| Summary: | 博士 === 國立臺灣師範大學 === 地球科學系 === 101 === We use several survey catalogues in different wavelengths, including the Two Micro All Sky Survey (2MASS), Six-degree Field Galaxy Survey (6dFGS), NRAO VLA Sky Survey (NVSS), ROSAT All Sky Survey (RASS), ROSAT-ESO Flux Limited X-ray (REFLEX) cluster catalogue, and galaxy velocity data from the the NASA Extragalactic Database (NED), to study how galaxy star formation and nuclear activity depend on the environmental effects from cluster/group mergers, halo mass, the intracluster medium (ICM) and local galaxy density as well as on galaxy stellar mass (indicated by the $K$-band absolute magnitude $M_\rm{k}$) in the Shapley supercluster (SSC).
By using the 4000 {\AA} break strength (D4000) as an indicator of star formation history, the relation between the D4000 and local galaxy density (D4000--density) shows that the D4000 is strongly dependent on the local galaxy density as well as the on galaxy stellar mass. The D4000--density correlation is strongest for density estimated on scale of 1 Mpc for all $M_\rm{k}$ ranges. As to the cluster/group environmental effects on the relations of D4000 and clustercentric distance (D4000--radius) for galaxies within 5 $r_{200}$, it is analyzed by dividing the 69 clusters/groups in the SSC into two populations based on the difference in $M_{200}$, X-ray flux, and merger features, respectively, as well as by dividing the selected galaxies into 4 subclasses based on their $M_\rm{k}$. Our results show that:
(1) The D4000--radius relation depend strongly on galaxy stellar mass for all cluster/group populations. (2) For more massive, X-ray detected clusters, the mean strength of 4000 {\AA} break is mostly stronger than the less massive, under X-ray detection ones, even out to 3--5 $r_{200}$ for all $M_\rm{k}$ ranges. (3) In merging clusters, the faint low-mass galaxies are younger (lower D4000) than those in non-merging systems from cluster center out to 3--5 $r_{200}$, and the enhanced star formation activity is possibly triggered by merging events. (4) The difference in the D4000--radius relations between the two divided populations shown in this study is most pronounced for low-mass galaxies, this means that low-mass, gas-rich galaxies are most sensitive to their environments.
The environmental effects on galaxy activities are also analyzed by the fractions of radio-selected star-forming galaxies (SFG) and absorption-line AGN (AA). Our results show that the fraction of SFG drops quickly towards high galaxy density regions, whereas the fraction of AA increases towards denser regions. The cluster/group environmental effects on the $f_\rm{SFG}$--radius relation show that there is an obvious enhanced fraction within 1--2 $r_{200}$. A possible scenario for this enhancement may be owing to the increasing galaxy-galaxy interactions when galaxies are infalling into clusters. The outskirt regions for this enhancement are mostly related to on-going cluster mergers with $\sigma_v \geq$ 500 km s$^{-1}$.
Finally, we investigate the possible connection between cluster dynamical state and the properties of its member galaxies for one cluster merger Abell S0721. In this cluster, it shows strong evidence of three substructures, and owns the highest fraction of radio-emitting galaxies among the clusters in the SSC. A total of three AA and two SFG are identified. For each AA, it is also the brightest galaxy in each substructure core. Assuming the gas supply of AA is frequently associated with the thermodynamic states of galactic coronae or host groups and clusters, therefore, the possible scenario from solutions of the equations of motion for this cluster is that the substructure at the north-east is either unbound or is collapsing at a current separation of
$\sim$1.4 Mpc, and the two clumps in the main structure are possibly turning to collapse after the maximum expansion.
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