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Postdoctoral Research Position in modelling of the disc-halo interface in star-forming galaxies

Embedding: This project is part of the ERC Advance Grant project "FLOWS": Gas flows in and out of galaxies: solving the cosmic baryon cycle

Group: The FLOWS group will consist of 3 PhD students, 2 postdocs and the PI

PI: Prof. F. Fraternali

Collaborators: Prof. K. Rubin (San Diego State University, USA) and Prof. F. Marinacci (University of Bologna, Italy)

Note that aside the specific project below the successful candidate will have some opportunity to pursue their own independent research

Scientific background


The majority of matter in the Universe is in the form of gas, found in galaxies and their surrounding environments. Gas flows, including accretion and ejection, play a crucial role in the growth, star formation, and overall properties of galaxies. However, our understanding of these gas flows is limited. Observationally, we have gathered a significant amount of data on gas in and outside of galaxies, but the interpretation is uncertain. Theoretical models also lack a clear understanding of how large-scale gas flows relate to gas accretion onto galaxies and the effectiveness of ejective feedback in removing gas. This project aims to address these gaps in knowledge by investigating all gas flows and incorporating observations into a coherent physical picture.

The key to understand gas accretion is hold by the processes that take place at the disc-halo interface of star-forming galaxies. There, a continuous circulation of gas powered by supernova feedback (galactic fountain) stirs and mixes the disc gas with inner circumgalactic medium (CGM) and redistributes gas, metals, and angular momentum. In previous works, evidence has been found that galactic fountains can contribute to gas accretion by promoting the cooling and subsequent accretion of ambient gas above the galactic disc. This is important because the hot CGM surrounding disc galaxies has a long cooling time and a low mass accretion rate, making it insufficient to feed star formation. However, the galactic fountain reduces the cooling time of the hot CGM by mixing it with high-metallicity cold gas from the disc, leading to its condensation and accretion onto the disc (Fig. 1). This mechanism is referred to as "fountain accretion" (Fraternali 2017). The process of fountain accretion can be a crucial way for galaxies to gather gas for star formation, particularly during the epoch of hot-mode accretion. This mechanism leaves a distinct signature in the kinematics of extraplanar gas, which can be easily identified in observations. However, the efficiency of fountain accretion, its universality across all star-forming galaxies, and its evolution over time are still unknown.

Figure 1. A sketch of the fountain accretion mechanism in the Milky Way. The cloud+wake systems are visible both through emission and absorption lines. From Fraternali et al. (2013).

Main goals of the project


The main goals of this project are to unveil and measure, for the first time, the gas accretion via fountain accretion in local and high-z galaxies. This will be achieved by building a new dynamical model of the galactic fountain, informed by ultra-high-resolution magneto-hydrodynamical simulations (Fig. 2), that will be fit directly to emission-line datacubes.


Figure 2. Temperature slice of a magneto-hydrodynamical simulation of a cloud moving (along x) through the hot CGM. The magnetic field is initially perpendicular to the motion and the simulation also includes radiative cooling and anisotropic thermal conduction.  All gas below 20000 K has been coloured dark blue, gas above a million degree is in red. From Kooij et al. (2021).


The starting point will be the "extragas" code developed in previous studies (Fraternali & Binney 2008; Marasco et al. 2012; Li et al. 2022). The code produces 3D mock observations from a fountain accretion model, taking into account the galactic potential and hydrodynamical effects. The models have few free parameters, such as ejection velocities of the fountain gas and the coronal condensation rate, which can be estimated by comparing with observations. The key observable is the gas kinematics at the disc-halo interface, specifically in the extraplanar gas layer. The presence of corona condensation can be detected through the modification of fountain gas trajectories.


We will modify the extragas software by implementing an MCMC algorithm for parameter estimation and improving the hydrodynamical interactions between fountain clouds and hot gas. This latter will be achieved through the use of a library of ultra-high-resolution magneto-hydrodynamical simulations. Approximately 100 simulations will be conducted to explore the parameter space, with at least half of them being run on our own computer cluster. These simulations will provide valuable data on condensation rates based on different fountain and corona parameters. The results obtained from these simulations will contribute to the development of the first simulation-informed analytic model (SIAM) for a galactic fountain.

The study will be applied to observations of local galaxies, particularly HI and Halpha datacubes. The availability of HI observations from surveys and upcoming observations from SKA pathfinders will provide ample data for the study. The goal is to determine the prevalence of fountain condensation in nearby galaxies, assess gas accretion profiles, and analyze the angular momentum of the accreting gas. Additionally, ionized gas can also be used as a tracer for accretion, and observations from the William Herschel Telescope (WHT) Enhanced Area Velocity Explorer (WEAVE) will be utilized for this component. At higher redshift we will use both VLT data and ALMA data, in particular [CII] observations that are already showing extraplanar gas layer in galaxies up to z~4.5 (Fig. 3).


Figure 3. Left: HI position-velocity (PV) diagram along the major axis of the nearby galaxy NGC 6946 showing extraplanar gas emission (arrows) outside the normal disc gas, roughly included in the green curve and dimmed (Boomsma+ 2008). Right: [CII] PV diagram of the galaxy SGP38326-1 at z=4.4 obtained with ALMA also showing the presence of extraplanar gas.


Boomsma R., Oosterloo T.A., Fraternali F., van der Hulst J.M., Sancisi R., 2008, A&A, 490, 555
Fraternali, F. & Binney, J.J., 2008, MNRAS, 386, 935

Fraternali, F., 2017, Springer, Astrophysics and Space Science Library, 430, 323
Kooij, R., Grønnow, A., and Fraternali, F. 2021, MNRAS, 502, 1263
Marasco, A., Fraternali, F., Binney, J.J. 2012, MNRAS, 419, 1107


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