We investigated environmental effects on galaxy evolution using the Sloan Digital Sky Survey ( SDSS ) data .
By developing a new , uniform galaxy cluster selection method ( the Cut & Enhance method ) , we have created one of the largest , most uniform galaxy cluster catalog with well determined selection function .
Based on this cluster catalog , we derived extensive observational evidence of cluster galaxy evolution .

Composite luminosity functions ( LF ) of these cluster galaxies show that cluster LFs have a brighter characteristic magnitude ( M ^ { * } ) and a flatter faint end slope than field LFs .
We also found that early-type galaxies always have flatter slopes than late-type galaxies .
These results suggest that cluster galaxies have a quite different evolutionary history from that of field galaxies .

We confirmed the existence of the Butcher-Oemler effect as an increase of fractions of blue cluster galaxies with increasing redshift .
This is direct evidence of spectral evolution of cluster galaxies .
Cluster galaxies evolve by changing their color from blue to red , perhaps reducing their star formation rate ( SFR ) .
We also found that fractions of morphologically spiral galaxies are larger in higher redshift .
This “ morphological Butcher-Oemler effect ” is shown for the first time using an automated galaxy classification , and is direct evidence of morphological cluster galaxy evolution .
Cluster galaxies change their morphology from spiral to early-type galaxies .
In addition to the redshift evolution , we found the slight dependence of blue/spiral fractions on cluster richness , in a sense that richer clusters have smaller fractions of blue/spiral galaxies .
This result has significant implication for the underlying physical mechanism since it is consistent with a theoretical prediction of a ram-pressure stripping model , where richer clusters have more effective ram-pressure .

While investigating the morphology-density relation in the SDSS , we found two characteristic environments where the morphology-density relation abruptly changes .
In the sparsest regions ( galaxy density below 2 galaxy Mpc ^ { -2 } or outside of 2 virial radius ) , the morphology-density relations become less notable , suggesting that the responsible physical mechanisms require denser environment .
In the intermediate density regions , ( galaxy density between 2 and 6 galaxy Mpc ^ { -2 } or virial radius between 0.3 and 2 ) , S0 fractions increase toward denser regions , whereas late-spiral fractions decrease .
Considering the median size of S0 galaxies are smaller than that of late-spiral galaxies and star formation rate radically declines in these regions , the mechanism that gradually reduces star formation might be responsible for morphological changes in these intermediate density regions ( e.g. , ram-pressure stripping , strangulation ) .
In the cluster core regions ( above 6 galaxy Mpc ^ { -2 } or inside of 0.3 virial radius ) , S0 fractions decreases radically and elliptical fractions increase .
This is a contrasting results to that in intermediate regions and it suggests that yet another mechanism is responsible for morphological change in these regions .

Finally , we found that passive spiral galaxies preferentially live in cluster infalling regions ( galaxy density 1 \sim 2 Mpc ^ { -2 } and 1 \sim 10 virial radius ) .
Thus the origins of passive spiral galaxies are likely to be cluster related .
The existence of passive spiral galaxies suggest that a physical mechanism that works calmly is preferred to dynamical origins such as major merger/interaction since such a mechanism can destroy spiral arm structures .

Considering all the observational results , we propose a new evolutionary scenario of cluster infalling galaxies .
Around 2 virial radius or galaxy density below 2 galaxy Mpc ^ { -2 } , infalling field spiral galaxies quiescently stop their star formation and are transformed into passive spiral galaxies calmly .
These passive spiral galaxies later become S0 galaxies .
Possible responsible physical mechanisms in this region include ram-pressure stripping , strangulation , thermal evaporation and minor mergers , perhaps mainly happening in sub-clump regions around a cluster .
In the cluster core regions , we speculate that S0 galaxies fade away to enhance the dominance of old bright elliptical galaxies .