The impact of cosmic filaments on starburst galaxies across cosmic times

Abstract

Cosmological simulations suggest that various galaxy properties depend on their location within the cosmic web. Yet direct observational evidence of the dependence of star formation activity on distance to filaments remains scarce and is missing at z>1. We investigate how starburst, main-sequence (MS), and quenched galaxies are distributed with respect to cosmic web filaments, and how this distribution evolves with redshift. We first use the SIMBA cosmological simulation to predict the redshift evolution of the mean distance to the closest filament from z=3 to z=0 for different galaxy populations after removing stellar-mass dependencies. We then measure the corresponding signal in the COSMOS field, using COSMOS2020 and COSMOS-Web data, where accurate photometric redshifts enable reconstruction of the projected cosmic web from z=2 to z=0.5, and starbursts are identified through far-infrared spectral energy distribution fitting. In agreement with the results from SIMBA, starburst galaxies are found closer to filaments at z>1 and at larger distances at z<1, MS galaxies occupy intermediate environments with little evolution, and quenched galaxies show progressively shorter distances to filaments toward low redshift, with a crossing between starburst and MS populations around z~1. In COSMOS-Web, the relative evolution in the average distance to filaments between starburst and MS galaxies is detected at a significance level of at least 5σ. We show that a minimal toy model in which the only environmental ingredient is the sSFR-filament distance modulation measured in simulations is sufficient to reproduce the observed differential evolution of the average filament distance between starburst and MS galaxies. These results show that the imprint of large-scale environmental effects on the star formation activity of galaxies, predicted by simulations, is detectable from z=2 down to z=0.5.

Figures (17)

• Figure 1: Upper panel: Separation of starbursts (purple), MS galaxies (blue) and quenched galaxies (red) in Simba using their locus in the $\mathrm{SFR} - M_{\star}$ diagram at $z=1$ with $\log(\rm SFR_{SB}/SFR_{MS})>0.2\, dex$ and $\log(\rm \tau_{SB}/\tau_{MS})<-0.2\, dex$. Lower panel: Separation in the $\tau - M_{\star}$ diagram with the same starburst-selection criteria. The blue solid lines indicate the best-fit MS relations, i.e. the SFR-$M_{\star}$ and $\tau$-$M_{\star}$ relations for SFG in the upper and lower panels, respectively.
• Figure 2: Upper panels: Mean distance to the cosmic web filaments, measured in projection (2D), for starburst galaxies (purple), MS galaxies mass-matched to starbursts (blue), and quenched galaxies mass-matched to the complete main sequence sample (red) as a function of redshift for four different selection criteria in Simba, with our reference case on the left. The dotted lines show the same quantities without mass-matching. The upper left boxes show the same trends for the median values. Lower panels: Fraction of starburst galaxies with respect to MS galaxies as a function of redshift. Starbursts are, on average, closer to filaments than MS galaxies at high redshift but exceed the MS distance at $z\simeq1$. The fraction of SFG rises with redshift and stabilises around $z\simeq1.5$, while the starburst fraction remains roughly constant at $\sim10\%$, with a modest enhancement for galaxies near filaments at $z>1$.
• Figure 3: Ratio of the mean projected (2D) distance to cosmic filaments for two galaxy populations as a function of redshift in Simba with $\log(\rm SFR_{SB}/SFR_{MS})>0.2\, dex$ and $\log(\rm \tau_{SB}/\tau_{MS})<-0.2\, dex$. The purple curve shows the ratio between starbursts and MS galaxies mass-matched to starbursts (solid purple line and solid blue line in Fig. \ref{['fig:d_z_prediction']}). The red curve shows the ratio between quenched galaxies mass-matched to the MS sample and MS galaxies (solid red line and dotted blue line in Fig. \ref{['fig:d_z_prediction']}). At late cosmic times, starbursts are, on average, located farther from filaments than their mass-matched MS counterparts. On the other hand, quenched galaxies lie, on average, closer to filaments than mass-matched MS galaxies, and the contrast between the two increases towards lower redshift.
• Figure 4: Example of FIR SED fitting for one galaxy in our sample. Black points show the observed photometric fluxes as a function of wavelength. The solid blue and purple curves correspond to the best-fit MS and starburst (SB) FIR templates, respectively. The inset shows the posterior marginal likelihood of the FIR luminosity $\mathcal{L}(L_{\rm IR})$ obtained for the MS (blue) and starburst (purple) solutions. Here, we show a case where both the SB-type and MS-type templates provide reasonable fits, with a slight preference for the SB-type one.
• Figure 5: Galaxies in the COSMOS field (grey dots) and 100 over-plotted realisations of the cosmic web for COSMOS-Web (filaments) in a slice centred around $z_{\rm phot}\simeq1.2$.