The Nature of Starbursts: I. The Star Formation Histories of Eighteen
  Nearby Starburst Dwarf Galaxies
  We use archival HST observations of resolved stellar populations to derive
the star formation histories (SFHs) of 18 nearby starburst dwarf galaxies. In
this first paper we present the observations, color-magnitude diagrams, and the
SFHs of the 18 starburst galaxies, based on a homogeneous approach to the data
reduction, differential extinction, and treatment of photometric completeness.
We adopt a star formation rate (SFR) threshold normalized to the average SFR of
the individual system as a metric for classifying starbursts in SFHs derived
from resolved stellar populations. This choice facilitates finding not only
currently bursting galaxies but also "fossil" bursts increasing the sample size
of starburst galaxies in the nearby (D<8 Mpc) universe. Thirteen of the
eighteen galaxies are experiencing ongoing bursts and five galaxies show fossil
bursts. From our reconstructed SFHs, it is evident that the elevated SFRs of a
burst are sustained for hundreds of Myr with variations on small timescales. A
long >100 Myr temporal baseline is thus fundamental to any starburst definition
or identification method. The longer lived bursts rule out rapid
"self-quenching" of starbursts on global scales. The bursting galaxies' gas
consumption timescales are shorter than the Hubble time for all but one galaxy
confirming the short-lived nature of starbursts based on fuel limitations.
Additionally, we find the strength of the H{\alpha} emission usually correlates
with the CMD based SFR during the last 4-10 Myr. However, in four cases, the
H{\alpha} emission is significantly less than what is expected for models of
starbursts; the discrepancy is due to the SFR changing on timescales of a few
Myr. The inherently short timescale of the H{\alpha} emission limits
identifying galaxies as starbursts based on the current characteristics which
may or may not be representative of the recent SFH of a galaxy.
