Star-forming galaxies in the cosmic web in the last 11 Gyr

Abstract

We investigate how the star formation activity of galaxies depends on their position within the cosmic web using the SIMBA cosmological simulation from redshift $z=3$ to $z=0$. While previous studies found that galaxies closer to filaments tend to be more massive and quenched, it remained unclear whether these trends reflect intrinsic environmental effects or changes in the galaxy population mix. To address this, we focus exclusively on star-forming galaxies, robustly selected using both the specific star formation rate (sSFR) and gas depletion timescale criteria, in order to isolate the direct impact of the cosmic web on star-forming galaxies. We reconstruct the 3D cosmic web skeleton using DisPerSE and compute each galaxy's distance to its nearest filament. After removing mass dependencies, we examine deviations in star formation rate (SFR), sSFR, molecular and atomic gas depletion timescales, and gas fractions as a function of this distance. We find a clear and redshift-dependent modulation of star formation with filament proximity: at high redshift ($z \gtrsim 2$), galaxies closer to filaments show enhanced SFR and gas accretion, reflecting efficient filament-fed growth. At $z=0$, we observe a V-shaped trend in the sSFR and depletion timescales, with minima at intermediate distances ($\sim 0.25$ cMpc) and a surprising upturn very close to the filament cores, suggesting a resumed accretion in the densest environments. These effects are not driven by mergers and are primarily associated with satellite galaxies at low redshift. Our results demonstrate that large-scale cosmic web proximity modulates star formation in star-forming galaxies through a combination of gas supply regulation and environmental processing, with different mechanisms dominating across cosmic time.

Figures (19)

• Figure 1: Separation of star-forming galaxies (blue dots) and green valley and quenched galaxies (red dots) using their locus in the $\tau_{\text{gas}} - M_{\star}$ diagram at $z=0$. The selection of $\text{sSFR} > 10^{-10 + 0.3z}$ yr$^{-1}$ has already been applied. Similar cuts are applied at $z=1, 2 \text{ and } 3$ where the fraction of quenched or transitioning galaxies is lower. This selection removes galaxies forming stars slowly compared to their gas mass reservoir.
• Figure 2: Stellar mass of star-forming galaxies (blue) and quenched and green-valley galaxies (red) in Simba at $z=0$, 1, and 2 (from left to right) as a function of the distance to the closest filament. Each dot represents a galaxy position in the stellar mass-$\mathrm{d}_{fil}$ parameter space (left side y-axis), and the solid lines are the corresponding smoothed and normalised distributions of distances for each population (right side y-axis). The visual inspection reveals the emergence of quenching at $z\sim1$ and only a marginal quenched galaxy population a $z\geq2$, with massive quenched galaxies lying typically closer to filaments than star-forming galaxies at all redshifts with a significant quenched population at $z=0$.
• Figure 3: Residuals deviation as a function of the distance to the filaments after correcting for stellar-mass effects. We show the SFR (top left), the sSFR (top right), the molecular hydrogen depletion timescale (middle left), the total gas depletion timescale (middle right), the molecular hydrogen fraction (bottom left), and the gas fraction (bottom right) for star-forming galaxies in Simba in bins of distance at $z=0$ (blue), 1 (green), 2 (red), and 3 (brown). The arrows pointing to the left represent galaxies located at smaller distances than the plotted range, grouped into a single bin. For each panel, the upper right box shows the deviation of the in-bin median from the best-fit relation. Star formation and gas properties vary systematically with filament proximity with enhancement near filaments at high z, but a V-shaped trend in sSFR and $\tau_{\rm H_{2}}$ at $z=0$.
• Figure 4: Deviation of the mean specific star-formation rate (top) and molecular hydrogen depletion timescale (bottom) from the respective best-fit relations in logarithmic scale as a function of $d_{\rm fil}$ in Simba at $z=0$, using different selection criteria for star-forming galaxies. For each panel, the upper right box shows the deviation of the in-bin median from the best-fit relation. The qualitative sSFR and $\tau_{\rm H_{2}}$ trends with filament distance at z = 0 are robust against variations in the star-forming selection criteria.
• Figure 5: Same as Fig. \ref{['fig:DeltaQ_arranged']} only for galaxies which underwent a recent major merging event, for sSFR and $\tau_{H_{2}}$ only. Star-forming galaxies with recent major mergers do not show the V-shaped sSFR trend, suggesting mergers are not the primary driver of environmental modulation.