اشترك بالحزمة الذهبية واحصل على وصول غير محدود شمرا أكاديميا
تسجيل مستخدم جديدThe HELLAS2XMM survey. V. Near-Infrared observations of X-ray sources with extreme X/O ratios
63
0
0.0
(
0
)
تأليف
M. Mignoli
اسأل ChatGPT حول البحث
ﻻ يوجد ملخص باللغة العربية
We present the results of deep near-infrared observations of 11 hard X-ray selected sources in the Hellas2XMM survey, with faint optical magnitude (R>24) and high X-ray-to-optical flux ratio. All but one of the sources have been detected in the Ks band, with bright counterparts (Ks<19) and red colors (R-K>5), and therefore belong to the ERO population. A detailed analysis of the surface brightness profiles allows us to classify all of the NIR counterparts. There are 2 point-like objects, 7 elliptical galaxies and one source with a disky profile. None of the extended sources shows any evidence for the presence of a central unresolved object tracing the putative X-ray emitting AGN. Using both the R-K colors and the morphological information, we have estimated for all the sources a ``minimum photometric redshift, ranging between 0.8 and 2.4; the elliptical hosts have zmin=0.9-1.4. We computed the X-ray properties using these redshifts: most of the sources have NH>10^{22}, with unabsorbed X-ray luminosities up to 10^{45}erg s^{-1}. These objects therefore belong to the population of obscured (Type II) quasars and, from a statistical point of view, they turn out to be a non-negligible fraction (~10%) of the most luminous AGN. Selecting the high X/O sources for a follow-up study in the NIR is therefore a powerful technique aimed at studying at high redshift the hosts of Type II AGN. Overall, our results seem to indicate that the hosts are mostly elliptical galaxies at z~1, and that these near-IR bright objects would be among the most massive spheroids at these epochs.
قيم البحث
اقرأ أيضاً
We describe a possible new class of X-ray sources that have robust detections in ultra-deep Chandra data, yet have no detections at all in our deep multi-band GOODS Hubble Space Telescope (HST) ACS images, which represent the highest quality optical
imaging obtained to date on these fields. These extreme X-ray / Optical ratio sources (EXOs) have values of Fx/Fopt at least an order of magnitude above those generally found for other AGN, even those that are harbored by reddened hosts. We thus infer two possible scenarios: (1) if these sources lie at redshifts z < 6, then their hosts need to be exceedingly underluminous, or more reddened, compared with other known sources; (2) if these sources lie above z ~ 6-7, such that even their Lyman-alpha emission is redshifted out of the bandpass of our ACS z(850) filter, then their optical and X-ray fluxes can be accounted for in terms of relatively normal L* hosts and moderate-luminosity AGN.
We present extensive optical, radio and infrared follow-up observations of a sample of 35 hard X-ray (2-10 keV) selected sources discovered serendipitously in the PV XMM-Newton observation of the radio-loud quasar PKS 0312-77 field, for which also an
archival Chandra observation is available. The observations have been carried out as part of the HELLAS2XMM survey, a program aimed to understand the nature of the sources responsible for the bulk of the hard X-ray Background (XRB). The identification of the optical counterparts greatly benefits from the positional accuracy obtained from Chandra and radio observations. As a consequence, the spectroscopic completeness of the present sample (80%) is limited only by the faintness of the optical counterparts. The multiwavelength coverage of our survey allows us to unveil a large spread in the overall properties of hard X-ray selected sources. At low redshift (z<1), the source breakdown includes Broad Line AGN, Narrow Emission-Line Galaxies, and optically ``normal galaxies. All the ten sources at z>1 are spectroscopically classified as Broad Line AGNs. A few of them show significant intrinsic X-ray absorption (N(H)>10^22 cm^-2), further supporting previous evidence of a decoupling between optical and X-ray properties at high luminosities and redshifts. Finally, a non negligible fraction (15%) of the hard X-ray sources are not detected down to the limiting magnitude of the optical images. The corresponding high X-ray to optical flux ratio, X-ray and optical-infrared colors strongly suggest that they are high redshift, obscured AGN.
Recent X-ray surveys have clearly demonstrated that a population of optically dull, X-ray bright galaxies is emerging at 2-10 keV fluxes of the order of 10^{-14} erg cm^{-2} s^{-1}. Although they might constitute an important fraction of the sources
responsible for the hard X-ray background, their nature is still unknown. With the aim to better understand the physical mechanisms responsible for the observed properties, we have started an extensive program of multiwavelength follow-up observations of hard X-ray, optically quiet galaxies discovered with XMM-Newton. Here we report the results of what can be considered the first example of this class of objects: CXOUJ031238.9-765134, originally discovered by Chandra, and optically identified by Fiore et al. (2000) with an apparently normal early-type galaxy at z=0.159, usually known as FIORE P3. The analysis of the broad-band energy distribution suggests the presence of a heavily obscured active nucleus.
Recent works have suggested that selection criteria based on MIR colors can be used to reveal a population of dust-enshrouded, extremely luminous quasars at z>1. However the X-ray spectral properties of these intriguing sources still remain largely u
nexplored. We report on an X-ray spectroscopic study of a sample of 44 very bright mid-IR galaxies with extreme mid-IR to optical flux ratios (MIR/O>2000). The X-ray coverage of the sample is highly inhomogeneous (from snap-shot 5 ks Chandra observations to medium-deep XMM exposures of 70 ks) and, consequently, a sizable fraction of them (~43%) remains undetected in the 0.5-10 keV band. The vast majority (95%) of the detected sources (23) show an absorption column density NH>10e22 cm-2 and, remarkably, we also find that 50% of them 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.
The Galactic Ridge X-ray Emission (GRXE) is apparently extended X-ray emission along the Galactic Plane. The X-ray spectrum is characterized by hard continuum with a strong Fe K emission feature in the 6-7 keV band. A substantial fraction (~80%) of t
he GRXE in the Fe band was resolved into point sources by deep Chandra imaging observations, thus GRXE is mostly composed of dim Galactic X-ray point sources at least in this energy band. To investigate the populations of these dim X-ray point sources, we carried out Near-Infrared (NIR) follow-up spectroscopic observations in two deep Chandra fields located in the Galactic plane at (l,b)=(0.1{arcdeg}, -1.4{arcdeg}) and (28.5{arcdeg}, 0.0{arcdeg}) using NTT/SofI and Subaru/MOIRCS. We obtained well-exposed NIR spectra from 65 objects and found that there are three main classes of Galactic sources based on the X-ray color and NIR spectral features: those having (A) hard X-ray spectra and NIR emission features such as HI(Br{gamma}), HeI, and HeII (2 objects), (B) soft X-ray spectra and NIR absorption features such as HI, NaI, CaI, and CO (46 objects), and (C) hard X-ray spectra and NIR absorption features such as HI, NaI, CaI and CO (17 objects). From these features, we argue that class A sources are Cataclysmic Variables (CVs), and class B sources are late-type stars with enhanced coronal activity, which is in agreement with current knowledge. Class C sources possibly belong to a new group of objects, which has been poorly studied so far. We argue that the candidate sources for class C are the binary systems hosting white dwarfs and late-type companions with very low accretion rates. It is likely that this newly recognized class of the sources contribute to a non-negligible fraction of the GRXE, especially in the Fe K band.
سجل دخول لتتمكن من نشر تعليقات
التعليقات
جاري جلب التعليقات
سجل دخول لتتمكن من متابعة معايير البحث التي قمت باختيارها
