Early Structure Formation from Primordial Density Fluctuations with a Blue, Tilted Power Spectrum: High-Redshift Galaxies

Abstract: Recent observations by the James Webb Space Telescope (JWST) discovered unexpectedly abundant luminous galaxies at high redshift, posing possibly a severe challenge to popular galaxy formation models. We study early structure formation in a cosmological model with a blue, tilted power spectrum (BTPS) given by $P(k) \propto k^{m_{\rm s}}$ with $m_{\rm s} > 1$ at small length scales. We run a set of cosmological $N$-body simulations and derive the abundance of dark matter halos and galaxies under simplified assumptions on star formation efficiency. The enhanced small-scale power allows rapid nonlinear structure formation at $z>7$, and galaxies with stellar mass exceeding $10^{10}\,M_\odot$ can be formed by $z=9$. Because of frequent mergers, the structure of galaxies and galaxy groups appears clumpy. The BTPS model reproduces the observed stellar mass density at $z=7-9$, and thus eases the claimed tension between galaxy formation theory and recent JWST observations. The large-scale structure of the present-day Universe is largely unaffected by the modification of the small-scale power spectrum. We conduct a systematic study by varying the slope of the small-scale power spectrum to derive constraints on the BTPS model from a set of observations of high-redshift galaxies.