IS DARK-MATTER IN SPIRAL GALAXIES COLD GAS .1. OBSERVATIONAL CONSTRAINTS AND DYNAMICAL CLUES ABOUT GALAXY EVOLUTION

Based on dynamical constraints about the Hubble sequence evolution, ob servational data and a number of ''conspiracies'', we propose that the dark matter around spiral galaxies is in the form of cold gas, essent ially in molecular form and rotationally supported. The existence of a much larger amount of cold gas in the outer discs of spirals is in ac cordance with dynamical studies taking into account the bar phenomenon and the secular growth of bulges, leading to the general conclusion t hat spiral galaxies have to evolve along the Hubble sequence from Sd t o Sa over a couple of Gyr's. If so, the varying MIL ratio along the Hu bble sequence suggests that dark matter is transformed into stars, i.e . dark matter should be in a sufficiently diluted form of gas. This hy pothesis sheds light on several puzzles: 1) the conspiracy of the flat rotation curves, 2) the estimated short gas consumption time of spira l galaxies, leading to the ''gas consumption problem'', 3) the constan t ratio of dark matter to HI mass in the outer spiral discs, 4) the la rger amount of visible gas in interacting galaxies with respect to iso lated ones, giving rise to gigantic starbursts, 5) the high frequency of absorption lines in front of quasars, together with the large sizes of the absorbing medium around galaxies. Several problems raised by a completely gaseous dark matter in the outer spiral discs are discusse d. In particular the stability and self-consistency of cold gaseous di scs is shown to be a less severe problem than commonly believed.