GLASS Paper VII: The Spatial Distribution of Star Formation
Using GLASS data, Vulcani et al. (2016a) analyzed the spatial distribution of star formation in 76 galaxies in 10 clusters and 85 galaxies in the field at 0.3< z <0.7. The samples are well matched in stellar mass (10^8-10^11 M⊙) and star formation rate (0.5-50 M⊙/yr). The authors have visually classified galaxies in terms of broad-band morphology, Hα morphology and likely physical process acting on the galaxy. They found that most Hα emitters have a spiral morphology (41±8% in clusters, 51±8% in the field), followed by mergers/interactions (28±8%, 31±7%, respectively) and early-type galaxies (remarkably as high as 29±8 in clusters and 15±6% in the field). A diversity of Hα morphologies have been detected, suggesting a diversity of physical processes. In clusters, 30±8% of the galaxies present a regular morphology, mostly consistent with star formation diffused uniformly across the stellar population (mostly in the disk component, when present). The second most common morphology (28±8%) is asymmetric/jellyfish, consistent with ram pressure stripping or other non-gravitational processes in 18±8% of the cases. Ram pressure stripping appears significantly less prominent in the field (2±2%), where the most common morphology/mechanism appears to be consistent with minor gas rich mergers or clump accretion.
Comparing the position of the peak of the Hα emission to that of the continuum, as traced by the F475W filter, the author found that in both environments, for most of the galaxies the displacement is smaller than 1.5 kpc and the average offset is ~0.5 kpc. The existence of the offset suggests that current star formation is not generally colocated with recent star formation, perhaps as the result of accretion of satellites or gas, or non gravitational interactions such as ram pressure stripping affecting the spatial distribution of the cold gas.
The emerging picture is that Hα emitters are a very heterogeneous population, characterized by a range of morphologies, sizes and SFRs, therefore, a simple explanation can not describe all the observations. Even though some small systematic differences between galaxies in the field and in clusters emerge, both populations present very mixed morphologies and experience a variety of processes.
This work demonstrates that the effects of cluster- specific mechanisms on galaxy evolution are detectable in GLASS unprecedented data. However, they are both subtle and complex. They are subtle in the sense that no dramatic trend is found between the morphology of the current star formation and the environment or other properties of the galaxy. Every trend is weak and there are always exceptions. A full understanding of this complexity requires larger samples and detailed and spatially resolved physical models.
GLASS Paper VIII: The Correlation Between Star Formation and Cluster Environment
Extending the study presented in Vulcani et al. (2016a) described above, Vulcani et al. (2016b) used the same GLASS data and the results from Vulcani et al. (2016a) to characterize the spatial distribution of star formation in 76 galaxies in 10 clusters at 0.3< z <0.7. They correlated the properties of Hα emitters to a number of tracers of the cluster environment to investigate its role in driving galaxy transformations. Hα emitters are found in the clusters out to 0.5 virial radii, the maximum radius covered by GLASS. The peak of the Hα emission is offset with respect to the peak of the UV-continuum. Decomposing this offsets into a radial and tangential component, the radial component points away from the cluster center in 60% of the cases, with 95% confidence. The decompositions agree with cosmological simulations, i.e. the Hα emission offset correlates with galaxy velocity and ram pressure stripping signatures. Our clusters span a wide range of morphologies. Trends between Hα emitters properties and surface mass density distributions and X-ray emissions emerge only for unrelaxed clusters.
The most statistically significant result is that galaxies with asymmetric Hα distribution, interpreted as signatures of recent ram pressure stripping, are preferentially found within 0.3 r500, at higher local density conditions and higher X-ray counts and have a negative radial projected offset, i.e. the peak of the Hα emission is pointing away from the cluster center with respect to the continuum emission.
The lack of strong correlations with the global environment does not allow to identify a unique environmental effect originating from the cluster center. In contrast, correlations between Hα morphology and local number density emerge.
This work concludes that local processes, such as ram pressure, strangulation and galaxy-galaxy interactions, rather than processes taking place on large scale, are the most important, or at least the most easily detectable, drivers of environmental evolution.
Further Details
For further details on the above studies please refer to: