The ROSAT North Ecliptic Pole Survey

Abstract

In the ROSAT All-Sky Survey, an 80.7 deg region around the north ecliptic pole (NEP) constitutes the deepest observation of the X-ray sky ever achieved with such a large, contiguous solid angle (W. Voges et al. 2001, ApJ, 553, L119). Here, 445 unique sources are detected with fluxes measured at greater than 4 j significance. The median and maximum exposure times are approximately 5 and 38 ks, respectively, and the minimum flux limit is ∼ ergs s cm 2 (0.5–2.0 keV). 14 1 2 # 10 We have identified the physical nature of 443 (99.6%) of the ROSAT NEP X-ray sources through a comprehensive program of imaging and spectroscopy primarily with the University of Hawaii 2.2 m, Canada-France-Hawaii 3.6 m, and Keck II 10 m telescopes (J. P. Henry et al. 2001, ApJ, 553, L109). The main constituents of the resulting catalog are 217 active galactic nuclei (AGNs; 49.0%), 152 stars (34.3%), and 64 groups and clusters of galaxies (14.4%). Minor components include eight BL Lacertae objects, one isolated elliptical galaxy, and one planetary nebula. We have obtained spectroscopic redshifts for all of the extragalactic objects. The median and maximum redshifts of the cluster sample are 0.205 and 0.811, whereas for the AGNs the corresponding values are 0.408 and 3.889. We have used the complete samples of extragalactic X-ray sources to examine the evolution and distribution of large-scale structure in the universe (I. M. Gioia et al. 2001, ApJ, 553, L105). The low-redshift X-ray luminosity function (XLF; 0.02 ! z ! ) of the NEP clusters is consistent with previous determi0.3 nations of the local XLF. Conversely, the high-redshift XLF ( ) shows significant deviations from the local XLF 0.3 ! z ! 0.85 at high luminosities. Similarly, a comparison of the number of observed clusters versus the number expected assuming noevolution models indicates a deficit of clusters at high redshift and high luminosity. Thus, the NEP data show negative evolution in the cluster population at ergs s and 44 1 L 1.5 # 10 z 1 X at greater than approximately 4.2 j. This result agrees with 0.3 the most simple form of hierarchical, bottom-up structure formation that predicts fewer luminous/massive objects in the past as compared to the present. Using the ROSAT NEP survey data, we have detected a supercluster at (C. R. Mullis et al. 2001, ApJ, 553, z p 0.087 L115). The X-ray data greatly improve our understanding of this supercluster’s characteristics, approximately doubling our knowledge of the structure’s spatial extent and tripling the cluster/group membership compared to the optical discovery data. The supercluster is a rich structure consisting of at least 21 galaxy clusters and groups, 12 AGNs, 61 IRAS galaxies, and various other objects. A majority of these components were discovered with the X-ray data, but the supercluster is also robustly detected in optical, IR, and UV wavebands. Extending ( Mpc), the 1 129 # 102 # 67 h50 NEP supercluster has a flattened shape oriented nearly edge-on to our line of sight. Owing to the softness of the ROSAT X-ray passband and the deep exposure over a large solid angle, we have detected for the first time a significant population of X-ray–emitting galaxy groups in a supercluster. Also relevant to large-scale structure, and in particular cluster formation, is the distant cluster RX J1716.6 6708 ( ), z p 0.81 which we discovered during the ROSAT NEP survey (J. P. Henry et al. 1997, AJ, 114, 1293; I. M. Gioia et al. 1999, AJ, 117, 2608). The member galaxies have a filamentary distribution, with the X-rays coming from the midpoint of the filament. The X-ray contours have an elongated shape that coincides roughly with weak gravitational lensing contours. This distant system of galaxies could be an example of a protocluster where matter is flowing along filaments and the X-ray flux is maximal at the impact point of the colliding streams of matter. Working beyond coherent structures, we have measured the degree of clustering in the ROSAT NEP data by calculating twopoint spatial correlation functions. Accounting for the effects of the NEP supercluster, the correlation function of galaxy clusters is consistent with previous measurements, albeit with large uncertainties. More importantly, our measurement of the spatial correlation function for X-ray–selected AGNs provides the best constraints (3.8 j) reported so far. The maximum likelihood determination of the power-law fit indicates best-fit values of Mpc and . If we set , the 2.5 1 1.40 r p 6.6 h g p 1.65 g p 1.8 0 5.4 1.07 correlation length is Mpc. These results suggest 1.5 1 r p 6.8 h 0 1.6 that X-ray–luminous AGNs are spatially clustered in a manner similar to that of normal galaxies and optically selected AGNs. Additional details concerning the ROSAT NEP survey can be found at http://www.ifa.hawaii.edu/∼mullis/nep.html.