Morphological Bias: How Ellipticals and Spirals Trace the Cosmic Web Differently

Abstract

We present a data-driven measurement of galaxy bias and scale-dependent relative bias for elliptical and spiral galaxies using angular auto- and cross-power spectra from DES and DESI Legacy Imaging Surveys DR8. We introduce the cross-tracer clustering ratio (CTCR), the ratio of auto- to cross-power spectra, which isolates the relative clustering of morphological tracers as a function of angular multipole $\ell$. Across both surveys, ellipticals are more strongly clustered than spirals; the cleanest constraints come from DESI, where the two redshift distributions are well matched. The difference is scale dependent: the relative bias is near unity on large scales ($\ell \lesssim 50$) and rises toward smaller scales, reaching $2.9$--$4.0\sigma$ per bin at $\ell =200$ with an inverse-variance-weighted amplitude $\Delta b=+0.56\pm 0.06$. A complementary linear-bias analysis confirms that ellipticals are positively biased relative to the matter field, while spirals are consistent with weak bias or anti-bias. The results are robust to two covariance estimators, contamination tests, luminosity splits, and comparisons with two N-body mock catalogs. Throughout, we present the CTCR as an \emph{effective} clustering ratio that is an upper bound on the physical bias ratio, adopting an assumption-light strategy appropriate for a first measurement; the joint multi-spectrum decomposition required to extract the physical bias ratio is identified as future work. These findings provide empirical evidence that galaxy morphology imprints both the amplitude and scale dependence of galaxy bias, and establish the CTCR as a useful observable for testing halo occupation and assembly-bias models with future imaging surveys.