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 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:
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FileName Lrecl Records Explanations
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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]
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Byte-by-byte Description of file: fig34567.dat
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Bytes Format Units Label Explanations
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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
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Byte-by-byte Description of file: table0.dat
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Bytes Format Units Label Explanations
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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- 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
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(End) Paul A. Scowen [Arizona State Univ.] 1995-Jan-17