Local tadpole galaxies: dynamics and metallicity
  Tadpole galaxies, with a bright peripheral clump on a faint tail, are
morphological types unusual in the nearby universe but very common early on.
Low mass local tadpoles were identified and studied photometrically in a
previous work, which we complete here analyzing their chemical and dynamical
properties. We measure Halpha velocity curves of seven local tadpoles,
representing 50% of the initial sample. Five of them show evidence for rotation
(sim 70%), and a sixth target hints at it. Often the center of rotation is
spatially offset with respect to the tadpole head (3 out of 5 cases). The size
and velocity dispersion of the heads are typical of giant HII regions, and
three of them yield dynamical masses in fair agreement with their stellar
masses as inferred from photometry. In four cases the velocity dispersion at
the head is reduced with respect to its immediate surroundings. The oxygen
metallicity estimated from [NII]6583/Halpha often shows significant spatial
variations across the galaxies (sim 0.5 dex), being smallest at the head and
larger elsewhere. The resulting chemical abundance gradients are opposite to
the ones observed in local spirals, but agrees with disk galaxies at high
redshift. We interpret the metallicity variation as a sign of external gas
accretion (cold-flows) onto the head of the tadpole. The galaxies are low
metallicity outliers of the mass-metallicity relationship. In particular, two
of the tadpole heads are extremely metal poor, with a metallicity smaller than
a tenth of the solar value. These two targets are also very young (ages smaller
than 5 Myr). All these results combined are consistent with the local tadpole
galaxies being disks in early stages of assembling, with their star formation
sustained by accretion of external metal poor gas.
