ORCID Profile
0000-0003-0573-1606
Current Organisation
University of Sydney
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Publisher: American Astronomical Society
Date: 11-08-2017
Publisher: Oxford University Press (OUP)
Date: 30-06-2015
Publisher: American Astronomical Society
Date: 21-09-2018
Publisher: Oxford University Press (OUP)
Date: 15-01-2021
Abstract: The large temperature difference between cold gas clouds around galaxies and the hot haloes that they are moving through suggests that thermal conduction could play an important role in the circumgalactic medium. However, thermal conduction in the presence of a magnetic field is highly anisotropic, being strongly suppressed in the direction perpendicular to the magnetic field lines. This is commonly modelled by using a simple prescription that assumes that thermal conduction is isotropic at a certain efficiency f & 1, but its precise value is largely unconstrained. We investigate the efficiency of thermal conduction by comparing the evolution of 3D hydrodynamical (HD) simulations of cold clouds moving through a hot medium, using artificially suppressed isotropic thermal conduction (with f), against 3D magnetohydrodynamical (MHD) simulations with (true) anisotropic thermal conduction. Our main diagnostic is the time evolution of the amount of cold gas in conditions representative of the lower (close to the disc) circumgalactic medium of a Milky-Way-like galaxy. We find that in almost every HD and MHD run, the amount of cold gas increases with time, indicating that hot gas condensation is an important phenomenon that can contribute to gas accretion on to galaxies. For the most realistic orientations of the magnetic field with respect to the cloud motion we find that f is in the range 0.03–0.15. Thermal conduction is thus always highly suppressed, but its effect on the cloud evolution is generally not negligible.
Publisher: Oxford University Press (OUP)
Date: 27-11-2021
Abstract: High-velocity clouds (HVCs) are believed to be an important source of gas accretion for star formation in the Milky Way. Earlier numerical studies have found that the Galactic magnetic field and radiative cooling strongly affects accretion. However, these effects have not previously been included together in the context of clouds falling through the Milky Way’s gravitational potential. We explore this by simulating an initially stationary cloud falling through the hot hydrostatic corona towards the disc. This represents an HVC that has condensed out of the corona. We include the magnetic field in the corona to examine its effect on accretion of the HVC and its associated cold gas. Remnants of the original cloud survive in all cases, although a strong magnetic field causes it to split into several fragments. We find that mixing of cold and hot gas leads to cooling of coronal gas and an overall growth with time in cold gas mass, despite the low metallicity of the cloud and corona. The role of the magnetic field is to (moderately to severely) suppress the mixing and subsequent cooling, which in turn leads to less accretion compared to when the field is absent. A stronger field leads to less suppression of condensation because it enhances Rayleigh–Taylor instability. However, magnetic tension in a stronger field substantially decelerates condensed cloudlets. These have velocities typically a factor 3–8 below the velocity of the main cloud remnants by the end of the simulation. Some of these cloudlets likely disperse before reaching the disc.
Publisher: Oxford University Press (OUP)
Date: 03-07-2023
Abstract: Galactic halos accrete material from the intergalactic medium (IGM) and part of this accretion is expected to be in the form of cool (T ∼ 104 K) gas. A signature of this process could reside in the detection of numerous clouds in the circumgalactic medium (CGM). However, whether this material is able to accrete onto the galaxies and feed their star formation or, instead, evaporates into the CGM hot phase (corona, T ∼ 106 K), is not yet understood. Here, we investigate the evolution of cool CGM clouds accreted from the IGM and falling through the hot corona of low-redshift disc galaxies, using 3D high-resolution hydrodynamical simulations. We include the effects of gravity due to the dark matter halo, isotropic thermal conduction, radiative cooling, and an ionizing UV background. We explored different values of parameters such as the halo mass, coronal mass, initial cloud velocity and strength of the thermal conduction. We find that the clouds lose the vast majority of their mass at distances larger than half of the galaxy virial radius and are completely dissolved in the corona before reaching the central galaxy. Resolving the Field length with at least 5–7 cells is crucial to correctly capture the evolution of the infalling cool gas. Our results indicate that cool IGM accretion can not feed star formation in z ∼ 0 star-forming galaxies in halos with masses of 1011.9 M⊙ or above. This suggests that present-day massive star-forming galaxies can sustain their star formation only via the spontaneous or induced cooling of their hot corona.
No related grants have been discovered for Asger Grønnow.