[Paper Review] Is Dark Matter in Spiral Galaxies Cold Gas? I. Observational Constraints and Dynamical Clues About Galaxy Evolution

D. Pfenniger, F. Combes+1 more|arXiv (Cornell University)|Nov 18, 1993

Galaxies: Formation, Evolution, Phenomena54 citations

TL;DR

The paper proposes that dark matter in spiral galaxies is primarily cold molecular gas, rotationally supported and dynamically stable, resolving key puzzles like flat rotation curves and rapid gas consumption. It argues that the observed dark matter-to-HI mass ratio constancy and gas-rich interactions stem from underestimated gas masses due to clumpiness and high optical depth, suggesting a gaseous dark matter origin that reconciles dynamical constraints with galaxy evolution models.

ABSTRACT

Based on dynamical constraints about the Hubble sequence evolution, observational data and a number of "conspiracies", we propose that the dark matter around spiral galaxies is in the form of cold gas, essentially in molecular form and rotationally supported. (full A&A paper (in press) available by anonymous ftp at obssd8.unige.ch in /pub/fractal as postscript file: dm_paper_I.ps (170k), or papers I & II + figures as a compressed tar file dm_papers.Z.tar (2.1 Mb)).

Motivation & Objective

• To resolve the longstanding puzzle of flat rotation curves in spiral galaxies by proposing a gaseous dark matter origin.
• To reconcile the observed constancy of the dark matter to HI mass ratio in outer discs with dynamical evolution models.
• To address the 'gas consumption problem'—where gas depletes faster than observed—by re-evaluating gas mass estimates.
• To investigate whether the observed conspiracy of dark matter and HI mass ratios can be explained by underestimated gas masses due to inhomogeneity.
• To assess the dynamical stability of cold, clumpy gas discs as a viable alternative to particle-based dark matter models.

Proposed method

• Analyzes Hubble sequence evolution, emphasizing secular evolution driven by bars and bulge growth to infer timescales of 1–2 Gyr for spiral galaxy transformation.
• Applies dynamical constraints from rotation curves and HI 21 cm observations to infer mass distribution and dark matter content.
• Uses N-body simulations and stability theory (e.g., Kalnajs 1987) to assess the role of halos and the viability of bar formation in gas-rich discs.
• Evaluates the impact of ISM inhomogeneity on gas mass estimation, particularly the effects of high optical depth and thermal equilibrium with the CMB.
• Models the transition from gas-rich, unstable discs to stable, exponential-profile systems via internal instabilities and star formation.
• Proposes that HI mass measurements may underestimate total gas mass by a factor of 10 or more due to clumpiness and high column densities.

Experimental results

Research questions

• RQ1 Can the flat rotation curves of spiral galaxies be explained by a rotationally supported, cold molecular gas disc rather than particle dark matter?
• RQ2 Why is the ratio of dark matter to HI mass approximately constant in the outer discs of spiral galaxies?
• RQ3 How can the rapid consumption timescale of interstellar gas be reconciled with the long-term stability of spiral galaxies?
• RQ4 To what extent does the inhomogeneous, clumpy structure of the ISM lead to systematic underestimation of total gas mass in HI surveys?
• RQ5 Is a cold, gaseous dark matter component dynamically stable and self-consistent over cosmological timescales?

Key findings

• The observed constancy of the dark matter to HI mass ratio in outer discs is naturally explained if dark matter is cold, molecular gas with high optical depth and clumpiness.
• Gas mass estimates from HI 21 cm observations may underestimate the true gas mass by a factor of 10 or more due to high column densities and inhomogeneous distribution.
• The dynamical stability of cold, clumpy gas discs is less problematic than previously assumed, especially when thermal equilibrium with the 3 K CMB is considered.
• Bar-driven secular evolution over 1–2 Gyr can transform gas-rich Sd-type spirals into bulge-dominated Sa-type galaxies, implying rapid internal evolution.
• The observed conspiracy of dark matter and HI mass ratios across the Hubble sequence suggests a transformation of dark matter into stars via a gaseous phase.
• Nucleosynthesis constraints allow for a baryonic dark matter contribution of Ωb ≈ 0.1, consistent with the proposed cold gas component in galactic discs.