Quantifying the impact of mergers on the angular momentum of simulated
  galaxies
  We use EAGLE to quantify the effect galaxy mergers have on the stellar
specific angular momentum of galaxies, $j_{\rm stars}$. We split mergers into:
dry (gas-poor)/wet (gas-rich), major/minor, and different spin alignments and
orbital parameters. Wet (dry) mergers have an average neutral gas-to-stellar
mass ratio of $1.1$ ($0.02$), while major (minor) mergers are those with
stellar mass ratios $\ge 0.3$ ($0.1-0.3$). We correlate the positions of
galaxies in the $j_{\rm stars}$-stellar mass plane at $z=0$ with their merger
history, and find that galaxies of low spins suffered dry mergers, while
galaxies of normal/high spins suffered predominantly wet mergers, if any. The
radial $j_{\rm stars}$ profiles of galaxies that went through dry mergers are
deficient by $\approx 0.3$~dex at $r\lesssim 10\,r_{50}$ (with $r_{50}$ being
the half-stellar mass radius), compared to galaxies that went through wet
mergers. Studying the merger remnants reveals that dry mergers reduce $j_{\rm
stars}$ by $\approx 30$\%, while wet mergers increase it by $\approx 10$\%, on
average. The latter is connected to the build-up of the bulge by newly formed
stars of high rotational speed. Moving from minor to major mergers accentuates
these effects. When the spin vectors of the galaxies prior to the dry merger
are misaligned, $j_{\rm stars}$ decreases to a greater magnitude, while in wet
mergers co-rotation and high orbital angular momentum efficiently spun-up
galaxies. We predict what would be the observational signatures in the $j_{\rm
stars}$ profiles driven by dry mergers: (i) shallow radial profiles and (ii)
profiles that rise beyond $\approx 10\,r_{50}$, both of which are significantly
different from spiral galaxies.
