  Abell 1758 was classified as a single rich cluster of galaxies by Abell, but
a ROSAT observation showed that this system consists of two distinct clusters
(A1758N and A1758S) separated by approximately $8\arcmin$ (a projected
separation of 2 Mpc in the rest frame of the clusters). Only a few galaxy
redshifts have been published for these two clusters, but the redshift of the
Fe lines in the Chandra and XMM-Newton spectra shows that the recessional
velocities of A1758N and A1758S are within 2,100 km s$^{-1}$. Thus, these two
clusters most likely form a gravitationally bound system, but our imaging and
spectroscopic analyses of the X-ray data do not reveal any sign of interaction
between the two clusters. The Chandra and XMM-Newton observations show that
A1758N and A1758S are both undergoing major mergers.
  A1758N is in the late stages of a large impact parameter merger between two 7
keV clusters. The two remnant cores have a projected separation of 800 kpc.
Based on the measured pressure jumps preceding the two cores, they are receding
from one another at less than 1,600 km s$^{-1}$. The two cores are surrounded
by hotter gas ($\mathrm{kT}=9$--12 keV) that was probably shock heated during
the early stages of the merger. The gas entropy in the two remnant cores is
comparable with the central entropy observed in dynamically relaxed clusters,
indicating that the merger-induced shocks stalled as they tried to penetrate
the high pressure cores of the two merging systems.Each core also has a wake of
low entropy gas indicating that this gas was ram pressure stripped without
being strongly shocked (abridged). (A copy of the paper with higher resolution
images is available at http://asc.harvard.edu/~lpd/a1758.ps).
