publications

2025

1. ApJ

   

   Deep Chandra X-Ray Observations of A2029: The Merger History of a Relaxed, Strong Cool Core Cluster

   Courtney B. Watson, Elizabeth L. Blanton, Scott W. Randall, and 2 more authors

   The Astrophysical Journal, 2025

   Abs DOI Bib HTML

   We present results from very deep (485 ks) Chandra X-ray observations of the relaxed, cool core cluster A2029 (z = 0.0767). A2029 hosts one of the longest, most continuous sloshing spirals ever observed, which we find extends nearly 600 kpc from the cluster core. In addition to providing detailed views of the sloshing spiral, imaging and spectroscopic analysis reveals intracluster medium (ICM) substructure related to the merger history, including a broad "splash" of cooler gas and a potential merger shock. The radio lobes of the central WAT source show evidence of alignment with the sloshing motions, consistent with ICM bulk flow, rather than host-galaxy motion, as the primary driver of lobe bending. Comparison to a 1:10 mass-ratio off-axis merger simulation indicates that the observed ICM structures are relics of a second core passage of a subcluster ∼4 Gyr after the start of the merger, where the "splash" feature is revealed to be a wake of cool gas trailing behind the subcluster. Overall, our results suggest that A2029 is still settling from past interactions—showing that even the initially most relaxed-looking clusters can be hiding a rich history of dynamical activity.

   @article{Watson2025b,
     author = {Watson, Courtney B. and Blanton, Elizabeth L. and Randall, Scott W. and Clarke, Tracy E. and ZuHone, John A.},
     doi = {10.3847/1538-4357/ae2026},
     journal = {The Astrophysical Journal},
     pages = {106},
     title = {Deep Chandra X-Ray Observations of A2029: The Merger History of a Relaxed, Strong Cool Core Cluster},
     url = {https://doi.org/10.3847/1538-4357/ae2026},
     volume = {996},
     year = {2025},
     dimensions = {true},
   }

2. ApJ

   

   HST Grism Observations of a z ∼ 1 . 8 Cluster Candidate from the Clusters Occupied by Bent Radio AGN ( COBRA ) Survey

   Courtney B Watson, Elizabeth L Blanton, Emmet Golden-marx, and 2 more authors

   The Astrophysical Journal, 2025

   Abs DOI Bib HTML

   We present new Hubble Space Telescope/Wide Field Camera 3 G141 grism observations for COBRA 1411+3415, originally identified as a high-redshift cluster candidate in the Clusters Occupied by Bent Radio AGN (COBRA) survey using radio, infrared, and optical data. We spectroscopically identify seven cluster members within a 0.5 Mpc radius with grism redshifts in the range 1.8006 ≤ zgrism ≤ 1.8175, consistent with COBRA 1411+3415 being a high-redshift cluster with a mean redshift of 〈zgrism〉 = 1.8106 ± 0.0006. The detection of seven galaxies within this small redshift range is significant above the background distribution of galaxies at the level of 5σ. The line-of-sight velocity dispersion of the cluster is found to be σ∥ = km s−1 with a virial mass of M200 ≈ × 1014 M⊙. However, the mass may be lower if the cluster is still in formation. In projected phase space, we also identify two possible infalling members of COBRA 1411+3415 and two additional structures at z ∼ 1.73 and z ∼ 1.88. The similar spatial distributions and small projected separation from the main cluster suggest they could be a part of the same large-scale filament and together may form a protocluster system that could eventually merge to form a single, massive cluster. COBRA 1411+3415 is the highest-redshift cluster to be spectroscopically confirmed using a bent, double-lobed radio source as a cluster tracer.

   @article{Watson2025a,
     author = {Watson, Courtney B and Blanton, Elizabeth L and Golden-marx, Emmet and Ashby, Matthew L N and Randall, Scott W},
     doi = {10.3847/1538-4357/adc410},
     issn = {1538-4357},
     journal = {The Astrophysical Journal},
     pages = {57},
     title = {HST Grism Observations of a z ∼ 1 . 8 Cluster Candidate from the Clusters Occupied by Bent Radio AGN ( COBRA ) Survey},
     url = {http://dx.doi.org/10.3847/1538-4357/adc410},
     volume = {984},
     year = {2025},
     dimensions = {true},
   }

2023

1. ApJ

   

   Chandra X-Ray Observations of Abell 119: Cold Fronts And A Shock In An Evolved Off-Axis Merger

   Courtney B. Watson, Elizabeth L. Blanton, Scott W. Randall, and 4 more authors

   The Astrophysical Journal, 2023

   Abs DOI Bib HTML

   We present \backslashchandra X-ray observations of the dynamically complex galaxy cluster Abell 119 (}z = 0.044}). A119 is host to two NAT radio sources (0053-015 \backslash& 0053-016) whose tails are oriented parallel to each other despite orthogonally oriented jet axes. Imaging and spectral analysis reveal X-ray emission elongated along the NE-SW axis along with the presence of complex structures, including surface brightness discontinuities, which suggest possible merger activity along this axis. From radial profiles of the X-ray surface brightness, temperature, pressure, and density, we identify two surface brightness edges which are found to be cold fronts, possibly associated with large-scale sloshing of ICM gas. We also identify a brightness edge to the south which is found to be a shock front with Mach number }M = 1.21 \backslashpm 0.11}, consistent with a merger shock. In addition, previous optical studies show alignment of optical substructures along the north-south direction. The elongated X-ray emission, orientations of the NAT tails, and alignment of optical substructure all suggest recent or on-going merger activity in the NE-SW direction.

   @article{Watson2023,
     author = {Watson, Courtney B. and Blanton, Elizabeth L. and Randall, Scott W. and Sarazin, Craig L. and Sarkar, Arnab and ZuHone, John A. and Douglass, E. M.},
     doi = {10.3847/1538-4357/acee74},
     issn = {15384357},
     journal = {The Astrophysical Journal},
     number = {2},
     pages = {103},
     title = {Chandra X-Ray Observations of Abell 119: Cold Fronts And A Shock In An Evolved Off-Axis Merger},
     url = {https://ui.adsabs.harvard.edu/abs/2023ApJ...955..103W},
     volume = {955},
     year = {2023},
     dimensions = {true},
   }

2020

1. ApJL

   

   Solar-cycle–related Changes in the Helium Ionization Zones of the Sun

   Courtney B. Watson and Sarbani Basu

   The Astrophysical Journal Letters, 2020

   Abs DOI Bib HTML

   Helioseismic data for solar cycles 23 and 24 have shown unequivocally that solar dynamics changes with solar activity. Changes in solar structure have been more difficult to detect. Basu & Mandel had claimed that the then-available data revealed changes in the He ii ionization zone of the Sun. The amount of change, however, indicated the need for larger than expected changes in the magnetic fields. Now that helioseismic data spanning two solar cycles are available, we have redone the analysis using improved fitting techniques. We find that there is indeed a change in the region around the He ii ionization zone that is correlated with activity. Because the data sets now cover two solar cycles, the time variation is easily discernible.

   @article{Watson2020,
     author = {Watson, Courtney B. and Basu, Sarbani},
     doi = {10.3847/2041-8213/abc348},
     issn = {20418213},
     journal = {The Astrophysical Journal Letters},
     number = {2},
     pages = {L29},
     publisher = {IOP Publishing},
     title = {Solar-cycle–related Changes in the Helium Ionization Zones of the Sun},
     url = {http://dx.doi.org/10.3847/2041-8213/abc348},
     volume = {903},
     year = {2020},
     dimensions = {true}
   }

2019

1. ApJ

   

   Galaxy Merger Fractions in Two Clusters at Z ∼ 2 Using the Hubble Space Telescope

   C. Watson, K.-V. Tran, A. Tomczak, and 9 more authors

   The Astrophysical Journal, 2019

   Abs DOI Bib HTML

   We measure the fraction of galaxy–galaxy mergers in two clusters at z ∼ 2 using imaging and grism observations from the Hubble Space Telescope. The two galaxy cluster candidates were originally identified as overdensities of objects using deep mid-infrared imaging and observations from the Spitzer Space Telescope, and were subsequently followed up with HST/WFC3 imaging and grism observations. We identify galaxy–galaxy merger candidates using high-resolution imaging with the WFC3 in the F105W, F125W, and F160W bands. Coarse redshifts for the same objects are obtained with grism observations in G102 for the z ∼ 1.6 cluster (IRC0222A) and G141 for the z ∼ 2 cluster (IRC0222B). Using visual classifications as well as a variety of selection techniques, we measure merger fractions of in IRC0222A and in IRC0222B. In comparison, we measure a merger fraction of for field galaxies at z ∼ 2. Our study indicates that the galaxy–galaxy merger fraction in clusters at z ∼ 2 is enhanced compared to the field population, but note that more cluster measurements at this epoch are needed to confirm our findings.

   @article{Watson2019,
     author = {Watson, C. and Tran, K.-V. and Tomczak, A. and Alcorn, L. and Salazar, I.V. and Gupta, A. and Momcheva, I. and Papovich, C. and Dokkum, P.V. and Brammer, G. and Lotz, J. and Willmer, C.N.A.},
     doi = {10.3847/1538-4357/ab06ef},
     issn = {15384357},
     journal = {The Astrophysical Journal},
     title = {Galaxy Merger Fractions in Two Clusters at Z ∼ 2 Using the Hubble Space Telescope},
     url = {https://doi.org/10.3847/1538-4357/ab06ef},
     volume = {874},
     year = {2019},
     dimensions = {true}
   }