Authors: Simona Mei, Brad P. Holden, John P. Blakeslee, Piero Rosati, Marc Postman, Myungkook J. Jee, Alessandro Rettura, Marco Sirianni, Ricardo Demarco, Holland C. Ford, Marijn Franx, Nicole L. Homeier, Garth D. Illingworth
First Author’s Institution: Johns Hopkins University
If we want to know why galaxies such as our own Milky Way look the way they do today, we need to know something about galaxy evolution- that is, how galaxies formed, clumped up into clusters, and collided with each other as the universe grew from the big bang to today. While this would be a tricky guessing game if we could only observe galaxies in the present era, thanks to relativity we can actually see the universe as it appeared billions of years ago when we look out over great distances. By comparing the galaxies we see at distant redshifts- which are generally smaller, weirder, and asymmetric– with our own local cluster, we can start to put together a picture of how it all went down.
Galaxy clusters naturally contain a range of galaxies of different types and ages. Early-type galaxies are those which haven’t developed bars or spirals- morphologically, we categorize them as elliptical or lenticular galaxies and they probably formed when smaller galaxies collided. In clusters, these types of galaxies represent the oldest and most massive in the universe. They usually have very little dust and very few hot, bright stars.
The Color-Magnitude Relation
One of the universal traits we observe in local (low-redshift) clusters is a relationship between the color and brightness of their early-type galaxies. This color-magnitude relation (CMR) is something like a Hertzsprung-Russel diagram for galaxies, and like the H-R diagram it indicates that there is some common mechanism at work that determines a galaxy’s color and mass. It is thought to arise from a relationship between a galaxy’s mass and metallicity – its relative abundance of heavier elements.
This CMR has been observed in clusters out to redshift ~1, and it has even been shown that the relationship evolves in time in a way consistent with passive evolution models. This means that the early-type galaxies on the CMR sequence seem to simply grow older without significantly forming new stars, causing them to gradually grow redder and dimmer. This is just one example of how such a feature helps us characterize galaxy evolution, but naturally we would want to probe this to even farther, earlier moments of the universe. When you consider that galaxies have been observed at redshifts out to 8 or even more, you can see that the CMR data we currently have gives us a picture of only the more recent developments in our universe.
The Lynx Clusters
A supercluster is a gravitationally-bound collection of galaxy clusters (a cluster of clusters) and the Lynx supercluster is the highest-redshift supercluster known to date. Within this supercluster, our authors are concerned with clusters Lynx W and Lynx E, at redshifts ~1.3. At this distance, we expect that clusters in general are still in the process of forming and merging- in fact, as Lynx W and E are 2 megaparsecs apart, they themselves might be merging. Therefore it should be extremely interesting to see if the CMR relation is still in place in such early galaxy clusters.
Lynx W seems to be younger- that is, less dynamically evolved- than Lynx E because of its sparse, filamentary distribution; Lynx E has already been compacted into a denser, spherical structure. What’s more, data from the Chandra X-Ray telescope shows Lynx E to have a brighter overall luminosity, and higher temperature. Therefore, we can reasonably say that these clusters represent very different environments. That being the case, should their galaxy populations also be different?
If the color-magnitude relation of early type galaxies is consistent across the two clusters, that would mean that a common population of early type galaxies has already formed at redshifts higher than 1, both in an evolved and less-evolved cluster, and before the two merge. This is of course consistent with a “hierarchical structure formation” model, wherein galaxies gradually clump together to form the massive structure of clusters.
Much of the data processing in this paper deals with the meticulous job of determining which galaxies belong to a cluster (and which are simply getting in the way when we try to observe Lynx E and W), whether they’re early-type or not, and trying to get accurate measures of their colors. After collecting colors and magnitudes for 40 galaxies across the two clusters, the authors then fit these data to separate CMR lines for each cluster and for each galaxy type, and then did the same for the combined sample.
In the end, they did find a consistent CMR across both clusters, despite their obvious structural differences! The authors conclude that “the bright red early-type galaxy population is already in place in both clusters at z ~ 1.26.” This is the deepest redshift at which this feature has been observed. If it’s true that elliptical galaxies form from the collision of smaller galaxies, then this means that galaxies had probably formed and merged before clusters started to take shape.
A CMR allows us to determine some other neat features of clusters. For example, they can be used to estimate galaxy ages when taken with models of stellar populations. Additionally, the authors show that their results are consistent with those of recent studies which showed that the brightest galaxies in clusters seem to have already evolved and shut off their star formation at redshifts beyond 1, while fainter galaxies are still evolving and forming stars.