Spiral Galaxies and Tracers of Mass Accretion：（螺旋星系和质量吸积的示踪剂）.pdfVIP

Spiral Galaxies and Tracers of Mass Accretion F. Combes 9 9 Observatoire de Paris, DEMIRM, Paris, France 9 1 r p Abstract A Can the present dynamics of spiral galaxies tell us something about the merging his- 2 tory, the formation and evolution of disks? Galaxy interactions thicken or destroy disks; 1 the simultaneous presence of thick and thin disks is a tracer of past accretion, and also 1 of recent disk re-formation. Observation of a large number of counter-rotating disks is v also evidence of past mergers, as well as the frequency of polar-ring galaxies. Finally, the 3 ubiquitous presence of warps in the outer parts of HI disks might also provide a clue of 3 1 how frequently disks accrete mass with different angular momentum. 4 0 9 9 1 Introduction / h p According to hierarchical cosmological scenarios, galaxies form through merging of smaller - o entities. At least the dark haloes of dissipationless matter hierarchically merge, and it is r expected that some of the visible galaxies also interact and exchange mass, while spiraling in t s a a common halo. If major mergers lead to the formation of ellipticals, and leave vestiges such : as shells, ripples and loops around present-day elliptical galaxies (Schweizer Seitzer 1988), v i signatures of accretion or merger are less easy to see in spiral galaxies. Yet, mass and gas X r accretion is required in spiral galaxies for several reasons: a • Metallicity distribution in the disk (the G-dwarf problem for instance, requires gas infall) • Spiral formation and maintenance: e