The H II regions of the galaxy M101

Spectrophotometrically calibrated CCD imagery of two overlapping 16 arcmin fields in the SAB(rs)cdI galaxy M101 in the emission of Hα, [O III] λ5007, Hβ and [S II] λ6723 are used to study various physical properties of the H II region population. Individual H II regions are identified and mapped using an automated algorithm, thus eliminating personal bias in the measurements. Characteristics of the population studied include the H II region luminosity function and radial variations in extinction, [O III]/Hβ, O/H, [S II]/Hα, ionization parameter, and numbers of ionizing photons. In addition, radial and azimuthal variations in the H II surface density are studied in comparison to that of H I and H₂ inferred from radio 21 cm and CO observations. It is demonstrated that the magnitude of trends and "gradients" found from such analyses depend on the surface-brightness threshold set in defining the H II region boundaries; so results are presented for both low (log F (Hα) = -15.7 ergs s^-1 cm^-2 pixel^-2; 625 H II regions) and high (-15.0; 248 H II regions) thresholds. Radial gradients in both extinction, C(Hβ), and O/H are seen in the H II region population - with a distinct flattening in the O/H gradient seen in the outer disk beginning at about 10 kpc. The luminosity distance. Current star-formation processes active in the disk of M101 are analyzed two dimensionally by comparison of the distribution of ionized and neutral hydrogen. The star-formation efficiency shows considerable variation across the disk, suggesting significant spatial variation in the critical density for cloud collapse across the disk of M101 and thus departures from the simple Toomre model.