ALMA-IMF I -- Investigating the origin of stellar masses: Introduction
  to the Large Program and first results
  The ALMA-IMF Large Program imaged a total noncontiguous area of 53pc2,
covering 15 extreme, nearby protoclusters of the Milky Way. They were selected
to span relevant early protocluster evolutionary stages. Our 1.3mm and 3mm
observations provide continuum images that are homogeneously sensitive to
point-like cores with masses of 0.2 and 0.6Msun, respectively, with a matched
spatial resolution of 2000au. We also detect lines that probe the protocluster
structure, kinematics, chemistry, and feedback over scales from clouds to
filaments to cores. We classify ALMA-IMF protoclusters as Young, Intermediate,
or Evolved based on the amount of dense gas in the cloud that has potentially
been impacted by HII regions. The ALMA-IMF catalog contains 700 cores that span
a mass range of 0.15-250Msun at a typical size of 2100au. We show that this
core sample has no significant distance bias and can be used to build core mass
functions at similar physical scales. Significant gas motions, which we
highlight here in the G353.41 region, are traced down to core scales and can be
used to look for inflowing gas streamers and to quantify the impact of the
possible associated core mass growth on the shape of the CMF with time. Our
first analysis does not reveal any significant evolution of the matter
concentration from clouds to cores or from the youngest to more evolved
protoclusters, indicating that cloud dynamical evolution and stellar feedback
have for the moment only had a slight effect on the structure of high-density
gas in our sample. Furthermore, the first-look analysis of the line richness
toward bright cores indicates that the survey encompasses several tens of hot
cores, of which we highlight the most massive in the G351.77 cloud. Their
homogeneous characterization can be used to constrain the emerging molecular
complexity in protostars of high to intermediate masses.
