SF Enhancement as a Function of Separation and Environment

We now split the sample by environment and separation instead of stellar mass and separation. The three environment bins are field (with halo mass 1010 _{to 10}13_M

4.1 Recent Star Formation in Close Pairs 108 group (with halo mass 1013to 1014M) and cluster (with halo mass 1014to 1015M).

Notice in Figure 4.2 that the NUV-r colour distribution shifts to bluer colours from the high density cluster environment to the low density field environment; we would expect this since more SF tends to take place in galaxies inhabiting lower density environments (e.g. Kauffmann et al., 2004).

Figure 4.2: Top: Median NUV-r colours for close and wide pairs binned by separation (0-15kpc, 15-30kpc, 30-60kpc and 90-130kpc) for pairs in field (black), group (blue) and cluster (red) environments. Bottom: SSFR difference between the separation bin in question and the widest separation bin, for each environment. The fractional ∆SSFR error is ∼20%.

SSFRs for each bin are shown in Table 4.2. We find a difference of 1.4×10−11yr−1 (i.e. a factor of 1.8 ± 0.5 increase) in SSFR from the widest to the smallest sepa- ration bin for pairs in field environments, and no significant increase for pairs in group and cluster environments. Since stellar mass and environment are correlated, such that increasingly massive galaxies tend to be found in higher density environ- ments (a consequence of the morphology-density relation), we attempt to break the degeneracy by splitting our sample by stellar mass and environment.

Figure 4.3 shows the environment/separation analysis, but now restricted to low mass galaxies (108_-1011_M

 -top) and higher mass galaxies (1011-1013M -bottom).

In the low mass analysis, NUV-r colours in field and group environments become noticeably bluer with decreasing projected separation; with no clear trend for cluster environments. We find an average rise in SSFR of 4.4×10−11yr−1(a factor of 2.4±0.7

4.1 Recent Star Formation in Close Pairs 109

Environment Projected Separation (kpc)

0-15 15-30 30-60 90-130

Field 2.98×10−11 1.91×10−11 2.04×10−11 1.63×10−11

Group 2.62×10−12 2.08×10−12 1.78×10−12 2.12×10−12

Cluster 1.76×10−12 2.14×10−12 1.37×10−12 1.71×10−12

Table 4.2: Median SSFR (yr−1) derived from NUV luminosity for each environ- ment and separation bin.

Figure 4.3: Median NUV-r and ∆SSFR values for each environment/separation bin are plotted for low stellar mass galaxies (108_-1011_M

 -top) and high stellar

4.2 Major and Minor Mergers 110 increase) for field pairs and an average rise of 1.2×10−11yr−1 (a factor of 3.3 ± 0.9 increase) for group pairs in the low mass sample. For high mass pairs the NUV-r range is much narrower and lies in the redder colour region, implying that less rSF is being triggered. We see an average rise of 2.2×10−12yr−1 (a factor of 2.5 ± 0.7 increase) in SSFR for high mass pairs in field environments.

Through splitting our sample by mass and environment, we have shown that both the close pair mass and the environment that the pairs are located in play an important role in the amount of resulting SF. The highest level of average rSF is seen for low mass close pairs in field environments. However, the largest relative increase in SSFR when compared with the wide pairs control sample is seen in low mass close pairs in group environments; here we saw an average rise of 1.2×10−11yr−1 (a factor of 3.3 ± 0.9 increase compared with the control sample).

4.2

Major and Minor Mergers

We now look to our major and minor mergers samples. The major mergers sample comprises pairs with mass ratio >1/3 and the minor mergers sample comprises pairs with mass ratio <1/3. The major/minor mergers terminology extends to our full close and wide pairs samples, including projected separations up to 150kpc; at wide separation the pairs will not actually be merging, but we refer to the full separation range as major/minor mergers for simplicity. As with Section 4.1, we take the widest separation bin as a control sample to compare the close pairs with, and we test pairs in different mass and environment bins to see how these properties impact rSF in major and minor pairs.

4.2.1

SF Enhancement as a Function of Separation and

Mass

Figure 4.4 (top) shows the galaxies in the major merger sample, and Figure 4.4 (bot- tom) shows the galaxies in the minor merger sample; we further split these samples by mass. For both the major and minor samples, the NUV-r colour distributions for lower mass galaxies are bluer in NUV-r as we would expect from Section 4.1.1 and due to cosmic downsizing.

There is a slightly bluer NUV-r distribution for high mass galaxies in major mergers and also a slightly redder NUV-r distribution for low mass galaxies in major mergers. This is likely because of the narrower mass range in the major mergers sample (see Figure 3.14 (bottom left)) leading to a slightly more heterogeneous

4.2 Major and Minor Mergers 111 NUV-r distribution.

Figure 4.4: Median NUV-r and ∆SSFR values are plotted for the major mergers sample (with pair mass ratio >1/3) -top, and and minor mergers sample (with pair mass ratio <1/3) -bottom.

For the major mergers sample, we find a difference of 5.3×10−11yr−1 in SSFR from the widest (90-130kpc) to the smallest separation bin (0-15kpc) for low stellar mass galaxies (an average factor of 4.6 ± 0.7 increase in SSFR), and a difference of 1.3×10−12yr−1 for high stellar mass galaxies (an average factor of 1.8±0.7 increase in SSFR). For the minor mergers sample, we find a difference of 7.1×10−11yr−1in SSFR from the widest (90-130kpc) to the smallest separation bin (0-15kpc) for low stellar

4.2 Major and Minor Mergers 112 mass galaxies (an average factor of 3.9 ± 0.5 increase in SSFR), and a difference of 2.2×10−12yr−1 for high stellar mass galaxies (an average factor of 3.0 ± 1.3 increase in SSFR).

Figure 4.5: Median NUV-r and ∆SSFR values are plotted for the minor mergers sample split into primary progenitors and secondary progenitors.

Galaxies in the major mergers sample have similar mass by definition, so we might expect similar SFRs for both galaxies in a major merger system. However, in a minor merger we might expect to see a stronger impact on the low mass progenitor (as Woods & Geller (2007) reported; see Section 1.4.3), since it is interacting with a significantly more massive progenitor that will have a strong gravitational influence. From now on, we refer to the higher mass galaxy in a minor merger as the ‘primary’ progenitor and the lower mass galaxy as the ‘secondary’ progenitor.

We split the minor mergers sample according to the primary and secondary pro- genitors in each system (see Figure 4.5). Notice that the median NUV-r colour for the smallest separation bin, 0-15kpc, becomes sharply bluer for the primary progen- itors as well as the secondary progenitors, indicating a significant enhancement in rSF for both the primary and secondary progenitors in low separation minor merger pairs.

We find a difference of 2.1×10−11yr−1 in SSFR from the widest (90-130kpc) to the smallest separation bin (0-15kpc) for primary galaxies (an average factor

4.2 Major and Minor Mergers 113 of 13.5 ± 3.8 increase in SSFR), and a difference of 11.5×10−11yr−1 for secondary galaxies (an average factor of 4.9 ± 1.4 increase in SSFR); i.e. we see a higher relative increase in SSFR in primary galaxies than in secondary galaxies in minor mergers. This is a surprising result since Woods & Geller (2007) found SSFR enhancements in minor merger secondaries, but no evidence for an enhancement in SSFR in primaries. We expect that this is because the level of SF in primaries is relatively low and since Woods and Geller used Hα as an SSFR diagnostic small changes have not been detected, whereas the NUV is particularly sensitive to measuring changes in rSFR.