Kavli Affiliate: Hu Zhan
| First 5 Authors: Hang Zhou, Hang Zhou, , ,
| Summary:
The MAMMOTH-MOSFIRE program is a deep Keck/MOSFIRE K-band spectroscopic
follow-up of emission-line galaxies identified in the MAMMOTH-Grism HST/WFC3
G141 slitless spectroscopic survey, targeting the core regions of three most
massive galaxy protoclusters at cosmic noon. To introduce this program, we
present a comprehensive analysis of the $rm [N,II]lambda$6584, $rm
[S,II]lambdalambda$6717,6731, and $rm [O,I]lambda$6300 BPT diagnostics
for a unique sample of 43 protocluster member galaxies at $zsim2$,
investigating how the overdense environment influences their interstellar
medium ionization conditions. We find that, similar to their field counterparts
at $zsim2$, protocluster galaxies exhibit a systematic offset in the $rm
[N,II]$ BPT diagram relative to the local star-forming sequence, but no such
offset in the $rm [S,II]$ BPT diagram. Notably, we detect significantly
elevated $rm [O,I]$/$rm H alpha$ ratios, which can be well reproduced by
photoionization models incorporating both $rm H II$ regions and low-velocity
shocks ($v$ $sim$ 200 km s$^-1$). We caution that neglecting shock
excitation can bias abundance measurements, leading to an overestimation of
nitrogen enrichment. This provides a potential explanation for the
long-standing puzzle of enhanced $rm [N,II]$/$rm H alpha$ ratios observed
in $zsim2$ galaxies. We further suggest that these shocks are likely
environmentally driven, e.g., by ram-pressure stripping or tidal interactions,
which requires future confirmation through direct observations of features such
as stripped gas tails.
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