Abstract
Spectroscopic surveys such as the Dark Energy Spectroscopic Instrument (DESI) and Euclid map the spatial distribution of millions of galaxies, with emission line galaxies (ELGs) serving as the dominant tracer in the redshift range 0.8 z
σ
8
and Ω
m
h
2
down to scales of 0.3
h
−1
Mpc on both samples. We find that including scales below
r
= 0.8
h
−1
Mpc – within the halo – is critical for mitigating projection effects and obtaining unbiased constraints on
σ
8
. Applied to the DESI One-Percent measurements, our analysis yields ∼6% constraints on
σ
8
= 0.81
+0.05
−0.06
and Ω
m
h
2
= 0.146
+0.009
−0.009
. Remarkably, the accuracy of these constraints is similar to that obtained from the combined full-shape analysis of all DESI DR1 tracers, yet using only 1% of the survey volume. A naive extrapolation of our results from the One-Percent to the full survey area suggests that the complete survey could deliver roughly an order-of-magnitude improvement in precision – a prospect that, while subject to significant practical challenges, illustrates the cosmological potential encoded in the nonlinear regime. The SHAMe-SF model is well positioned to exploit this information and disentangle cosmological parameters from the galaxy–halo connection through small-scale ELG clustering.