What made discy galaxies giant?

Abstract

I studied giant discy galaxies with optical radii more than 30 kpc. The comparison of these systems with discy galaxies of moderate sizes revealed that they tend to have higher rotation velocities, B-band luminosities, HI masses and dark-to-luminous mass ratios. The giant discs follow the trend $\log(M_{\rm HI})(R_{25})$ found for normal size galaxies. It indicates the absence of the peculiarities of evolution of star formation in these galaxies. The HI mass to luminosity ratio of giant galaxies appears not to differ from that of normal size galaxies, giving evidences in favor of similar star formation efficiency. I also found that the bars and rings occur more frequently among giant discs. I performed mass-modelling of the subsample of 18 giant galaxies with available rotation curves and surface photometry data and constructed $\chi^2$ maps for the parameters of their dark matter haloes. These estimates indicate that giant discs tend to be formed in larger more massive and rarified dark haloes in comparison to moderate size galaxies. However giant galaxies do not deviate significantly from the relations between the optical sizes and dark halo parameters for moderate size galaxies. These findings can rule out the catastrophic scenario of the formation of at least most of giant discs, since they follow the same relations as normal discy galaxies. The giant sizes of the discs can be due to the high radial scale of the dark matter haloes in which they were formed.