ALMA Observations of the Spatial Distribution of three C$_2$H$_4$O$_2$
  Isomers towards Sgr B2(N)
  The C$_2$H$_4$O$_2$ isomers have been previously investigated primarily via
disparate sets of observations involving single dish and array measurements.
The only attempt at using a uniform set of observations was performed with the
IRAM 30 m observation in 2013 (Belloche et al. 2013). In this study, we present
an intensive and rigorous spectral and morphological analysis of the
C$_2$H$_4$O$_2$ isomers towards Sgr B2(N) with interferometers, ALMA Band 3
observations. We propose a quantitative selection method, which automates the
determination of the most uncontaminated transitions and allows us to report
the discovery of previously undetected transitions of the three isomers. With
the least contaminated transitions, the high spatial-resolution millimeter maps
of the C$_2$H$_4$O$_2$ isomers reveal that HCOOCH$_3$ and CH$_2$OHCHO each
display two different velocity components, while only one velocity component of
CH$_3$COOH is resolved. Moreover, the distribution of HCOOCH$_3$ is extended
and offset from the continuum emission, unlike CH$_2$OHCHO and CH$_3$COOH, for
which the low-velocity component is found to be compact and concentrated toward
the continuum emission peak of Sgr B2(N). The distinct morphologies of these
C$_2$H$_4$O$_2$ isomeric species indicate that HCOOCH$_3$ have significant
differences in chemical processes than CH$_2$OHCHO and CH$_3$COOH, which
display similar spatial distributions.
