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The origin of the disk and spheroid of galaxies has been a key open questionin understanding their morphology. Using the high-resolution cosmologicalsimulation, New Horizon, we explore kinematically decomposed disk andspheroidal components of 144 field galaxies with masses greater than $\rm10^9\,M_{\odot}$ at $z=0.7$. The origins of stellar particles are classifiedaccording to their birthplace (in situ or ex situ) and their orbits at birth.Before disk settling, stars form mainly through chaotic mergers betweenproto-galaxies and become part of the spheroidal component. When disk settlingstarts, we find that more massive galaxies begin to form disk stars fromearlier epochs; massive galaxies commence to develop their disks at $z\sim1-2$,while low-mass galaxies do after $z\sim1$. The formation of disks is affectedby accretion as well, as mergers can trigger gas turbulence or inducemisaligned gas infall that prevents galaxies from forming co-rotating diskstars. The importance of accreted stars is greater in more massive galaxies,especially in developing massive spheroids. A significant fraction of thespheroids comes from the disk stars that are perturbed, which becomes moreimportant at lower redshifts. Some ($\sim12.5\%$) of our massive galaxiesdevelop counter-rotating disks from the gas infall misaligned with the existingdisk plane, which can last for more than a Gyr until they become the dominantcomponent, and flip the angular momentum of the galaxy in the oppositedirection. The final disk-to-total ratio of a galaxy needs to be understood inrelation to its stellar mass and accretion history. We quantify thesignificance of the stars with different origins and provide them as guidingvalues.

New Horizon: On the origin of the stellar disk and spheroid of field galaxies at $z=0.7$