On the Formation of Spiral Arms in Dwarf Galaxies

Abstract

Spiral structure (both flocculent and Grand Design types) is very rarely observed in dwarf galaxies because the formation of spiral arms requires special conditions. In this work we analyze the sample of about 40 dS-galaxies found by scanning by eye the images of late-type galaxies with and and photometric diameter kpc. We found that apart from the lower average gas (HI) fraction the other properties of dS-galaxies including the presence of a bar and the isolation index do not differ much from those for dwarf Irr or Sm-types of similar luminosity and rotation velocity (or specific angular momentum).There are practically no dS-galaxies with rotation velocity below 50-60 km s -1 . To check the conditions of formation of spiral structure in dwarf galaxies we carried out a series of N-body/hydrodynamic simulations of lowmass stellar-gaseous discy galaxies by varying the model kinematic parameters of discs, their initial thickness, relative masses and scale lengths of stellar and gaseous disc components, and stellar-to-dark halo masses. We came to conclusion that the gravitational mechanism of spiral structure formation is effective only for thin stellar discs, which are non-typical for dwarf galaxies, and for not too slowly rotating galaxies. Therefore, only a small fraction of dwarf galaxies with stellar/gaseous discs have spiral or ring structures. The thicker stellar disc, the more gas is required for the spiral structure to form. The reduced gas content in many dS-galaxies compared to non-spiral ones may be a result of more efficient star formation due to a higher volume gas density thank to the thinner stellar/gaseous discs.

Formation of Transitional cE/UCD Galaxies through Massive/Dwarf Disc Galaxy Mergers

“…The dark halo dominated only at the periphery of the disc, as we were limited to dark-matter profile with a scale of 0.62 kpc without a cusp, in contrast to the NFW model [113]. The maximum gas-rotation velocity in the satellite was about 80 km s −1 , which is typical for dwarf S-galaxies [114]. The models of the satellite had different gas contents, from complete absence to mass M…”