J/AJ/104/92 J/AJ/104/92 HII Regions Properties in M101 P A Scowen R J Dufour J J Hester Astron. J. 104 92 1992 1992AJ....104...92S Abundances Galaxies, nearby Galaxies, optical HII regions Interstellar matter Spectrophotometrically calibrated CCD imagery of two overlapping 16 arc min fields in the SAB(rs)cdI galaxy M101 in the emission of H alpha, [O III] lambda 5007, H beta, and [S II] lambda 6723 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]/H beta, O/H, [S II]/H alpha, ionization parameter, and numbers of ionizing photons. In addition, radial and azimuthal variations in the H II surface density are studied in comparision to that of H I and H_2 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 S(H alpha) = -15.7 ergs s^-1 cm^-2 arcsec^-2; 625 H II regions) and high (-15.0; 248 H II regions) thresholds. Radial gradients in both extinction, C(H beta), 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 function of the H II region population found is similar to previous studies, though both the high and low ends are sensitive to the threshold levels set. Other parameters, such as the ionization parameters, luminosities, and size distribution, show no evidence for systematic variations with galactocentric 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.

Rnum HII Region Reference Number --- CHb Balmer Decrement as C(H beta) --- O3Hb Ratio of [O III] to H beta --- S2Ha Ratio of [S II] to H alpha --- OH Oxygen Abundance as 12+log(O/H) --- U Ionization Parameter --- Q Number of Ionizing photons /s 10+45ct/s Rnum HII Region Reference Number --- RAh Region RA Hour (1950.0) h RAm Region RA Minute (1950.0) min RAs Region RA Second (1950.0) s DE- Sign of declination --- DEd Region Dec Degree (1950.0) deg DEm Region Dec Minute (1950.0) arcmin DEs Region Dec Second (1950.0) arcsec A Region Area, 1 pix = 1.2 arcsec pix2 Ha H alpha surf.brightness, 1pix=1.2" mW/m2/pix Hb H beta surf.brightness, 1pix=1.2" mW/m2/pix O3 surf.brightness, [OIII], 1pix=1.2" mW/m2/pix S2 surf.brightness, [S II], 1pix=1.2" mW/m2/pix Rad Radial distance of region in galaxy pc Paul A. Scowen Arizona State Univ. 1995Jan17 J_AJ_104_92.xml