The Mass Structure of a Galaxy Depends on Where it Lives

By Caro Derkenne
[email protected]

Can the region in which a galaxy resides – its environment – influence how the mass within it is distributed? This is the question we set out to answer with some of the first data from the Middle Ages Galaxy Properties with Integral field spectroscopy (MAGPI) Survey. The survey uses MUSE data with ground-layer adaptive optics to observe resolved stellar kinematics of targets at a lookback time of 3-4 billion years, but with the resolution of local Universe surveys such as SAMI.

We measured the internal mass structure of MAGPI galaxies by using the motion of their stars to determine their gravitational potentials. This method is powerful because it is sensitive to any mass that contributes to the gravitational potential – stellar and dark matter – without making assumptions about stellar populations or how light converts to mass. Our measurements then inform us whether a particular galaxy is centrally compact or relatively diffuse, quantified by a parameter called the “total density slope” and labelled as “gamma” in the figure below.

Total density slopes
Figure 1: The total density slopes of many different studies, shown against redshift and lookback time. Focus on the black MAGPI circles and the dark blue Frontier Fields cluster squares. On average, the MAGPI total density slopes are more negative (steeper), indicating they are more centrally compact than the Frontier Fields cluster galaxies at similar redshifts.

The MAGPI galaxies we studied are found in a mixture of settings, from isolated field galaxies to those in larger groups. To further supplement what environments we probed, we also measured the internal mass structures of Frontier Fields cluster galaxies. The Frontier Fields clusters are some of the most extreme environments in the Universe, and were originally observed as ideal candidates for gravitational lensing due to their astonishing density. The two samples combined then bookend the span of galaxy environments in the Universe, from field galaxies to clusters.

When we compared the mass structure of MAGPI galaxies to that of Frontier Fields galaxies, we found that MAGPI galaxies are typically more compact in how their stellar and dark matter is distributed compared to their Frontier Field counterparts. In other words, we found that cluster galaxies tend to be more “puffed-up” in their total mass compared to field galaxies or galaxies in groups. This was surprising, as the Frontier Fields galaxies tend to have smaller half-light radii than MAGPI galaxies, and from that it could be inferred they would be more compact in their total mass structure, too. That they are not raises the intriguing possibility that galaxies in cluster environments have more dark matter than galaxies in the field or in groups, and this may be driving the observed differences in mass distribution – but more work is needed to confirm or refute this! For now, all the details of this study can be found in our recently published paper in MNRAS.


Michael Murphy is the Australian representative on the ESO Science Technical Committee. Contact: [email protected]

Sarah Sweet is the Australian representative on the ESO Users Committee. Contact: [email protected]

Stuart Ryder is a Program Manager with AAL. Contact: [email protected]

Guest posts are also welcome – please submit these to [email protected]