[Paper Review] A Heavy Baryonic Galactic Disc

J. I. Davies|arXiv (Cornell University)|Apr 20, 2012

Galaxies: Formation, Evolution, Phenomena3 references3 citations

TL;DR

This paper proposes that flat rotation curves in spiral galaxies can be explained by a massive, extended baryonic disc—primarily cold atomic and molecular hydrogen—without requiring non-baryonic dark matter. By extending Mestel's disc models and invoking a factor of ~7 more baryonic mass than currently observed, the authors show that such a disc naturally reproduces observed rotation curves and the Tully-Fisher relation, resolving long-standing puzzles like the disc-halo conspiracy and maximum disc fitting.

ABSTRACT

We investigate the possibility that the observed rotation of galaxies can be accounted for by invoking a massive baryonic disc with no need for non-baryonic dark matter or a massive halo. There are 5 primary reasons for suggesting this: 1. there are well known disc surface mass density distributions that naturally produce the observed rotation curves of galaxies. 2. there are a number of rotation curve `puzzles' that cannot be explained by a massive dark matter halo i.e. the success of maximum disc fitting, HI gas scaling to the observed rotation, the disc/halo conspiracy and the interpretation of the Tully-Fisher relation. 3. recent 21cm observations show an almost constant HI surface density and a distinct `cut-off' or edge to galactic discs. We explain this constant surface density in terms of either an optical depth effect or the onset of molecular gas formation and hence the possibility of considerably more gas existing in galaxies. We suggest that the HI cut-off does indeed mark the edge of the galactic disc. 4. there have been an increasing number of recent observations that imply that X_CO may be ten times higher in the outer Galaxy. This `dark' gas may provide adequate mass to account for galaxy rotation. 5. we show that the additional baryonic mass required to account for the rotation of galaxies is just that required to reconcile observed baryons with those predicted by big bang nucleosynthesis. Mestel discs can be used to straight forwardly explain the scaling laws of galaxies, particularly the observed relation between rotation velocity and radius and the oft used Tully-Fisher relation. We discuss observations of the baryonic content of galactic discs and where sufficient `hidden' baryons might be found to account for the rotation.

Motivation & Objective

• To challenge the necessity of non-baryonic dark matter in explaining flat galaxy rotation curves.
• To investigate whether a massive baryonic disc—especially with hidden atomic and molecular gas—can account for observed kinematics.
• To resolve longstanding puzzles in galaxy dynamics, such as the disc-halo conspiracy and maximum disc fitting, through baryonic mass alone.
• To reconcile observed baryonic masses with predictions from big bang nucleosynthesis by accounting for a hidden baryonic reservoir in galactic discs.
• To explore whether the monolithic collapse of isothermal gas clouds can naturally produce observed scaling laws like the Tully-Fisher relation.

Proposed method

• Uses Mestel’s analytical disc models—specifically the constant linear velocity (Mestel) disc and constant angular velocity disc—to derive surface mass density distributions that produce flat rotation curves.
• Applies the surface mass density profiles from Mestel’s collapse model of isothermal, constant-density spherical clouds to explain observed rotation curves via gravitational potential calculations.
• Introduces a modified interpretation of HI 21cm observations, suggesting the observed constant HI surface density arises from optical depth effects or the onset of molecular gas formation, implying higher total gas mass than previously assumed.
• Proposes that the HI 'cut-off' marks the true edge of the galactic disc, not a truncation of gas, and that this edge corresponds to the transition to molecular gas.
• Reinterprets gamma-ray observations suggesting a tenfold increase in the X_CO factor in outer galaxies, implying a large reservoir of undetected molecular gas.
• Uses the Tully-Fisher relation as a diagnostic: the predicted logarithmic slope of 1/3 from Mestel’s model matches observations, supporting the baryonic disc hypothesis.

Experimental results

Research questions

• RQ1 Can flat galaxy rotation curves be explained by a massive baryonic disc without invoking non-baryonic dark matter?
• RQ2 What surface mass density distribution naturally produces observed flat rotation curves, and can it arise from gravitational collapse of a gaseous protogalaxy?
• RQ3 Why do observed HI surface densities appear constant, and could this reflect a hidden reservoir of cold, opaque atomic and molecular gas?
• RQ4 How does the observed Tully-Fisher relation's slope of ~1/3 emerge from baryonic disc dynamics, and does it support the absence of a dark matter halo?
• RQ5 Is the discrepancy between observed baryonic mass and big bang nucleosynthesis predictions resolved by including a hidden baryonic component in the disc?

Key findings

• A massive baryonic disc with a surface mass density profile matching Mestel’s constant linear velocity model can naturally produce flat rotation curves without requiring a dark matter halo.
• The observed constant HI surface density is consistent with a combination of optical depth effects and the onset of molecular gas formation, suggesting up to a factor of seven more baryonic mass than previously accounted for.
• The HI 'cut-off' at large radii likely marks the true edge of the galactic disc, not a truncation of gas, supporting the idea of a continuous, massive baryonic disc.
• The Tully-Fisher relation’s observed slope of approximately 1/3 is naturally reproduced by Mestel’s model, which assumes constant-density proto-galactic clouds collapsing with preserved angular momentum distribution.
• The required additional baryonic mass to explain rotation curves is precisely the amount needed to reconcile observed baryons with big bang nucleosynthesis predictions, eliminating the need for a warm intergalactic medium.
• The model resolves several 'conspiracies' in galaxy dynamics—such as the disc-halo conspiracy and maximum disc fitting—by removing the need for a dark matter halo altogether.