IDCS J1426.5+3508: The Most Massive Galaxy Cluster at z>1.5z > 1.5

We present a deep (100 ks) Chandra observation of IDCS J1426.5+3508, a spectroscopically confirmed, infrared-selected galaxy cluster at $z = 1.75$. This cluster is the most massive galaxy cluster currently known at $z > 1.5$, based on existing Sunyaev-Zel'dovich (SZ) and gravitational lensing detections. We confirm this high mass via a variety of X-ray scaling relations, including $T_X$-M, $f_g$-M, $Y_X$-M and $L_X$-M, finding a tight distribution of masses from these different methods, spanning M$_{500}$ = 2.3-3.3 $\times 10^{14}$ M$_{\odot}$, with the low-scatter $Y_X$-based mass $M_{500,Y_X} = 2.6^{+1.5}_{-0.5} \times 10^{14}$ M$_\odot$. IDCS J1426.5+3508 is currently the only cluster at $z > 1.5$ for which X-ray, SZ and gravitational lensing mass estimates exist, and these are in remarkably good agreement. We find a relatively tight distribution of the gas-to-total mass ratio, employing total masses from all of the aforementioned indicators, with values ranging from $f_{gas,500}$ = 0.087-0.12. We do not detect metals in the intracluster medium (ICM) of this system, placing a 2$\sigma$ upper limit of $Z(r < R_{500}) < 0.18 Z_{\odot}$. This upper limit on the metallicity suggests that this system may still be in the process of enriching its ICM. The cluster has a dense, low-entropy core, offset by $\sim$30 kpc from the X-ray centroid, which makes it one of the few "cool core" clusters discovered at $z > 1$, and the first known cool core cluster at $z > 1.2$. The offset of this core from the large-scale centroid suggests that this cluster has had a relatively recent ($\lesssim$500 Myr) merger/interaction with another massive system.Comment: Minor changes to match accepted version, results unchanged; ApJ in pres

Spectroscopic survey of faint planetary-nebula nuclei. I. Six new "O VI" central stars

We report initial results from an ongoing spectroscopic survey of central stars of faint planetary nebulae (PNe), obtained with the Low-Resolution Spectrograph on the Hobby-Eberly Telescope. The six PN nuclei (PNNi) discussed here all have strong emission at the O VI 3811-3834 A doublet, indicative of very high temperatures. Five of them--the nuclei of Ou 2, Kn 61, Kn 15, Abell 72, and Kn 130--belong to the hydrogen-deficient PG 1159 class, showing a strong absorption feature of He II and C IV at 4650-4690 A. Based on exploratory comparisons with synthetic model-atmosphere spectra, and the presence of Ne VIII emission lines, we estimate them to have effective temperatures of order 170,000 K. The central star of Kn 15 has a Wolf-Rayet-like spectrum, with strong and broad emission lines of He II, C IV, N V, and O V-VI. We classify it [WO2], but we note that the N V 4604-4620 A emission doublet is extremely strong, indicating a relatively high nitrogen abundance. Several of the emission lines in Kn 15 vary in equivalent width by factors as large as 1.5 among our four observations from 2019 to 2022, implying significant variations in the stellar mass-loss rate. We encourage spectroscopic monitoring. Follow-up high-time-resolution photometry of these stars would be of interest, given the large fraction of pulsating variables seen among PG 1159 and [WO] PNNi.Comment: Accepted by MNRA