2025ApJ...980L...2A

We identify a chain of galaxies along an almost straight line in the nearby Universe with a projected length of ∼5 Mpc. The galaxies are distributed within projected distances of only 7-105 kpc from the axis of the identified filament. They have redshifts in a very small range of z = 0.0361-0.0370 so that their radial velocities are consistent with galaxy proper motions. The filament galaxies are mainly star forming and have stellar masses in a range of 10^9.1-10^10.7M_⊙. We search for systems with similar geometrical properties in the full-sky mock galaxy catalog of the MillenniumTNG simulations and find that, although such straight filaments are unusual and rare, they are predicted by ΛCDM simulations (4% incidence). We study the cold H I gas in a 1.3 Mpc section of the filament through H I 21 cm emission line observations and detect 11 H I sources, many more than expected from the H I mass function in a similar volume. They have H I masses 10^8.5-10^9.5M_⊙ and are mostly within ∼120 kpc projected distance from the filament axis. None of these H I sources has a confirmed optical counterpart. Their darkness together with their large H I 21 cm line widths indicates that they contain gas that might not yet be virialized. These clouds must be marking the peaks of the dark matter and H I distributions over large scales within the filament. The presence of such gas clouds around the filament spines is predicted by simulations, but this is the first time that the existence of such clouds in a filament is observationally confirmed.