The Green Valley is a Red Herring
Great news everybody! The latest Galaxy Zoo 1 paper has been accepted by MNRAS and has appeared on astro-ph: http://arxiv.org/abs/1402.4814
In this paper, we take a look at the most crucial event in the life of a galaxy: the end of star formation. We often call this process “quenching” and many astrophysicists have slightly different definitions of quenching. Galaxies are the place where cosmic gas condenses and, if it gets cold and dense enough, turns into stars. The resulting stars are what we really see as traditional optical astronomers.
Not all stars shine the same way though: stars much more massive than our sun are very bright and shine in a blue light as they are very hot. They’re also very short-lived. Lower mass stars take a more leisurely pace and don’t shine as bright (they’re not as hot). This is why star-forming galaxies are blue, and quiescent galaxies (or “quenched” galaxies) are red: once star formation stops, the bluest stars die first and aren’t replaced with new ones, so they leave behind only the longer-lived red stars for us to observe as the galaxy passively evolves.
Blue Ellipticals & Red Spirals
The received wisdom in galaxy evolution had been that spirals are blue, and ellipticals are red, meaning that spirals form new stars (or rather: convert gas into stars) and ellipticals do not form new stars (they have no gas to convert to stars). Since you’re taking part in Galaxy Zoo, you know that this isn’t entirely true: there are blue (star-forming) ellipticals and red (passive) spirals. It’s those unusual objects that we started Galaxy Zoo for, and in this paper they help us piece together how, why and when galaxies shut down their star formation. You can already conclude from the fact that blue ellipticals and red spirals exist that there is no one-to-one correlation between a galaxy’s morphology and whether or not it’s forming stars.
Blue, Red and…. Green?
A few years back, astronomers noticed that not all galaxies are either blue and star forming or red and dead. There was a smaller population of galaxies in between those two, which they termed the “green valley” (the origin of the term is rather interesting and we talk about it in this Google+ hangout). So how do these “green” galaxies fit in? The natural conclusion was that these “in between” galaxies are the ones who are in the process of shutting down their star formation. They’re the galaxies which are in the process of quenching. Their star formation rate is dropping, which is why they have fewer and fewer young blue stars. With time, star formation should cease entirely and galaxies would become red and dead.
The Green Valley is a Red Herring
Ok, why is this green valley a red herring you ask? Simple: the green valley galaxies aren’t a single population of similar galaxies, but rather two completely different populations doing completely different things! And what’s the biggest evidence that this is the case? Some of them are “green spirals” and others are “green ellipticals”! (Ok, you probably saw that coming from a mile away).
So, we have both green spirals and green ellipticals. First: how do we know they must be doing very different things? If you look at the colour-mass diagram of only spirals and only ellipticals, we start to get some hints. Most ellipticals are red. A small number are blue, and a small number are green. If the blue ellipticals turn green and then red, they must do so quickly, or there would be far more green ellipticals. There would be a traffic jam in the green valley. So we suspect that quenching – the end of star formation – in ellipticals happens quickly.
In the case of spirals, we see lots of blue ones, quite a few green one and then red ones (Karen Masters has written several important Galaxy Zoo papers about these red spirals). If spirals slowly turn red, you’d expect them to start bunching up in the middle: the green “valley” which is revealed to be no such thing amongst spirals.
Galaxy Quenching time scales
We can confirm this difference in quenching time scales by looking at the ultraviolet and optical colours of spirals and ellipticals in the green valley. What we see is that spirals start becoming redder in optical colours as their star formation rate goes down, but they are still blue in the ultraviolet. Why? Because they are still forming at least some baby stars and they are extremely bright and so blue that they emit a LOT of ultraviolet light. So even as the overall population of young stars declines, the galaxy is still blue in the UV.
Ellipticals, on the other hand, are much redder in the UV. This is because their star formation rate isn’t dropping slowly over time like the spirals, but rather goes to zero in a very short time. So, as the stellar populations age and become redder, NO new baby stars are added and the UV colour goes red.
It’s all about gas
Galaxies form stars because they have gas. This gas comes in from their cosmological surroundings, cools down into a disk and then turns into stars. Galaxies thus have a cosmological supply and a reservoir of gas (the disk). We also know observationally that gas turns into stars according to a specific recipe, the Schmidt-Kennicutt law. Basically that law says that in any dynamical time (the characteristic time scale of the gas disk), a small fraction (around 2%) of that gas turns into stars. Star formation is a rather inefficient process. With this in mind, we can explain the behaviour of ellipticals and spirals in terms of what happens to their gas.
Spirals are like Zombies
Spirals quench their star formation slowly over maybe a billion years or more. This can be explained by simply shutting off the cosmological supply of gas. The spiral is still left with its gas reservoir in the disk to form stars with. As time goes on, more and more of the gas is used up, and the star formation rate drops. Eventually, almost no gas is left and the originally blue spiral bursting with blue young stars has fewer and fewer young stars and so turns green and eventually red. That means spirals are a bit like zombies. Something shuts off their supply of gas. They’re already dead. But they have their gas reservoir, so they keep moving, moving not knowing that they’re already doomed.
Ellipticals life fast, die young
The ellipticals on the other hand quench their star formation really fast. That means it’s not enough to just shut off the gas supply, you also have to remove the gas reservoir in the galaxy. How do you do that? We’re not really sure, but it’s suspicious that most blue ellipticals look like they recently experienced a major galaxy merger. There are also hints that their black holes are feeding, so it’s possible an energetic outburst from their central black holes heated and ejected their gas reservoir in a short episode. But we don’t know for sure…
So that’s the general summary for the paper. Got questions? Ping me on twitter at @kevinschawinski
Spheroidal Post Merger Systems at the AAS
I think Chris said it best – any session which is ended by a guy in a bowtie went well. And for our AAS Galaxy Zoo session, that guy in a bowtie was Alfredo Carpineti from UCL, who talked about his work on the properties of spheroidal post-merger systems selected with the help of the Galaxy Zoo merger classifications, and using a control sample of non-merging spheroidals (or ellipticals) also selected from Galaxy Zoo.
Alfredo provided me with the below description, and his slides are available to download at Carpineti_AAS218talk.pdf.
In this talk we discuss the properties subset of galaxies from the GZ mergers catalogue that are spheroidal ‘post-mergers’, where a single remnant is in the final stages of relaxation after the collision and shows evidence for a dominant bulge, making them plausible progenitors of early-type galaxies.
The Anatomy of Galaxies
Following on from my post about the Hubble diagram, I thought I’d mention a bit about the main types of galaxies that are out there. Galaxies come in three basic types: spirals, ellipticals and irregulars. Each of these three broad morphologies of galaxy tells us a little about what is going on inside the galaxy itself. They are all structured differently.
The spiral arms of a galaxy contain most of the interstellar medium – dust and other material between stars – within a galaxy. It is in the spiral arms that new stars are forming, hence their usually bright, blueish or white colour. Spirals are made of about 10-20% dust and gas. This is the source material for the stars that are forming within the spiral arms. It is the dust that obscures background light to create the dark lanes you see in spiral galaxies. You can the arms and the dust lanes very well in this artistic impression of our own galaxy, the Milky Way from Nick Risinger / NASA.
The central bulge of spiral galaxy contains older, redder stars and often also contains a invisible, massive black hole. Some, but by no means all, central bulges have the appearance of a mini elliptical galaxy.
The central bulge and spiral arms vary greatly in appearance from galaxy-to-galaxy. But of course, you know this from working on Galaxy Zoo!
Spiral galaxies are also made up of a third component: the galactic halo. This is an almost spherical fuzz of stars and globular clusters surrounding the galaxy, trapped by gravity. You can see the halo quite well in the above image of the Sombrero Galaxy, which is a spiral seen almost edge-on. This image is from Hubble Heritage
Elliptical galaxies are essentially all bulge and nothing else! In an elliptical galaxy the stars tend to be older and there is less gas and dust around. The stars orbit around the centre of mass of the galaxy in a more random way – their orbits are not constrained to a disk shape. There is very little star formation going on in elliptical galaxies and so they usually appear reddish in colour: dominated by older, cooler stars.
There is obviously little to say about the structure of irregular galaxies because they are irregular. They make up about a quarter of all galaxies. It is thought that many irregulars were once ellipticals or spirals and have been distorted by interactions or collisions with other galaxies. Irregular galaxies can have very high star formation rates and can contain a lot of dust and gas – often more than spiral galaxies.
Galaxy Zoo: Hubble has a whole new branch of questions to try and help classify these clumpy galaxies.
You could add this fourth category to the list of galaxy types. Dwarf galaxies might appear to be just smaller versions of the above types, but they are the most common type of galaxy. There are more dwarfs than any of the others, if you just count them up.
The Large and Small Magellanic Clouds – the LMC and SMC, which are visible in the Southern Hemisphere – are actually two small galaxies, orbiting around our own larger Milky Way. The image below, from Mr. Eclipse, shows both of these objects. The LMC is an irregular galaxy and the SMC is a dwarf.
We’ll continue talking about the different types of galaxies – and how they all fit together – in the next post in this series. In the meantime might I suggest yet another type of galaxy, perhaps with a coffee and a bit of classification?