Dependence of merging features of host clusters/groups

To understand the effects of cluster/group mergers on the D4000 of member galaxies, the cluster sample is divided into two populations based on their available DS-test probabilities

Table 4.3 Total number of galaxies with M_k > -24.8 and clustercentric distance ranges for host clusters with or without X-ray detection

M₂₀₀(0.8 − 4 × 10¹⁴M)

clustercentric X-ray (N_cl=10) Non X-ray (N_cl=16) distance < M_k> N_gal < M_k> N_gal

r≤ r₂₀₀ -24.2 75 -24.0 97

1 < r ≤ 5r₂₀₀ -24.1 98 -23.8 155

Velocity dispersion (230 − 600 km s⁻¹)

clustercentric X-ray (N_cl=7) Non X-ray (N_cl=38) distance < M_k> N_gal < M_k> N_gal

r≤ r₂₀₀ -24.2 35 -24.0 152

1 < r ≤ 5r₂₀₀ -24.0 73 -23.9 269

Figure 4.9 The mean D4000 (left panel) and the fractions of young galaxies (right panel) are shown as a function of clustercentric distance (0–0.5, 0.5–1, 1–2, 2–

3 and 3–5 r200) for galaxies in the four M_k ranges. The populations of the host clusters/groups with and under X-ray detection are represented by the red and blue colors, respectively. The error bars are derived through bootstrapping (left) and Poisson statistics (right).

4.3 Relation of D4000 and Clustercentric Distance 67

Figure 4.10 The mean D4000 in each clustercentric distance range of Figure 4.9 is shown as a function of the local galaxy number density in the left panel. The red and blue lines represent the populations of host clusters/groups with and under X-ray detection, respectively. By comparison with the mean D4000 of the whole D4000 sample in the same M_k range (solid lines), for all clustercentric distance bins, the deviations of D4000 at the same density are plotted in the right panel.

The galaxy number density is calculated by using a projected radius of 1 Mpc and a velocity difference 1000 km s⁻¹, the error bars are derived through bootstrapping.

Figure 4.11 The mean D4000 is shown as a function of M₂₀₀ (left panel) and ve-locity dispersion (right panel) for galaxies with M_k≥−24.8. The populations of the host clusters/groups with and under X-ray detection are represented by the red and blue lines, respectively. The solid lines are for galaxies within r200, the dotted lines are for galaxies within 1–5 r₂₀₀. For comparison purposes, only clusters/groups having similar M₂₀₀ and velocity dispersion in each population are included. The total number of galaxies in each population is equally divided into three bins. The error bars are derived through bootstrapping.

Figure 4.12 The mean D4000 in Figure 4.11 is compared with the mean D4000 of the whole D4000 sample with M_k≥−24.8. The deviations in D4000 at the same density are plotted according to M₂₀₀and the velocity dispersion in the left and right panel, respectively. The subpopulations of host clusters/groups with and without X-ray detection are represented by the red and blue colors, respectively. The solid and dotted lines are for galaxies within r₂₀₀ and 1–5 r₂₀₀, respectively. The error bars are derived through bootstrapping.

4.3 Relation of D4000 and Clustercentric Distance 69

and ellipticities. The merging candidates are selected from clusters with DS-test probabili-ties of less than 0.05 or ellipticiprobabili-ties greater than 0.6. There are a total of 28 clusters/groups in the population of merging candidates, and 41 in the population of non-merging candidates.

The merging population consists of ten X-ray detected clusters, several of them are massive, such as A3528N, A3528S, A3558, and A3562.

The mean D4000 and the fractions of young galaxies of these two populations are shown as a function of the clustercentric distance for the four M_kranges in Figure 4.13. The clustercentric distance is also divided into five ranges. The total number of galaxies in each bin is shown in Table 4.4. The most pronounced difference between the two populations is for the low-mass galaxies with −23.8 < M_k. Those low-mass galaxies in the merging population has obviously smaller mean D4000 values and higher fractions of young galaxies out to 3 − 5r200; whereas the differences between the two populations are not apparent in the other M_k ranges. The obvious increase in D4000 at 2 − 3r₂₀₀ of M_k≤−24.8 is not seen in the related fraction of young galaxies. This may be just bias owing to the poor sample number. This result is also examined by the relation between the mean D4000 and the local galaxy number density (see Figure 4.14). For galaxies with M_k>−23.8, the local densities are nearly identical for both populations in all radius ranges but clearly differ in the mean D4000, whereas for the other M_kranges, the deviations are small and without specific trend.

As previous analyses for populations divided by the X-ray detection show, the mean D4000 is higher for galaxies with host clusters detected by X-ray. Here the decrease in the mean D4000 for the merging population is also examined to see whether it is caused by the smaller fraction of galaxies related to the X-ray detected clusters/groups. Although the fractions of the X-ray detected clusters in the merging and non-merging populations are similar (36% and 29%), but the fraction of galaxies related to the X-ray host clusters for each radius bin has a larger variation. The fractions are 44, 32, 38, 21, and 11% for the merging population and 36, 48, 40, 44, and 48% for non-merging population, from the inner to outer radius. In comparison with the distribution of the mean D4000 in the two populations, the correlation between the fractions and the mean D4000 is weak. For the innermost radius, the merging population even has a higher fraction of galaxies in the X-ray detected host

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Table 4.4 Total number of galaxies with different M_k and clustercentric distance ranges for non-merging and merging host clusters

Non Merging Candidates (N_cl=41) Merging Candidates (N_cl=28) clustercentric distance (r₂₀₀) clustercentric distance (r₂₀₀) M_k 0 − 0.5 0.5 − 1 1 − 2 2 − 3 3 − 5 0 − 0.5 0.5 − 1 1 − 2 2 − 3 3 − 5

After these examinations, the mechanisms responsible for the decrease in the D4000 of the low-mass galaxies in merging clusters/groups are possibly related to merging events.

For low-mass gas-rich galaxies involved in a merger, those infalling into clusters along the filamentary structure (Braglia et al. 2009; Mahajan et al. 2012), as well as those in merging substructures (groups) with relative low velocity and high-density galaxy environments, are suggested to have higher opportunities for galaxy-galaxy interactions (Gnedin 2003), and thus enhance star formation activity and decrease the D4000. Alternatively, mergers could dramatically increase the pressure of the ICM surrounding the cluster member galaxies, to the extent that a significant fraction of galaxies can be simultaneously affected (Bekki et al.

2010).