Methylamine and other simple N-bearing species in the hot cores NGC
  6334I MM1-3
  In the search for the building blocks of life, nitrogen-bearing molecules are
of particular interest since nitrogen-containing bonds are essential for the
linking of amino acids and ultimately the formation of larger biological
structures. The elusive molecule methylamine (CH$_3$NH$_2$) is thought to be a
key pre-biotic species but has so far only been securely detected in the giant
molecular cloud Sgr B2. We identify CH$_3$NH$_2$ and other simple
nitrogen-bearing species towards three hot cores in NGC 6334I. Column density
ratios are derived in order to investigate the relevance of the individual
species as precursors of biotic molecules. Observations obtained with ALMA were
used to study transitions of CH$_3$NH$_2$, CH$_2$NH, NH$_2$CHO, and the
$^{13}$C- and $^{15}$N-methyl cyanide (CH$_3$CN) isotopologues. Column
densities are derived for each species assuming LTE and excitation temperatures
in the range 220-340 K for CH$_3$NH$_2$, 70-110 K for the CH$_3$CN
isotopologues, and 120-215 K for NH$_2$CHO and CH$_2$NH. We report the first
detections of CH$_3$NH$_2$ towards NGC 6334I with column density ratios with
respect to CH$_3$OH of 5.9$\times$10$^{-3}$, 1.5$\times$10$^{-3}$, and
5.4$\times$10$^{-4}$ for the three hot cores MM1, MM2, and MM3, respectively.
These values are slightly lower than the values derived for Sgr B2 but higher
by more than order of magnitude as compared with the values derived for the
low-mass protostar IRAS 16293-2422B. The detections of CH$_3$NH$_2$ in the hot
cores of NGC 6334I hint that CH$_3$NH$_2$ is generally common in the
interstellar medium, albeit high-sensitivity observations are essential for its
detection. The good agreement between model predictions of CH$_3$NH$_2$ ratios
and the observations towards NGC 6334I indicate a main formation pathway via
radical recombination on grain surfaces.
