探討薛普利超星系團中環境對星系活動的影響
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2013
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在這篇研究中，我們利用許多不同波段的巡天資料和從NASA河外資料庫(NED)收集的星系紅移資料，來探討在薛普利超星系團中，星系的恆星形成及星系核心的活動是如何受到來自星系團合併、星系團質量、星系團內介質(ICM)及星系密度等環境因素和星系本身恆星質量的影響。這些巡天資料目錄包括2微米巡天(2MASS)，6度視野星系巡天(6dFGS)，國家電波天文台甚大天線陣巡天(NVSS)，倫琴衛星巡天(RASS)和倫琴衛星-歐南天文台X射線星系團目錄(REFLEX)。
藉著利用星系光譜的4000Å轉折強度(4000Å break strength，D4000)做為星系恆星形成歷史的指標，並代表星系中恆星的平均年齡。我們在4000Å轉折強度與星系密度的關係中，我們發現星系中恆星的平均年齡與周圍星系的密度以及星系本身的恆星質量大小都有很強的相關性。特別是當星系密度的計算是以1 Mpc的尺度為半徑所得到的密度值，其相關性最好。
為了探討星系團的環境對於其成員星系在4000Å轉折強度和該星系至星系團核心距離的關係(D4000-radius)上的影響，我們將69個在薛普利超星系團中的星系團和星系群依其維里質量(virial mass，以M200表示)高低 、X射線的強度、以及是否顯示星系團合併的特徵等特性各分成2個族群，並且將距離星系團核心5倍維里半徑(virial radius,以r200表示)內的星系依其在Ｋ波段的絕對星等高低（代表星系的恆星質量）分成4個範圍，結果顯示：（1）對所有不同特性的星系團族群，其4000Å轉折強度隨星系至星系團核心距離的變化，都與成員星系的恆星質量有關。（2）對分別在4個星系恆星質量範圍的星系來說，在較高質量及Ｘ射線強度的星系團中的星系，其平均星系年齡都比在較低質量、X射線的星系團中的星系老。（3）對於位於合併中的星系團5倍維里半徑內的低質量星系，其平均年齡都較在非合併中的星系團中的低質量星系年輕。（4）對依星系團特性而區分的2個星系團族群來說，兩者間的差異以低質量星系最明顯，這表示低質量、氣體豐度高的星系對環境的影響最敏感。
另外，我們也利用由電波源所辨識的恆星形成星系(star-forming galaxy，SFG)及吸收線活躍星系核(absorption-line AGN，AA)來了解環境對星系活動的影響。結果顯示，星系中恆星形成星系的比率隨著環境星系密度的增加而快速降低；反之，吸收線活躍星系核的比率隨著星系密度的增加而增加。另外，我們也發現，在合併中且其速度瀰散度(velocity dispersion)大於500公里/秒的星系團外圍，距離星系團核心1-2倍半徑的地方，恆星形成星系的比率特別高。這可能是因為當星系從絲狀結構落入星系團的時候，因星系密度增加且星系彼此間的相對速度低，使得星系間因互相作用而觸發恆星的形成。
最後，我們利用Abell S0721這個在薛普利超星系團中，擁有最高比率吸收線活躍星系核、並且是正在合併中的星系團，來研究其動力狀態與成員星系活動的關係。由於吸收線活躍星系核的電波發射來源被認為與星系本身或周圍的星系團熱氣體之熱動力狀態有關，我們因此推測，在這個星系團中，吸收線活躍星系核所在的次結構應該是在較穩定的狀態。因此我們可以利用運動方程式的解(equations of motion)來推測目前S0721可能的合併狀態。
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.
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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Keywords
超星系團, 4000Å轉折強度, 恆星形成, 星系團合併, supercluster, 4000Å break strength, star formation, cluster merger