The majority of galaxy mergers are expected to be minor mergers. The
observational signatures of minor mergers are not well understood, thus there
exist few constraints on the minor merger rate. This paper seeks to address
this gap in our understanding by determining if and when minor mergers exhibit
disturbed morphologies and how they differ from the morphology of major
mergers. We simulate a series of unequal-mass moderate gas-fraction disc galaxy
mergers. With the resulting g-band images, we determine how the time-scale for
identifying galaxy mergers via projected separation and quantitative morphology
(the Gini coefficient G, asymmetry A, and the second-order moment of the
brightest 20% of the light M20) depends on the merger mass ratio, relative
orientations and orbital parameters. We find that G-M20 is as sensitive to 9:1
baryonic mass ratio mergers as 1:1 mergers, with observability time-scales ~
0.2-0.4 Gyr. In contrast, asymmetry finds mergers with baryonic mass ratios
between 4:1 and 1:1 (assuming local disc galaxy gas-fractions). Asymmetry
time-scales for moderate gas-fraction major disc mergers are ~ 0.2-0.4 Gyr, and
less than 0.06 Gyr for moderate gas-fraction minor mergers. The relative
orientations and orbits have little effect on the time-scales for morphological
disturbances. Observational studies of close pairs often select major mergers
by choosing paired galaxies with similar luminosities and/or stellar masses.
Therefore, the various ways of finding galaxy mergers (G-M20, A, close pairs)
are sensitive to galaxy mergers of different mass ratios. By comparing the
frequency of mergers selected by different techniques, one may place empirical
constraints on the major and minor galaxy merger rates.Comment: 16 pages; resubmitted to MNRA
