News

“LyC escape from SPHINX galaxies in the Epoch of Reionization” by Rosdahl et al. has been accepted for publication in MNRAS. The paper explores correlations of the escape fraction of LyC radiation, f_esc, with various galaxy properties in tens of thousands of SPHINX galaxies and how f_esc evolves with redshift.  The escape fraction is found to fluctuate enormously in individual galaxies due to its regulation by stellar feedback. It is found to be highest in intermediate-mass, metallicity, and luminosity galaxies and drops strongly for the most massive, metal-rich, and brightest galaxies. The global average f_esc is found to drop with time, in sync with a decreasing intensity of star formation in the expanding Universe. 

A new SPHINX paper titled “Mg II in the JWST Era: a Probe of Lyman Continuum Escape?” by Katz et al. has been accepted for publication in MNRAS. The authors use SPHINX to probe whether there exists a correlation between the escape of ionizing radiation from galaxies and their MgII emission and absorption. If it exists, such a correlation could be used to identify the galaxies that power reionization. Statistically there is a tendency for galaxies with high LyC escape fractions to be MgII emitters, but with a lot of scatter. In short it’s complicated and line emission is difficult to interpret, especially since the same galaxy can exhibit very different spectral properties along different lines of sight.

“Predicting LyC emission of galaxies using their physical and Lyα emission properties” by Moupiya Maji et al. has been accepted for publication in A&A. This work uses the SPHINX simulations to study what observational properties can be used to predict the escaping luminosity of ionising Lyman-continuum (LyC) radiation from galaxies in the Epoch of Reionization, which is by itself very difficult to estimate observationally. A strong correlation between Lyman-alpha (Lyα) and Lyman-continuum escape fractions is found in Lya-bright galaxies is found, as in observations of low-redshift galaxies, and these Lya-bright galaxies are found to be the ones that predominantly reionize the Universe. However, this correlation breaks down in Lya-dim galaxies. The most important predictors of LyC luminosity are found to be Lyα luminosity, galaxy gas mass, galaxy metallicity, and star formation rate.

“Introducing SPHINX-MHD: the impact of primordial magnetic fields on the first galaxies, reionization, and the global 21-cm signal” studies the effects of primordial magnetic fields on high-redshift galaxy formation, using new SPHINX runs with radiative magneto-hydrodynamics. The simulations show that realistic-magnitude primordial fields do not have a large impact on star formation, but stronger magnetic fields tend to make galaxies somewhat more compact and have slightly stronger ionising escape fractions. Primordial field spectral indices of about -2.6 can be ruled out as they produce electron scattering optical depths for cosmic microwave background radiation which is higher than observational constraints.

“Cosmological Magnetogenesis: The Biermann battery during the Epoch of Reionization” proposes a new numerical scheme to model the generation of magnetic fields via the Biermann battery effect — currents generated by misaligned gradients in the electron pressure and density. The new numerical scheme is tested in a SPHINX volume and simulation setup. As expected, a weak volume-filling magnetic field is generated by expanding ionization fronts in the simulation volume. However, a much stronger inter-galactic magnetic field is also generated via the Biermann battery by expanding galactic winds, which are powered by supernova feedback in the first galaxies.

 

 

A new SPHINX paper, “Lyman-𝛼 as a tracer of cosmic reionisation in the SPHINX radiation-hydrodynamics cosmological simulation” by Thibault Garel et al. has been accepted for publication in MNRAS. The paper addresses the question of whether the increase in Lyman-alpha (Lya) emitters with decreasing redshift is due to an evolution in galaxy properties or due to the decreasing Lya absorption by neutral gas in the inter-galactic medium (IGM). Little or no evolution is found in internal Lya emission or escape from galaxies, so the evolution in Lya emitters is pinned on the reionizing IGM.

The second SPHINX paper has now been accepted for publication!
Katz et al. (2020) describes how the process of reionization suppresses the accretion of gas filaments onto dwarf galaxies and hence quenches their growth.

Thanks to PRACE , we have received 54 million core-hours to perform the next SPHINX simulation on the JUWELS supercomputer at JSC. The SPHINX20 volume has a width of 20 co-moving Mpc, and is 8 times larger than our previous largest volume. This will give us a much better sample of galaxies to study the variations in the escape of ionizing radiation from galaxies, up to ten times higher galaxy masses than we could before.

SPHINX20 is now running and has reached redshift 11. Stay tuned!

The first project paper has been submitted for publication and posted on ArXiv. The paper is named “The SPHINX Cosmological Simulations of the First Billion Years: the Impact of Binary Stars on Reionisation“. Here, we present the setup and physics of our simulation suite and address the question of whether and how binary stars affect reionisation. It turns out that due to mass transfer and mergers between binary companion stars, the escape fraction of ionising radiation from galaxies is significantly higher than without binary stars, and this leads to early reionisation (redshift >6), while the Universe fails to reionise before redshift 6 with single stars only.