The Population of Giant Clumps in Simulated High-z Galaxies: In-situ and Ex-situ, Migration and Survival

Abstract

We study the properties of giant clumps and their radial gradients in high-$z$ disc galaxies using AMR cosmological simulations. Our sample consists of 770 snapshots in the redshift range $z=4-1$ from 29 galaxies that at $z=2$ span the stellar mass range $(0.2-3)\times 10^{11}M_{\odot}$. Extended gas discs exist in 83% of the snapshots. Clumps are identified by gas density in 3D and their stellar and dark matter components are considered thereafter. While most of the overdensities are diffuse and elongated, 91% of their mass and 83% of their star-fromation rate (SFR) are in compact round clumps. Nearly all galaxies have a central, massive bulge clump, while 70% of the discs show off-center clumps, 3-4 per galaxy. The fraction of clumpy discs peaks at intermediate disc masses. Clumps are divided based on dark-matter content into $\textit{in-situ}$ and $\textit{ex-situ}$, originating from violent disc instability (VDI) and minor mergers respectively. 60% of the discs are in a VDI phase showing off-center $\textit{in-situ}$ clumps, which contribute 1-7% of the disc mass and 5-45% of its SFR. The $\textit{in-situ}$ clumps constitute 75% of the off-center clumps in terms of number and SFR but only half the mass, each clump containing on average 1% of the disc mass and 6% of its SFR. They have young stellar ages, $100-400 {\rm Myr}$, and high specific SFR (sSFR), $1-10 {\rm Gyr}^{-1}$. They exhibit gradients resulting from inward clump migration, where the inner clumps are somewhat more massive and older, with lower gas fraction and sSFR and higher metallicity. Similar observed gradients indicate that clumps survive outflows. The \exsitu clumps have stellar ages $0.5-3 {\rm Gyr}$ and sSFR $\sim 0.1-2 {\rm Gyr}^{-1}$, and they exhibit weaker gradients. Massive clumps of old stars at large radii are likely \exsitu mergers, though half of them share the disc rotation.