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J/AJ/104/92     HII Regions Properties in M101 (Scowen+ 1992)

The HII Regions of the Galaxy M101 Scowen P.A., Dufour R.J., Hester J.J. <Astron. J. 104, 92 (1992)> =1992AJ....104...92S
ADC_Keywords: Galaxies, nearby ; Galaxies, optical ; Interstellar medium ; H II regions ; Abundances Abstract: 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 comparison 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/cm2/arcsec2; 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. File Summary:
FileName Lrecl Records Explanations
ReadMe 80 . This file fig34567.dat 67 248 Table of derived properties of HII regions used in Figures 3,4,5,6 and 7. Higher threshold used. [Rnum,CHb,O3Hb,S2Ha,OH,U,Q] table0.dat 97 248 Table of observed properties of HII regions used in paper. Not explicitly presented in paper - see last paragraph of Summary section in paper. [Rnum,RAh,RAm,RAs,DEd,DEm,DEs,A, Ha,Hb,O3,S2,Rad]
Byte-by-byte Description of file: fig34567.dat
Bytes Format Units Label Explanations
1- 5 I5 --- Rnum [2/323]+ HII Region Reference Number 8- 13 F6.2 --- CHb Balmer Decrement as C(H beta) 17- 23 F7.2 --- O3Hb Ratio of [O III] to H beta 26- 31 F6.2 --- S2Ha Ratio of [S II] to H alpha 36- 39 F4.2 --- OH Oxygen Abundance as 12+log(O/H) 43- 51 E9.3 --- U Ionization Parameter 59- 67 E9.3 10+45ct/s Q Number of Ionizing photons /s
Byte-by-byte Description of file: table0.dat
Bytes Format Units Label Explanations
1- 3 I3 --- Rnum [2/323]+ HII Region Reference Number 8- 9 I2 h RAh Region RA Hour (1950.0) 11- 12 I2 min RAm Region RA Minute (1950.0) 14- 17 F4.1 s RAs Region RA Second (1950.0) 22 A1 --- DE- [+] Sign of declination 23- 24 I2 deg DEd Region Dec Degree (1950.0) 26- 27 I2 arcmin DEm Region Dec Minute (1950.0) 32- 35 F4.1 arcsec DEs Region Dec Second (1950.0) 39- 41 I3 pix2 A Region Area, 1 pix = 1.2 arcsec 45- 53 E9.3 mW/m2/pix Ha H alpha surf.brightness, 1pix=1.2" 57- 65 E9.3 mW/m2/pix Hb H beta surf.brightness, 1pix=1.2" 69- 77 E9.3 mW/m2/pix O3 surf.brightness, [OIII], 1pix=1.2" 81- 89 E9.3 mW/m2/pix S2 surf.brightness, [S II], 1pix=1.2" 93- 97 I5 pc Rad Radial distance of region in galaxy

(End) Paul A. Scowen [Arizona State Univ.] 1995-Jan-17
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