arXiv subjects

Joseph A. O'Leary

Publications and source records attributed to Joseph A. O'Leary.

6 recordsLinked to original sources

arXiv2026

Blue Monsters and Dusty Descendants: Reconciling UV and IR Emission from Galaxies from z=7, up to z= 14

Recent JWST observations reveal massive, UV-bright galaxies at $z > 10$ with little apparent dust attenuation, whereas ALMA detections at $z \simeq 7$ show similarly massive systems that are already dust-rich and IR-luminous. This raises a fundamental question: can a single physical model of star formation and dust production explain both populations across cosmic time? We address this using a minimal framework with only two free parameters--the instantaneous star formation efficiency ($ε_\star$) and the dust yield per Type II supernova ($y_d$)--and predict the rest-frame UV and IR luminosity functions (LFs) from $z \simeq 14$ to 7. For a uniform ISM, we find a UV-IR tension at the bright end of the LFs at $z \ge 7$. The UV LF requires low dust yields ($y_d \lesssim 0.01\,M_\odot$), whereas the $z=7$ IR LF requires higher yields ($y_d \sim 0.1\,M_\odot$) unless the star formation efficiency is boosted above $ε_\star \approx 5$-10%. We show that incorporating a porous, turbulent ISM largely resolves this tension: turbulence opens low-column-density sightlines that enhance the UV escape fraction while leaving the total absorbed energy--and thus the IR luminosity--nearly unchanged once radiative-transfer--induced flattening of the attenuation curve is included. Large-grain dust distributions, while reducing UV opacity, play a secondary role once ISM porosity and radiative transfer are taken into account. At $z > 10$, however, even strong turbulence cannot reproduce the bright end of the UV LF at high dust yield. This could be resolved either by efficient dust removal in early massive systems or by substantial ISM dust growth by $z \simeq 7$. Our results highlight dust physics as a key lever for interpreting the rapidly growing UV and IR constraints within the broader context of early galaxy formation.

astro-ph.GA↗

arXiv2023

Predictions on the stellar-to-halo mass relation in the dwarf regime using the empirical model for galaxy formation EMERGE

One of the primary goals when studying galaxy formation is to understand how the luminous component of the Universe, galaxies, relates to the growth of structure which is dominated by the gravitational collapse of dark matter haloes. The stellar-to-halo mass relation probes how galaxies occupy dark matter haloes and what that entails for their star formation history. We deliver the first self-consistent empirical model that can place constraints on the stellar-to-halo mass relation down to log stellar mass $\log_{10}(m^*/\mathrm{M}_{\odot}) \leq 5.0$ by fitting our model directly to Local Group dwarf data. This is accomplished by penalising galaxy growth in late-forming, low-mass haloes by mimicking the effects of reionization. This process serves to regulate the number density of galaxies by altering the scatter in halo peak mass $M^{\mathrm{peak}}_{h}$ at fixed stellar mass, creating a tighter scatter than would otherwise exist without a high-$z$ quenching mechanism. Our results indicate that the previously established double-power law stellar-to-halo mass relation can be extended to include galaxies with $\log_{10}(M^{\mathrm{peak}}_{\mathrm{h}}/\mathrm{M}_{\odot})\gtrsim 10.0$. Furthermore, we show that haloes with $\log_{10}(M^{\mathrm{peak}}_{\mathrm{h}}/\mathrm{M}_{\odot})\lesssim 9.3$ by $z=4$ are unlikely to host a galaxy with $\log_{10}(m^*/\mathrm{M}_{\odot}) > 5.0$.

astro-ph.GA↗

arXiv2021

EMERGE: Constraining merging probabilities and timescales of close galaxy pairs

Theoretical models are vital for exploring the galaxy merger process, which plays a crucial role in the evolution of galaxies. Recent advances in modelling have placed tight constraints on the buildup of stellar material in galaxies across cosmic time. Despite these successes, extracting the merger rates from observable data remains a challenge. Differences in modelling techniques, combined with limited observational data, drive conflicting conclusions on the merging timescales of close pairs. We employ an empirical model for galaxy formation that links galaxy properties to the growth of simulated dark matter halos, along with mock lightcone galaxy catalogues, to probe the dependencies of pair merging probabilities and merging timescales. In this work, we demonstrate that the pair merging probabilities are best described by a logistic function and that mean merging timescales can be well approximated by a linear relation in the projected separation and line of sight velocity difference in observed pairs. Together, our fitting formulae can accurately predict merger rates from galaxy pairs to at least $z\sim4$ under a wide variety of pair selection criteria. Additionally, we show that some commonly used pair selection criteria may not represent a suitable sample of galaxies to reproduce underlying merger rates. Finally, we conclude from our analysis that observation timescales are primarily driven by dynamics and are not strongly impacted by the star formation properties of the component galaxies.

astro-ph.GA↗

arXiv2021

Globular cluster numbers in dark matter haloes in a dual formation scenario: an empirical model within EMERGE

We present an empirical model for the number of globular clusters (GCs) in galaxies based on recent data showing a tight relationship between dark matter halo virial masses and GC numbers. While a simple base model forming GCs in low-mass haloes reproduces this relation, we show that a second formation pathway for GCs is needed to account for observed younger GC populations. We confirm previous works that reported the observed linear correlation as being a consequence of hierarchical merging and its insensitivity to the exact GC formation processes at higher virial masses, even for a dual formation scenario. We find that the scatter of the linear relation is strongly correlated with the relative amount of smooth accretion: the more dark matter is smoothly accreted, the fewer GCs a halo has compared to other haloes of the same mass. This scatter is smaller than that introduced by halo mass measurements, indicating that the number of GCs in a galaxy is a good tracer for its dark matter mass. Smooth accretion is also the reason for a lower average dark matter mass per GC in low-mass haloes. Finally, we successfully reproduce the observed general trend of GCs being old and the tendency of more massive haloes hosting older GC systems. Including the second GC formation mechanism through gas-rich mergers leads to a more realistic variety of GC age distributions and also introduces an age inversion in the halo virial mass range $\log M_\mathrm{vir}/\mathrm{M}_\odot = 11{-}13$.

astro-ph.GA↗

arXiv2021

EMERGE: Empirical predictions of galaxy merger rates since $z\sim6$

We explore the galaxy-galaxy merger rate with the empirical model for galaxy formation, Emerge. On average, we find that between $2$ per cent and $20$ per cent of massive galaxies ($\log_{10}(m_{*}/M_{\odot}) \geq 10.3$) will experience a major merger per Gyr. Our model predicts galaxy merger rates that do not scale as a power-law with redshift when selected by descendant stellar mass, and exhibit a clear stellar mass and mass-ratio dependence. Specifically, major mergers are more frequent at high masses and at low redshift. We show mergers are significant for the stellar mass growth of galaxies $\log_{10}(m_{*}/M_{\odot}) \gtrsim 11.0$. For the most massive galaxies major mergers dominate the accreted mass fraction, contributing as much as $90$ per cent of the total accreted stellar mass. We reinforce that these phenomena are a direct result of the stellar-to-halo mass relation, which results in massive galaxies having a higher likelihood of experiencing major mergers than low mass galaxies. Our model produces a galaxy pair fraction consistent with recent observations, exhibiting a form best described by a power-law exponential function. Translating these pair fractions into merger rates results in an inaccurate prediction compared to the model intrinsic values when using published observation timescales. We find the pair fraction can be well mapped to the intrinsic merger rate by adopting an observation timescale that decreases linearly with redshift as $T_{\mathrm{obs}} = -0.36(1+z)+2.39$ [Gyr], assuming all observed pairs merge by $z=0$.

astro-ph.GA↗

arXiv2020

GalaxyNet: Connecting galaxies and dark matter haloes with deep neural networks and reinforcement learning in large volumes

We present the novel wide & deep neural network GalaxyNet, which connects the properties of galaxies and dark matter haloes, and is directly trained on observed galaxy statistics using reinforcement learning. The most important halo properties to predict stellar mass and star formation rate (SFR) are halo mass, growth rate, and scale factor at the time the mass peaks, which results from a feature importance analysis with random forests. We train different models with supervised learning to find the optimal network architecture. GalaxyNet is then trained with a reinforcement learning approach: for a fixed set of weights and biases, we compute the galaxy properties for all haloes and then derive mock statistics (stellar mass functions, cosmic and specific SFRs, quenched fractions, and clustering). Comparing these statistics to observations we get the model loss, which is minimised with particle swarm optimisation. GalaxyNet reproduces the observed data very accurately ($χ_\mathrm{red}=1.05$), and predicts a stellar-to-halo mass relation with a lower normalisation and shallower low-mass slope at high redshift than empirical models. We find that at low mass, the galaxies with the highest SFRs are satellites, although most satellites are quenched. The normalisation of the instantaneous conversion efficiency increases with redshift, but stays constant above $z\gtrsim0.7$. Finally, we use GalaxyNet to populate a cosmic volume of $(5.9~\mathrm{Gpc})^3$ with galaxies and predict the BAO signal, the bias, and the clustering of active and passive galaxies up to $z=4$, which can be tested with next-generation surveys, such as LSST and Euclid.

astro-ph.GA↗