Evolution of the X-ray Luminosity in Young HII Galaxies
  In an effort to understand the correlation between X-ray emission and present
star formation rate (SFR), we obtained XMM-Newton data to estimate the X-ray
luminosities of a sample of actively starforming HII galaxies. The obtained
X-ray luminosities are compared to other well known tracers of star formation
activity such as the far infrared and the ultraviolet luminosities. We also
compare the obtained results with empirical laws from the literature and with
recently published analysis applying synthesis models. We use the time delay
between the formation of the stellar cluster and that of the first X-ray
binaries, in order to put limits on the age of a given stellar burst. We
conclude that the generation of soft X-rays, as well as the Ha or infrared
luminosities is instantaneous. The relation between the observed radio and hard
X-ray luminosities, on the other hand, points to the existence of a time delay
between the formation of the stellar cluster and the explosion of the first
massive stars and the consequent formation of supernova remnants and high mass
X-ray binaries (HMXB) which originate the radio and hard X-ray fluxes
respectively. When comparing hard X-rays with a star formation indicator that
traces the first million years of evolution (e.g. Ha luminosities) we found a
deficit in the expected X-ray luminosity. This deficit is not found when the
X-ray luminosities are compared with infrared luminosities, a star formation
tracer that represents an average over the last 10^8 years. The results support
the hypothesis that hard X-rays are originated in X-ray binaries which, as
supernova remnants, have a formation time delay of a few mega years after the
starforming burst.
