Exploring Galaxy Evolution with CANDELS Morphologies

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.

It has recently been discovered that massive galaxies at z>1 have surprisingly smaller sizes than similarly massive galaxies in the local Universe and in the most extreme cases are up to a factor of 4 times smaller. In fact, it has been shown that the most compact of these high redshift galaxies also display the least amount of ongoing star-formation, which raises the question of how such compact systems at z>1 can grow to reach the sizes of comparably massive local galaxies if they have very little continued star-formation. There are several proposed mechanisms which may explain how these galaxies can grown in size, with very little mass increase from high redshift to the present day, such as through minor merger events or from AGN driven gas expulsion, which causes the system to expand. The exact physics responsible for this required growth remains debated, but it is clear that the morphologies of these massive galaxies can provide us with important information about their evolution.

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>$1x10$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. 

Size-mass relations for the separate bulge and disk components. Left: bulge components over-plotted by the red solid line with the size-mass relation found in the local Universe for spheroidal galaxies, and by the red-dashed line which is the corresponding error on the local relation. Right: disk components over-plotted in blue by the size-mass relation found in the local Universe for disk galaxies. From these plots we can see that while some bulge components lie on the local relation, the majority of them fall below it, whereas for disks a larger fraction are consistent with the local relation, and for those components which fall below the difference in size with local values is not as great as for the bulge components.

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.

In previous CANDELS posts the issue of what triggers the switching off of star-formation in galaxies to make them passive has been discussed, and it has been suggested that while the presence of a prominent bulge may best correlate with passivity, some passive galaxies with significant disks have also been observed. By utilising our decomposition of objects into their separate bulge and disk components we  directly addressed this question by including star-formation activity for the objects in our sample. When we did this, we found that the majority of star-forming galaxies are disks, and passive galaxies are bulges, but interestingly, a significant fraction (~40%) of passive galaxies have disk-dominated morphologies, i.e. where less than 50% of the total light from the galaxy is contained in the bulge, where the advantage of our decomposition technique allows us to explicitly assess how much of the overall light from the system is associated to the different components. Moreover, we also found that some of our passive galaxies appear to be pure disks. As discussed in previous posts, this suggests that while the traditional star-formation quenching (i.e., shutting down) mechanism of major mergers may indeed be important for some massive galaxies, there may also be additional physical processes which can quench star-formation in a galaxy but leave a massive disk intact.

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.