D. Christopher Martin

Fast radio bursts (FRBs) are millisecond-duration radio transients from extragalactic sources. Their frequency and polarization properties are affected by plasma and magnetic fields along the line of sight to Earth. We report radio observations of FRB 20240304B and near-infrared follow-up to identify its host galaxy. The host is a low-mass, clumpy, star-forming galaxy at redshift 2.148 ± 0.0013, corresponding to 3 billion years after the Big Bang. This FRB occurred during the peak of cosmic star formation and probes ionized gas over approximately 80% of cosmic history. The highly scattered burst emission implies that there is a population of FRBs that are undetected by current surveys.

The Wisconsin H α Mapper surveyed the entire Galactic sky in H α (∣ v _LSR ∣ ≲ 100 km s ⁻¹ ) to approximately 0.1 Rayleigh ( R ), albeit with a 1° beam. Reynolds et al. (2005) reported “point sources” which stood out against the Galactic background in space and velocity. Half of the sources are associated with plausible planetary nebulae and OB stars. Reynolds et al. suggested sub dwarfs for one quarter of the sources. Here, we investigate one such source, WPS 46, for which Reynolds et al. suggested the sub-dwarf PG 0931+691 to provide the source of ionization. With the Keck Cosmic Web Imager we found numerous nebular emission lines within the vicinity of WPS 46, but we failed to find H α emission in the arc-minute vicinity of PG 0931+691. The line ratios (BPT diagram and [S ii ]/H α ) combined with the morphology are more consistent with AGN or LI(N)ER-like ionization than with pure warm ionized medium or H ii region-like photoionization. Separately, we offer compelling reasons to argue that PG 0931+691 cannot be the source of ionizing power for WPS 46. We suggest that WPS 46 is associated with an intermediate velocity complex (IVC) and that H α and nebula emission may arise as a result of a shock. We conclude by outlining a plan of action of using SDSS’s Local Volume Mapper along with deep narrow band imagery obtained by amateur astronomers to explore and study the ionized sky on sub-degree scales, in general, and specifically studies of IVC and high-velocity complexes.

Detecting spatially resolved emission at surface-brightness levels of ∼10 ⁻²⁰ –10 ⁻¹⁹ erg cm ⁻² s ⁻¹ arcsec ⁻² Å ⁻¹ in optical integral-field spectroscopy requires reduction procedures limited not by photon statistics, but by systematic effects such as sky subtraction, cosmic-ray (CR) contamination, correlated noise, and detection methodology. We present kcwiulb , a modular postprocessing framework for ultralow surface-brightness mapping developed for the Keck Cosmic Web Imager (KCWI). Built on top of the standard KCWI data reduction pipeline outputs, the framework addresses four principal challenges: (1) residual sky subtraction for nod-and-shuffle observations at the subpercent level, (2) component-based sky subtraction with iterative decoupling of CR contamination, (3) covariance-aware coaddition with explicit propagation of interpolation-induced noise correlations, and (4) statistically controlled adaptive smoothing with empirical variance calibration. We quantify the observational constraints imposed by temporal sky variability, demonstrate that the pipeline attains the sky-subtraction fidelity required for diffuse emission studies at the 10 ⁻¹⁹ level, and show that it approaches 10 ⁻²⁰ sensitivity in favorable cases. The propagated uncertainties are validated using signal-to-noise statistics in blank sky regions. While developed for KCWI, this framework enables reproducible, systematics-aware recovery of faint extended emission and provides a general methodology applicable to integral-field spectroscopic observations operating in the systematics-limited regime.

Accurate chemical compositions of star-forming regions provide a critical diagnostic tool for characterizing the star formation history and gas flows that regulate galaxy formation. However, the abundance discrepancy factor (ADF) between measurements derived from the “direct” optical electron temperature ( T _e ) method and those from recombination lines (RLs) introduces a ∼0.2 dex systematic uncertainty in the oxygen abundance. The degree of uncertainty for other elements is unknown. We conduct a comprehensive analysis of O ⁺⁺ and N ⁺ ion abundances using optical and far-infrared (far-IR) spectra of a star-forming region within the nearby dwarf galaxy Haro 3, which exhibits a typical ADF. Assuming homogeneous conditions, the far-IR emission indicates an oxygen abundance higher than that derived using the T _e method and consistent with the RL value, as expected from temperature fluctuations, whereas the far-IR nitrogen abundance is too large to be explained by temperature fluctuations. A two-phase analytical model reveals that differential dust obscuration associated with temperature inhomogeneity is likely required to explain all the emission-line ratios, and that the total oxygen metallicity of two phases is consistent with the RL metallicity. Our findings underscore the critical importance of resolving the cause of abundance discrepancies and understanding the biases between different metallicity methods. This work presents a promising methodology, and we identify further approaches to address the dominant sources of uncertainty.

We present the first detailed, spatially resolved study of the circumgalactic medium (CGM) surrounding the low-mass, low-redshift star-forming galaxy J0910b (SDSS J091458.61+110845.1; M _⋆ ∼ 10 ^9.2 M _⊙ , z = 0.096), focusing on its gas dynamics, ionization mechanisms, and kinematics. J0910b is the lowest-mass galaxy in our low-redshift CGM survey using the Keck Cosmic Web Imager (KCWI). Using KCWI integral field spectroscopy and CLOUDY modeling, we map the ionization structure and velocity field of the CGM through H α , [O III ] λ 5007, and [O II ] λλ 3727, 3729 emission. We detect a predominantly cool ( T ∼ 10 ⁴ K) ionized halo extending to ∼35 kpc (∼0.35 R _vir ), with a complex structure including a counterrotating component and two filamentary inflows. The counterrotating CGM contains ∼7.4 × 10 ⁹ M _⊙ of ionized gas—nearly 5 times the stellar mass—and exhibits a positive radial metallicity gradient, rising from 0.2 Z _⊙ at 9 kpc to 0.65 Z _⊙ at 16 kpc. Velocity dispersion maps reveal a shell-like structure near the interstellar medium-CGM interface, suggesting feedback-driven outflows that redistributed enriched gas. In contrast, the filaments show low metallicities (∼0.07 Z _⊙ ), high specific angular momentum ( j ∼ 3.7 × 10 ³ kpc km s ⁻¹ ), and an accretion rate of 0.05 M _⊙ yr ⁻¹ , consistent with an intergalactic origin. Together, these components reveal a dynamically rich CGM. The counterrotating gas dominates the halo’s mass, accounts for ∼33% of the cosmic baryon budget, and must be continuously replenished ( t _cool ∼ 0.3–2 Myr). These results suggest a long-lived, regulated CGM that governs star formation through angular momentum exchange and gas recycling.

The recently discovered Pa 30 nebula, the putative type Iax supernova remnant associated with the historical supernova of 1181 AD, shows puzzling characteristics that make it unique among known supernova remnants. In particular, Pa 30 exhibits a complex morphology, with a unique radial and filamentary structure, and it hosts a hot stellar remnant at its center, which displays oxygen-dominated, ultrafast winds. Because of the surviving stellar remnant and the lack of hydrogen and helium in its filaments, it has been suggested that Pa 30 is the product of a failed thermonuclear explosion in a near- or super-Chandrasekhar white dwarf, which created a subluminous transient, a rare subtype of the Ia class of supernovae called type Iax. We present here a detailed study of the 3D structure and velocities of a full radial section of the remnant. The Integral Field Unit observations, obtained with the new red channel of the Keck Cosmic Web Imager spectrograph, reveal that the ejecta are consistent with being ballistic, with velocities close to the free-expansion velocity. Additionally, we detect a large cavity inside the supernova remnant and a sharp inner edge to the filamentary structure, which coincides with the outer edge of a bright ring detected in infrared images. Finally, we detect a strong asymmetry in the amount of ejecta along the line of sight, which might hint at an asymmetric explosion. Our analysis provides strong confirmation that the explosion originated from SN 1181.

We present SN 2023zaw—a subluminous (M _r = −16.7 mag) and rapidly evolving supernova (t _1/2,r = 4.9 days), with the lowest nickel mass (≈0.002 M _⊙) measured among all stripped-envelope supernovae discovered to date. The photospheric spectra are dominated by broad He i and Ca near-infrared emission lines with velocities of ∼10,000−12,000 km s⁻¹. The late-time spectra show prominent narrow He i emission lines at ∼1000 km s⁻¹, indicative of interaction with He-rich circumstellar material. SN 2023zaw is located in the spiral arm of a star-forming galaxy. We perform radiation-hydrodynamical and analytical modeling of the lightcurve by fitting with a combination of shock-cooling emission and nickel decay. The progenitor has a best-fit envelope mass of ≈0.2 M _☉ and an envelope radius of ≈50 R _⊙. The extremely low nickel mass and low ejecta mass (≈0.5 M _⊙) suggest an ultrastripped SN, which originates from a mass-losing low-mass He-star (zero-age main-sequence mass < 10 M _⊙) in a close binary system. This is a channel to form double neutron star systems, whose merger is detectable with LIGO. SN 2023zaw underscores the existence of a previously undiscovered population of extremely low nickel mass (M _☉) stripped-envelope supernovae, which can be explored with deep and high-cadence transient surveys.

Theory suggests that there are two primary modes of accretion through which dark matter halos acquire the gas to form and fuel galaxies, hot and cold mode accretion. In cold mode accretion, gas streams along cosmic web filaments to the center of the halo, allowing for the efficient delivery of star-forming fuel. Recently, two QSO-illuminated HI Lyman alpha (Ly{\alpha}) emitting objects were reported to have properties of cold, rotating structures (Martin et al. 2015, Martin et al. 2016). However, the spatial and spectral resolution available was insufficient to constrain radial flows associated with connecting filaments. With the Keck Cosmic Web Imager (KCWI) we now have eight times the spatial resolution, permitting the detection of these in-spiraling flows. In order to detect these inflows, we introduce a suite of models which incorporate zonal radial flows, demonstrate their performance on a numerical simulation that exhibits coldflow accretion, and show that they are an excellent match to KCWI velocity maps of two Ly{\alpha} emitters observed around high-redshift quasars. These Multi-Filament Inflow models kinematically isolate zones of radial inflow that correspond to extended filamentary emission. The derived gas flux and inflow path is sufficient to fuel the inferred central galaxy star formation rate and angular momentum. Thus, our kinematic emission maps provide strong evidence for the inflow of gas from the cosmic web building galaxies at the peak of star formation.