Addressing the [O III]/Hb{eta} Offset of Dwarf Galaxies in the RESOLVE Survey. (arXiv:1904.05912v1 [astro-ph.GA])
<a href="http://arxiv.org/find/astro-ph/1/au:+Richardson_C/0/1/0/all/0/1">Chris T. Richardson</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Polimera_M/0/1/0/all/0/1">Mugdha S. Polimera</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Kannappan_S/0/1/0/all/0/1">Sheila J. Kannappan</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Moffett_A/0/1/0/all/0/1">Amanda J. Moffett</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Bittner_A/0/1/0/all/0/1">Ashley S. Bittner</a>
Metal poor dwarf galaxies in the local universe, such as those found in the
RESOLVE galaxy survey, often produce high [O III]/Hb{eta} ratios close to the
star forming demarcation lines of the diagnostic BPT diagram. Modeling the
emission from these galaxies at lower metallicities generally underpredicts
this line ratio, which is typically attributed to a deficit of photons >35 eV.
We show that applying a model that includes empirical abundances scaled with
metallicity strongly influences the thermal balance in HII regions and
preserves the [O III]/Hb{eta} offset even in the presence of a harder
radiation field generated by interacting binaries. Additional heating
mechanisms are more successful in addressing the offset. In accordance with the
high sSFR typical of dwarf galaxies in the sample, we demonstrate that cosmic
ray heating serves as one mechanism capable of aligning spectral synthesis
predictions with observations. We also show that incorporating a range of
physical conditions in our modeling can create even better agreement between
model calculations and observed emission line ratios. Together these results
emphasize that both the hardness of the incident continuum and the variety of
physical conditions present in nebular gas clouds must be accurately accounted
for prior to drawing conclusions from emission line diagnostic diagrams.
Metal poor dwarf galaxies in the local universe, such as those found in the
RESOLVE galaxy survey, often produce high [O III]/Hb{eta} ratios close to the
star forming demarcation lines of the diagnostic BPT diagram. Modeling the
emission from these galaxies at lower metallicities generally underpredicts
this line ratio, which is typically attributed to a deficit of photons >35 eV.
We show that applying a model that includes empirical abundances scaled with
metallicity strongly influences the thermal balance in HII regions and
preserves the [O III]/Hb{eta} offset even in the presence of a harder
radiation field generated by interacting binaries. Additional heating
mechanisms are more successful in addressing the offset. In accordance with the
high sSFR typical of dwarf galaxies in the sample, we demonstrate that cosmic
ray heating serves as one mechanism capable of aligning spectral synthesis
predictions with observations. We also show that incorporating a range of
physical conditions in our modeling can create even better agreement between
model calculations and observed emission line ratios. Together these results
emphasize that both the hardness of the incident continuum and the variety of
physical conditions present in nebular gas clouds must be accurately accounted
for prior to drawing conclusions from emission line diagnostic diagrams.
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