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  • Haines, C. P.; Finoguenov, A.; Smith, G. P.; Babul, A.; Egami, E.; Mazzotta, P.; Okabe, N.; Pereira, M. J.; Bianconi, M.; Mcgee, S. L.; Ziparo, F.; Campusano, L. E.; Loyola, C. (2018)
    Galaxy clusters are expected to form hierarchically in a Lambda cold dark matter (Lambda CDM) universe, growing primarily through mergers with lower mass clusters and the continual accretion of group-mass haloes. Galaxy clusters assemble late, doubling their masses since z similar to 0.5, and so the outer regions of clusters should be replete with accreting group-mass systems. We present an XMM-Newton survey to search for X-ray groups in the infall regions of 23 massive galaxy clusters (<M-200 > similar to 10(15)M(circle dot)) at z similar to 0.2, identifying 39 X-ray groups that have been spectroscopically confirmed to lie at the cluster redshift. These groups have mass estimates in the range 2 x 10(13)-7 x 10(14)M(circle dot), and group-to-cluster mass ratios as low as 0.02. The comoving number density of X-ray groups in the infall regions is similar to 25x higher than that seen for isolated X-ray groups from the XXL survey. The average mass per cluster contained within these X-ray groups is 2.2 x 10(14)M(circle dot), or 19 +/- 5 per cent of the mass within the primary cluster itself. We estimate that similar to 10(15)M(circle dot) clusters increase their masses by 16 +/- 4 per cent between z = 0.223 and the present day due to the accretion of groups with M-200 >= 10(13.2)M(circle dot). This represents about half of the expected mass growth rate of clusters at these late epochs. The other half is likely to come from smooth accretion of matter not bound within haloes. The mass function of the infalling X-ray groups appears significantly top heavy with respect to that of 'field' X-ray systems, consistent with expectations from numerical simulations, and the basic consequences of collapsed massive dark matter haloes being biased tracers of the underlying large-scale density distribution.
  • Marchesi, S.; Lanzuisi, G.; Civano, F.; Iwasawa, K.; Suh, H.; Comastri, A.; Zamorani, G.; Allevato, V.; Griffiths, R.; Miyaji, T.; Ranalli, P.; Salvato, M.; Schawinski, K.; Silverman, J.; Treister, E.; Urry, C. M.; Vignali, C. (2016)
    We present the X-ray spectral analysis of the 1855 extragalactic sources in the Chandra COSMOS-Legacy survey catalog having more than 30 net counts in the 0.5-7 keV band. A total of 38% of the sources are optically classified type 1 active galactic nuclei (AGNs), 60% are type 2 AGNs, and 2% are passive, low-redshift galaxies. We study the distribution of AGN photon index Gamma and of the intrinsic absorption N-H,N-z based on the sources' optical classification: type 1 AGNs have a slightly steeper mean photon index Gamma than type 2 AGNs, which, on the other hand, have average N-H,N-z similar to 3 times higher than type 1 AGNs. We find that similar to 15% of type 1 AGNs have N-H,N-z > 10(22) cm(-2), i.e., are obscured according to the X-ray spectral fitting; the vast majority of these sources have L2-10 (keV) > 10(44) erg s(-1). The existence of these objects suggests that optical and X-ray obscuration can be caused by different phenomena, the X-ray obscuration being, for example, caused by dust-free material surrounding the inner part of the nuclei. Approximately 18% of type 2 AGNs have N-H,N-z <10(22) cm(-2), and most of these sources have low X-ray luminosities (L2-10 (keV) <10(43) erg s(-1)). We expect a part of these sources to be low-accretion, unobscured AGNs lacking broad emission lines. Finally, we also find a direct proportional trend between N-H,N-z and host-galaxy mass and star formation rate, although part of this trend is due to a redshift selection effect.
  • LaMassa, Stephanie M.; Glikman, Eilat; Brusa, Marcella; Rigby, Jane R.; Ananna, Tonima Tasnim; Stern, Daniel; Lira, Paulina; Urry, C. Megan; Salvato, Mara; Alexandroff, Rachael; Allevato, Viola; Cardamone, Carolin; Civano, Francesca; Coppi, Paolo; Farrah, Duncan; Komossa, S.; Lanzuisi, Giorgio; Marchesi, Stefano; Richards, Gordon; Trakhtenbrot, Benny; Treister, Ezequiel (2017)
    We present results of a ground-based near-infrared campaign with Palomar TripleSpec, Keck NIRSPEC, and Gemini GNIRS to target two samples of reddened active galactic nucleus (AGN) candidates from the 31 deg(2) Stripe 82 X-ray survey. One sample, which is similar to 89% complete to K <16 (Vega), consists of eight confirmed AGNs, four of which were identified with our follow-up program, and is selected to have red R - K colors (> 4, Vega). The fainter sample (K > 17, Vega) represents a pilot program to follow-up four sources from a parent sample of 34 that are not detected in the single-epoch SDSS catalog and have WISE quasar colors. All 12 sources are broad-line AGNs (at least one permitted emission line has an FWHM exceeding 1300 km s(-1)) and span a redshift range 0.59 <z <2.5. Half the (R - K)-selected AGNs have features in their spectra suggestive of outflows. When comparing these sources to a matched sample of blue Type 1 AGNs, we find that the reddened AGNs are more distant (z > 0.5), and a greater percentage have high X-ray luminosities (L-X,L- full > 10(44) erg s(-1)). Such outflows and high luminosities may be consistent with the paradigm that reddened broad-line AGNs represent a transitory phase in AGN evolution as described by the major merger model for black hole growth. Results from our pilot program demonstrate proof of concept that our selection technique is successful in discovering reddened quasars at z > 1 missed by optical surveys.