The satellite galaxy plane of NGC 4490 in light of {\Lambda}CDM - Sparsity of similarly extreme analogs and a possible role of satellite pairs

TL;DR

This paper analyzes the satellite galaxy plane around NGC4490, finding it to be an extremely flattened and kinematically coherent structure that is rare in simulations, raising questions about the standard cosmological model.

Contribution

It provides the first quantitative assessment of the tension between observed satellite planes and $ mf ext{Λ}$CDM expectations, highlighting the role of satellite pairs and large-scale alignments.

Findings

The NGC4490 satellite system is highly flattened and kinematically coherent.

Such extreme satellite arrangements are very rare in IllustrisTNG-50 simulations.

Presence of satellite pairs may be significant for understanding satellite plane formation.

Abstract

The galaxy system around NGC4490 was recently highlighted to display a flattened, kinematically correlated structure reminiscent of satellite galaxy planes around other hosts. Since known satellite planes are in tension with $\Lambda$CDM expectations from cosmological simulations, we quantitatively assess for the first time the tension posed by the NGC4490 system. We measure the on-sky flattening as the major-to-minor axis ratio b/a of the satellite distribution and their line-of-sight kinematic correlation. Analogs are selected in the IllustrisTNG-50 simulation and their flattening and correlation are similarly measured. We confirm the strong kinematic coherence of all 12 observed objects with available line-of-sight velocities (of 14 in total): the northern ones approach and the southern ones recede relative to the host. The spatial distribution of all 14 objects is substantially flattened with b/a=0.38 (0.26 considering only the 12 objects with velocities). Such extreme arrangements are rare in the simulation at 0.21-0.35%. This would drop further if at least one of the two satellite objects without velocities is confirmed to follow the kinematic trend, and would become zero if both are rejected as non-members. We also identify a likely galaxy pair in the observed system, and find a similar pair in the best-matching simulated analog. Our findings establish NGC4490 as another strong example of a satellite plane in the Local Volume. This emphasizes that planes of satellites are a more general issue faced by $\Lambda$CDM also beyond the Local Group. The tension with typical systems in simulations suggests that the observed one requires a specific formation scenario, potentially connected to the larger-scale galaxy alignment in its vicinity. The presence of galaxy pairs in the observed and a simulated system hints at the importance such groupings may have to understand satellite planes.