Although giant clumps of stars are thought to be crucial to galaxy formation and evolution , the most basic demographics of clumps are still uncertain , mainly because the definition of clumps has not been thoroughly discussed .
In this paper , we carry out a study of the basic demographics of clumps in star-forming galaxies at 0.5 < z < 3 , using our proposed physical definition that UV-bright clumps are discrete star-forming regions that individually contribute more than 8 % of the rest-frame UV light of their galaxies .
Clumps defined this way are significantly brighter than the HII regions of nearby large spiral galaxies , either individually or blended , when physical spatial resolution and cosmological dimming are considered .
Under this definition , we measure the fraction of star-forming galaxies that have at least one off-center clump ( f _ { clumpy } ) and the contributions of clumps to the rest-frame UV light and star formation rate ( SFR ) of star-forming galaxies in the CANDELS/GOODS-S and UDS fields , where our mass-complete sample consists of 3239 galaxies with axial ratio q > 0.5 .
The redshift evolution of f _ { clumpy } changes with the stellar mass ( { M _ { * } } ) of the galaxies .
Low-mass ( { log ( M _ { * } / M _ { \odot } ) } < 9.8 ) galaxies keep an almost constant f _ { clumpy } of \sim 60 % from z \sim 3 to z \sim 0.5 .
Intermediate-mass and massive galaxies drop their f _ { clumpy } from 55 % at z \sim 3 to 40 % and 15 % , respectively , at z \sim 0.5 .
We find that ( 1 ) the trend of disk stabilization predicted by violent disk instability matches the f _ { clumpy } trend of massive galaxies ; ( 2 ) minor mergers are a viable explanation of the f _ { clumpy } trend of intermediate-mass galaxies at z < 1.5 , given a realistic observability timescale ; and ( 3 ) major mergers are unlikely responsible for the f _ { clumpy } trend in all masses at z < 1.5 .
The clump contribution to the rest-frame UV light of star-forming galaxies shows a broad peak around galaxies with { log ( M _ { * } / M _ { \odot } ) } \sim 10.5 at all redshifts .
The clump contribution in the intermediate-mass and massive galaxies is possibly linked to the molecular gas fraction of the galaxies .
The clump contribution to the SFR of star-forming galaxies , generally around 4–10 % , also shows dependence on the galaxy { M _ { * } } , but for a given galaxy { M _ { * } } , its dependence on the redshift is mild .