Local Group analogues in a cosmological context -- I. Relating velocity structure to the cosmic web

TL;DR

This work investigates how the large-scale cosmic web environment influences Local Group analogues (LGAs) in ΛCDM using the ABACUSSUMMIT N-body simulations. By constructing a statistically robust LGA catalog and defining the coupling energy $E_{ m coupling}$ and orbital angular momentum $\mathbf j$, the authors link small-scale pair dynamics to large-scale overdensity and anisotropy, including redshift-space corrections. They find that LGAs in underdense regions tend to have negative $E_{ m coupling}$ and polar anisotropy aligned with filaments, while those in denser regions show positive $E_{ m coupling}$ and azimuthal anisotropy with large-scale tracers in the orbital plane; velocity distributions differ between coupled and uncoupled populations. The Local Group, though typical in its $E_{ m coupling}$ and $\|\,\mathbf j\|$, resides in a relatively underdense environment, highlighting the critical role of the cosmic web in assessing the rarity and properties of LG-like systems and guiding future observational tests with deeper surveys.

Abstract

Our Local Group, dominated in mass by the Milky Way (MW) and M31, provides a unique laboratory for testing $Λ$CDM cosmology on small scales owing to its proximity. However, its connection to the surrounding large-scale environment, which is essential for interpreting its properties, is inadequately understood. In this work, we explore the connection between Local Group analogues (LGAs) and their surrounding large-scale environments using the ABACUSSUMMIT simulation suite, highlighting the key role of the coupling energy of the MW-M31 orbit, $E_{\rm coupling}$. We find that LGAs with high $E_{\rm coupling}$ preferentially reside in denser regions, whereas those with low $E_{\rm coupling}$ tend to occupy low-density environments. Furthermore, LGAs with low $E_{\rm coupling}$ exhibit strong alignment with cosmic filaments, manifested as a pronounced polar anisotropy in the distribution of tracer haloes. By contrast, LGAs with high $E_{\rm coupling}$ show a weaker polar anisotropy but an enhanced azimuthal anisotropy, with large-scale tracer haloes preferentially lying in the plane spanned by the halo pair and the orbital spin vector. Within this framework, our Local Group is characterised by typical $E_{\rm coupling}$ residing in a relatively under-dense environment, yet it remains consistent with the 95\% range of analogue systems identified in the simulation.

Figures (15)

• Figure 1: A schematic of the LGA selection criterion. A pair of haloes (two blue filled circles) is classified as a LGA if they both satisfy the following four criteria: (1) the virial mass $M_{\rm vir}$ is within $10^{12\pm 0.3}\xspace h^{-1}\text{M}_\odot$; (2) there is only one halo that satisfy the same mass criterion within $400-700 h^{-1}\rm kpc$ (gray filled band); (3) there are no other haloes above $10^{11.7}h^{-1}\rm M_\odot$ within $700h^{-1}\rm kpc$ of either halo (red filled circle on the left); (4) either halo is away from haloes above $10^{11.7}h^{-1}\rm M_\odot$ (red filled circle ont the right) by at least $5R_{\rm vir}$, where $R_{\rm vir}$ is the virial radius of the neighbor halo.
• Figure 2: The fraction of LGAs among all MW-size haloes ($M_{\rm halo}\in 10^{12\pm 0.3}h^{-1}{\rm M_{\rm \odot}}$) with gradually stricter selection criterion applied. The fraction is about 8.6 percent when we require at least one MW-size companion within $400-700h^{-1}{\rm kpc}$ and goes to about 7.4 percent when we require only one MW-size companion. The fraction drops to about 6.1 percent when we require no other haloes more massive than $10^{12\pm0.3}h^{-1}{\rm M_\odot}$ within $700 h^{-1}{\rm kpc}$ of either halo. Finally, we require both haloes to be out of the exclusion region, $R_{\rm exclusion}$, of massive haloes above $10^{12 + 0.3}h^{-1}\rm M_\odot$, where the exclusion region is $fR_{\rm vir}$ for a given dark matter halo, and the fraction of LGAs as a function of $f$ is shown in red circles. Here we choose $f=5$ and the eventual fraction is 4.2 percent. The magenta and orange pentagons show the fraction of LGAs with $E_{\rm coupling} < 0$ and $E_{\rm coupling} > 0$ (see equation \ref{['eq:ecoupling']}), respectively.
• Figure 3: The joint distribution of the coupling energy, $E_{\rm coupling}$, and the specific orbital angular momentum, $\|{\bf j}\|$, for all LGAs (left) and LGAs in different large-scale environmental bins (panels on the right, with the environment range indicated in each panel; see § \ref{['sec:large_scale_environment_in_real_space']} for the definition of environment). In the panels, the contour lines enclose 68 and 95 per cent of the sample and the remaining data points are shown with cyan crosses. The bottom right grey region shows the forbidden region separated by $\|\mathrm d\mathbf v_{\rm r}\| = 0$. Spearman's rank correlation coefficients between $E_{\rm coupling}$ and $\|\mathbf j\|$ are shown in the lower right corner of each panel for the corresponding sample. The result for our LG is presented according to the values in Table \ref{['tab:localgroup']}.
• Figure 4: Top panel: The blue circles and orange squares show $f\langle \Delta\rangle$ and $f\langle \Delta^{\rm RS} - c\rangle$ as a function of halo mass, where $\Delta$ and $\Delta^{\rm RS}$ are the number overdensity of tracer haloes within $5-15\xspace h^{-1}\text{Mpc}$ (see equations \ref{['eq:delta']} and \ref{['eq:delta_rs']}), and $\langle\cdot\rangle$ means the mean value over all haloes in the corresponding halo mass bin. The halo bias relation in tinkerLargescaleBiasDark2010 is shown in gray solid curve. The small embedded panels show the comparison between $f\langle \Delta\rangle$ and $f\langle \Delta^{\rm RS} - c\rangle$ in selected halo mass bins. Bottom panel: The fractional residual of $f\langle \Delta \rangle$ and $f\langle \Delta^{\rm RS} - c\rangle$ with respect to the halo bias relation in tinkerLargescaleBiasDark2010 as a function of halo mass. Halo bias scales linearly with the mean overdensity of tracer haloes at $5-15\xspace h^{-1}\text{Mpc}$, while an additional constant shift is required for the mean overdensity measured in the redshift space.
• Figure 5: The spatial distribution of LGAs in the cosmic web for the lowest 20% (left) and highest 20% (right) of $E_{\rm coupling}$, projected on the $X-Y$ panel over a depth of $10h^{-1}\rm Mpc$ along the $Z$-axis. In each panel, the blue line segments indicate the orientation of the halo pair in each LGA, and the length proportional to their projected separation by a factor of 10. The background color map shows the distribution of all tracer haloes. LGAs are preferentially distributed in filamentary and cluster structures, while maintaining an alignment to the filaments, especially for those with low $E_{\rm coupling}$. Comparing these two panels, LGAs with low $E_{\rm coupling}$ are more sparsely distributed, and those with high $E_{\rm coupling}$ exhibit stronger clustering.