Abstract
We investigate the ISM properties of 13 star-forming galaxies within the z~2
COSMOS cluster. We show that the cluster members have NII/Ha and OIII/Hb
emission-line ratios similar to z~2 field galaxies, yet systematically
different emission-line ratios (by ~0.17 dex) from the majority of local
star-forming galaxies. We find no statistically significant difference in the
NII/Ha and OIII/Hb line ratios or ISM pressures among the z~2 cluster
galaxies and field galaxies at the same redshift. We show that our cluster
galaxies have significantly larger ionization parameters (by up to an order of
magnitude) than local star-forming galaxies. We hypothesize that these high
ionization parameters may be associated with large specific star formation
rates (i.e. a large star formation rate per unit stellar mass). If this
hypothesis is correct, then this relationship would have important implications
for the geometry and/or the mass of stars contained within individual star
clusters as a function of redshift.
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