The Environmental Effects of Cluster Galaxy Sub-Populations

Hello Astrogeeks,

Well, here I am with an actual new post for you to have a good gaze at. Well, I am currently sat down with a tea in hand writing this after spending all day working. So, my first full dive for 2019 will be on a topic quite close to my heart at the moment. Now, I do literally love all areas of astronomy, but, please bear with me for once and allow me to indulge on my current biases.

Get on with it man!

Right! Yes… Well, my current area of research is involved in trying to better understand the physics of galaxy clusters. More specifically, the kinematics of the member galaxy sub-populations, in an attempt to witness any effects differing cluster environments has on their kinematics. Making strides to study this can lead us to bettering our knowledge in the evolutionary histories of these galaxy clusters and their constituent galaxies. I am not going to delve into complicated specifics here, but, I will try my best to break down the thinking process when it comes to research in general as well as the tasks, tools and key skills modern astronomers employ to try and answer the questions they ask.


In our universe, we are pretty lucky to be able to observe light from (very) distant objects provided you have the right instruments. As a result, we are now well aware of the sea of galaxies that seemingly pervade every patch of sky evenly. Although, not all galaxies are truly spread out evenly in the sky as isolated islands of stars, gas and (potentially) dust.  We can observe that galaxies have coalesced, or are coalescing, onto large clusterings of galaxies known as (unsurprisingly) ‘galaxy clusters’.

Considering we cannot watch galaxies infall onto these clusters of large gravitational potentials in real-time. We have to rely on our current catalogue of knowledge in how isolated galaxies evolve, and compare this with a multitude of cases of non-isolated galaxies interacting with one another; what differences are there in the colour, shape/morphology, chemical composition/metallicities*, rates of star formation or their stellar masses? These comparisons can come from modelling the physics derived from the observations as well as the raw observational data.

The crux is that galaxy cluster environments can play host to an increased number of interactions, leading to most cluster galaxies to evolve prematurely than if they were still in the field and isolated. For example, such interactions can be either via increased contact with other galaxies via galaxy-galaxy tidal interactions; the hot diffuse gas in-between the galaxies, known as the Intra-Cluster Medium (ICM), can help form a pressure wave against an in-falling galaxy that strips any cold gas surrounding the in-falling galaxy; Strangulation of the in-falling galaxy due to its own halo of hot diffuse gas being stripped, preventing a replenishment of the gas needed to form new stars.

From this, we therefore ask, ‘How do the sub-populations of galaxies in clusters, determined from their different properties, kinematically respond to different galaxy cluster environments?’… Well, hopefully I can summarise what I did and how below.

*The Metallicity is defined as the ratio between an element heavier than hydrogen and hydrogen, commonly iron and hydrogen.

The Data and Analysis

Now, because I am an observational astronomer (hence the name of this webpage), I analyse observational data taken from a telescope. In this particular case, this is archival data taken from observing runs of the Sloan Digital Sky Survey (SDSS). The telescope itself has a 2.5m diameter primary mirror and is still in operation, with a few upgrades/add-ons, at Apache point in New Mexico, USA. Covering large swathes of the northern hemisphere, SDSS has been a remarkable (and relatively) little telescope that is still being data-mined to this day by people like myself.

Using the SDSS archival data suite, I grab all galaxy information (e.g. colours, stellar masses rates of star formation etc.) and marry this with a database of galaxy clusters, the one I use is known as the X-Ray Galaxy Cluster Database (BAX). This allows me to build the galaxy data from SDSS into the clusters defined by BAX. With the galaxies now assigned to their respective galaxies, they can then be split into two differing environments. In my case, we define ‘merging’ and ‘non-merging’ environments based of a statistical test known as the ‘Dressler & Schectman Test‘. This results in two sub-samples of clusters of merging and non-merging.

One the sub-samples of clusters have been formulated I can then begin to focus on analysing how the sub-population kinematics varies as a function of radius between these two cluster environments. The kinematics of the sub-populations are determined by computing Velocity Dispersion Profiles (VDPs), which is the spread/scale of the velocities at an increasing radius from the centre. An example of such results can be shown below with merging clusters on the top row and non-merging at the bottom row.

Basic RGB

As it can be seen, there is a clear difference in the shapes of the VDPs from the top row merging clusters, compared to the bottom row non-merging clusters. These VDPs are reproduced to determine the average effects of each environment for different sub-populations. For example, see the below VDP plots on morphology and colour.



Below is a quick snapshot of the key results from the work shown here.

  • In merging systems, a mix of red and blue elliptical galaxies appear to be driving the rising VDP at large radii. This may be the result of galaxy tidal-tidal interactions within galaxy groups.
  • Non-merging systems commonly display little variation in kinematics throughout their VDPs, however, there are consistently higher σP(R) values from the VDPs associated with a younger population of galaxies.
  • Spiral galaxy VDPs in merging systems present a strong segregation in their dispersion of velocities, with the blue spiral galaxies possessing a high velocity dispersion that is indicative of an in-falling subpopulation of field galaxies.

Well, fellow astrogeeks, if you want to know more and understand the methodology used and the science explored, you may find my paper either on the arxiv preprint server, or, the official publication on the MNRAS webpages.

Until next time!

-The ObAstro-



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