Unveiling a Thin Filament of the Cosmic Web in the Ursa Major Supergroup

TL;DR

The paper tackles the challenge of observing small-scale filaments in the cosmic web and tests predictions of gas accretion along filaments. It leverages FAST's high-sensitivity HI observations from the FASHI survey to identify a thin, ~$L\approx0.9$ Mpc HI filament in the Ursa Major supergroup, comprising 17 galaxies and 5 starless gas clumps, with a clear velocity gradient and galaxy-spin alignment indicating cold accretion along the structure. Mass analysis yields a total filament mass of $M_{tot}\approx3.5\times10^{12} M_\odot$ and a baryonic mass of $M_{bar}\approx8.8\times10^{10} M_\odot$, implying that dark matter dominates by about a factor of 40; the filament’s thickness is comparable to a galaxy diameter, and its spine hosts a regular spacing of $\sim71.6$ kpc in at least one sub-filament. This discovery provides observational evidence for ultra-thin filaments in the local universe and informs models of hierarchical structure formation and galaxy growth via cold accretion flows along filaments.

Abstract

Filaments are crucial components of the cosmic web, representing the extensive and aligned distributions of galaxies and gas. Using the Five-hundred-meter Aperture Spherical radio Telescope (FAST), we report the detection of a filament in the Ursa Major supergroup using atomic-hydrogen (HI) observations. This filament consists of sixteen various types of galaxies and five starless gas clumps, spanning a length of approximately 0.9 Mpc. Notably, it is extremely thin, with a thickness comparable to the diameter of a galaxy. We observed a galaxy-filament spin alignment and a velocity gradient within the filament. These findings strongly suggest a cold accretion flow along the filament, potentially contributing to the formation and growth of the galaxies. The thin filament, as a small group, is likely to be merged into the Ursa Major supergroup in the context of hierarchical structure formation.

Figures (4)

• Figure 1: Overview of the galaxy filament. Left, H i column-density map shown in the grey scale and blue contours for each galaxy. The contours begin at 3.2$\times$10$^{17}$ cm$^{-2}$ (3$\sigma$) in steps of 3.8$\times$10$^{17}$ cm$^{-2}$. The black contours indicate several gas clumps. The red dashed and dotted lines are used to link two adjacent galaxies. The cyan lines on each galaxy indicate the direction of spin. The FAST beam in a black circle is shown in the bottom-right corner. Right, the SDSS RGB (z, r, and g bands) composite image of a small region shown by a cyan dashed box in the H i column-density map.
• Figure 2: Overview of the Ursa Major supergroup from the optical observations 2013MNRAS.429.2264K2014MNRAS.445..630P. The center and size of the supergroup are indicated by the red solid and dashed circles, respectively. The identified filament is marked with a red dashed box. The black dots represent the positions of the member galaxies in the supergroup, while the blue and green dots indicate the member galaxies and H i gas clumps in the filament. N, north; W, west.
• Figure 3: Detected filamentary pattern in a 22-source volume within a pattern of galaxies and clumps (points). The initial linking length values. The short black line represents two galaxies that comply with the FoF algorithm.
• Figure 4: Velocity--distance plot. Each black dot represents a galaxy. The starting dot is the galaxy SBS 1147 located at the bottom of the galaxy filament. Each two galaxies are linked by a black dashed line. The red dashed lines are the fitted curves for the galaxies in the SW, NE, and M smaller filaments. a, the small filaments in the SW and M sections of the galaxy filament. b, the small filaments in the SW and NE sections of the galaxy filament.