The galaxy-environment connection revealed by constrained simulations

TL;DR

This paper develops a density-centric framework to quantify how galaxy properties relate to environment by leveraging constrained CSiBORG simulations to recover the underlying dark matter field. Environment is characterized both by local density smoothed on multiple scales and by distances to cosmic-web filaments identified with DisPerSE. Cross-matching with NSA and ALFALFA reveals that galaxy colour, star formation rate, and HI content correlate more strongly with environmental density than with filament proximity, with low-mass galaxies showing stronger environmental quenching. The use of multiple constrained realizations enables robust uncertainty quantification and highlights the importance of the dark matter density in shaping galaxy evolution, offering a principled approach to compare models against observations.

Abstract

The evolution of galaxies is known to be connected to their position within the large-scale structure and their local environmental density. We investigate the relative importance of these using the underlying dark matter density field extracted from the Constrained Simulations in BORG (CSiBORG) suite of constrained cosmological simulations. We define cosmic web environment through both dark matter densities averaged on a scale up to 16 Mpc/h, and through cosmic web location identified by applying DisPerSE to the CSiBORG haloes. We correlate these environmental measures with the properties of observed galaxies in large surveys using optical data (from the NASA-Sloan Atlas) and 21-cm radio data (from ALFALFA). We find statistically significant correlations between environment and colour, neutral hydrogen gas (HI) mass fraction, star formation rate and Sérsic index. Together, these correlations suggest that bluer, star forming, HI rich, and disk-type galaxies tend to reside in lower density areas, further from filaments, while redder, more elliptical galaxies with lower star formation rates tend to be found in higher density areas, closer to filaments. We find analogous trends with the quenching of galaxies, but notably find that the quenching of low mass galaxies has a greater dependence on environment than the quenching of high mass galaxies. We find that the relationship between galaxy properties and the environmental density is stronger than that with distance to filament, suggesting that environmental density has a greater impact on the properties of galaxies than their location within the larger-scale cosmic web.

Figures (10)

• Figure 1: The dark matter distribution from a single realisation of CSiBORG, shown without and then with Gaussian smoothing of standard deviation $\sigma = 2$ and $8$ Mpc/$h$ (middle and right panel). Each panel represents a 2D slice through the Z-axis of the simulation box, centred on the Milky Way. The high-completeness central region of radius 155 Mpc/$h$ is visible. As smoothing increases the smaller scale structures are smoothed out. See Section \ref{['sec:csiborg']} for further details of CSiBORG, and Section \ref{['sec:density_stats']} for further details of the smoothing methodology.
• Figure 2: The distribution of NSA galaxies (left) and ALFALFA galaxies (right), shown on Mollweide projections of right ascension and declination. The NSA catalogue, based on optical data, contains information for 640,000 galaxies out to a redshift of 0.15. The ALFALFA catalogue provides $\mathrm{H}\mathrm{I}$ 21 cm line measurements for 31,500 galaxies out to a redshift of 0.06, covering nearly 7000 deg$^2$ of high galactic latitude sky. See Section \ref{['sec:NSA_data']} and \ref{['sec:alfalfa']} for further information on the NSA and ALFALFA surveys respectively.
• Figure 3: Distribution of dark matter haloes (black) derived from CSiBORG, overlaid with NSA galaxies (purple; left panel), ALFALFA galaxies (magenta; right panel) and cosmic web filaments (blue), shown in a 15 Mpc slice along the z-axis centred on the Milky Way. The cosmic web filaments are calculated using DisPerSE, applied to the CSiBORG haloes. NSA and ALFALFA galaxies predominantly cluster along the filamentary structures, with some galaxies distributed within the voids of the cosmic web. See Section \ref{['sec:csiborg']} for details on the CSiBORG haloes, and Section \ref{['sec:cosmic_web_stats']} for further details on the DisPerSE methodology and filament extraction.
• Figure 4: The relationship between colour ($u-r$) and stellar mass, with the solid black line showing the LOWESS fit. The bimodal distribution of colour has been separated into "star-forming" (left) and "quenched" (right). The contour lines show the points contained within $0.5\sigma$, $1\sigma$, $1.5\sigma$, and $2\sigma$, which represents 11.8$\%$, 39.3$\%$, 67.5$\%$, and 86.4$\%$ regions of each distribution, respectively.
• Figure 5: The bimodal distribution of the star formation rate of NSA galaxies (from the MPA/JHU catalogue), classifying galaxies as star-forming (blue; above the dividing line) and quenched (red; below the dividing line). We use Equation \ref{['eq:quenching']} to divide the two populations. See Section \ref{['sec:quenched_fraction']} for further details on the methodology of defining a quenched galaxy.