GIANT H II REGIONS IN M101. I. X-RAY ANALYSIS OF HOT GAS

We performed a Chandra X-ray study of three giant H II regions (GHRs), NGC 5461, NGC 5462, and NGC 5471, in the spiral galaxy M101. The X-ray spectra of the three GHRs all contain a prominent thermal component with a temperature of similar to 0.2 keV. In NGC 5461, the spatial distribution of the soft (<1.5 keV) X-ray emission is generally in agreement with the extent of H1105, the most luminous H II region therein, but extends beyond its southern boundary, which could be attributed to outflows from the star cloud between H1105 and H1098. In NGC 5462, the X-ray emission is displaced from the H II regions and a ridge of blue stars; the H alpha filaments extending from the ridge of star cloud to the diffuse X-rays suggest that hot gas outflows have occurred. The X-rays from NGC 5471 are concentrated at the B-knot, a "hypernova remnant" candidate. Assuming a Sedov-Taylor evolution, the derived explosion energy, on the order of 10(52) erg, is consistent with a hypernova origin. In addition, a bright source in the field of NGC 5462 has been identified as a background active galactic nucleus, instead of a black hole X-ray binary in M101.