Simeon Bird

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 ∼ 4 - 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 ( Φ ∼ 2 × 10 − 4 M p c − 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 () 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 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 galaxy luminosity functions and photometric predictions for the Vera C. Rubin Observatory Legacy Survey of Space and Time using the ASTRID cosmological hydrodynamical simulation. Galaxy magnitudes combine stellar population synthesis with a dust attenuation model whose optical depth scales with metal surface density. We calibrate the dust model at z = 0 using SDSS luminosity functions and test it at z = 0.5, 1.0, and 1.5 in rest frame B , V , R , I bands, finding good agreement with observations. We then construct LSST ready mock photometric catalogs over 0 ≤ z ≤ 2 in steps of Δ z = 0.1, containing about 378 million galaxies, including about 47 million above the LSST Year 10 coadded i -band limit. We provide apparent magnitude luminosity functions in ugrizy , best-fit Schechter parameters, and differential and cumulative galaxy counts as a function of survey depth.

We present the first results from AMBRA simulation ( A STRID with M BH seeding from BRA HMA) evolved to z = 8. AMBRA combines the large cosmological volume and statistical power of ASTRID with the physically motivated gas-based black hole (BH) seeding models from BRAHMA . Motivated by James Webb Space Telescope’s (JWST) discoveries of massive BHs at z ≳ 9, AMBRA adopts a lenient heavy-seed prescription from the BRAHMA suite, allowing for the formation of 4 × 10 ⁴⁻⁵ M _⊙ seeds in halos with star-forming, metal-poor gas. The seeding model is motivated by scenarios in which heavy seeds form through stellar collisions in star clusters or from the rapid growth of Population III remnants. The improved seeding model enables AMBRA to form BH seeds much earlier and more efficiently compared to ASTRID . This significantly enhances early BH growth, producing a z = 8 BH number density more than an order of magnitude higher than that in ASTRID over the mass range 10 ⁵⁻⁷ M _⊙ . BHs reaching masses consistent with GN-z11 and CEERS-1019 typically originate in highly compact density peaks and undergo multiple early mergers. In these systems, ∼50% of BH masses by z = 11 is from BH mergers, after which gas accretion becomes the dominant growth channel. Without this early merger-driven assembly, ASTRID cannot reproduce the high-mass BH detected by JWST. Our results indicate that abundant early seed formation combined with frequent mergers can explain several JWST massive BH candidates without requiring sustained super-Eddington accretion. As a testable prediction, AMBRA yields ≈4 Laser Interferometer Space Antenna (LISA) detectable BH merger events per year at z ≥ 8, which is three orders of magnitude higher than that in ASTRID .

Dual active galactic nuclei (DAGN) mark the observable stage of massive black hole (MBH) pairing during galaxy mergers and are the progenitors of the MBH binaries that generate low-frequency gravitational waves. Using the large-volume ASTRID cosmological simulation, we construct mock DAGN catalogs tailored to the selection functions of current (COSMOS-Web and DESI) and forthcoming (Nancy Grace Roman Space Telescope (Roman) and Lynx X-ray Observatory (Lynx)) surveys, enabling direct comparisons between simulations and observations. With realistic observational selections, ASTRID reproduces the observed dual fractions, projected separations, and host-galaxy properties across redshifts. We predict a substantial population of small-separation ( 1, as the environments most likely to host imminent MBH binaries. These results establish a unified cosmological framework connecting DAGN demographics, MBH binary evolution, and gravitational-wave sources, while identifying high-priority targets for coordinated electromagnetic and multimessenger observations in the coming decade.

Baryonic feedback from active galactic nuclei (AGN) is often invoked as a major source of suppression in the matter power spectrum, with implications for precision cosmology and the S ₈ tension. We present Astrid-DMO , the dark-matter-only counterpart to the large-volume Astrid hydrodynamical simulation, and measure baryonic effects through P _hydro ( k )/ P _DMO ( k ). We find no significant suppression at z = 0 and mild suppression at z = 0.2, weaker than in other state-of-the-art simulations. Using controlled small-volume runs, we identify a key driver of this discrepancy: the treatment of black hole (BH) dynamics. The widely used BH repositioning scheme artificially enhances BH mergers and boosts kinetic AGN feedback (e.g., by a factor of 2 at z = 1.5), leading to overly strong suppression. By contrast, a more physical dynamical friction model reduces feedback efficiency and weakens clustering suppression. Consequently, reconciling large-scale structure measurements with cosmic microwave background (CMB)-inferred ΛCDM cosmology, while matching observed halo gas fractions, becomes more challenging. Although strengthening AGN feedback can increase suppression, in our model this induces tensions with the observed galaxy stellar mass and AGN luminosity functions. These results sharpen the need for novel mechanisms that can efficiently eject gas from halos without compromising other galaxy properties.

We present a new cosmological analysis of the small-scale Lyman- α forest 1D flux power spectrum (P1D) using high-resolution quasar spectra from XQ100 and KODIAQ-SQUAD, interpreted through the PRIYA emulator. PRIYA is a suite of galaxy formation simulations spanning a range of cosmological and inhomogeneous He ii reionization parameters, enabling few-percent-level predictions of the P1D. These datasets, probing down to k ∼ 6 h Mpc ^-1 at z = 2-5, offer access to non-linear scales inaccessible to large-volume surveys like eBOSS. We find that the XQ100 P1D yields constraints on the primordial power spectrum parameters ( A _P , n _P ) at pivot scale k ₀ = 0.78 Mpc ^-1 that are consistent with PRIYA results from eBOSS DR14 and Planck CMB, albeit with broader uncertainties. Notably, this is achieved without external IGM temperature data, showing that XQ100 alone provides stronger constraints on thermal history than eBOSS DR14. In contrast, the KODIAQ-SQUAD P1D favors a significantly higher A _P value, driven by the selection bias toward high-column density absorbers (HCDs). We also find that the P1D at k > 0.045 s/km is more sensitive to Lyman limit system contamination and thermal history. When imposing a prior on ( A _P , n _P ), the reduced χ ² remains unchanged and the inferred mean IGM temperature is unaffected, suggesting that cosmological and thermal parameters are largely sensitive to different scales. The XQ100 P1D therefore provides complementary information on thermal nuisance parameters, which can be jointly fit with eBOSS or DESI P1D measurements to improve cosmological constraints.

We study the effects of varying different active galactic nucleus (AGN) feedback parameters on the Ly α forest 1D transmitted flux power spectrum (P1D). We use the Cosmology and Astrophysics with MachinE Learning Simulations suite to explore variations on the Simba simulation AGN feedback model. The parameters explored include the AGN momentum flux, AGN jet speed, supermassive black hole (SMBH) radiative efficiency, jet velocity threshold, and minimum SMBH mass needed to produce jet feedback. Although all parameters affect the P1D, this work explores the radiative efficiency, jet velocity threshold, and minimum SMBH mass in this context for the first time and finds the following results. Primarily, the most massive SMBHs impact the Ly α forest through the jet feedback mode. While heating AGN jets to the virial temperature at injection aids in the removal of neutral hydrogen from the Ly α forest, this heating also inhibits further jet feedback. Similar behaviors are seen when varying the SMBH radiative efficiency, with higher values resulting in a suppression of the SMBH growth and thus a later reduction in the AGN feedback and lower values directly reducing the impact of the AGN feedback on the Ly α forest P1D. These results imply that increasing the AGN feedback strength in the Simba simulation model suppresses the Ly α forest P1D, but only if the feedback does not impact the number of massive-jet-producing BHs. Future studies of AGN feedback models will require careful exploration of the unique aspects of the specific subgrid model, and how they interact with one another, for a complete understanding of the potential astrophysical impacts of SMBH feedback.

We present the z = 0 results for the cosmological simulation ASTRID . Hosting 2 × 5500 ³ ≈ 0.33 trillion particles in a box of 370 Mpc per side, ASTRID is one of the largest cosmological hydrodynamic simulations evolved to z = 0. ASTRID features a large population of massive black holes (MBHs), covering a wide mass range 4 × 10 ⁴ ∼ 2 × 10 ¹¹ M _⊙ . The adopted dynamical friction model provides a relatively accurate description of MBH dynamics, making ASTRID a powerful tool to study MBH growth and mergers in a cosmological context. ASTRID successfully captures the coevolution of MBHs and their host galaxies, producing M _BH – M _⋆ and M _BH – σ relations in good agreement with observations. Notably, ASTRID generates scatter in these relations that is more consistent with observations than previous simulations, indicating a more realistic MBH diversity. The galaxy stellar mass function at z = 0 is generally consistent with observational constraints. When dust attenuation is applied, the galaxy luminosity function also agrees well with observations, and the bimodality in galaxy colors is reproduced as well. ASTRID hosts a large population of massive galaxy groups and clusters: seven halos have M _200c > 10 ¹⁵ M _⊙ , and 9709 halos have M _200c > 10 ¹³ M _⊙ . We quantify the stellar mass content in these halos, and find that the correlations between the stellar and halo mass match well with observational constraints. Finally, we present the z = 0 power spectra of MBH and galaxies, as well as their bias with respect to the matter power spectrum. We find that MBHs with M _BH ≥ 10 ⁸ M _⊙ and galaxies with M _⋆ ≥ 10 ^10.5 M _⊙ serve as good tracers of large-scale structure.