Nashwan Sabti
UV luminosity functions provide a wealth of information on the physics of galaxy formation in the early Universe. Given that this probe indirectly tracks the evolution of the mass function of dark matter halos, it has the potential to constrain alternative theories of structure formation. One of such scenarios is the existence of primordial non-Gaussianity at scales beyond those probed by observations of the Cosmic Microwave Background. Through its impact on the halo mass function, such small-scale non-Gaussianity would alter the abundance of galaxies at high redshifts. In this work we present an application of UV luminosity functions as measured by the Hubble Space Telescope to constrain the non-Gaussianity parameter f_NL for wavenumbers above a cut-off scale k_cut. After marginalising over the unknown astrophysical parameters and accounting for potential systematic errors, we arrive at a 2sigma bound of f_NL = 71^{+426}_{-237} for a cut-off scale k_cut = 0.1 Mpc^-1 in the bispectrum of the primordial gravitational potential. Moreover, we perform forecasts for the James Webb Space Telescope, as well as future global-signal and interferometric 21-cm experiments, finding an expected improvement of a factor 3-4 upon the current bound.
A departure from Gaussianity in the primordial fluctuations alters the abundance of dark matter halos, and thus the UV luminosity function (UV LF) measured by, for example, the Hubble Space Telescope (HST). Here we show that this makes the UV LF a powerful probe of PNG, enabling us to search for it at small scales, which are difficult to access by cosmic microwave background (CMB) and large-scale structure (LSS) observations.
In the early Universe, galaxies contain young stars that emit in the ultra-violet part of the spectrum. This radiation gets redshifted due to the expansion of the Universe and can be observed today with telescopes such as the HST. The abundance of galaxies in the early Universe can thus be indirectly tracked through their luminosity function, which describes the relation between the observed number density of galaxies and their magnitude.
The LF consists of two parts: The first is the halo mass function, which describes how many halos of each mass there are, and is mainly influenced by Cosmology. The second is the halo-galaxy connection, driven by astrophysical processes and which allows us to relate the halo mass to the observed emission.
Through a simple phenomenological approach, we are able to link the abundance of galaxies to their luminosity.
The deviation from Gaussianity in the distribution of matter perturbations in the early Universe will become imprinted onto the abundance and distribution of galaxies. In principle, this means that the halo mass function will become dependent on the non-Gaussianity parameter f_NL (think about it as the strength of PNG).
Having this formalism established, we can now move on to comparing with data.
The high-redshift UV LF has been observed by the Hubble Space Telescope over a decades-long endeavour. This has resulted in two main data catalogs dubbed the Hubble Legacy Fields (HLF) and the Hubble Frontier Fields (HFF). The first consists of several deep-field surveys and has robustly probed the UV LF at the bright end, while the latter consists of observations of six cluster lenses, where faint background galaxies are magnified enough to become observable. The impact of primordial non-Gaussianities will be mainly visible at the bright end of the LF. Therefore, we perform our main analysis with the data obtained from the HLF catalog.
We perform an MCMC analysis and find constraints at 2sigma of f_NL = 71^{+426}_{-237}. We have also studied how well future data from the epochs of cosmic dawn and reionisation will be able to constrain small-scale PNG. We focused on two probes. The first is the upcoming James Webb Space Telescope, which will significantly improve upon the UV LFs of the HST. The second is 21-cm measurements during cosmic dawn. A set of forecasts shows that such experiments would be able to further improve upon the current bound by a factor 3-4.
This work establishes the UV LF as a powerful probe of the fundamental processes that were at play in the early Universe. Upcoming cosmological surveys will offer an exciting possibility to unveil the origin of structures in our cosmos and in which process the UV LF will play a prominent role.