  Tadpole galaxies have a head-tail shape with a large clump of star formation
at the head and a diffuse tail or streak of stars off to one side. We measured
the head and tail masses, ages, surface brightnesses, and sizes for 66 tadpoles
in the Hubble Ultra Deep Field (UDF), and we looked at the distribution of
neighbor densities and tadpole orientations with respect to neighbors. The
heads have masses of 10^7-10^8 Msun and photometric ages of ~0.1 Gyr for z~2.
The tails have slightly larger masses than the heads, and comparable or
slightly older ages. The most obvious interpretation of tadpoles as young
merger remnants is difficult to verify. They have no enhanced proximity to
other resolved galaxies as a class, and the heads, typically less than 0.2 kpc
in diameter, usually have no obvious double-core structure. Another possibility
is ram pressure interaction between a gas-rich galaxy and a diffuse
cosmological flow. Ram pressure can trigger star formation on one side of a
galaxy disk, giving the tadpole shape when viewed edge-on. Ram pressure can
also strip away gas from a galaxy and put it into a tail, which then forms new
stars and gravitationally drags along old stars with it. Such an effect might
have been observed already in the Virgo cluster. Another possibility is that
tadpoles are edge-on disks with large, off-center clumps. Analogous lop-sided
star formation in UDF clump clusters are shown.
