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

1. Problem Statement

Previous observational and theoretical studies have established that galaxies closer to cosmic filaments tend to be more massive and more frequently "quenched" (star formation suppressed). However, a critical ambiguity remained: Do these environmental trends reflect intrinsic changes in the star formation activity of individual galaxies, or are they merely a statistical artifact caused by a shift in the population mix (i.e., a higher fraction of quenched galaxies near filaments)?

To resolve this, the authors aim to isolate the direct impact of the cosmic web on star-forming galaxies by excluding quenched systems and investigating how proximity to filaments modulates star formation rates (SFR), gas content, and accretion histories across cosmic time ($z=3$ to $z=0$).

2. Methodology

Data Source:

• Simulation: The study utilizes the Simba cosmological hydrodynamical simulation (Davé et al. 2019), which includes state-of-the-art physics for black hole growth, AGN feedback (X-ray, radiative, and jet modes), stellar feedback, and dust evolution.
• Volume & Resolution: A comoving volume of $100 h^{-1} \text{cMpc}^3$ with $1024^3$ particles.
• Redshifts: Snapshots at $z = 0, 1, 2, 3$.
• Control Runs: Three "no-feedback" variants (no X-ray, no AGN, no AGN+stellar) were used to disentangle the role of feedback mechanisms.

Galaxy Selection:
To ensure a robust sample of strictly star-forming galaxies, the authors applied a dual-criterion selection:

1. Specific Star Formation Rate (sSFR): $sSFR > 10^{-10 + 0.3z} \text{ yr}^{-1}$.
2. Gas Depletion Timescale ($\tau_{gas}$): $\tau_{gas} < 10^{19.5 - \log(M_*/M_\odot)} \text{ yr}$.
   This selection removes galaxies in the "green valley" or those transitioning to quiescence, ensuring the sample represents active star formation.

Cosmic Web Reconstruction:

• Algorithm: The DisPerSE code (based on discrete Morse theory) was used to reconstruct the 3D cosmic web skeleton from the distribution of all galaxies with $M_* > 10^9 M_\odot$.
• Metric: The distance to the nearest filament ($d_{fil}$) was calculated for each galaxy using orthogonal projection via a k-d tree algorithm.

Analysis Technique:
To isolate environmental effects from mass-dependent trends, the authors computed the residuals ($\Delta \log Q$) of various physical quantities (SFR, sSFR, gas fractions, depletion timescales) relative to a best-fit double power-law relation with stellar mass ($M_*$). This allows for the examination of deviations caused purely by filament proximity.

3. Key Contributions

• Isolation of Star-Forming Population: Unlike previous studies that analyzed mixed populations, this work explicitly isolates star-forming galaxies to determine if the cosmic web directly modulates their internal physics.
• Discovery of a V-Shaped Trend: The paper identifies a non-monotonic, V-shaped relationship between specific star formation rate (sSFR) and filament distance at low redshift ($z=0$), characterized by a minimum at intermediate distances and a recovery near filament cores.
• Disentanglement of Drivers: The study systematically rules out major mergers as the primary driver of these trends and identifies satellite galaxies and gas accretion rates as the dominant factors.
• Feedback Analysis: By utilizing "no-feedback" simulation runs, the authors quantify how AGN and stellar feedback amplify, rather than create, the environmental modulation of star formation.

4. Key Results

Redshift Evolution of Star Formation:

• High Redshift ($z \gtrsim 2$): Galaxies closer to filaments show enhanced SFR and gas accretion. This reflects efficient, filament-fed cold gas inflows (the "cosmic web" feeding mechanism) dominating over environmental suppression.
• Low Redshift ($z = 0$): A distinct V-shaped trend emerges for sSFR and gas depletion timescales ($\tau_{H2}$):
  • Intermediate Distances ($\sim 0.25 \text{--} 0.5 \text{ cMpc}$): sSFR and gas accretion reach a minimum. This region is dominated by environmental suppression (pre-processing).
  • Filament Cores ($d_{fil} < 0.25 \text{ cMpc}$): Surprisingly, sSFR and gas accretion increase again (the "upturn"), suggesting a resumption of cold gas accretion even in the densest environments.

Drivers of the Trends:

• Mergers: Major mergers do not drive the V-shaped trend. Galaxies with recent major mergers show a monotonic decline in sSFR toward filaments, lacking the core upturn seen in the general population.
• Centrals vs. Satellites:
  • Centrals: Show a relatively flat sSFR trend with distance, except for a mild upturn near cores. They are less affected by the environment.
  • Satellites: Are the primary drivers of the V-shaped trend. Satellites of quenched centrals exhibit the strongest suppression at intermediate distances (due to ram-pressure stripping and hot halo interaction) but show a recovery near filament cores.
• Gas Accretion (MAR): The Mass Accretion Rate (MAR) of gas mirrors the sSFR trends. MAR drops at intermediate distances (suppression by shock heating/tidal stripping) but rises again near filament cores, indicating that the densest parts of filaments can still channel significant cold gas onto galaxies.
• Feedback Role:
  • AGN Feedback: Crucial for suppressing accretion in central galaxies at low $z$, but the filament-core upturn persists even without it, suggesting large-scale dynamics play a role.
  • Stellar Feedback: Essential for maintaining the sSFR excess in satellites near filaments. Removing all feedback flattens the trends entirely, indicating that feedback mechanisms modulate the amplitude of the environmental effect but the underlying trend is driven by large-scale gas dynamics.

5. Significance

This study provides a unified physical picture of galaxy evolution within the cosmic web:

1. Multi-Scale Regulation: Star formation is regulated by a competition between large-scale gas supply (filamentary inflows) and local environmental processing (stripping, heating).
2. The "Pre-processing" vs. "Re-accretion" Balance: At low redshift, galaxies at intermediate filament distances suffer from "pre-processing" (gas stripping/heating in group outskirts), while those in the very cores of filaments benefit from renewed, coherent cold inflows that can overcome feedback suppression.
3. Satellite Dominance: The environmental signal in the local universe is largely a satellite phenomenon. The "V-shape" is a signature of satellites navigating the complex intra-halo environment of groups and clusters aligned with filaments.
4. Methodological Rigor: By rigorously removing mass dependencies and isolating star-forming populations, the authors demonstrate that the cosmic web imprints a measurable, redshift-dependent modulation on star formation efficiency that is distinct from simple population mixing.

In conclusion, the paper establishes that the cosmic web does not merely host galaxies but actively regulates their star formation histories through a complex interplay of gas supply, environmental stripping, and feedback, with distinct mechanisms dominating at different cosmic epochs and for different galaxy types (centrals vs. satellites).