From astrobites: “It’s a tough life for a small galaxy”

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Astrobites

Jun 2, 2017
Paddy Alton

Title: The hELENa project – I. Stellar populations of early-type galaxies linked with local environment and galaxy mass
Authors: A. Sybilska, T. Lisker, H. Kuntschner, A. Vazdekis, G. van de Ven, R. Peletier, J. Falcón-Barroso, R. Vijayaraghavan, and J. Janz
First Author’s Institution: European Southern Observatory

Status: Accepted for publication by Monthly Notices of the Royal Astronomical Society, [open access]

It’s a jungle out there, and it’s all about survival of the biggest.

As our understanding of how galaxies form has developed, we’ve come to realise that galaxies tend to start small and grow over time by gobbling up smaller galaxies: a kind of cosmic food chain. The biggest galaxies – the predators of this vicious ecosystem – can usually look after themselves. It’s pretty rare for them to encounter an equally big and bad galaxy and for the most part they barely pause as they tear unfortunate smaller galaxies to shreds.

(Quite literally – no exaggeration here! When the galaxies are similar sizes it gets pretty messy…)

In the end, most massive galaxies generally end up all looking fairly similar, because there’s not much that the galaxies around them can do to make them evolve differently to how they would if they were left alone. But for smaller galaxies it’s a very different story.

The trials and tribulations of dwarf ellipticals

Today’s featured paper is the first in a series all about such galaxies and the lives they lead. The hELENa project (that’s The role of Environment in shaping Low-mass Early-type Nearby galaxies) may not be doing the astronomy community any favours in terms of our reputation for egregious acronym abuse, but we shouldn’t let that detract from some great science.

The authors are taking a closer look at the stellar populations of dwarf elliptical galaxies. These galaxies at first sight look a lot like the massive galaxies I’ve already mentioned, only smaller – but because they are smaller, they are much more susceptible to external sources of disruption. The stars they contain can bear the scars of these encounters, so we can learn a lot about the large scale physics of galaxy formation by taking a closer look.

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The Virgo cluster of galaxies. In the left panel the locations on the sky of the residents of this galactic metropolis are marked with dots, colour-coded by galaxy type. The hELENa targets are shown with black squares, and the monstrous galaxy Messier 87 is given a special marker. The other panels show the sample superimposed on a map of the cluster’s density (computed two different ways in the middle and right-hand panels), so you can see that some of the sample lie in the cluster’s quiet outskirts, whilst others reside in the busy central regions. Figure 1 from the paper.

Many galaxies can be found in clusters, giant gravitationally-bound agglomerations of galaxies. In some ways clusters are like cities for galaxies, with densely populated regions with lots of action in the centres and more relaxed suburban areas around the edge. The type of environment a dwarf galaxy finds itself in can matter a great deal. In particular, processes such as ram-pressure stripping can become important for galaxies falling into denser regions, tearing out their gas and abruptly curtailing star formation. Likewise, galaxy harassment (close encounters with other galaxies) can disturb the structure of small galaxies. In short, it’s not easy being a dwarf elliptical. In figure 1 (above) the target galaxies from this paper – all from the Virgo cluster – are shown along with the environment

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The effect of environment

The authors’ goal is to look for correlations between the stellar populations in their target sample and the environments their samples live in. Information about stellar populations is encoded in the light from these galaxies and can be accessed via spectroscopy. Briefly, the idea is that certain spectral features (things like absorption bands at particular wavelengths) get stronger or weaker in the light from stars depending on how old they are, whether they are more/less enriched in heavy elements, more/less massive etc. Measuring these features – in particular, measuring how they vary within each galaxy – can tell us lots of useful things about the history of the galaxy, for example how quickly it formed its stars. Some examples of these measurements are shown in figure 2 (below).

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Some spectroscopic maps of a few target galaxies (a comprehensive version can be found in the paper’s Figure 2). Taking spectra from each pixel in the image, it’s possible to learn a lot about each galaxy. The three left-most columns show measurements of key spectral absorption features, colour-coded by strength. From these the age and chemical composition of the stars in each galaxy can be inferred. The other three columns give information about how bright parts of the galaxy are and the velocities of the stars.

From these measurements the authors can estimate things like the ages of the stars in different parts of the galaxy, their chemical composition, whether the galaxy is rotating, and more.

So what can be learnt from this bounty of information? The authors find a number of interesting effects, for example that the stars in galaxies residing in denser regions of the cluster tend to be older. This suggests that the effect of the denser environment is indeed to help shut off star formation earlier. Likewise, the properties of galaxies deep in the cluster seem to correlate with their size more tightly than those in the outskirts. This shouldn’t be too surprising: the effects of many encounters on these galaxies will drive them towards a common, average outcome. By contrast those galaxies which lie further out will have a variety of histories.

The authors are able to confirm that whereas the properties of massive galaxies are principally determined by their size, dwarf ellipticals show a much wider variation in their properties. In other words, massive galaxies mostly ignore external effects and only their internal physics matters, but for dwarf galaxies the varied external encounters they have experienced are more important.

See the full article here .

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