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We present new and archival atomic hydrogen (H i) observations of 15 of the most massive spiral galaxies in the local Universe (${M_{\star }}\gt 10^{11} \, {\rm M}_\odot$). From 3D kinematic modeling of the datacubes, we derive extended H i rotation curves, and from these, we estimate masses of the dark matter halos and specific angular momenta of the discs. We confirm that massive spiral galaxies lie at the upper ends of the Tully–Fisher relation (mass vs velocity, M ∝ V4) and Fall relation (specific angular momentum vs mass, j ∝ M0.6), in both stellar and baryonic forms, with no significant deviations from single power laws. We study the connections between baryons and dark matter through the stellar (and baryon)-to-halo ratios of mass fM ≡ M⋆/Mh and specific angular momentum fj, ⋆ ≡ j⋆/jh and fj, bar ≡ jbar/jh. Combining our sample with others from the literature for less massive disc-dominated galaxies, we find that fM rises monotonically with M⋆ and Mh (instead of the inverted-U shaped fM for spheroid-dominated galaxies), while fj, ⋆ and fj, bar are essentially constant near unity over four decades in mass. Our results indicate that disc galaxies constitute a self-similar population of objects closely linked to the self-similarity of their dark halos. This picture is reminiscent of early analytical models of galaxy formation wherein discs grow by relatively smooth and gradual inflow, isolated from disruptive events such as major mergers and strong active galactic nuclei feedback, in contrast to the more chaotic growth of spheroids.

 

ABSTRACT We study the gas kinematics of a sample of six isolated gas-rich low surface brightness galaxies, of the class called ultra-diffuse galaxies (UDGs). These galaxies have recently been shown to be outliers from the baryonic Tully–Fisher relation (BTFR), as they rotate much slower than expected given their baryonic mass, and to have a baryon fraction similar to the cosmological mean. By means of a 3D kinematic modelling fitting technique, we show that the H i in our UDGs is distributed in ‘thin’ regularly rotating discs and we determine their rotation velocity and gas velocity dispersion. We revisit the BTFR adding galaxies from other studies. We find a previously unknown trend between the deviation from the BTFR and the exponential disc scale length valid for dwarf galaxies with circular speeds ≲ 45 km s−1, with our UDGs being at the extreme end. Based on our findings, we suggest that the high baryon fractions of our UDGs may originate due to the fact that they have experienced weak stellar feedback, likely due to their low star formation rate surface densities, and as a result they did not eject significant amounts of gas out of their discs. At the same time, we find indications that our UDGs may have higher-than-average stellar specific angular momentum, which can explain their large optical scale lengths.