2021ApJ...923...78S -
Astrophys. J., 923, 78-78 (2021/December-2)
Limits to ionization-parameter mapping as a diagnostic of HII region optical depth.
SAWANT A.N., PELLEGRINI E.W., OEY M.S., LOPEZ-HERNANDEZ J. and MICHEVA G.
Abstract (from CDS):
We employ ionization-parameter mapping (IPM) to infer the optical depth of H II regions in the northern half of M33. We construct [O III]λ5007/[O II]λ3727 and [O III]λ5007/[S II]λ6724 ratio maps from narrowband images continuum-subtracted in this way, from which we classify the H II regions by optical depth to ionizing radiation, based on their ionization structure. This method works relatively well in the low-metallicity regime, 12+log(O/H)≤ 8.4, where [O III]λλ4959, 5007 is strong. However, at higher metallicities, the method breaks down due to the strong dependence of the [O III]λλ4959, 5007 emission lines on the nebular temperature. Thus, although O++ may be present in metal-rich H II regions, these commonly used emission lines do not serve as a useful indicator of its presence, and hence the O ionization state. In addition, IPM as a diagnostic of optical depth is limited by spatial resolution. We also report a region of highly excited [O III] extending over an area ∼1 kpc across and [O III]λ5007 luminosity of 4.9 ± 1.5 x 1038 erg s–1, which is several times higher than the ionizing budget of any potential sources in this portion of the galaxy. Finally, this work introduces a new method for continuum subtraction of narrowband images based on the dispersion of pixels around the mode of the diffuse-light flux distribution. In addition to M33, we demonstrate the method on C III]λ1909 imaging of Haro 11, ESO 338-IG004, and Mrk 71.
Abstract Copyright:
© 2021. The American Astronomical Society. All rights reserved.
Journal keyword(s):
Disk galaxies - Irregular galaxies - Starburst galaxies - Emission line galaxies - Interstellar line emission - Nebulae - Direct imaging - Radiative transfer - Young massive clusters - Star formation - Astrostatistics distributions
VizieR on-line data:
<Available at CDS (J/ApJ/923/78): table2.dat>
Simbad objects:
124
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