What Drives the Redshift Evolution of Strong Emission Line Ratios?
  We study the physical mechanisms that cause the offset between low-redshift
and high-redshift galaxies on the [OIII]/H$\beta$ versus [NII]/H$\alpha$
``Baldwin, Phillips & Terlevich'' (BPT) diagram using a sample of local
analogues of high-redshift galaxies. These high-redshift analogue galaxies are
selected from the Sloan Digital Sky Survey. Located in the same region on the
BPT diagram as the ultra-violet selected galaxies at $z\sim2$, these
high-redshift analogue galaxies provide an ideal local benchmark to study the
offset between the local and high-redshift galaxies on the BPT diagram. We
compare the nitrogen-to-oxygen ratio (N/O), the shape of the ionising radiation
field, and ionisation parameters between the high-redshift analogues and a
sample of local reference galaxies. The higher ionisation parameter in the
high-redshift analogues is the dominant physical mechanism driving the BPT
offset from low- to high-redshift, particularly at high {\nii/\ha}.
Furthermore, the N/O ratio enhancement also plays a minor role to cause the BPT
offset. However, the shape of the ionising radiation field is unlikely to cause
the BPT offset because the high-redshift analogues have a similar hard ionising
radiation field as local reference galaxies. This hard radiation field cannot
be produced by the current standard stellar synthesis models. The stellar
rotation and binarity may help solve the discrepancy.
