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Register a new userA Limit on the Cosmological Mass Density and Power Spectrum from the Rotation Curves of Low Surface Brightness Galaxies
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S. McGaugh
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The concentrations of the cuspy dark matter halos predicted by simulations of cold dark matter are related to the cosmology in which the halos form. Observational constraints on halo concentration therefore map into constraints on cosmological parameters. In order to explain the observed concentrations of dark matter dominated low surface brightness galaxies, we require a cosmology with rather little power on galaxy scales. Formally, we require s_8 G_0.6 < 0.23, where G_0.6 is a modified shape parameter appropriate to this problem. Practically, this means either Omega_m < 0.2 or s_8 < 0.8. These limits apply as long as we insist that the cuspy halos found in simulations must describe the halos of low surface brightness galaxies. A low density cosmology helps with the low observed concentrations, but it offers no explanation of the many cases where the shape of the density profile deviates from the predicted cuspy form. These cases must have suffered very extensive mass redistribution if the current halo formation picture is not to fail outright. It is far from clear whether any of the mass redistribution mechanisms which have been suggested (e.g., feedback) are viable.
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A recent study has claimed that the rotation curve shapes and mass densities of Low Surface Brightness (LSB) galaxies are largely consistent with $Lambda$CDM predictions, in contrast to a large body of observational work. I demonstrate that the method used to derive this conclusion is incapable of distinguishing the characteristic steep CDM mass-density distribution from the core-dominated mass-density distributions found observationally: even core-dominated pseudo-isothermal haloes would be inferred to be consistent with CDM. This method can therefore make no definitive statements on the (dis)agreement between the data and CDM simulations. After introducing an additional criterion that does take the slope of the mass-distribution into account I find that only about a quarter of the LSB galaxies investigated are possibly consistent with CDM. However, for most of these the fit parameters are so weakly constrained that this is not a strong conclusion. Only 3 out of 52 galaxies have tightly constrained solutions consistent with $Lambda$CDM. Two of these galaxies are likely dominated by stars, leaving only one possible dark matter dominated, CDM-consistent candidate, forming a mere 2 per cent of the total sample. These conclusions are based on comparison of data and simulations at identical radii and fits to the entire rotation curves. LSB galaxies that are consistent with CDM simulations, if they exist, seem to be rare indeed.
The Rastall gravity is a modification of Einsteins general relativity, in which the energy-momentum conservation is not satisfied and depends on the gradient of the Ricci curvature. It is in dispute whether the Rastall gravity is equivalent to the general relativity (GR). In this work, we constrain the theory using the rotation curves of Low Surface Brightness (LSB) spiral galaxies. Through fitting the rotation curves of LSB galaxies, we obtain the parameter $beta$ of the Rastall gravity. The $beta$ values of LSB galaxies satisfy Weak Energy Condition (WEC) and Strong Energy Condition(SEC). Combining the $beta$ values of type Ia supernovae and gravitational lensing of elliptical galaxies on the Rastall gravity, we conclude that the Rastall gravity is equivalent to the general relativity.
We present high-resolution rotation curves of a sample of 26 low surface brightness galaxies. From these curves we derive mass distributions using a variety of assumptions for the stellar mass-to-light ratio. We show that the predictions of current Cold Dark Matter models for the density profiles of dark matter halos are inconsistent with the observed curves. The latter indicate a core-dominated structure, rather than the theoretically preferred cuspy structure.
Dwarf and low surface brightness galaxies are ideal objects to test modified Newtonian dynamics (MOND), because in most of these galaxies the accelerations fall below the threshold below where MOND supposedly applies. We have selected from the literature a sample of 27 dwarf and low surface brightness galaxies. MOND is successful in explaining the general shape of the observed rotation curves for roughly three quarters of the galaxies in the sample presented here. However, for the remaining quarter, MOND does not adequately explain the observed rotation curves. Considering the uncertainties in distances and inclinations for the galaxies in our sample, a small fraction of poor MOND predictions is expected and is not necessarily a problem for MOND. We have also made fits taking the MOND acceleration constant, a_0, as a free parameter in order to identify any systematic trends. We find that there appears to be a correlation between central surface brightness and the best-fit value of a_0, in the sense that lower surface brightness galaxies tend to have lower a_0. However, this correlation depends strongly on a small number of galaxies whose rotation curves might be uncertain due to either bars or warps. Without these galaxies, there is less evidence of a trend, but the average value we find for a_0 ~ 0.7*10^-8 cm s^-2 is somewhat lower than derived from previous studies. Such lower fitted values of a_0 could occur if external gravitational fields are important.
We present Very Large Array ({sc vla}) and Westerbork Synthesis Radio Telescope ({sc wsrt}) 21-cm H{sc i} observations of 19 late-type low surface brightness (LSB) galaxies. Our main findings are that these galaxies, as well as having low surface brightnesses, have low H{sc i} surface densities, about a factor of $sim 3$ lower than in normal late-type galaxies. We show that LSB galaxies in some respects resemble the outer parts of late-type normal galaxies, but may be less evolved. LSB galaxies are more gas-rich than their high surface brightness counterparts. The rotation curves of LSB galaxies rise more slowly than those of HSB galaxies of the same luminosity, with amplitudes between 50 and 120~km~s$^{-1}$, and are often still increasing at the outermost measured point. The shape of the rotation curves suggests that LSB galaxies have low matter surface densities. We use the average total mass surface density of a galaxy as a measure for the evolutionary state, and show that LSB galaxies are among the least compact, least evolved galaxies. We show that both $M_{rm HI}/L_B$ and $M_{rm dyn}/L_B$ depend strongly on central surface brightness, consistent with the surface brightness--mass-to-light ratio relation required by the Tully-Fisher relation. LSB galaxies are therefore slowly evolving galaxies, and may well be low surface density systems in all respects.
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