Galaxies 09: Assembly, Gas Content and Star Formation History of Galaxies
Jennifer Lotz
NOAO
The Effect of Gas on Observations of the Galaxy Merger Rate
Gas-rich mergers are more easily identified by their disturbed morphologies than mergers with less gas. Because gas-rich galaxies and mergers are expected to be more common at high redshift, the under-counting of gas-poor mergers
may bias current estimates of the evolution of galaxy merger rate. To understand the magnitude of this bias, we explore the effect of gas fraction on the optical morphologies of simulated disk galaxy mergers. We determine how the timescales for identifying galaxy mergers via quantitative morphology depend on baryonic gas fraction $f_{gas}$. Strong asymmetries last significantly longer in high gas-fraction mergers, with timescales ranging from $\leq 300$ Myr for $f_{gas}$ $\sim$ 20\% to $\geq$ 1 Gyr for $f_{gas}$ $\sim$ 50\%. Therefore the strong evolution observed in the fraction of asymmetric galaxies may reflect a strong evolution in the gas properties of merging galaxies rather than the global galaxy merger rate. On the other hand, the timescale for identifying a galaxy merger via double nuclei is not a strong function of $f_{gas}$, consistent with the weak evolution observed for $G-M_{20}$ merger candidates. In the coming decade, ALMA will be crucial for measuring the evolution of gas-rich mergers and their contribution to galaxy assembly.
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