Revisiting Marked Galaxy Clustering from a Joint Point Process Perspective

TL;DR

This paper introduces a joint point process framework for galaxy clustering that captures mark-dependent interactions, providing a more detailed analysis than traditional summary statistics.

Contribution

It formulates galaxies as points with marks in a product space and defines a joint correlation function and diagnostics to analyze mark-dependent clustering.

Findings

The joint pair correlation function $g(r;m_1,m_2)$ describes mark-dependent clustering.

The diagnostic $ riangle_{ind}(r;m_1,m_2)$ quantifies deviations from independence.

Common diagnostics are shown as averaged projections of the joint structure.

Abstract

Marked correlation functions, in which galaxy properties such as luminosity or stellar mass are treated as marks, are widely used to test models of galaxy formation. In astronomy, however, these statistics are typically implemented as summary measures that do not preserve the joint structure of mark pairs conditioned on separation. In this work, we formulate galaxies as points $(x,m)$ on the product space $\mathbb{R}^3\times\mathcal{M}$, where $x$ denotes position and $m$ a mark, and introduce the joint pair correlation function $g(r;m_1,m_2)$ as the fundamental quantity describing mark-dependent clustering. We further define a diagnostic quantity $\Delta_{\mathrm{ind}}(r;m_1,m_2)$ that locally quantifies deviations from the independence hypothesis relative to spatial clustering alone, thereby providing a projection-free description of which mark pairs are over- or underrepresented at a given separation scale. Within this framework, commonly used diagnostics such as the inhomogeneous cross-$J$ function are naturally interpreted as summary statistics obtained through averaging over mark sets and geometric-event-based reductions of the joint structure. This perspective clarifies that previously discussed marked effects, including assembly bias, correspond to projections of an underlying joint dependence, and that observationally accessible information is the existence of non-factorizable joint structure itself. The present formulation provides both a fundamental quantity and practical diagnostics for its characterization.