Chandra and optical/IR observations of CXO J1415.2+3610, a massive, newly discovered galaxy cluster at z ~ 1.5

¹ INAF-Osservatorio Astronomico di Trieste, via Tiepolo 11, 34133 Trieste, Italy
² INFN-National Institute for Nuclear Physics, via Valerio 2, 34127 Trieste, Italy
e-mail: tozzi@oats.inaf.it
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

Received: 26 October 2012
Accepted: 12 December 2012

Abstract

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 (JK_s), 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 K_α line complex, from which we obtain z_X = 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 z_phot = 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 Z_Fe = 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.