  Urea, NH2C(O)NH2, is a molecule of great importance in organic chemistry and
biology. Two searches for urea in the interstellar medium were reported in the
past, but neither were conclusive. We want to take advantage of the increased
sensitivity and angular resolution provided by ALMA to search for urea toward
the hot cores embedded in the high-mass star forming region Sgr B2(N). We use
the new spectral line survey ReMoCA performed toward Sgr B2(N) with ALMA in its
observing cycle 4. The spectra are analyzed under the LTE approximation. We
construct a full synthetic spectrum that includes all the molecules identified
so far. We use new spectroscopic predictions for urea in its vibrational ground
state and first vibrationally excited state to search for this complex organic
molecule in the ReMoCA data set. We employ the gas-grain chemical kinetics
model MAGICKAL to interpret the astronomical observations. We report the secure
detection of urea toward the hot core Sgr B2(N1) at a position called N1S
slightly offset from the continuum peak, which avoids obscuration by the dust..
We derive a column density of 2.7x10^16 cm-2 for urea, two orders of magnitude
lower than formamide, and one order of magnitude below methyl isocyanate,
acetamide, and N-methylformamide. The latter molecule is reliably identified
toward N1S with 60 clearly detected lines, confirming an earlier claim of its
tentative interstellar detection. We report the first interstellar detections
of NH2CH18O and 15NH2CHO. We also report the nondetection of urea toward the
secondary hot core Sgr B2(N2) with an abundance relative to the other four
species at least one order of magnitude lower than toward the main hot core.
Our chemical model roughly reproduces the relative abundances of formamide,
methyl isocyanate, acetamide, and N-methylformamide, but it overproduces urea
by at least one order of magnitude.
