arXiv subjects

Ryan Jeffrey Farber

Publications and source records attributed to Ryan Jeffrey Farber.

6 recordsLinked to original sources

arXiv2025

Little Red Dots: The Assembly of Early Supermassive Black Holes in the JWST Era

Since the launch of James Webb Space Telescope (JWST) in late 2021, our understanding of high-redshift objects has faced several upheavals. JWST has discovered much more massive galaxies and supermassive black holes (SMBH) than cosmological models had expected. Furthermore, JWST observations have revealed an entirely novel population of high-redshift objects. Characterized by a dominant red rest-frame component and point-like morphology, these ``little red dots'' (LRD) have set off a flurry of observational and theoretical follow-up. The current identity of LRD is highly debated, yet falling into two main scenarios: active galactic nuclei (i.e., SMBH) or compact star-forming regions. If star-forming, LRD would represent the highest stellar densities ever observed. If SMBH, their high Eddington fractions, and already high masses, help elucidate the growth of the most massive SMBH found by JWST in the early Universe ($z \gtrsim4)$. In this mini-review, we present the observational evidence accumulated to date, including sub-millimeter probes of LRD dust masses, constraints on radio and X-ray emission from stacking, and rest-frame ultraviolet \& optical measurements provided by JWST. Furthermore, we highlight how identifying additional LRD that are truly primarily SMBH-driven may help to shed light on the formation of `overly massive' SMBH discovered by JWST within the first billion years since the Big Bang.

astro-ph.GA↗

arXiv2025

Multi-cloud crushing -- the collective survival of cold clouds in galactic outflows

The ram-pressure acceleration of cold gas by hot outflows plays a crucial role in the dynamics of multiphase galactic winds. Recent numerical studies incorporating radiative cooling have identified a size threshold for idealized cold clouds to survive within high-velocity outflows. This study extends the investigation to a more complex morphology of cold gas as observed in the interstellar medium. We conduct three-dimensional hydrodynamic simulations of ensembles of individual spherical clouds to systematically explore under which conditions the cold clouds can survive. We find that cloud ensembles can survive collectively -- even when individual clouds, if isolated, would be rapidly destroyed. Our results indicate that, besides the morphology, factors such as tight packing, small inter-cloud distance and higher fragmentation facilitate survival. We propose a novel multi-cloud survival criterion that accounts for collective properties of the cloud system, including total gas mass and the geometric configuration based on an effective volume filling fraction of the cold gas $F_V$. This fraction is computed by constructing a composite volume from individual enclosing conical boxes aligned with the wind, incorporating spatial overlap and cloud-tail spreading. The box dimensions scale with the critical survival radius $r_{\rm crit}$ from the single-cloud criterion. We find a universal threshold $F_{V,{\rm crit}}\approx 0.24$ that robustly separates surviving from destroyed systems across diverse geometric configurations. Our findings emphasize the critical importance of initial cloud distribution and fragmentation in governing the long-term evolution and survival of cold gas structures, providing insight into observed multiphase outflows and CGM dynamics.

astro-ph.GA↗

arXiv2023

Better Together: The Complex Interplay Between Radiative Cooling and Magnetic Draping

Rapidly outflowing cold H-I gas is ubiquitously observed to be co-spatial with a hot phase in galactic winds, yet the ablation time of cold gas by the hot phase should be much shorter than the acceleration time. Previous work showed efficient radiative cooling enables clouds to survive in hot galactic winds under certain conditions, as can magnetic fields even in purely adiabatic simulations for sufficiently small density contrasts between the wind and cloud. In this work, we study the interplay between radiative cooling and magnetic draping via three dimensional radiative magnetohydrodynamic simulations with perpendicular ambient fields and tangled internal cloud fields. We find magnetic fields decrease the critical cloud radius for survival by two orders of magnitude (i.e., to sub-pc scales) in the strongly magnetized ($β_{\rm wind}=1$) case. Our results show magnetic fields (i) accelerate cloud entrainment through magnetic draping, (ii) can cause faster cloud destruction in cases of inefficient radiative cooling, (iii) do not significantly suppress mass growth for efficiently cooling clouds, and, crucially, in combination with radiative cooling (iv) reduce the average overdensity by providing non-thermal pressure support of the cold gas. This substantially reduces the acceleration time compared to the destruction time (more than due to draping alone), enhancing cloud survival. Our results may help to explain the cold, tiny, rapidly outflowing cold gas observed in galactic winds and the subsequent high covering fraction of cold material in galactic halos.

astro-ph.GA↗

arXiv2022

Molecular Shattering

Recent observations suggest galaxies may ubiquitously host a molecular component to their multiphase circumgalactic medium (CGM). However, the structure and kinematics of the molecular CGM remains understudied theoretically and largely unconstrained observationally. Recent work suggests molecular gas clouds with efficient cooling survive acceleration in hot winds similar to atomic clouds. Yet the pressure-driven fragmentation of molecular clouds when subjected to external shocks or undergoing cooling remains unstudied. We perform radiative, inviscid hydrodynamics simulations of clouds perturbed out of pressure equilibrium to explore the process of hydrodynamic fragmentation to molecular temperatures. We find molecular clouds larger than a critical size can shatter into a mist of tiny droplets, with the critical size deviating significantly from the atomic case. We find that cold clouds shatter only if the sound crossing time exceeds the local maximum of the cooling time ~8000 K. Moreover, we find evidence for a universal mechanism to 'shatter' cold clouds into a 'mist' of tiny droplets as a result of rotational fragmentation -- a process we dub 'splintering.' Our results have implications for resolving the molecular phase of the CGM in observations and cosmological simulations.

astro-ph.GA↗

arXiv2022

Stress-Testing Cosmic Ray Physics: The Impact of Cosmic Rays on the Surviving Disk of Ram Pressure Stripped Galaxies

Cluster spiral galaxies suffer catastrophic losses of the cool, neutral gas component of their interstellar medium due to ram pressure stripping, contributing to the observed quenching of star formation in the disk compared to galaxies in lower density environments. However, the short term effects of ram pressure on the star formation rate and AGN activity of galaxies undergoing stripping remain unclear. Numerical studies have recently demonstrated cosmic rays can dramatically influence galaxy evolution for isolated galaxies, yet their influence on ram pressure stripping remains poorly constrained. We perform the first cosmic-ray magneto-hydrodynamic simulations of an $L_{*}$ galaxy undergoing ram pressure stripping, including radiative cooling, self-gravity of the gas, star formation, and stellar feedback. We find the microscopic transport of cosmic rays plays a key role in modulating the star formation enhancement experienced by spirals at the outskirts of clusters compared to isolated spirals. Moreover, we find that galaxies undergoing ram pressure stripping exhibit enhanced gas accretion onto their centers, which may explain the prevalence of AGN in these objects. In agreement with observations, we find cosmic rays significantly boost the global radio emission of cluster spirals. Although the gas removal rate is relatively insensitive to cosmic ray physics, we find that cosmic rays significantly modify the phase distribution of the remaining gas disk. These results suggest observations of galaxies undergoing ram pressure stripping may place novel constraints on cosmic-ray calorimetry and transport.

astro-ph.GA↗

arXiv2021

The Survival of Multiphase Dusty Clouds in Hot Winds

Much progress has been made recently in the acceleration of $\sim10^{4}$\,K clouds to explain absorption-line measurements of the circumgalactic medium and the warm, atomic phase of galactic winds. However, the origin of the cold, molecular phase in galactic winds has received relatively little theoretical attention. Studies of the survival of $\sim10^{4}$\,K clouds suggest efficient radiative cooling may enable the survival of expelled material from galactic disks. Alternatively, gas colder than 10$^4$\,K may form within the outflow, including molecules if dust survives the acceleration process. We explore the survival of dusty clouds in a hot wind with three-dimensional hydrodynamic simulations including radiative cooling and dust modeled as tracer particles. We find that cold $\sim10^{3}$\,K gas can be destroyed, survive, or transformed entirely to $\sim 10^4\,$K gas. We establish analytic criteria distinguishing these three outcomes which compare characteristic cooling times to the system's `cloud crushing' time. In contrast to typically studied $\sim10^{4}$\,K clouds, colder clouds are entrained faster than the drag time as a result of efficient mixing. We find that while dust can in principle survive embedded in the accelerated clouds, the survival fraction depends critically on the time dust spends in the hot phase and on the effective threshold temperature for destruction. We discuss our results in the context of polluting the circumgalactic medium with dust and metals, as well as understanding observations suggesting rapid acceleration of molecular galactic winds and ram pressure stripped tails of jellyfish galaxies.

astro-ph.GA↗