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
Great news everybody!
We applied for radio observations with the e-MERLIN network of radio telescopes in the UK. The e-MERLIN network can link up radio dishes across the UK to form a really, really large radio telescope using the interferometry technique. Linking all these radio dishes means you get the resolution equivalent to a country-sized telescope. You don’t alas get the sensitivity, as the collecting area is still just that of the sum of the dishes you are using.
Our proposal was to observe the Voorwerpjes. We wanted to take a really high resolution look at what the black holes are doing right now by looking for nuclear radio jets. The Voorwerpjes, like their larger cousin, Hanny’s Voorwerp, tell us that black holes can go from a feeding frenzy to a starvation diet in a short time scale (for a galaxy, that is). We really want to see what happens to the central engine of the black hole as that happens. There’s a suspicion that as the black hole stops gobbling matter as fast as it can, it starts “switching state” and launches a radio jet that starts putting a lot of kinetic energy (think hitting the galaxy with a hammer).
So, we want to look for such radio jets in the Voorwerpjes. We asked for a LOT of time, and the e-MERLIN time allocation committee approved our request…
… partially. Rather than giving us the entire time, they gave us time for just one source to prove that we can do the observations, and that they are as interesting as we claimed. So, we’re trying to decide which target to pick (argh! so hard).
3000 astronomers will bring down the wireless in any building, so I have been a bit behind in posting from the American Astronomical Society meeting in Long Beach CA…
Yesterday, Bill Keel presented a poster with the latest Hubble observations of the Voorwerpjes in the Giant Room Full of Posters, where astronomers, pretty much ALL of who work on absolutely cool stuff, present their results. So, anything you can do to get peoples’ attention helps! I decided to bring along some chocolates from Switzerland. If any unwary astronomer walked past and took one, they then had to at least look at the poster… ; )
AAS meeting update!
The last 24 hours have been good for Zoo team member Bill Keel (@ngc3314) is based at the University of Alabama. Not only did his University football team win some sort of championship (they all look the same to Europeans) last night, but the Hubble Space Telescope observed the final Voorwerpje in our approved programme! That means Bill was probably glued to the TV and downloading and reducing the data at the same time!
He’ll add the reduced image to his poster at the AAS meeting, so if you want to see the image, come join us at the poster tomorrow! He may also blog it some time later, but for the FIRST look, you’ll have to come to the poster! There may be chocolates too….
The poster is: 339.47. HST Imaging of Giant Ionized Clouds Around Fading AGN, up all of Wednesday from 9-6.
It’s January and that means that astronomers from all over the world flock to the American Astronomical Society‘s annual winter meeting (Jan 6-10 2013). This year, the 221st meeting, is in Long Beach CA. Quite a few of the team members and collaborators are going to the meeting and we’ll keep you posted on the exciting results that we’ll be presenting here on the blog and via our twitter account at @galaxyzoo.
The following talks by the team will be happening:
I’ll (@kevinschawinski) be talking about how blue galaxies turn into red ones (I needed all the blue ellipticals and red spirals you all found!) and how the two processes are completely independent. The talk is based on a paper in progress that I’m hoping to get ready for submission soon.
Kyle Willett (@kwwillett) will present an update on the reduction and analysis of the Galaxy Zoo 2 data. How do we turn your clicks into galaxy classification? This is the talk that will explain it!
Brooke Simmons (@vrooje) is going to update us on how bulgeless galaxies spotted by you! managed to grow enormous 10 millions solar mass black holes at their centers entirely through `gentle’ or `secular’ processes. No major mergers here!
Sugata Kaviraj will talk about the formation of early-type galaxies in the first half of cosmic time and discuss how your clicks from the ongoing Hubble Zoo might help uncover their secrets. And about how to get short term loans when you’re a student of the universe.
Finally, Bill Keel (@ngc3314) will present a poster with the latest analysis of the Hubble data of the Voorwerpjes, the light echoes of dying black holes.
Our friends from the Planethunters team are also going and may have some exciting news up their sleeve as well!
We’re looking for:
AAS ad: http://jobregister.aas.org/job_view?JobID=43152
The postdoc position is for two plus one year and comes with support for travel, computing, publishing etc. Research will include work with Galaxy Zoo data, especially the new Hubble Zoo data from CANDELS and also include the hunt for the first black holes in the universe.
Two Ph.D Students
AAS ad: http://jobregister.aas.org/job_view?JobID=43155
The Ph.D positions are fully funded for four years and also come with support for all things a student needs. Both thesis ideas are based on Zoo data and ideas. For the Ph.D position, you’ll need a Master’s degree in physics, astronomy, or related field.
Both postdoc and the students can get involved in the Zooniverse and getting more people to engage with science online.
Zurich is usually ranked in the top ten cities in the world in terms of quality of life and ETH is the highest ranked European university in the world. ETH ASTRO has expertise from planet formation to cosmology and is involved in a number of large projects and surveys. Also, there’s unlimited espresso.
The deadline for both is December 7 2012! For further details, please see the AAS ads.
Posting again for Karen Masters who is still in China: