Title: The Evolution of the Red Sequence Slope in Massive Galaxy Clusters
Authors: J. P. Stott, K. A. Pimbblet, A. C. Edge, G. P. Smith, J. L. Wardlow
First Author’s Institution: Astrophysics Research Institute, Liverpool John Moores University
In this post, we’ll examine a recent paper on the ongoing and controversial investigation into the evolution of the galactic color-magnitude relation. The color-magnitude relation is a phenomenon specific to galaxies in clusters, and I’ve written a post on the subject before. Studying the time evolution of this relationship is a powerful means with which to probe galaxy evolution, which means it can provide us with a picture of how structure formed in our universe.
The Color-Magnitude Relation
We’ve known about the color-magnitude relation in some capacity since the 1950s. De Vaucouleurs noticed the effect in 1961 during his study of morphology in the Virgo cluster, and before him Stebbins and Whitford had found suggestive results in a handful of their galaxies. However, it wasn’t until Bower’s work in 1992 that the relation was conclusively demonstrated.
So what is it? To put it simply, the color-magnitude relation just means that an elliptical or lenticular galaxy of a certain redness in a cluster will have a certain absolute magnitude, or brightness. A bluer galaxy is fainter. There are several things to note about the phenomenon:
For one, the relation isn’t so exact that the galaxy must have the indicated magnitude, but Bower demonstrated that the relationship was in fact very good, with intrinsic scatter on the order of a few hundredths of magnitude for his sample. In fact, since we can determine a galaxy’s absolute magnitude to such precision, we can get a fairly good estimate of its distance through this method. If we know an object’s absolute brightness from its color, and compare with how bright it appears to us on Earth, the distance can be inferred through the inverse-square power law of radiation. A similar trick is done with the red sequence of stars in globular clusters, in fact.
Secondly, this relation only applies to “early-type” galaxies: that is, lenticulars and ellipticals. Spirals, on the whole, tend to be bluer and don’t demonstrate such a trend. Further, this relationship has only been observed in cluster galaxies so far, and controversy rages over whether it is present in field galaxies.
What causes the color-magnitude relation? Nobody knows. However, since a galaxy’s color is a direct result of its population of stars, we can infer that the relation is telling us something about the star formation history of the galaxies in question. This phenomenon in particular gives us a foothold toward understanding galaxy evolution.
The paper we are discussing in this post is concerned with how the color-magnitude relation evolves in time. This is an active, open field of inquiry in extragalactic astronomy. We know that the galactic red sequence is in place, for example, in the Coma and Virgo clusters and many others which are in our cosmic back yard. What does the sequence look like in clusters at much farther redshift, when the universe was younger? The sample in this paper extends back to redshift one, which covers more than half the age of the universe.
Previous studies have shown that the red sequence slope is comparable between galactic clusters at the same redshift, and changes with redshift. Thus, if this is a real effect and not the result of our analysis, it means that the physical phenomenon responsible for the color-magnitude relation is changing in time. Perhaps more galaxies are evolving onto the red sequence, or perhaps it reflects the natural progression of galaxy evolution.
The authors have assembled Hubble data for the X-ray selected MAssive Cluster Survey (MACS) sample. The fact that this sample is X-ray selected simply means that the team found their galaxy clusters by looking at variation in X-ray luminosity due to hot cluster gas. Although this is a robust technique for discovering clusters, it is always good to consider the selection method when dealing with a survey; for instance, an X-ray selected sample tends to pick out more massive clusters than other methods. This could mean that the findings of this paper ultimately only reflect the behavior of the most massive clusters.
The authors perform the usual galaxy photometry calculations to determine total galactic magnitude and color for each galaxy. Although an ideal color parameter is independent of redshift, we must take into account the fact that redshift, as it pushes a galaxy’s spectral energy distribution into larger wavelengths, will actually affect the luminosity measured through our set of filters. To take this into account, we need to convolve our filter response functions with assumed spectral energy distributions to see how a galaxy of a given color will appear to our instruments at different redshifts. This is known as a K-correction, and if done carelessly it can have a definite impact on the appearance of our red sequence slope. The authors are very careful with this correction in particular.
They have found a red sequence slope for the clusters in their sample, and then plot this slope as a function of cluster redshift in figures 5, 6 and 7. They write that they “have found a significant evolution in the rest-frame slope of the red sequence in rich galaxy clusters between z ~ 0.5 and z ~ 0.1,” which is at odds with some previous results by other teams and in agreement with yet others.
The authors also demonstrate the robustness of their result by attempting to correlate the change in red sequence slope with parameters other than redshift. For instance, they plot it versus total X-ray luminosity (which traces cluster mass), galactic velocity dispersion, and the luminosity of the brightest cluster galaxy (BCG), finding no significant correlations. Apparently, the perceived evolution of the red sequence slope is due to something changing with redshift, and therefore with time.
So what does it mean? As stated before, this result would indicate some ongoing galactic evolutionary process related to whatever is responsible for the red sequence in the first place, which is still an open question. The fact that redder galaxies tend to be brighter could mean that massive galaxies have older stellar populations, or it could mean that they have higher metallicities (more heavy elements.) Recent work by Kodama and Arimoto suggests the latter.
The authors submit that the changing color-magnitude relation is probably due to smaller, younger galaxies falling into the cluster core from the outer filaments as the cluster evolves in time. The in-falling galaxies would be star-forming and therefore blue to begin with, but their star formation would be rapidly quenched (or “strangled”) as they pass into the denser cluster environment. Therefore, they would redden and move onto the red sequence, reddening faster than their older, luminous counterparts and thus flattening the red sequence slope at lower redshifts.
Such a result describes a pretty specific mechanism of cluster formation and galaxy evolution (specifically favoring hierarchical formation instead of monolithic collapse, for instance) and is an example of the sort of reasoning we can bring to bear on the topic when we utilize cluster surveys such as the one in this paper. Although the jury is still out on red sequence evolution, we can see that our results will have the effect of constraining models and do in fact tell us something about the history of the universe we inhabit.