  We study galaxy shapes in the Illustris cosmological hydrodynamic simulation.
We find that massive galaxies have a higher probability of being prolate. For
galaxies with stellar mass larger than $10^{11}\rm M_{\odot}$, 35 out of total
839 galaxies are prolate. For 21 galaxies with stellar mass larger than
$10^{12}\rm M_{\odot}$, 9 are prolate, 4 are triaxial while the others are
close to being oblate. There are almost no prolate galaxies with stellar mass
smaller than $3\times10^{11}\rm M_{\odot}$. We check the merger history of the
prolate galaxies, and find that they are formed by major dry mergers. All the
prolate galaxies have at least one such merger, with most having mass ratios
between $1:1$ and $1:3$. The gas fraction (gas mass to total baryon mass) of
the progenitors is 0-3 percent for nearly all these mergers, except for one
whose second progenitor contains $\sim 15\%$ gas mass, while its main
progenitor still contains less than $5\%$. For the 35 massive prolate galaxies
that we find, 18 of them have minor axis rotation, and their angular momenta
mostly come from the spin angular momenta of the progenitors (usually that of
the main progenitor). We analyse the merger orbits of these prolate galaxies
and find that most of them experienced a nearly radial merger orbit. Oblate
galaxies with major dry mergers can have either radial or circular merger
orbits. We further discuss various properties of these prolate galaxies, such
as spin parameter $\lambda_{\rm R}$, spherical anisotropy parameter $\beta$,
dark matter fraction, as well as inner density slopes for the stellar, dark
matter and total mass distributions.
