2 slow, 2 furious

1. The MASSIVE survey – VII. The relationship of Angular Momentum, Stellar Mass and Environment of Early-Type Galaxies
2. The SAMI galaxy survey: mass as the driver of the kinematic morphology – density relation in clusters

1. Melanie Veale, Chung-Pei Ma, Jenny E. Greene, et al.
2. Sarah Brough, Jesse van de Sande, Matt S. Owers, et al.

First Authors’ Institutions:
1. University of California, Berkeley, USA
2. University of New South Wales, Australia

1. Submitted to Monthly Notices of the Royal Astronomical Society [open access]
2. Submitted to the Astrophysical Journal [open access]



Scientific papers are a bit like buses. Sometimes you wait for ages waiting for one to take you where you want to go, then – surprise, surprise – two come along at once. This is, of course, a fundamental physical law, to which even astrophysicists are not immune.

In today’s article I’m going to break with tradition a little bit and highlight not one, but two papers, released weeks apart and with similar goals. This happens reasonably often, principally because if the science is both exciting and possible, chances are more than one team are looking into it! It’s always interesting to see independent groups take on the same question – and of course, the replicability of results is at the core of the scientific method. So for those reasons, and in the interests of fairness, let’s look at two takes on the origin of fast and slow rotating elliptical galaxies.

Fast and Slow Rotators

In the last decade the terms ‘fast rotator’ and ‘slow rotator’ entered astrophysical parlance as detailed studies revealed important differences among nearby galaxies. At first sight all elliptical galaxies look much alike, being more-or-less featureless red-ish blobs (see figure). However, a closer look reveals that they exhibit two quite distinct types of kinematic behaviour (the term kinematic in this context refers to the movement of stars within a galaxy, in other words its internal motions). This important detail has been highlighted by Astrobites before.

Elliptical galaxies: big and red, but not to be confused with buses.

The terminology here is not particularly imaginative: the principal difference between fast and slow rotators is, well, that the former rotate faster than the latter. But let us go into a bit more depth. Galaxies are collisionless systems, meaning that the gulf separating stars is sufficiently vast relative to their size that head-on collisions never happen in practice. Instead, all interactions are through gravity; stars whip around their host galaxy, their motions governed by its gravitational potential well. The orbits of the stars can be correlated, so that they are mostly orbiting around the same axis and in the same direction – or messy, with disordered orbits. Moreover, while all closed orbits are ellipses, there’s a big difference between a nearly circular orbit (like the Earth going round the sun) and a highly elongated orbit (like that of a long-period comet). These extremes are sometimes respectively referred to as tangential and radial orbits.

If the orbits of stars in a galaxy are mostly correlated and tangential, the galaxy ends up as a flattened, more oblate rotating system. By contrast, disordered radial orbits give you blob-like systems without much rotation. In the first case, we might say that the system is ‘rotation supported’ (it doesn’t collapse down to a point under its own gravity because it’s rotating and can’t shed its angular momentum) and in the second that it is ‘pressure supported’ (stars falling in towards the centre are balanced by stars that have already passed through the centre and are now travelling outwards). This gets to the crux of the matter: most elliptical galaxies are rotation-supported fast rotators, but a significant fraction (about 15%) are pressure-supported slow rotators. The stark difference in their kinematics has led to suggestions that despite their apparent similarities, an alternate formation channel is required to create slow rotators.

Today’s papers

In order to get to the bottom of this, the two teams conducted similar investigations. Both used data from large surveys of many galaxies, the MASSIVE survey and the SAMI galaxy survey respectively. Both surveys provide detailed spectroscopy of many galaxies – large samples are necessary since the aim is to draw statistical conclusions about the population of slow rotator galaxies as a whole. From this data, the kinematics of each target can be inferred (I explained how that works in some detail in a previous article, but it’s not essential to recap all that here).

Encouragingly, both studies hold some conclusions in common. As was already believed to be the case, both find that slow rotators are preferentially found among the most massive galaxies. Both teams looked at the effect of galaxy environment (i.e. whether the galaxy is isolated or contained in a cluster with many nearby neighbours). Massive galaxies do tend to be more commonly found in clusters, so given the dependence on mass already established such a trend must exist. What’s important is that when mass is controlled for there is no additional dependence on environment: both teams concur on this point.


Galaxies tend to grow via a series of mergers – collisions – between smaller galaxies, a process that takes place faster in dense environments where the chance of encountering another galaxy is much higher. This is the explanation for the point made above, that galaxies in clusters tend to be more massive than their isolated counterparts.

In the past it has been suggested that slow rotators might form due to a major collision between two similar sized galaxies, a highly disruptive event that would of course tend to leave behind a particularly massive galaxy. This kind of event would be much more common in the centre of a cluster of galaxies. However, neither of the studies presented here find strong evidence for a ‘special’ formation channel like this!

It’s certainly true that slow rotator galaxies tend to be particularly massive, but they don’t seem to care how they were put together (i.e. by many minor mergers or one big major merger): whether minor or major, galaxy mergers will tend to add mass and (usually) decrease the angular momentum of a galaxy. The more mergers that occur (i.e. the more massive a galaxy gets), the slower it will tend to rotate. In other words, fast rotators that grow large enough will eventually transition to become slow rotators instead.

About Paddy Alton

I am a fourth year PhD student at Durham University's Centre for Extragalactic Astronomy, where I work with Dr John Lucey and Dr Russell Smith. My research is on the stellar populations of other galaxies - with a specific focus on those of the largest elliptical galaxies, whose stars formed under radically different conditions to those in our own Milky Way. I graduated in 2013 from the University of Cambridge with an MSci in Natural Sciences, having specialised in Astrophysics. Out of the office I enjoy a variety of sports, but particularly rowing (whenever Durham's fickle river Wear allows it).

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