Gradients of Stellar Population Properties and Evolution Clues in a
  Nearby Galaxy M 101
  Multi-band photometric images from ultraviolet and optical to infrared are
collected to derive spatially resolved properties of a nearby Scd type galaxy M
101. With evolutionary stellar population synthesis models, two-dimensional
distributions and radial profiles of age, metallicity, dust attenuation, and
star formation timescale in the form of the Sandage star formation history are
obtained. When fitting with the models, we use the IRX-$A_\mathrm{FUV}$
relation, found to depend on a second parameter of birth rate b (ratio of
present and past-averaged star formation rate), to constrain the dust
attenuation. There are obvious parameter gradients in the disk of M101, which
supports the theory of an "inside-out disk" growth scenario. Two distinct disc
regions with different gradients of age and color are discovered, similar to
another late-type galaxy NGC 628. The metallicity gradient of the stellar
content is flatter than that of H {\sc ii} regions. The stellar disk is
optically thicker inside than outside and the global dust attenuation of this
galaxy is lower, compared with galaxies of similar and earlier morphological
type. We highlight that a variational star formation timescale describes the
real star formation history of a galaxy. The timescale increases steadily from
the center to the outskirt. We also confirm that the bulge in this galaxy is a
disk-like pseudobulge, whose evolution is likely to be induced by some secular
processes of the small bar with relatively young age, rich metal, and much
dust.
