Suppression of low-mass galaxy formation during reionisation
18 April 2016
The Universe experienced an "Epoch of Reionisation" (EoR) at redshift z ≥ 6 during which the cosmic diffuse neutral hydrogen (HI) was "photo-ionised" by a background of ultraviolet (UV) and X-ray radiation. This radiation was produced by the first generation of stars and galaxies that formed from HI. Otherwise inefficient in their star formation, the gas from the shallow potential wells of low-mass dark matter haloes also got heated and expelled by this radiation. The questions remain whether there is a critical dark matter halo mass below which galaxy formation was suppressed during reionisation, and what the precise value of the halo circular velocity is.
A recent publication by CAASTRO members in the Evolving Universe theme, led by University of Melbourne Affiliate Dr Hansik Kim, used data from the HI Parkes All-Sky Survey (HIPASS) and Arecibo Legacy Fast ALFA Survey (ALFALFA) to estimate the HI mass function down to masses of a few times 106 h-2 Solar Masses in the local Universe. The researchers tried to reproduce the observed HI mass functions using the semi-analytic galaxy formation model GALFORM – previously developed by CAASTRO member Dr Claudia Lagos at ICRAR-UWA – and to probe the physics of low-mass galaxy formation.
By default, GALFORM models the effect of photo-ionisation feedback by restricting galaxy formation in haloes of smaller effective circular velocities than a fixed value at redshifts following the end of reionisation. The predicted HI mass function from this model shows good agreement with observations for masses greater than ~ 108 h-2 Solar Masses but it does not explain the abundance of galaxies with HI masses between 106 h-2 and 108 h-2 Solar Masses. In the current study, the research team built on the default model by varying parameters such as supernova feedback, efficiency of star formation, the cosmological model, redshift and critical velocity of suppression of galaxy formation by photo-ionisation feedback. All of these effects failed to explain the shape and abundance of the low-mass end of HI mass function though.
They show that redshift-dependent modelling of feedback from photo-ionisation on low-mass galaxy formation was needed to match the shape and abundance of the observed HI mass function in the local Universe. They also found that the HI mass function was more sensitive to the redshift evolution of photo-ionisation feedback at high redshift. Future measurements of HI clustering in low-mass galaxies and the relation between the HI mass and the stellar mass of galaxies will further constrain the form of ionising feedback.