Abstract
We discuss the observational evidences of the morphological transformation of Spirals into S0 galaxies in the cluster environment exploiting two big databases of galaxy clusters: WINGS (0.04<z<0.07)andEDisCS(0.4<z<0.8). The most important results are: 1) the average number of S0 galaxies in clusters is almost a factor of ∼ 3 − 4 larger today than at redshift z ∼ 1; 2) the fraction of S0’s to Spirals increases on average by a factor ∼ 2 every Gyr; 3) the average rate of transformation for Spirals (not considering the infall of new galaxies from the cosmic web) is: ∼ 5 Sp→S0’s per Gyr and ∼ 2 Sp→E’s per Gyr; 4) there are evidences that the interstellar gas of Spirals is stripped by an hot intergalactic medium; 5) there are also indirect hints that major/minor merging events have played a role in the transformation of Spiral galaxies. In particular, we show that: 1) the ratio between the number of S0’s and Spirals (NS0/NSp) in the WINGS clusters is correlated with their X-ray luminosity LX ; 2) that the brightest and massive S0’s are always close to the cluster center; 3) that the mean Se rsic index of S0’s is always larger than that of Spirals (and lower than E’s) for galaxy stellar masses above 10^9.5M⊙; 4) that the number of E’s in clusters cannot be constant; 5) that the largest difference between the mean mass of S0’s and E’s with respect to Spirals is observed in clusters with low velocity dispersion. Finally, by comparing the properties of the various morphological types for galaxies in clusters and in the field, we find that the most significant effect of the environment is the stripping of the outer galaxy regions, resulting in a systematic difference in effective radius and Se rsic index.
Highlights
The morphological class of S0 galaxies has been largely debated since Hubble (1936) supposed their existence in “Realm of the Nebulae,” and Sandage (1961) placed them as transition types in the tuning fork diagram. Spitzer and Baade (1951) first suggested that S0’s might originate from Spirals stripped out of gas and that this class of objects could form a dustless sequence that parallels the Sa-Sb-Sc sequence in the Hubble tuning fork
In particular the fraction S0/Sp is nearly double every Gyr; 2. starting from the mean number of morphological types observed in EDisCS and ending to those measured in WINGS, we calculated a mean rate of transformation of Spirals into S0’s of ∼ 5 objects per Gyr, while the mean transformation of Spirals into E’s is at least ∼ 2 objects per Gyr; 3. the S0 population in clusters appears to grow at the expenses of the Spiral one by a factor of ∼ 3 − 4 since z ∼ 1
S0’s are more frequent in the central region of nearby clusters, while Spirals are preferentially found in the outer regions of high redshift clusters; 5. the S0 family at z ∼ 0 is on average quite different from that of present day Spirals: in general at a given distance from the cluster center they are more massive, 25% smaller in size, with an higher Sérsic index and effective surface brightness, and are redder in B−V colors
Summary
The morphological class of S0 galaxies has been largely debated since Hubble (1936) supposed their existence in “Realm of the Nebulae,” and Sandage (1961) placed them as transition types in the tuning fork diagram. Spitzer and Baade (1951) first suggested that S0’s might originate from Spirals stripped out of gas and that this class of objects could form a dustless sequence that parallels the Sa-Sb-Sc sequence in the Hubble tuning fork. The merging hypothesis is quite popular and old (see e.g., Biermann and Tinsley, 1975; mechanism defined at that time “collision between galaxies”) and often advocated for explaining the existence of S0 galaxies in low density environments (e.g., groups and field) where the mechanisms typical of clusters are less effective This possibility found support in the discovery of “Polar Ring S0’s,” i.e., lenticular galaxies hosting structures likely due to a second event such as the transfer of mass from a companion galaxy during a close encounter as well as a true merger (Schweizer et al, 1983).
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