Satellite compaction pathways: environmental drivers shaping dwarf galaxy corpulence in the TNG50 simulation
We explore the physical mechanisms driving dwarf galaxy corpulence, focusing on those that end up as compact satellites. We select dwarf galaxies at $z=0$ with $\log(M_\star/{\rm M}_\odot)$ between 8.4 and 9.2 from the TNG50 hydrodynamical simulation after excluding systems flagged as potentially spurious. Compact dwarfs are defined according to the $z=0$ size-mass relation as those on the lower envelope of its main branch or on its lower-size secondary branch, while "Normal" lie on the main branch spine. We identify two robust compaction pathways and a third, more tentative, channel: 1) Compact satellites that remain rich in dark matter (DM) inhabit poorer environments having fewer mergers, favouring the accretion of lower-angular-momentum gas. This allows gas inflows that drive concentrated inner star formation and compaction, as previously found for centrals. 2) Most DM-poor satellites (which typically end up red and metal-rich for their stellar mass) undergo compaction mainly caused by tidal stripping of outer stars. Their compaction is faster when gas is present, by at least 15 per cent after correcting for the stronger tidal field. 3) For most of our few very metal-rich DM-poor Compact satellites, the major compaction phase begins with a starburst driven by ram pressure compression near first pericentre, even if much of the compaction often occurs during subsequent tidal stripping. As a result, compact dwarf satellites in TNG50 arise through distinct pathways. We discuss how numerical effects can affect this conclusion.