A 15 Mpc rotating galaxy filament at redshift z = 0.032

Abstract

Understanding the cold atomic hydrogen gas (HI) within cosmic filaments has the potential to pin down the relationship between the low density gas in the cosmic web and how the galaxies that lie within it grow using this material. We report the discovery of a cosmic filament using 14 HI-selected galaxies that form a very thin elongated structure of 1.7 Mpc. These galaxies are embedded within a much larger cosmic web filament, traced by optical galaxies, that spans at least $\sim 15$~Mpc. We find that the spin axes of the HI galaxies are significantly more strongly aligned with the cosmic web filament ($\langle\lvert \cos ψ\rvert\rangle = 0.64 \pm 0.05$) than cosmological simulations predict, with the optically-selected galaxies showing alignment to a lesser degree ($\langle\lvert \cos ψ\rvert\rangle = 0.55 \pm 0.05$). This structure demonstrates that within the cosmic filament, the angular momentum of galaxies is closely connected to the large-scale filamentary structure. We also find strong evidence that the galaxies are orbiting around the spine of the filament, making this one of the largest rotating structures discovered thus far, and from which we can infer that there is transfer of angular momentum from the filament to the individual galaxies. The abundance of HI galaxies along the filament and the low dynamical temperature of the galaxies within the filament indicates that this filament is at an early evolutionary stage where the imprint of cosmic matter flow on galaxies has been preserved over cosmic time.

Figures (9)

• Figure 1: Normalised histogram of the optical galaxies from DESI in the redshift range of $0.03 <$ z $< 0.034$ as a function of stellar mass.
• Figure 2: Top left: the on-sky distribution of H i galaxies (blue squares), SDSS and DESI optical galaxies (green circles and line, depending on the availability of optical PA measurements), and the cosmic filament. The MIGHTEE COSMOS footprint is shown in a grey block. Other panels show the DESI multi-band cutout image and the H i moment-1 map of each H i-selected galaxy. The size of the images and moment maps is fixed to 16 arcsec across each panel. The green dashed ellipse in each DESI image panel shows the ellipticity and the size (tripled for visual purposes) of the optical counterpart of each H i galaxy. Note that Galaxy 1 does not have an optical DESI counterpart. The grey colour arrow in the moment-1 map is the H i spin axis.
• Figure 3: The rotation velocity of the filament galaxies which have a projected distance to filament within 1 Mpc. The colour bar shows the relative velocity to the filament. The black dots denote the non-filament galaxies in the DESI observations in the redshift interval $0.03 < z < 0.034$. The thin gray lines represent the bootstrapped filament iterations.
• Figure 4: Number density of galaxies as a function of distance-to-filament. The blue line represents the modelled data assuming a cylinder of uniform density (see text).
• Figure 5: (top) Number density of galaxies as a function of distance-to-filament, with Poissonian uncertainties. The black (grey) line denotes the 6th (5th) order polynomial fits to the data, with residual uncertainties denoted by shaded regions. (bottom) The differential of the polynomial fits as a function of distance-to-filament. The vertical dotted lines show the minima (black for the 5th order derivative, gray for the 4th order derivative), corresponding to the characteristic radius of the filament.