Authors: Jeffrey C.C. Chan, Gillian Wilson, Michael Balogh, Gregory Rudnick, Remco F. J. van der Burg, et al.
First Author’s Institution: Department of Physics & Astronomy, University of California, Riverside, CA, USA
Status: Accepted for publication in the Astrophysical Journal, available on arXiv
Galaxies in our Universe can generally be separated into two categories. First, we have beautiful, intricate spiral galaxies, the galaxies that astronomers want you to see. These galaxies have flat, disky shapes (like a dinner plate), delicate spiral arms, and rich supplies of gas, meaning that they are in the process of forming new stars (giving them a blue colour). On the other hand, we have elliptical galaxies. These less-glamorous cousins of star-forming spirals have a spheroidal, featureless shape, and are typically “quiescent”, meaning that they contain very little gas and therefore very little star formation (making them appear red). Nevertheless, studying them still leads to exciting discoveries…
One of the most important findings in the field of galaxy evolution is that the proportion of galaxies in each of these categories depends on where in the Universe we are looking. In cosmic fields, the majority of galaxies are star-forming. However, when we peer into dense galaxy clusters, we find an abundance of quiescent galaxies. This relation tells us that, for some reason, star-forming galaxies in clusters evolve into quiescent ones — that is, they stop forming stars (they “quench”), and they become rounder. Sounds simple enough, right?
Alas, it’s not so simple. In order to fully understand this relationship between a galaxy’s properties and the environment in which it lives, we need to understand the mechanisms that can remove gas from a galaxy, and the mechanisms that can change its shape (or “morphology”). Moreover, we need to understand which mechanisms are the most important in driving the evolution of galaxies. Today’s paper helps us take a step towards this, by finding an unexpected twist in the relationship between morphology and environment.
GOGREEN or go home
The authors of today’s paper use data from the GOGREEN (Gemini Observations of Galaxies in Rich Early ENvironments) survey, a large survey of galaxy clusters at redshifts between 1.0 and 1.5, carried out using the two Gemini Observatory telescopes in Hawaii and Chile. This work uses 832 galaxies from 11 of these clusters, and compares these to 6471 field galaxies, taken from the CANDELS and 3D-HST surveys. These surveys provide detailed information on the colours of these galaxies (indicating whether they are star-forming or quiescent), and the galaxy shapes.
First, the galaxies in each sample are grouped by colour, to distinguish between red quiescent galaxies, and blue star-forming galaxies. 58% of the cluster galaxies are quiescent, compared to only 16% in the field — this is what we’d expect, as we know that clusters can quench star formation.
However, the results get really interesting when we look at the shapes of these galaxies. The galaxy shapes are described by the axis ratio, q, the ratio of the minor axis of each galaxy to the major axis. For example, an axis ratio of q=1 would refer to a circle, q=0.5 would be an ellipse (shaped like an egg), and q=0.1 would be a long, thin shape, like a pencil (shown in Figure 1).
In both the clusters and the field, quiescent galaxies have a greater axis ratio than star-forming galaxies, meaning that they appear rounder. This is because quiescent galaxies are usually spheroidal in shape, whereas disky star-forming galaxies can appear long and thin if looked at from the side. Furthermore, star-forming galaxies in clusters have the same shapes as those in the field.
However, the really surprising result is that quiescent galaxies in the clusters do not have the same shape as those in the field, as shown in the right panel of Figure 2. Intermediate-mass quiescent galaxies in the clusters have a lower axis ratio, indicating that these quiescent galaxies are flatter in clusters than those in the field. Conversely, high-mass quiescent galaxies in clusters are actually rounder than their cosmic field counterparts.
Elliptical galaxies ain’t so elliptical
The fact that star-forming galaxies look the same in these two environments, but quiescent galaxies have different shapes in the field and in clusters, is puzzling. What it tells us is that there are different processes at play in these environments: the processes that cause field galaxies to quench and become spheroidal are different in the field to those in clusters. Although galaxies in both of these environments cease their star formation, the change in their morphologies is markedly different.
In fact, the results of this work are consistent with a scenario in which the shape of intermediate-mass galaxies does not change at all when they are quenched by a cluster! One example scenario the authors provide is galaxy starvation; essentially, when an external supply of gas for a galaxy is removed, meaning that it cannot form any new stars. This process can quench a galaxy’s star formation without changing its morphology. But questions remain: why would this only be the case for intermediate galaxies? And what quenches field galaxies?
This work drives home a fact that has been long known to astronomers: there are a lot of factors to consider when it comes to galaxy evolution. However, it provides further exciting evidence that many processes are at play, and that the connection between star formation and morphology may be even more complex than we’d previously thought.
Astrobite edited by Mitchell Cavanagh
Featured image credit: GOGREEN Survey