Liang Dai

Astrometric perturbations of lensed arcs behind galaxy clusters have been recently suggested as promising probes of small-scale (≲ 10⁹M_⊙) dark matter substructure. Populations of cold dark matter (CDM) subhalos, predicted in hierarchical structure formation theory, can break the symmetry of arcs near the critical curve, leading to positional shifts in the observed images. We present a novel statistical method to constrain the average subhalo mass fraction (f_sub) in clusters that takes advantage of this induced positional asymmetry. Focusing on CDM, we extend a recent semi-analytic model of subhalo tidal evolution to accurately simulate realistic subhalos within a cluster-scale host. We simulate the asymmetry of lensed arcs from these subhalo populations using Approximate Bayesian Computation. Using mock data, we demonstrate that our method can reliably recover the simulated f_sub to within 68% CI in 73% of cases, regardless of the lens model, astrometric precision, and image morphology. We show that the constraining power of our method is optimized for larger samples of well observed arcs, ideal for recent JWST observations of cluster lenses. As a preliminary test, we apply our method to the MACSJ0416 Warhol arc and AS1063 System 1. For Warhol we constrain the upper limit on |$\log f_{\rm sub} < -3.48^{+1.00}_{-0.91}$|⁠, while for AS1063 System 1 we constrain |$\log f_{\rm sub} = -2.44^{+0.61}_{-0.86}$| (both at 68% CI), consistent with CDM predictions. We elaborate on our method’s limitations and its future potential to place stringent constraints on dark matter properties in cluster environments.

The lensed Sunburst Arc ( z = 2.369) hosts a young (∼2–4 Myr), massive ( M _⋆ ∼ 10 ⁷ M _⊙ ), compact ( R _eff ∼ 8 pc) Lyman-continuum leaking super star cluster, which powers a compact (Z _⊙ and with an anomalously elevated nitrogen-to-oxygen ratio log ( N / O ) ∼ − 0.2 . We present semianalytic models and 3D magnetohydrodynamic simulations in an attempt to reproduce this system. The results indicate that the progenitor giant molecular cloud (GMC) may have M _cloud ≳ 3 × 10 ⁷ M _⊙ and R _cloud ∼ 70 pc, corresponding to a surface density ∼10 ³ –10 ⁴ M _⊙ pc ⁻² . Incorporating feedback from individual very massive stars (VMSs; ≥100 M _⊙ ) sampled from the Kroupa initial mass function, we find that their winds rapidly enrich ∼10 ⁴ M _⊙ of nearby gas with nitrogen (∼1 dex) and helium (∼0.1–0.2 dex). In the first 1–3 Myr, some cold gas falls to the system center where a central cluster builds up from subcluster mergers. There, the gas is photoionized, pressurized, and chemically enriched by the newly formed VMSs, before being radiatively expelled in the next ∼1 Myr. We find that both VMS feedback and a high-surface-density progenitor GMC are necessary to reproduce the observed nebular properties, such as high N/O, high pressure, and stellar proximity. Low metallicity ( Z ≤ 0.004) may be essential to avoid overproduction of carbon from WC stars. Such enrichment processes localized to compact starbursts may have caused strong nitrogen emission from dense ionized gas as observed in high-redshift galaxies such as GN-z11 and GS_3073.

In star-forming disk galaxies, the radio continuum emission (1–10GHz) powered by star formation has an integrated polarization direction imperfectly aligned with the apparent disk minor axis. This polarization-shape alignment effect was previously observed in a small sample of local spirals. If this is prevalent for disk galaxies out to cosmological redshifts, novel measurements of cosmic birefringence and cosmic shear will be enabled by leveraging radio continuum surveys such as the Square Kilometre Array synergized with galaxy shape measurements. We calculate the polarization-shape misalignment angle for star-forming galaxies in the IllustrisTNG50 simulation at 0 < z < 2, assuming that additional polarized radio emission from an active galactic nucleus is negligible in at least a sizable fraction of the star-forming galaxies. The alignment found for z = 0 is consistent with local spiral data, but significantly deteriorates as redshift increases. Moreover, it degrades toward lower frequencies due to internal Faraday depolarization. Thanks to cosmic redshifting, observing higher- z galaxies at a fixed frequency greatly mitigates degradation due to reduced Faraday depolarization at the source-frame frequency. We present analytic fits to the non-Gaussian misalignment angle distribution, and evaluate Fisher information per galaxy for measuring a polarization rotation angle induced by cosmic birefringence. For observation at 4.8 GHz, the effective root-mean-square misalignment angle σ _{α ,eff} is 18°, 23° and 33° at z = 0, 1 and 2, respectively. Analyzing N independent galaxies reduces the uncertainty on an isotropic cosmic birefringence signal to σ _{α ,eff} /√( N ), providing competitive sensitivity once large samples are available. As accurate observation-driven models are not yet available for cosmological galaxy samples, our results motivate pilot observations to empirically characterize polarization-shape alignment, and can facilitate forecasts of cosmology and fundamental physics applications that exploit this effect.

The strongly lensed z ∼ 6 Sunrise galaxy offers an incredible opportunity to investigate star formation in the early Universe on parsec or smaller scales. The highly magnified object Earendel within the Sunrise was previously identified as a candidate star or binary owing to size constraints placed by the lensing magnification; however, recent works have suggested that this constraint may be relaxed to even the size of star clusters. Here we explore the hypothesis that Earendel may actually be a star cluster, and we simultaneously evaluate other star clusters within the host galaxy. Leveraging deep, archival James Webb Space Telescope NIRSpec PRISM spectroscopy, we determine a spectroscopic redshift for the Sunrise galaxy z = 5.926 ± 0.013, and we fit simple stellar population (SSP) models from three premier libraries to evaluate the physical parameters of Earendel and another distinct star cluster in the Sunrise dubbed “1b.” We find the rest-UV through optical continuum of Earendel to be well described by an SSP, nearly equivalently to 1b, which is confidently a star cluster. We infer that they have intermediate ages t _age ∼ 30–150 Myr, are metal-poor (Z _⋆ ≲ 0.1 Z _⊙), and are consistent with the formation age–metallicity trend seen in local globular clusters. Such intermediate-age clusters are seldom probed spectroscopically in the high-redshift Universe, and we explore the extent to which these clusters can be characterized via the spectroscopic continuum.

Highly magnified stars (μ>100) are now routinely identified as transient events at cosmological distances thanks to microlensing by intra-cluster stars near the critical curves of galaxy clusters. Using the James Webb Space Telescope (JWST) in combination with the Hubble Space Telescope, we outline here an analytical framework that is applied to the Warhol arc (at z = 0.94) in the MACS 0416 galaxy cluster (at z = 0.396), where over a dozen microlensed stars have been detected to date. This method is general and can be applied to other lensed arcs. Within this lensed galaxy, we fit the spatially resolved spectral energy distribution spanned by eight JWST-NIRCam filters combined with three ACS filters, for accurate lensed star predictions in 2D. With this tool we can generate 2D maps of microlensed stars for well-resolved arcs in general, incorporating wavelength dependence and limiting apparent magnitude. These maps can be directly compared with planned cadenced campaigns from JWST and Hubble, offering a means to constrain the initial mass function and the level of dark matter substructure.

Wave-optics phenomena in gravitational lensing occur when the signal’s wavelength is commensurate to the gravitational radius of the lens. Although potentially detectable in lensed gravitational waves, fast radio bursts and pulsars, accurate numerical predictions are challenging to compute. Here we present novel methods for wave-optics lensing that allow the treatment of general lenses. In addition to a general algorithm, specialized methods optimize symmetric lenses (arbitrary number of images) and generic lenses in the single-image regime. We also develop approximations for simple lenses (point-like and singular isothermal sphere) that drastically outperform known solutions without compromising accuracy. These algorithms are implemented in (o); an accurate, flexible, and fast code. o efficiently computes the frequency-dependent amplification factor for generic lens models and arbitrary impact parameters in O ( 1 ms ) to O ( 10 ms ) depending on the lens configuration and complexity. o is readily applicable to model lensing diffraction on gravitational-wave signals, offering new means to investigate the distribution of dark-matter and large-scale structure with signals from ground and space detectors.

We study the formation of a partially ionized zone behind the usual H II zone in photoionized gas for a density intermediate between ordinary H II regions and active galactic nucleus broad-line regions, and without X-rays. While Lyman continuum photons are depleted before reaching this zone, Balmer continuum photons can ionize the n = 2 hydrogen atoms as those are pumped by scattering-trapped Lyα photons. Lyα photons need to be replenished in a steady state, but fast radiative cooling at high gas temperature makes replenishment through electron collisional excitation of n = 2 inefficient. Independent of the temperature, Raman scattering of incident continuum photons on the Lyman-series damping wings injects Lyα photons in the line core and pumps the n = 2 state, which helps realize a partially ionized equilibrium at 5000–7000 K. Since the ratio between the incident radiation flux and gas density is limited by the dynamic effects of radiation pressure, significant ionization fraction is only realized at densities n _H ≳ 10⁷ cm⁻³. A large Hα/Hβ line ratio results from such partially ionized gas, as large Balmer line optical depths enhance the n = 3 population relative to higher n states. Partial ionization also provides the ideal condition for internally forming an intense and very broad Lyα line, which can power strong fluorescent metal emission lines observed from the Weigelt blobs of η Carinae in the Galaxy, and from a candidate young super star cluster in the Cosmic Noon Sunburst galaxy. This phenomenon may also have implications for understanding the spectra of high-z galaxies that show unusually dense ionized gas.