The dichotomy concerns the so-called angular momentum (per unit mass) that in physics is a measure of size and rotation velocity. Spiral galaxies are found to be strongly rotating, with an angular momentum higher by a factor of about five than ellipticals. What is the origin of such a difference? An international research team investigated the issue in a study just published in the Astrophysical Journal. The team was led by SISSA Ph.D. student JingJing Shi under the supervision of Prof. Andrea Lapi and Luigi Danese, and in collaboration with Prof. Huiyuan Wang from USTC (Hefei) and Dr. Claudia Mancuso from IRA-INAF (Bologna). The researchers inferred from observations the amount of gas fallen into the central region of a developing galaxy, where most of the star formation takes places.
The outcome is that in elliptical galaxies, only about 40 percent of the available gas fell into that central region. More relevantly, this gas fueling star formation was characterized by a rather low angular momentum. This is in stark contrast with the conditions found in spirals, where most of the gas that ends up in stars has an appreciably higher angular momentum. The researchers have traced the dichotomy in the angular momentum of spiral and elliptical galaxies to their different formation histories. Elliptical galaxies form most of their stars in a fast collapse in which angular momentum is dissipated. This process is likely stopped early on by powerful gas outflows from supernova explosions, stellar winds and possibly even from the central supermassive black hole. For spirals, on the other hand, the gas fell slowly, conserving its angular momentum, and stars formed steadily along a timescale comparable to the age of the universe.
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