The power spectrum of galaxies from large to small scales: a line-intensity mapping perspective

ABSTRACT We present a model for the power spectrum of the density field of galaxies weighted by their star formation rate. This weighting is relevant in line-intensity mapping (lim) when the observed line luminosity is strongly correlated with star formation, as is the case for the H $\alpha$ line. Our model reproduces the measured power spectrum in the illustris tng simulation to within a few per cent across all scales, with fitting parameters that have clear physical interpretations. On scales of tens of megaparsecs, the model accounts for the weighted non-linear bias of galaxies as well as halo exclusion (two-halo term). On smaller scales, it incorporates the weighted distribution of satellite galaxies within haloes (one-halo term). The random sampling of satellite galaxies introduces a galaxy shot noise term to the power spectrum on small scales, and their confinement to haloes introduces a halo shot noise term on large scales. Omitting satellite galaxies from the analysis results in an underestimation of both the large-scale bias and the mean intensity by $\sim 30$ per cent each at $z \sim 1.5$. Assigning the intensity of satellites to the centre of their respective haloes affects the power spectrum on scales $k \gtrsim 0.3\ {\rm h\, Mpc^{-1}}$. Our fitting function provides a well-motivated parametrization that can be used to interpret data from upcoming lim surveys.