Halo Lightcones with Optimised Orientation and Interpolation in Cosmological Simulations -- an application to mock H$α$ selected galaxies

TL;DR

HolCon delivers a scalable, post-processing framework to generate realistic halo lightcones and galaxy mocks for Stage IV surveys. It combines an orientation-optimizing mask with linear halo interpolation and distributed computing to efficiently produce multiple lightcone realizations. The Uchuu-based lightcone is validated against halo mass functions and clustering, and is used to build SciPIC-based galaxy catalogs extended to $z\leq 10$, including $\mathrm{H}\alpha$-emitter tracers. The resulting mocks, calibrated with COSMOS2020 and extended LF evolution, provide valuable predictions for halo-galaxy connections and the clustering of emission-line galaxies in future cosmological surveys.

Abstract

A critical step to create realistic mock catalogs that support large-scale photometric and spectroscopic sky surveys is the production of cosmological simulations that accurately model the survey observables taking into account the redshift-dependent galaxy formation and evolution processes. Here we develop an efficient framework, HOLCon (Halo Optimised Lightcone Constructor), for post-facto construction of dark matter halo lightcones from simulations and use them to generate a mock galaxy catalogue. HOLCon includes a module to optimise the lightcone's orientation within the simulation box, minimising repeated structures when the survey volume exceeds a single box -- a common challenge in modern surveys. A linear interpolation scheme tracks the evolution of halo properties across snapshots. Applied to the publicly available Uchuu simulation, we construct a lightcone of 50 ${\rm deg}^2$ and extending up to $z = 10$, providing representative coverage of deep fields of Stage IV surveys. We validate the lightcone for cosmological applications by comparing the dark matter halo clustering in the lightcone with those from the original simulation snapshots. Subsequently, we make the galaxy-halo connection on the lightcone with a redshift extended version of the SciPIC algorithm producing a comprehensive set of descriptive galaxy attributes. HOLCon leverages Dask, a scalable parallel computing pythonic framework for fast construction of dark matter halo lightcones enabling rapid creation of multiple statistical realizations essential for robust cosmological inference. The produced galaxy mock makes predictions for clustering of H$α$ emitters, making it a useful cosmology resource.

Figures (16)

• Figure 1: Diagram of the lightcone placed with the observer at the origin. It is oriented at an angle with the direction vector (${i},{j},{k}$) where $i,j,k$ are integers and point to one of the vertices of the primary simulation cube and subsequent repeated cubes. Here, $\theta$ is the angular radius and $l$ is the length of the lightcone. The diagram shows the case where the lightcone is oriented at a limiting angle $\Phi_{i,j,k}$ with respect to the direction $(i,j,k)$. Any angle less than the limiting angle results in repetition of structures and an angle larger is free of repeated structures associated with this direction.
• Figure 2: Two-dimensional lattice points representing the corners of the periodic simulation box and the subsequent repeated boxes placed side-by-side. The maximum length that probes unique structures along a direction or 'the repetition length' is shown in blue, purple and green colors corresponding to the directions (1,1), (2,1), (4,1) respectively.
• Figure 3: Mollweide projection of the sky regions of repetition inside the periodic box: the left panel shows the numerically computed fraction of volume that is repeated and the right panel shows the analytic mask that demarcates lightcone orientations with repeated volumes from unique volumes. The top, middle and bottom panel corresponds to three different configurations of area coverage and $l/L_{\rm box}$. We can see that the region with unique structures (yellow) progressively decreases as we increase the sky coverage or increase $l/L_{\rm box}$. We have used \ref{['eq:main']} to plot the right panels. The red cross mark in the middle right panel denotes the chosen orientation for constructing a lightcone later in the text (See \ref{['sec:lcuchuu']}). An interactive version of the mask function is available at https://rsujatha.github.io/HOLCon.
• Figure 4: Schematic representation of the lightcone configuration: The sky coverage is 50 $\mathrm{deg}^2$ going up to depth of $z\sim 10$. The red cross in the middle panel of \ref{['fig:config1']}, marking a right ascension of $\alpha = 31 \degree$ and a declination of $\delta = 10 \degree$, indicates the chosen orientation for this lightcone with respect to a cartesian coordinate system oriented along the sides of the periodic box. Although 4 box repetitions are required to reach the highest redshift, the chosen orientation guarantees that the lightcone probes unique large-scale structures and haloes.
• Figure 5: Relation between halo mass and luminosity at various redshifts resulting from abundance matching the cumulative galaxy function and the cumulative unscattered luminosity function.