  The formation of high-mass stars is tightly linked to that of their parental
clouds. We here focus on the high-density parts of W43, a molecular cloud
undergoing an efficient event of formation. The cloud structure is studied with
a column density image derived from Herschel continuum maps obtained at 70,
160, 250, 350, and 500 micron. We identify two high-column density filamentary
clouds, quoted as the W43-MM1 and W43-MM2 ridges, which both account for
1.5x10^4 Msun gas mass above 10^23 cm-2 and within areas of 5 and 14pc^2,
respectively. We used the N_2H^+ 1--0 line to confirm that the W43-MM1 and
W43-MM2 ridges are structures coherent in velocity and gravitationally bound,
despite their large velocity dispersion and ~5 kms line widths. The most
intriguing result of the W43 large program is the bright wide-spread SiO 2--1
emission: 1--11 K kms$ stretching an area of ~28 pc^2. Concentrated toward the
W43-MM1 and W43-MM2 ridges and their immediate surroundings, it leads to a
total luminosity of L_SiO 2-1 ~4 10^4 K kms kpc^2pc^2. We measured a steep
relation between the luminosity and velocity extent of the SiO~2--1 lines and
propose to use it to distinguish the low-velocity shocks observed here from the
more classical high-velocity ones associated with outflows of high-mass young
stellar objects. We used state-of-the-art shock models to demonstrate that
low-velocity (<10 kms^-1) shocks with a small amount (10%) of Si atoms
initially in gas phase or in grain mantles can explain the observed SiO column
density in W43. The spatial and velocity overlaps between the ridges
high-density gas (n_H2>10^4-10^5 cm^-3) and the shocked SiO gas suggests that
ridges could be forming via colliding flows driven by gravity and accompanied
by low-velocity shocks. This mechanism may be the initial conditions for the
formation of young massive clusters in these ridges.

Astrophysics of Galaxies;Cosmology and Nongalactic Astrophysics