The most massive galaxies in the Universe are important probes of galaxy formation as they provide insight into the physical processes which govern the evolution of galaxies at the extreme high mass limit. By studying these most massive systems across cosmic time we can provide rigorous tests for our understanding of how mass is assembled in the Universe.
|Example 6x6 arcsecond image stamps of the |
bulge+disk decomposition of one of our
objects with significant bulge and disk
components. The residual image illustrates
the goodness of fit of the combined model.
In the local Universe galaxy morphologies can be classified by the Hubble sequence and they display a well-known correlation between colour and morphology, with spheroidals being predominantly red in colour due to the fact that they have very little ongoing star-formation and, conversely, disks being blue, but at higher redshifts the case is more complicated.
The unparalleled high resolution H-band data from CANDELS has allowed us to conduct a detailed study of ~200 of the most massive galaxies with M>11 (i.e. 100 billion) solar masses at 1<z<3 (when the Universe was 1/2 to 1/6 its current age) in the UDS field, where we were able to decompose the rest-frame optical morphologies of galaxies into their separate bulge and disk components for the first time at these high redshifts for such a large sample size (see the CANDELS paper here). By conducting this decomposition we were able to explicitly explore how the sizes of the different components evolve within this redshift range, and compare this to studies in the local Universe. In doing so we found that the bulge components appear to display a more dramatic evolution in size than the disks. This can be seen in the figure below both from the number of bulges which have sizes significantly smaller than objects in the lower Universe, and in the difference between these sizes, which is more extreme for bulges than disks.
In addition to how massive galaxies evolve in size, decomposing objects into their separate bulges and disks also allows us to explore how the overall morphologies of galaxies evolve with redshift. In the local Universe, the majority of massive galaxies are pure bulge systems, but from this study we found that not only do massive galaxies become increasingly mixed systems with significant bulge and disk components with higher redshift, but that by z~2, they have predominantly disk-dominated morphologies. This suggests that not only is 1<z<3 a crucial era in cosmic time when global star-formation in the Universe peaked, but that is also marks a key phase in morphological evolution, where galaxies undergo a dramatic transformation from disk to eventually bulge-dominated systems.
|The redshift evolution of the morphological fractions in our galaxy sample, after binning into redshift bins of width z = 0:5, using three alternative cuts in morphological classification.|
The next step in our work is to extend our analysis to the CANDELS-COSMOS field to allow greater area coverage, and we are currently implementing a new technique to extend our decomposition of bulge and disk components to the other 3 CANDELS bands (F125W, F814W and F606W) in order to provide photometry for separate components to conduct individual SED fitting, with the aim of generating separate stellar mass and age estimates for the different components. This will add an extra dimension to our morphology decompositions and shed new light on the properties of high redshift massive galaxies.