Current and forthcoming galaxy surveys will map the observable Universe with unprecedented depth, sky coverage, and precision. These maps are affected by relativistic redshift-space distortions (RSDs), which become increasingly relevant on ultralarge scales. Accurate modeling of these relativistic RSDs is essential to avoid systematic biases in key cosmological measurements, such as primordial non-Gaussianity (PNG). To address this, we introduce an updated implementation of the liger method, liger4gal, which incorporates all linear-order relativistic RSDs directly at the tracer level of high-resolution N-body simulations. We demonstrate that liger4gal improves upon previous iterations of the liger method by reproducing the expected nonlinear clustering while maintaining accuracy for relativistic RSDs on large scales. We use the updated code to generate a DESI-like sample of luminous red galaxies from the Huge Multidark Planck Simulation. By measuring the power spectrum multipoles of this sample with and without the imprint of relativistic RSDs, we assess the impact of relativistic effects on measurements of the local PNG signal ($f_{\mathrm{nl}}$). We find that the omission of the “finger-of-the-observer” (sourced by the peculiar velocity of the observer) effect in the power spectrum modeling can bias measurements of $f_{\mathrm{nl}}$ by more than 1 (0.25) ${\sigma }_{f_{\mathrm{nl}}}$ in 40% (80%) of the possible realizations of the universe if scales down to $k_{\min }=0.0015h/\mathrm{Mpc}$ are included.

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• Cosmology
• Large scale structure of the Universe
• Relativistic aspects of cosmology