ROSATX‐Ray Observations of the Dwarf Galaxy Holmberg II

Abstract

We present a study of the irregular dwarf galaxy Holmberg II based on ROSAT Position Sensitive Proportional Counter observations (total exposure time: 22 ks). Holmberg II is a nearby (3.2 Mpc), well-studied dwarf irregular galaxy. It is famous for its interstellar medium (ISM), which is dominated by expanding structures such as H I holes and shells. We search for X-ray emission from point sources as well as for diffuse emission, down to the detection limit of the ROSAT data. Using X-ray hardness ratio diagrams we differentiate between thermal plasma and power-law X-ray spectra, which helps to determine the nature of the individual sources. Correlating the X-ray data with complementary observations ranging from the far-ultraviolet to the radio regime, we increase the probability of correctly identifying sources belonging to Holmberg II. We did not detect soft X-ray emission originating from hot gas within supergiant H I shells above our luminosity sensitivity limit of Llimit(0.1-2.1 keV) ≥ 1037ergs s-1. This finding can probably be attributed to blowout in the case of the largest holes and insufficient sensitivity (owing to strong photoelectric absorption) in the case of the smaller H I holes. However, we find faint X-ray sources well beyond the stellar body but within the H I distribution of Holmberg II, which suggests the presence of X-ray binaries. This indicates that star formation has taken place across the entire gaseous disk of Holmberg II in the past, some of which may have created the structures seen in the ISM at large galactocentric radii.