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Astron. Astrophys. 345, 36-42 (1999)

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1. Introduction

The modified dynamics proposed by Milgrom (1983), MOND, remains unaccepted by the scientific community because of the absence of a relativistic theory that reduces to MOND in the weak-field limit. Many efforts have been made to test MOND in different astrophysics scenarios (Hernquist & Quinn 1987; Lake 1989; Gerbal et al. 1992; Lo et al. 1993; Gerhard 1993; van der Kruit 1995). However, these results are controversial or inconclusive (see McGaugh & de Blok 1998b for a review), and there is not any definite observational falsification of MOND despite the obvious interest to find it.

On the contrary, MOND can account for the shape and magnitude of a sample of half hundred rotation curves including nearby dwarf galaxies and low surface brightness galaxies, and is able to explain in a natural way the Tully-Fisher relation and other observational trends (Sanders 1996; McGaugh & de Blok 1998b). These facts are striking when one considers that this sample ranges from galaxies with asymptotic rotational velocities between 50 and 300 km[FORMULA]s and covers a range of [FORMULA] magnitudes in luminosity. Only the rotation curve for NGC 2841 is badly fitted and requires a distance twice as large as the Hubble law distance for an acceptable fit. Lake (1989) attempted to test MOND with dwarf galaxies and concluded that the constant of acceleration is not universal and should vary systematically with the maximum rotational velocity in the galaxy. However, Milgrom (1991) refuses to be convinced by this sample because of several uncertainties in inclination corrections and distance estimates.

The question that arises is whether is possible a definite test of MOND from galactic dynamics or one has to resort to larger scales such as clusters of galaxies. Deviations from MOND fitting to the observed rotation curves can be justified appealing to various data uncertainties such as non-circular motions, distance errors, imprecise inclination corrections, bulge-disc decomposition, warps, physical variations of the mass-to-light ratio ([FORMULA]) of the stellar population with radii, etc.

In this paper, we study the consistency of the application of Milgrom's theory to dwarf disc galaxies for which the uncertainty in [FORMULA] is removed because of the dominant contribution of the gas to the potential. For them, MOND prescription successfully reproduces the remarkable structure in the rotation curves of dwarf disc galaxies that dark-halo models are not able to (Begeman et al. 1991). However, these analyses are restricted to reproducing the shape of the rotation curves regardless of the consistency with the internal dynamics given by the theory of spiral structure or of stability against gravitational perturbations.

In this work it is reported that small gas-rich galaxies in MOND would suffer from catastrophic ring instabilities over almost the whole galactic radius. In particular, we focus on the low-mass galaxy IC 2574 for several reasons, such as its high inclination, the existence of HI velocity dispersion data of high accuracy, and other evidence which suggests the absence of strong spiral density waves. Since this galaxy presents a Toomre parameter in MOND less than unity at most galactocentric radii, it should have evolved violently from the present configuration of gas to another more stable one (Sect. 3.1). Generally speaking, it is possible to increase the value of the Toomre parameter by varying the adopted distance for gas-rich galaxies (Sect. 3.2). At present, however, we have not been able to find any satisfactory solution to the problem of stability even invoking magnetic fields (Sect. 4). Indeed, the high degree of instability predicted by MOND in the tidal tail between NGC 4485/4490 seems also to be in conflict with observations (Sect. 5). Overcoming these open questions is necessary for MOND to continue being a real alternative to the dark matter hypothesis in galaxy discs.

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© European Southern Observatory (ESO) 1999

Online publication: April 12, 1999