out of the total 50 deg 2 of the SWIRE survey has been surveyed by XMM-Newton or Chandra.…”

Exploring the active galactic nuclei population with extreme X-ray-to-optical flux ratios (fx/fo > 50)

“…It is also worth noting that, among the EXO50 sources defined here, about 40% can be classified as EDOGs (∼ 60% excluding the BL Lac objects). Unfortunately, two of our EXO50 EDOGs are still unidentified, preventing us to make a proper comparison with the work of Lanzuisi et al (2009). We can only say that the absorption properties of the only identified EXO50 EDOG (source number 5) are fully consistent with the main results obtained from Lanzuisi et al (2009): the source is an absorbed QSO with an intrinsic absorbing column density ∼ 3×10 22 cm −2 .…”

“…These 3 sources can be classified as Extreme Dust Obscured Galaxies (EDOGs; f 24 µm / f R 2000; Lanzuisi et al 2009), i.e. extreme examples of Dust Obscured Galaxies (DOG, f 24 µm / f R 1000 see Dey et al 2008), a class of galaxies firstly discovered with the Spitzer Space Telescope.…”

“…A sample of 44 EDOGs from the SWIRE survey having X-ray coverage (with XMM-Newton or Chandra) has been selected and studied by Lanzuisi et al (2009); the source redshifts (both spectroscopic and photometric), are in the range between ∼0.7 and ∼2.5. About 95% of the detected sources 9 (23 objects) are consistent with being obscured by neutral gas with an intrinsic column density (mostly derived using X-ray hardness ratios) typically in the range between 10 22 to few times 10 23 cm −2 .…”