The impact of cosmic filaments on the abundance of satellite galaxies

TL;DR

This paper investigates how cosmic filaments influence satellite galaxy abundance using the IllustrisTNG hydrodynamical simulation. Filaments are identified with the DisPerSE algorithm using galaxies as tracers, and centrals in filaments are compared to field centrals to quantify differences in satellite counts. The authors show that much of the observed enhancement in satellite abundance in filaments arises from differences in host halo mass distributions; after matching $M_{200c}$ (and further matching $M_{r,\rm cen}$), the disparity is greatly reduced, and the choice of filament tracer (galaxies vs dark matter) markedly biases the results, with galaxy tracers amplifying the signal. The findings highlight the critical roles of halo mass and tracer biases in interpreting environmental effects on satellites, and they provide a framework to account for these factors in comparisons to observations.

Abstract

The impact of cosmic web environments on galaxy properties plays a critical role in understanding galaxy formation. Using the state-of-the-art cosmological simulation IllustrisTNG, we investigate how satellite galaxy abundance differs between filaments and the field, with filaments identified using the DisPerSE algorithm. When filaments are identified using galaxies as tracers, we find that, across all magnitude bins, central galaxies in filaments tend to host more satellite galaxies than their counterparts in the field, in qualitative agreement with observational results from the Sloan Digital Sky Survey. The average ratios between satellite luminosity functions in filaments and the field are $3.49$, $2.61$, and $1.90$ in the central galaxy $r$-band magnitude bins of $M_{r, {\rm cen}} \sim -22$, $-21$, and $-20$, respectively. We show that much of this excess can be attributed to the higher host halo masses of galaxies in filaments. After resampling central galaxies in both environments to match the halo mass distributions within each magnitude bin, the satellite abundance enhancement in filaments is reduced by up to $79 \%$. Additionally, the choice of tracers used to identify filaments introduces a significant bias: when filaments are identified using the dark matter density field, the environmental difference in satellite abundance is reduced by more than $70 \%$; after further resampling in both magnitude and halo mass, the difference is further suppressed by another $\sim 60$--$95 \%$. Our results highlight the importance of halo mass differences and tracer choice biases when interpreting and understanding the impact of environment on satellite galaxy properties.

Figures (10)

• Figure 1: Filaments in a $30$ Mpc-thick slice of the TNG100-1 simulation, identified by the DisPerSE algorithm using galaxies with stellar masses $M_{\rm star} \geq 10^{9}~M_\odot$ as tracers. The grayscale background shows the dark matter density field. White circles mark the $3\times R_{\rm 200c}$ vicinities of galaxy clusters with $M_{\rm 200c} \geq 10^{13.5}~M_\odot$, which are defined as the knot environment. Red dots indicate the centers of galaxies residing in knots, which are excluded from our analysis. Black solid line segments show the filament spines defined by DisPerSE. Orange dots mark the centers of galaxies located within cylinders of radius of $R_{\rm filament} = 1$ Mpc around the filament spines (indicated by the orange shaded regions), and defined as filament galaxies. The remaining galaxies, shown as blue dots, are field galaxies. Visually, despite some minor discrepancies, the filament spines overall capture the massive filamentary structures in the underlying dark matter density field.
• Figure 2: Environmental dependence of satellite LFs. From left to right, each column shows the results for central galaxies in the bins of $M_{r,{\rm cen}} \sim -23$ (blue), $-22$ (red), $-21$ (green), and $-20$ (yellow). In the upper panel of each column, solid and dashed lines represent simulated satellite LFs for centrals in filaments and fields, respectively. Shaded regions indicate Poisson errors computed using Eq. (\ref{['eq:poisson_error']}). Filled squares and open circles show observational results from 2015ApJ...800..112G. Bottom panels display the ratio of the filament to field satellite LFs, with errors propagated accordingly. The satellite LF in the $M_{r,{\rm cen}} \sim -23$ bin for field galaxies is sparsely sampled due to the limited number of objects, and is therefore excluded from the following analysis. For the three remaining bins, central galaxies in filaments tend to host more satellites than their counterparts in the field, in qualitative agreement with the observational results of 2015ApJ...800..112G.
• Figure 3: Top: Distributions of central galaxy magnitudes ($M_{r,{\rm cen}}$). From left to right, we show the distributions of the original samples, the samples after resampling $M_{r,{\rm cen}}$, and the samples after resampling both $M_{r,{\rm cen}}$ and $M_{\rm 200c}$. In each panel, as in other figures in this study, the $M_{r,{\rm cen}} \sim -22$, $-21$, and $-20$ bins are plotted in red, green, and yellow, respectively. Central galaxies in filaments and the field are distinguished using solid and dashed lines, respectively. For each $M_{r,{\rm cen}}$ bin, the Kolmogorov–Smirnov (KS) test $p$-value is shown in the panel, quantifying the similarity between the filament and field $M_{r,{\rm cen}}$ distributions. Bottom: Same as the top panels, but showing the distributions of the host halo virial masses ($M_{\rm 200c}$) for central galaxies. After resampling, the filament and field samples exhibit similar distributions in both $M_{r,{\rm cen}}$ and $M_{\rm 200c}$. See the main text for details of the resampling procedure.
• Figure 4: Impact of magnitude and halo mass distributions on the environmental differences in satellite LFs. Left: The upper panel summarizes the satellite LFs for the $M_{r,{\rm cen}} \sim -22$ (red), $-21$ (green), and $-20$ (yellow) bins from Figure \ref{['fig:R1']}. Results for filament and field central galaxies are shown with solid and dashed lines, respectively. The lower panel shows the corresponding ratios between filament and field satellite LFs. Middle: Same as the left panels, but for galaxy samples resampled to have similar $M_{r,{\rm cen}}$ distributions. Right: Same as the middle panels, but for galaxy samples resampled to match both $M_{r,{\rm cen}}$ and $M_{\rm 200c}$ distributions. After resampling $M_{r,{\rm cen}}$, the average ratios decrease by less than $17 \%$. However, after additionally resampling $M_{\rm 200c}$, the average ratios drop by $78 \%$, $79 \%$, and $53 \%$ for the $M_{r,{\rm cen}} \sim -22$, $-21$, and $-20$ bins, respectively, relative to the original unresampled samples. This indicates that differences in halo mass distributions between filament and field central galaxies contribute significantly to the environmental differences in satellite abundance, especially in the brighter central galaxy bins.
• Figure 5: Similar to Figure \ref{['fig:M1']}, but for filaments identified using dark matter density as tracers. The spines of dark matter-traced filaments are shown as purple solid lines, while the galaxy-traced filament spines from Figure \ref{['fig:M1']} are overlaid using black dotted lines for comparison. Filaments identified by these two tracers are qualitatively similar, both following the underlying filamentary matter distribution, but they differ in detailed structures. We investigate these differences and their impact in Section \ref{['sec:tracer']}.