We show that the outskirts of HI rich galaxies obey a linear radius (R) versus rotational velocity (Vrot) relationship. This means they behave like clocks: they have the same orbital time of ~800 Myr. The relationship is valid over the full range for which we have data - a factor of 30 from dwarf galaxies with R ~ 1 kpc and Vrot ~ 10 km/s to giant spirals with R = 30 kpc and Vrot = 300 km/s with an intrinsic scatter smaller than 40%. A linear R -- Vrot relationship is expected for Cold Dark Matter (CDM) dominated halos. The fact that the collapsed baryons of disk galaxies obey this relationship can be readily understood within the CDM paradigm. We show what is required for the situation to occur. The mean density within the outer radius is 3e-3 Msun/pc^3, requiring that the baryonic component of disk galaxies to have collapsed by a factor of ~40. We outline the practical uses of the relationship and the implications for galaxy evolution.
|Publication status||Published - Jan 2014|
|Event||223rd American Astronomical Society Meeting - Gaylord National Resort and Convention Center, United States|
Duration: 5 Jan 2014 → 9 Jan 2014
|Conference||223rd American Astronomical Society Meeting|
|Period||5/01/14 → 9/01/14|