On the compact HII galaxy UM 408 as seen by GMOS-IFU: Physical
  conditions
  We present Integral Field Unit GMOS-IFU data of the compact HII galaxy UM408,
obtained at Gemini South telescope, in order to derive the spatial distribution
of emission lines and line ratios, kinematics, plasma parameters, and oxygen
abundances as well the integrated properties over an area of 3"x4".4(~750x1100
pc) located in the central part of the galaxy. The starburst in this area is
resolved into two giant regions of ~375 and 250pc diameter, respectively. The
ages of these two regions, estimated using Hb equivalent widths, suggest that
they are coeval events of ~5Myr with stellar masses of ~10^4M_o. We have also
used [OIII]/Hb and [SII]/Ha ratio maps to explore the excitation mechanisms in
this galaxy. The Ha emission line was used to measure the radial velocity and
velocity dispersion. We derived an integrated oxygen abundance of
12+log(O/H)=7.87 summing over all spaxels in our field of view. An average
value of 12+log(O/H)=7.77 and a difference of D(O/H)=0.47 between the minimum
and maximum values (7.58+-0.06-8.05+-0.04) were found, considering all data
points where the oxygen abundance was measured. The spatial distribution of
oxygen abundance does not show any significant gradient across the galaxy. On
the other hand, the bulk of data points are lying in a region of +-2sigma
dispersion (with sigma=0.1 dex) around the average value, confirming that this
compact HII galaxy as other previously studied dwarf irregular galaxies is
chemically homogeneous. Therefore, the new metals processed and injected by the
current star formation episode are possibly not observed and reside in the hot
gas phase, whereas the metals from previous events are well mixed and
homogeneously distributed through the whole extent of the galaxy.
