Stellar Populations and the Star Formation Histories of LSB Galaxies: V.
  WFC3 Color-Magnitude Diagrams
  We present WFC3 observations of three low surface brightness (LSB) galaxies
from the Schombert et. al LSB catalog that are within 11 Mpc of the Milky Way.
Deep imaging at F336W, F555W and F814W allow the construction of the V-I
color-magnitude diagrams (CMD) to M_I = -2. Overall 1869, 465 and 501 stellar
sources are identified in the three LSB galaxies F415-3, F608-1 and F750-V1.
The spatial distribution of young blue stars matches the H-alpha maps from
ground-based imaging, indicating that star formation in LSB galaxies follows
the same style as in other irregular galaxies. Several star complexes are
identified, matching regions of higher surface brightness as seen from
ground-based imaging. The color-magnitude diagrams for each LSB galaxy has the
similar morphology to Local Volume (LV) dwarf galaxies, i.e. a blue main
sequence, blue and red He burning branches and asymptotic giant branch (AGB)
stars. The LSB CMD's distinguish themselves from nearby dwarf CMD's by having a
higher proportional of blue main sequence stars and fewer AGB stars than
expected from their mean metallicities. Current [Fe/H] values below -0.6 are
deduced from the position of the red helium-burning branch (rHeB) stars in the
V-I diagram. The distribution of stars on the blue helium-burning branch (bHeB)
and rHeB from the U-V and V-I CMD indicate a history of constant star formation
for the last 100 Myrs.
