arXiv subjects

Nina Akerman

Publications and source records attributed to Nina Akerman.

6 recordsLinked to original sources

arXiv2026

Great Balls of Fire: Star Formation in Gas Clouds Accelerated by a Hot Wind

Satellite galaxies undergo ram pressure stripping, in which their gas is directly removed by a hydrodynamical interaction with the surrounding host halo gas. In clusters, ram pressure stripped tails of gas have been observed to be multiphase, even forming stars within the stripped material. Some observations find a specific age gradient along the tail, with old stars closer to the galaxy disk, and a ``fireball'' toy model has been proposed in which a gas cloud being accelerated away from a galaxy continuously forms stars. In this paper, we simulate individual gas clouds (with masses of ~10$^6$ M$_\odot$ and radii of a few-100 pc) interacting with an intracluster medium wind, and include star formation. We find that our accelerating clouds do generally produce a stellar age gradient with younger stars formed farther along the wind direction and with higher velocities. However, our simulations are more physically accurate than an empirical model of monolithic cloud acceleration, leading to strongly nonmonotonic age gradients. First, the evolution of the gas cloud, both from cloud compression and collapse as well as from the shredding of cloud material into downwind filaments, can lead to stars formed simultaneously at a range of heights and velocities. Second, the gravity from the gas and stars of the cloud can lead to velocity evolution of newly-formed stars. We conclude that the most distinct fireball stellar age gradients are formed from star-forming clouds that are rapidly accelerated and shredded by their surroundings.

astro-ph.GA↗

arXiv2025

What goes around comes around: the fate of stars in stripped tails of gas

We conduct high-resolution wind-tunnel simulations to study in-situ star formation in the stripped tails of two massive ($M_\text{star}=10^{11}M_\odot$) galaxies undergoing time-evolving ram-pressure stripping: one is stripped face-on (W0) and the other is subject to an angled wind (W45). We find that the majority of stars in the tail are formed close to the galaxy disc at the beginning of stripping. Most stars have ages that reflect outside-in stripping -- older stars are found at larger radii than younger stars. The velocities and metallicities of stars indicate that ICM mixing both increases the velocity and decreases the metallicity of star-forming gas, leading to faster, lower metallicity stars at larger distances from the galaxies. However, not all stars follow this simple model, even in the case of face-on stripping. Indeed, a considerable number (15--25 per cent) of tail stars are formed with negative velocities, indicating the fallback of star-forming gas on to the galaxy. Almost all of the tail stars formed within 20 kpc from the disc will eventually fall back on to the galaxy, and their contribution to the intracluster light is negligible. The orbits of the stars formed in the tail result in an extended (and asymmetric in the case of W45) stellar distribution around the disc. Mock UV images reveal that the observed vertical distribution of its stars is not significantly broader than in an undisturbed galaxy, indicating that more stars would need to form in the stripped tail than we find in our simulations to observably impact the UV disc width of ram pressure stripped galaxies.

astro-ph.GA↗

arXiv2024

Investigating the intracluster medium viscosity using the tails of GASP jellyfish galaxies

The microphysics of the intracluster medium (ICM) in galaxy clusters is still poorly understood. Observational evidence suggests that the effective viscosity is suppressed by plasma instabilities that reduce the mean free path of particles. Measuring the effective viscosity of the ICM is crucial to understanding the processes that govern its physics on small scales. The trails of ionized interstellar medium left behind by the so-called jellyfish galaxies can trace the turbulent motions of the surrounding ICM and constrain its local viscosity. We present the results of a systematic analysis of the velocity structure function (VSF) of the H$\alpha$ line for ten galaxies from the GASP sample. The VSFs show a sub-linear power law scaling below 10 kpc which may result from turbulent cascading and extends to 1 kpc, below the supposed ICM dissipation scales of tens of kpc expected in a fluid described by Coulomb collisions. Our result constrains the local ICM viscosity to be 0.3-25$\%$ of the expected Spitzer value. Our findings demonstrate that either the ICM particles have a smaller mean free path than expected in a regime defined by Coulomb collisions, or that we are probing effects due to collisionless physics in the ICM turbulence.

astro-ph.CO↗

arXiv2023

The Surprising Lack of Effect from Stellar Feedback on the Gas Stripping Rate from Massive Jellyfish Galaxies

We study the role of star formation and stellar feedback in a galaxy being ram pressure stripped on its infall into a cluster. We use hydrodynamical wind-tunnel simulations of a massive galaxy ($M_\text{star} = 10^{11} M_\odot$) moving into a massive cluster ($M_\text{cluster} = 10^{15} M_\odot$). We have two types of simulations: with and without star formation and stellar feedback, SF and RC respectively. For each type we simulate four realisations of the same galaxy: a face-on wind, edge-on wind, $45^\circ$ angled wind, and a control galaxy not subject to ram pressure. We directly compare the stripping evolution of galaxies with and without star formation. We find that stellar feedback has no direct effect on the stripping process, i.e. there is no enhancement in stripping via a velocity kick to the interstellar medium gas. The main difference between RC and SF galaxies is due to the indirect effect of stellar feedback, which produces a smoother and more homogeneous interstellar medium. Hence, while the average gas surface density is comparable in both simulation types, the scatter is broader in the RC galaxies. As a result, at the galaxy outskirts overdense clumps survive in RC simulation, and the stripping proceeds more slowly. At the same time, in the inner disc, underdense gas in the RC holes is removed faster than the smoothly distributed gas in the SF simulation. For our massive galaxy, we therefore find that the effect of feedback on the stripping rate is almost negligible, independent of wind angle.

astro-ph.GA↗

arXiv2023

Ram pressure stripping in the EAGLE simulation

Ram pressure stripping of satellite galaxies is thought to be a ubiquitous process in galaxy clusters, and a growing number of observations reveal satellites at different stages of stripping. However, in order to determine the fate of any individual galaxy, we turn to predictions from either simulations or analytic models. It is not well-determined whether simulations and analytic models agree in their predictions, nor the causes of disagreement. Here we investigate ram pressure stripping in the reference EAGLE hydrodynamical cosmological simulation, and compare the results to predictions from analytic models. We track the evolution of galaxies with stellar mass $M_{*} > 10^{9} \, \mathrm{M_{\odot}}$ and initial bound gas mass $M_{\mathrm{gas}} > 10^{9} \, \mathrm{M_{\odot}}$ that fall into galaxy clusters ($M_{\mathrm{200c}} > 10^{14} \, \mathrm{M_{\odot}}$) between $z = 0.27$ and $z = 0$. We divide each galaxy into its neutral gas disk and hot ionized gas halo and compare the evolution of the stripped gas fraction in the simulation to that predicted by analytic formulations for the two gas phases, as well as to a toy model that computes the motions of gas particles under the combined effects of gravity and a spatially uniform ram pressure. We find that the analytic models generally underpredict the stripping rate of neutral gas and overpredict that of ionized gas, with significant scatter between the model and simulation stripping timescales. This is due to opposing physical effects: the enhancement of ram pressure stripping by stellar feedback, and the suppression of stripping by the compaction of galactic gas.

astro-ph.GA↗

arXiv2023

How Ram Pressure Drives Radial Gas Motions in the Surviving Disk

Galaxy evolution can be dramatically affected by the environment, especially by the dense environment of a galaxy cluster. Recent observational studies show that massive galaxies undergoing strong ram pressure stripping (RPS) also show an enhanced frequency of nuclear activity. Here, we investigate this topic using a suite of wind-tunnel hydrodynamical simulations of an individual massive $M_\text{star} = 10^{11} M_\odot$ disk galaxy with 39 pc resolution and including star formation and stellar feedback. We find that RPS increases the inflow of gas to the galaxy centre regardless of the wind impact angle. This increase is driven by the mixing of interstellar and non-rotating intracluster media at all wind angles, and by increased torque on the inner disk gas, mainly from local pressure gradients when the ICM wind has an edge-on component. In turn, the increase in pressure torques is driven by rising gradient of ram pressure. We estimate the black hole (BH) accretion using Bondi-Hoyle and torque models, and compare it with the mass flux in the central 140 pc region. We find that the torque model estimates much less accretion onto the BH of a RPS galaxy than the Bondi-Hoyle estimator. However, we argue that both models are incomplete because the commonly used torque model does not account for torques caused by the gas distribution or local pressure gradients and the Bondi-Hoyle estimator depends on the the sound speed of the hot gas, which includes the ICM in stripped galaxies, thus a new estimator would be required.

astro-ph.GA↗