Chandra and optical/IR observations of CXO J1415.2+3610, a massive, newly discovered galaxy cluster at z ~ 1.5
INAF-Osservatorio Astronomico di Trieste, via Tiepolo 11,
2 INFN-National Institute for Nuclear Physics, via Valerio 2, 34127 Trieste, Italy
3 European Space Astronomy Centre (ESAC)/ESA, Madrid, Spain
4 European Southern Observatory, Karl-Schwarzschild Strasse 2, 85748 Garching, Germany
5 Università di Trieste, Dipartimento di Fisica, via Valerio, 2, 34127 Trieste, Italy
6 University Observatory, Ludwig-Maximillians University Munich, Scheinerstrasse 1, 81679 Munich, Germany
7 Excellence Cluster Universe, Boltzmannstrasse 2, 85748 Garching, Germany
Accepted: 12 December 2012
Aims. We report the discovery of CXO J1415.2+3610, a distant (z ~ 1.5) galaxy cluster serendipitously detected as an extended source with a very high significance level (S/N ~ 11) in a deep, high-resolution Chandra observation targeted to study the cluster WARP J1415.1+3612 at z = 1.03. This is the highest-z cluster discovered with Chandra so far. Moreover, the total exposure time of 280 ks with ACIS-S provides the deepest X-ray observation currently achieved on a cluster at z ≥ 1.5.
Methods. We perform an X-ray spectral fit of the extended emission of the intracluster medium (ICM) with Xspec assuming a single-temperature thermal mekal model. We use optical and infrared (IR) observations from Subaru-Suprime (BVRiz), Moircs (JKs), and Spitzer-IRAC (3.6 μm) to confirm the presence of an overdensity of red galaxies matching the X-ray extended emission. We use optical and IR data to investigate the color−magnitude relation of the candidate member galaxies.
Results. From a preliminary X-ray spectral analysis, we detect at a 99.5% confidence level the rest frame 6.7−6.9 keV Iron Kα line complex, from which we obtain zX = 1.46 ± 0.025. Our X-ray redshift measurement is supported by the optical and IR data. The analysis of the z − 3.6 μm color−magnitude diagram shows a well-defined sequence of red galaxies within 1′ from the cluster X-ray emission peak with a color range [5 < z − 3.6 μm < 6]. The photometric redshift obtained by spectral energy distribution fitting is zphot = 1.52 ± 0.06. After fixing the redshift to z = 1.46, we perform the final spectral analysis and measure the average gas temperature with a 20% error, kT = 5.8-1.0+1.2 keV, and the Fe abundance ZFe = 1.3-0.5+0.8 Z⊙. We fit the background-subtracted surface brightness with a single beta-model out to 35 arcsec (the maximum radius where the X-ray emission is detected), and derive the deprojected electron density profile. The ICM mass is 1.09-0.2+0.3 × 1013 M⊙ within 300 kpc. Under the assumption of hydrostatic equilibrium, the total mass is M2500 = 8.6-1.7+2.1 × 1013 M⊙ for R2500 = (220 ± 55) kpc. Extrapolating the profile at larger radii, we find M500 = 2.1-0.5+0.7 × 1014 M⊙ for R500 = 510-50+55 kpc. This analysis establishes CXOJ1415.2+3610 as one of the best characterized distant galaxy clusters based on X-ray data alone.
Key words: galaxies: clusters: individual: CXO J1415.2+3610 / galaxies: clusters: intracluster medium / X-rays: galaxies: clusters
© ESO, 2013