In search of the oldest star forming regions in Holmberg II

We present an X-ray study of the irregular dwarf galaxy Holmberg II based on deep ROSAT PSPC observations. Holmberg II is one of the most famous examples of an irregular dwarf galaxy with a disrupted interstellar medium (ISM): expanding Hi holes are present across the entire face of the galaxy. Huge Hi cavities of kpc extent are found even outside current starforming regions. We search for faint Xray sources (stellar end points as well as hot gas), touching the limits of the ROSAT PSPC detector and link the newly detected X-ray sources to features detected in other wavelengths. Using an X-ray hardness ratio color-color diagram we show that it is possible to differentiate between thermal plasma and power-law X-ray spectra, which helps to track down the physical nature of the individual sources. At the X-ray flux limit of the analyzed ROSAT PSPC data, the giant Hi holes appear as devoid of coronal gas. This can be used as a measure for the minimum age of the Hi holes to about t ≥ 10 – 10 years. Far outside the stellar body and positionally coincident with some Hi holes we find faint, point like X-ray sources which may be associated with low mass X-ray binaries (LMXBs). This old stellar population is undetectable in the optical regime because they are too faint. However, in the X-ray regime they are prominent X-ray sources, well known from Population I objects located within the bulge of the Milky Way and globular clusters. The search for LMXBs might be the key for identifying the earliest stellar population in distant galaxies. In case of Holmberg II their presence indicates that star formation took place across the entire gaseous disk in the past.