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1. Characterizing the Fast Radio Burst Host Galaxy Population and its Connection to Transients in the Local and Extragalactic Universe

   https://doi.org/10.3847/1538-3881/ac3aec  

   Bhandari, Shivani; Heintz, Kasper E.; Aggarwal, Kshitij; Marnoch, Lachlan; Day, Cherie K.; Sydnor, Jessica; Burke-Spolaor, Sarah; Law, Casey J.; Xavier Prochaska, J.; Tejos, Nicolas; et al (January 2022, The Astronomical Journal)

   Abstract We present the localization and host galaxies of one repeating and two apparently nonrepeating fast radio bursts (FRBs). FRB 20180301A was detected and localized with the Karl G. Jansky Very Large Array to a star-forming galaxy at z = 0.3304. FRB20191228A and FRB20200906A were detected and localized by the Australian Square Kilometre Array Pathfinder to host galaxies at z = 0.2430 and z = 0.3688, respectively. We combine these with 13 other well-localized FRBs in the literature, and analyze the host galaxy properties. We find no significant differences in the host properties of repeating and apparently nonrepeating FRBs. FRB hosts are moderately star forming, with masses slightly offset from the star-forming main sequence. Star formation and low-ionization nuclear emission-line region emission are major sources of ionization in FRB host galaxies, with the former dominant in repeating FRB hosts. FRB hosts do not track stellar mass and star formation as seen in field galaxies (more than 95% confidence). FRBs are rare in massive red galaxies, suggesting that progenitor formation channels are not solely dominated by delayed channels which lag star formation by gigayears. The global properties of FRB hosts are indistinguishable from core-collapse supernovae and short gamma-ray bursts hosts, and the spatial offset (from galaxy centers) of FRBs is mostly inconsistent with that of the Galactic neutron star population (95% confidence). The spatial offsets of FRBs (normalized to the galaxy effective radius) also differ from those of globular clusters in late- and early-type galaxies with 95% confidence. 

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2. ICM-SHOX. I. Methodology Overview and Discovery of a Gas–Dark Matter Velocity Decoupling in the MACS J0018.5+1626 Merger

   https://doi.org/10.3847/1538-4357/ad3fb5  

   Silich, Emily M; Bellomi, Elena; Sayers, Jack; ZuHone, John; Chadayammuri, Urmila; Golwala, Sunil; Hughes, David; Montaña, Alfredo; Mroczkowski, Tony; Nagai, Daisuke; et al (June 2024, The Astrophysical Journal)

   Galaxy cluster mergers are rich sources of information to test cluster astrophysics and cosmology. However, cluster mergers produce complex projected signals that are difficult to interpret physically from individual observational probes. Multi-probe constraints on the gas and dark matter (DM) cluster components are necessary to infer merger parameters that are otherwise degenerate. We present Improved Constraints on Mergers with SZ, Hydrodynamical simulations, Optical, and X-ray (ICM-SHOX), a systematic framework to jointly infer multiple merger parameters quantitatively via a pipeline that directly compares a novel combination of multi-probe observables to mock observables derived from hydrodynamical simulations. We report a first application of the ICM-SHOX pipeline to MACS J0018.5+1626, wherein we systematically examine simulated snapshots characterized by a wide range of initial parameters to constrain the MACS J0018.5+1626 merger geometry. We constrain the epoch of MACS J0018.5+1626 to the range 0–60 Myr post-pericenter passage, and the viewing angle is inclined ≈27°–40° from the merger axis. We obtain constraints for the impact parameter (≲250 kpc), mass ratio (≈1.5–3.0), and initial relative velocity when the clusters are separated by 3 Mpc (≈1700–3000 km s⁻¹). The primary and secondary clusters initially (at 3 Mpc) have gas distributions that are moderately and strongly disturbed, respectively. We discover a velocity space decoupling of the DM and gas distributions in MACS J0018.5+1626, traced by cluster-member galaxy velocities and the kinematic Sunyaev–Zel'dovich effect, respectively. Our simulations indicate this decoupling is dependent on the different collisional properties of the two distributions for particular merger epochs, geometries, and viewing angles. 

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3. The Massive and Quiescent Elliptical Host Galaxy of the Repeating Fast Radio Burst FRB 20240209A

   https://doi.org/10.3847/2041-8213/ad9de2  

   Eftekhari, T; Dong_董, Y 雨欣; Fong, W; Shah, V; Simha, S; Andersen, B C; Andrew, S; Bhardwaj, M; Cassanelli, T; Chatterjee, S; et al (January 2025, The Astrophysical Journal Letters)

   Abstract The discovery and localization of FRB 20240209A by the Canadian Hydrogen Intensity Mapping Fast Radio Burst (CHIME/FRB) experiment marks the first repeating FRB localized with the CHIME/FRB Outriggers and adds to the small sample of repeating FRBs with associated host galaxies. Here we present Keck and Gemini observations of the host that reveal a redshiftz = 0.1384 ± 0.0004. We perform stellar population modeling to jointly fit the optical through mid-IR data of the host and infer a median stellar mass log(M_*/M_⊙) = 11.35 ± 0.01 and a mass-weighted stellar population age  ~11 Gyr, corresponding to the most massive and oldest FRB host discovered to date. Coupled with a star formation rate  <0.31M_⊙yr⁻¹, the specific star formation rate  <10^−11.9yr⁻¹classifies the host as quiescent. Through surface brightness profile modeling, we determine an elliptical galaxy morphology, marking the host as the first confirmed elliptical FRB host. The discovery of a quiescent early-type host galaxy within a transient class predominantly characterized by late-type star-forming hosts is reminiscent of short-duration gamma-ray bursts, Type Ia supernovae, and ultraluminous X-ray sources. Based on these shared host demographics, coupled with a large offset as demonstrated in our companion Letter, we conclude that preferred sources for FRB 20240209A include magnetars formed through merging binary neutron stars/white dwarfs or the accretion-induced collapse of a white dwarf, or a luminous X-ray binary. Together with FRB 20200120E localized to a globular cluster in M81, our findings provide strong evidence that some fraction of FRBs may arise from a process distinct from the core collapse of massive stars. 

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4. Decoding the early Universe: exploring a merger scenario for the high-redshift cluster JKCS041 using numerical models

   https://doi.org/10.1093/mnras/stae2338  

   Felix, Sharon; Gogoi, Antareep; Shavelle, Kaitlyn; Sike, Brandon; King, Lindsay; Andreon, Stefano; Chadayammuri, Urmila; ZuHone, John; Romero, Charles (October 2024, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT JKCS041 ($z=1.8$) is one of the most distant galaxy cluster systems known, seen when the Universe was less than 4 billion years old. Recent Sunyaev–Zeldovich (SZ) observations show a temperature decrement that is less than expected based on mass estimates of the system from X-ray, weak gravitational lensing, and galaxy richness measurements. In this paper, we seek to explain the observables – in particular the low SZ decrement and single SZ peak, the projected offset between the X-ray and SZ peaks of $$\approx$$220 kpc, the gas mass measurements and the lensing mass estimate. We use the gamer-2 hydrodynamic code to carry out idealized numerical simulations of cluster mergers and compare resulting synthetic maps with the observational data. Generically, a merger process is necessary to reproduce the observed offset between the SZ and X-ray peaks. From our exploration of parameter space, seen a few tenths of a Gyr after first core passage, two components with total mass of $$\approx 2\times 10^{14} \,\text{M}_\odot$$, mass ratio of $$\approx$$2:3, gas fraction of $0.05-0.1$, and Navarro, Frenk and White mass density profile concentrations c$$\approx$$ 5 are scenarios that are consistent with the observational data. For consistency with the SZ and X-ray measurements, our simulations exclude total mass in excess of $$\approx 3\times 10^{14} {\rm M}_{\odot }$$, primarily based on the SZ signal. The mass ratio is constrained by the SZ–X-ray offset and magnitude of the SZ signal, ruling out systems with equal and vastly different masses. 

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5. A Catalog of Local Universe Fast Radio Bursts from CHIME/FRB and the KKO

   https://doi.org/10.3847/1538-4365/addbda  

   Amiri, Mandana; Amouyal, Daniel; Andersen, Bridget C; Andrew, Shion; Bandura, Kevin; Bhardwaj, Mohit; Boyle, P J; Brar, Charanjot; Cassity, Alyssa; Chatterjee, Shami; et al (August 2025, The Astrophysical Journal Supplement Series)

   Abstract We present the first catalog of fast radio burst (FRB) host galaxies from CHIME/FRB Outriggers, selected uniformly in the radio and the optical by localizing 81 new bursts to 2″ × ∼ 60″ accuracy using CHIME and the k’niʔatn k’l ⌣ stk’masqt Outrigger station, located 66 km from CHIME. Of the 81 localized bursts, we use the probabilistic association of transients to their hosts algorithm to securely identify 21 new FRB host galaxies, and compile spectroscopic redshifts for 19 systems, 15 of which are newly obtained via spectroscopic observations. The most nearby source is FRB 20231229A, at a distance of 90 Mpc. One burst in our sample is from a previously reported repeating source in a galaxy merger (FRB 20190303A). Three new FRB host galaxies (FRBs 20230203A, 20230703A, and 20231206A) are found toward X-ray and optically selected galaxy clusters, potentially doubling the sample of known galaxy cluster FRBs. A search for radio counterparts reveals that FRB 20231128A is associated with a luminous persistent radio source (PRS) candidate with high significance (P_cc ∼ 10⁻²). If its compactness is confirmed, it would be the nearest known compact PRS atz= 0.1079. Our catalog significantly increases the statistics of the Macquart relation at low redshifts (z < 0.2). In the near future, the completed CHIME/FRB Outriggers array will produce hundreds of FRBs localized with very long baseline interferometry (VLBI). This will significantly expand the known sample and pave the way for future telescopes relying on VLBI for FRB localization. 

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