We investigate the origin of Little Red Dots (LRDs) using a realization of the BRAHMA cosmological hydrodynamic simulation that produces overmassive black holes at $z \sim 4\text{-}8$. We generate mock JWST observations by modeling galaxy spectral energy distributions (including dust attenuation) together with AGN emission using cloudy-based templates, and test whether such systems reproduce the observed LRD population. We consider both a standard low-redshift AGN emission model and a gas-enshrouded AGN emission model, in which the AGN is embedded within unresolved dense gas that produces a strong Balmer break, and find that LRDs emerge in significantly larger numbers with the latter model. Assuming that all AGN reside in this gas-enshrouded phase, we recover LRD number densities ($\Phi \sim 2 \times 10^{-4}~\mathrm{Mpc}^{-3}$), close to the observed abundance. This implies an effective gas-enshrouded duty cycle approaching unity, which is not predicted by current theoretical models. Current JWST constraints on the high-z black hole mass function suggest that this tension may arise from a deficit of low-mass ($M_{\rm BH}\lesssim10^7~M_\odot$) LRDs in BRAHMA. Applying the same gas-enshrouded model to the ASTRID simulation, which lacks overmassive black holes, we find that the LRD abundance remains $\gtrsim2~\rm dex$ below observations. Overall, while the overmassive black holes in this BRAHMA realization substantially enhance the production of LRD-like systems, reproducing the observed LRD abundance requires them to reside in dense gas environments with unrealistically high duty cycles. We therefore conclude that overmassive black holes are unlikely to be the sole origin of the observed JWST LRD population, and may instead account for only a subset of these systems.
We present an Australian Square Kilometre Array Pathfinder (ASKAP) Evolutionary Map of the Universe (EMU) radio-continuum detection of the nearby spiral Seyfert I type galaxy NGC 7213 which we nickname ‘sand-timer’ due to its morphological appearance. We find an intriguing and peculiar emission extending from the central supermassive black hole (SMBH), which we refer to as galaxy hyperbubbles and/or active galactic nucleus (AGN) jets. Finally, we find two previously undetected kiloparsec-scale features in the polarised intensity image, which appear most likely also related to the central SMBH, as restarted bipolar radio jets, though somewhat obscured in the radio-continuum total intensity image. They resemble the bow shocks often seen in Fanaroff-Riley Class II (FR-II) radio galaxies, and so we propose they most likely represent an early episode of the ‘Sand-timer’s’ AGN jet activity. An alternative scenario is that the core of NGC 7213 may be a super massive binary black hole (SMBBH) which is undergoing a tidal disruption event (TDE), potentially causing the peculiar radio-continuum emission.
We present a new radio continuum study of the Large Magellanic Cloud supernova remnant N 132D using observations from the Australia Telescope Compact Array (ATCA) and MeerKAT. The radio morphology shows a clear spatial correlation with the X-ray emission in the central and left regions of the remnant, as well as in the right portion of the shell, indicating a close connection between the thermal and non-thermal plasma components. Polarisation measurements reveal predominantly radial magnetic fields, with mean fractional polarisations of 1.2 per cent, 3.0 per cent, and 4.0 per cent at 2100, 5500, and 9000 MHz, respectively. There is also a complex structure in the Faraday Depth spectrum, showing a double-peaked profile that indicates regions with differing magnetisation properties or varying magnetic field geometry. The rotation measure derived from ATCA data has an average value of 2 rad m^-2, with a maximum of 338 rad m^-2. We estimate a constant partition magnetic field of 90 microG and a minimum energy of 3 x 10^50 erg, which is in a constant partition between the energy density of electrons and the energy density of the magnetic field. The integrated radio spectrum follows a power law with a spectral index of -0.74 +- 0.01. These results provide new constraints on the magnetic field structure and energetic properties of N 132D and offer insight into particle acceleration and shock evolution in young supernova remnants.
We study the binary black hole (BBH) population observed through gravitational waves (GWs) using 256 events from the latest release of GWTC-5.0. The inferred chirp-mass distribution shows prominent peaks at approximately $7.5M_{\odot}$, $14M_{\odot}$, and $27M_{\odot}$, with subsequent peaks spaced by approximately a factor of two. A parsimonious explanation for this structured distribution is a hierarchical merger scenario, in which the first peak arises from mergers of black holes of stellar origin, and higher-mass peaks arise from repeated mergers. Notably, with the addition of new observations, an intermediate peak near $19M_{\odot}$ emerges. This feature was anticipated in earlier work as a consequence of intergenerational mergers involving second- and third-generation (G) black holes, highlighting the predictive expectations of the hierarchical-merger interpretation. Furthermore, two groups of $1G+2G$ mergers recently reported in separate studies can be understood as distinct rungs---$1G+2G$ and $3G+4G$---within this hierarchical chirp-mass ladder, a unification that describes both spin transitions with a single mechanism. Although we observe expected correlations between mass ratios and spins in multiple events across the mass range, the lack of clear signatures across all rungs invites investigation into the role of hierarchical mergers in shaping the BBH population.
The ultraviolet (UV) spectral slope, β, serves as a fundamental probe of stellar population properties and interstellar medium conditions in galaxies. At high redshift, a small sample of galaxies have been confirmed to have extremely blue spectral slopes of β<-2.8, as measured directly from spectroscopy. One explanation for such galaxies is that they are leaking copious amounts of ionizing photons such that these photons are never processed into the nebular continuum and we are observing the bluer intrinsic slopes of the stellar populations. Here, we use a detailed set of radiation hydrodynamics simulations with parsec-scale resolution and non-equilibrium chemistry to elucidate the physical origin of extremely blue UV slopes at high redshift. We show that, while some extremely blue galaxies (EBGs) do indeed have high escape fractions, a second population of EBGs has f_ esc<1%. In this second population, the ISM gas densities tend to be much lower, and the timescale for ionization fronts to reach their Strömgren radii can be longer than the main-sequence lifetime of massive stars, leading to a lag in nebular emission. Our results demonstrate that extremely blue UV slopes do not uniquely imply high escape fractions, which highlights the importance of time-dependent (non-equilibrium) nebular emission for interpreting the spectra of galaxies in the early Universe.
