Tag Archive | Papers

Post-starburst galaxies paper accepted!

Great news everybody!

The post-starburst galaxies paper has now been accepted by MNRAS. You can find the full paper for download on astro-ph.

How to Navigate the Astro Literature, Part 1

So you want to learn about current astrophysics research? You’re in luck! Not only are there many excellent blogs, pretty much all of the peer reviewed literature is out there accessible for free. In many areas of science, the actual papers are behind paywalls and very expensive to access. Astrophysics, like a few other areas of physics and mathematics, puts most papers on the arxiv.org preprint server where they are all available for download form anywhere. In addition, we have a very powerful search tool in the form of the NASA Astrophysics Data System which allows you to perform complex searches and queries across the literature.

Suppose you wanted to learn more about the green peas, one of our citizen science-led discoveries. Your first stop could be the ADS:

ADS, like any search engine, will now scour the literature for papers with the words “green peas”, “green” and “peas” in it, and return the results:

As you can see, the discovery paper of the peas, “Cardamone et al. (2009)” is not the first hit. That’s because in the meantime there has been another paper with “green peas” in the title. You can click on Cardamone et al. and find out more about the paper:

This is just the top of the page but it already contains a ton of information. Most importantly, the page has a link to the arxiv (or astro-ph) e-print (highlighted). Clicking there will get you to the arxiv page of the paper where you can get the full paper PDF.

Also there is a list of paper which are referencing Cardamone et all, at the moment 23 papers do so. By clicking on this link you can get a list of these papers. Similarly, just below, you can get a list of paper that Cardamone et al. is referencing.

Lower still are links to NED and SIMBAD, two databases of astronomy data. The numbers in the brackets indicate that SIMBAD knows 90 objects mentioned in the paper, and NED knows 88. By clicking on them, you can go find out what those databases know about the objects in Cardamone et al. (i.e. the peas).

Obviously there’s a lot more, but just with the arxiv and NASA ADS you can search and scour the astrophysics literature with pretty much no limits. Happy resarching!

Radio Peas on astro-ph

Today on astro-ph the Peas radio paper has come out!   I discussed the details of the radio observations in July, after the paper had been submitted.  The refereeing process can take several months, from the original submission until the paper is accepted.

The paper is very exciting to all of us that worked on the original Peas paper, because it is a great example on how these exciting young galaxies (not too far away) are giving us insights into the way galaxies form and evolve.  In the case of the Radio Peas, the observed radio emission suggests that perhaps galaxies start out with very strong magnetic fields.

Star formation rate vs. color in galaxy groups

Toady’s guest blog is from Andrew Wetzel, a postoc at Yale University. We asked Andrew to write this blog since he and his collaborators had used the public Galaxy Zoo 1 data in their own work (that is, they weren’t part of the team). Without any further ado, here’s Andrew’s experience with the Zoo data:

Recently, Jeremy Tinker, Charlie Conroy, and I posted a paper to the arXiv (click the link to access the paper) in which we sought to understand why galaxies located in groups and clusters have significantly lower star formation rates, and hence significantly redder colors, than galaxies in the field. Among the interesting things we found is that the likelihood of a galaxy to have its star formation quenched increases with group mass and increases towards the center of the group. Furthermore, galaxies are more likely to be quenched even if they are in groups as low in mass as 3 x 10^{11} Msol (for comparison, the `group’ comprised of the Milky Way and its satellites has a mass of about 10^{12} Msol). All together, these results place strong constraints on what quenches star formation in group galaxies. However, many of the above results disagree with what some other authors have found recently, and here is where Galaxy Zoo has been useful for us.

Because galaxies that are actively forming stars have a significant population of young, massive, blue stars, while galaxies that have very little star formation retain just long-lived, low-mass, red stars, astronomers often differentiate between star-forming and quenched galaxies based on their observed color. But using observed color can be dangerous, because if a galaxy contains a significant amount of gas and dust, it can appear red even if it is actively star-forming (analogous to how the sun appears redder on the horizon as the light passes through more of Earth’s atmosphere). To get a more robust measurement of a galaxy’s star formation, we used star formation rates derived from their spectra, because spectroscopic features are fairly immune to dust attenuation. But, we wanted to check how these spectroscopically-derived star formation rates compare with the color-based selection that many previous authors have used. What we found was striking: in lower mass galaxies, over 1/3 of those that appear red and dead actually have high star formation rates!

What is going on? Here is where Galaxy Zoo provided us with insight. We examined the Galaxy Zoo morphologies of these red-but-star-forming galaxies, and the result was telling: 70% of these galaxies are spirals (which have particularly high gas/dust content) and furthermore, 50% are edge-on-spirals (for which the dust attenuation is particularly strong). The image shows a good example of a galaxy which has a high star formation rate but appears red. You can even see the dust lane.

So, Galaxy Zoo helped to confirm our suspicion that many spiral galaxies that appear red are in fact actively forming stars, but their colors are reddened via dust (Karen Masters has done a lot of work in this direction as well). This gave us further confidence in our spectroscopic star formation rates and insight into why previous authors, using observed color, came to such different conclusions. Thanks to the Galaxy Zoo team and all the volunteers.

The Sudden Death of the Nearest Quasar

When I told Bill Keel the results of the analysis of the X-ray observations by the Suzaku and XMM-Newton space observatories, he summed up the result with a quote from a famous doctor:

“It’s dead, Jim.”

The black hole in IC 2497, that is.


To recap what we know: the Voorwerp is a bit of a giant hydrogen cloud next to the galaxy IC 2497. The supermassive black hole at the heart of IC 2497 has been munching on vast quantities of gas and dust and, since black holes are messy eaters, turned the center of IC 2497 into a super-bright quasar. The Voorwerp is a reflection of the light emitted by this quasar. The only hitch is that we don’t see the quasar. While the team at ASTRON has spotted a weak radio source in the heart, that radio source alone is far too little to power the Voorwerp. It’s like trying to light up a whole sports pitch with a single light bulb – what you really need is a floodlight (quasar).

Now it is possible to hide such a floodlight. You just put a whole bunch of gas and dust in front of it. If there’s enough material, no light even from a powerful floodlight will get through. Imagine pointing it at a solid wall – even the brightest floodlight in the world will be completely blocked by the wall. In the realm of quasars, such a barrier is usually made up of the torus of material (gas and dust) spiralling in towards the black hole and settling into an accretion disk. So you can have quasars that are feeding at enormous rates and being correspondingly enormously bright, but our line of sight is blocked.

So there are two possibilities of what could be going on with IC 2497 and the Voorwerp:

1) The quasar is “on” but hidden by lots of gas and dust, or

2) The quasar switched off recently, but because the Voorwerp is 70,000 light yeas away, the Voorwerp is still seeing the quasar –  after all, even light takes a while to travel 70,000 light years. This would make the Voorwerp a “light echo.”

So how do we distinguish between the two possibilities? The best way is to look at a part of the electromagnetic spectrum that generally has no trouble penetrating even thick walls: X-rays!

If the quasar in IC 297 is feeding, then we should see the X-ray light it is emitting even through the thickest barriers. That’s why we asked for observations with Suzaku and XMM-Newton. It took many months to gather and analyze the data before we were ready to write up a paper and submit it to the Astrophysical Journal as a Letter. The referee report was challenging but positive, and the Letter got accepted rapidly. The pre-print is now out on arxiv: http://arxiv.org/abs/1011.0427

So what did we find? We found something, but it isn’t a quasar. With the X-ray data, we can definitely rule out the presence of a quasar in IC 2497 powerful enough to light up the Voorwerp. We do however see some very weak X-ray emission that most likely comes from the black hole feeding at a very low level. Compared to what you need to light up the Voorwerp (the floodlight), the black hole currently puts out 1/10,000 of the required luminosity. That’s like trying to illuminate a sports stadium at night with a candle.

We can therefore conclude that the black hole in IC 2497 dropped in luminosity by a factor of ~10,000 at some point in the last 70,000 years. This implies a number of very exciting things:

1) A mere 70,000 years ago (a blink of an eye, cosmologically speaking), IC 2497 was a powerful quasar. Since it’s at a redshift of only z=0.05, it’s the nearest such quasar to us. Since IC 2497 is so close to us, and the quasar has switched off, it means that images of IC 2496 are the best images of a quasar host galaxy we will ever get.

2) Quasars can just switch off very quickly! We didn’t know they could do this before, and the fact that they can is very exciting.

3) Maybe the quasar didn’t just switch off, but rather switched state, and is now putting out all its energy not as light (i.e. a quasar), but as kinetic energy. That’s an extremely intriguing possibility and something I want to investigate.


We put out a press release via Yale. You can find it here.

Red spirals at night, astronomers' delight

We heard a few days ago that our paper on red spirals has been accepted by the journal. Not only is this another success for Galaxy Zoo science, but it’s a tribute to the hard work of Karen who led the effort. What with the first Zoo 2 paper being submitted and a few other distractions as well it’s been a very busy week for her.

Red spirals paper title

The paper itself is another variation on what should be becoming a very familiar theme for those who have followed Galaxy Zoo science: colour and shape are not the same, and tell us different things. To recap slightly, as young, massive and short-lived stars are blue, colour is a measure of what’s happened recently. The blue spiral arms in the galaxy pictured below, for example, mark sites of recent star formation.M51 from SDSS

It was known long before Galaxy Zoo that most of the star formation in our local Universe takes place in spiral galaxies, and so they tend to be blue whereas ellipticals are often red. In looking at the blue ellipticals and now the red spirals, it’s clear that interesting things happen when this rule is broken.

Before we can work out what’s going on though, we have to find our red spirals, and this is trickier than it sounds. If we weren’t careful, then our sample would get contaminated by edge-on systems, which appear redder because of the effect of the dust that scatters light which travels through the disk. As this paper uses only Zoo 1 data, we just selected the roundest spirals assuming that this would get rid of those pesky edge-on systems; we also insist that Zooites were able to identify a direction to the spiral arms.

It turns out that 6% of spiral galaxies are red, which I think is higher than most would have guessed before this project. So how did a substantial number of spiral galaxies come to turn red? What caused them to cease forming stars and become what the paper title calls them : ‘Passive red spirals’?

Selection of red and blue spirals

One important clue is understanding where this process happens. It turns out that the greater the density of the environment a spiral finds itself in (that is, the more neighbours it has) the more likely it is to be red…but only up to a point. Once we find ourselves near the core of a cluster of galaxies, the number and fraction of red spirals drops dramatically. So whatever it is that is causing the spirals to turn red must be more likely in the outskirts of galaxy clusters, but relatively rare outside this particular environment.

The story is, as ever, a little more complicated than that. If it was the environment that was driving the dramatic change from blue to red, then we’d expect the properties of the red spirals to depend on the environment. We might find that those in the densest environments were redder than their (still quite red) counterparts further out, for example. But we don’t. We don’t see any connection between the properties of the red spiral and the environment they find themselves in.

In my next blog, I’ll look at what we do know about this mysterious population of galaxies unearthed by your hard work. Until then, if you want the gory details, you can find the latest version of the paper over here.

Merger Papers Accepted for Publication in MNRAS

Thanks for everyone’s work – both papers should soon be appearing in the Monthly Notices of the Royal Astronomical Society 🙂


Galaxy Zoo motivation study paper accepted!

Our paper on the motivations of Galaxy Zoo users has been accepted for publication in the journal Astronomy Education Review! Now that the paper has been accepted, I have posted it on the arXiv system. Head on over and read it if you’re interested in hearing more about the interviews we did with some of you to learn what makes Galaxy Zoo appeal to you. I wrote a summary of the paper for this blog a while back, but now you can read the paper itself.

The paper should appear in the Galaxy Zoo Library in the forum soon, and Pamela, Georgia, or I would be glad to answer any questions you have about the paper there. The next step in this research is to analyze the data from the survey that many of you took, and we’re working on that step now. Updates on that will come soon. Thanks to my lovely co-authors, and of course to all of you, without whom this none of this research would be possible!

What happens next… Peer Review

With the first Galaxy Zoo paper submitted (kudos to Kate and Anze!), we’d like to describe to you what happens next. What’s scientific publishing all about? How does it work? If you’ve followed the blog and the forum, you have a pretty good idea of the first part of the scientific process: discovery!

We set out on the Galaxy Zoo project in part to test whether spiral galaxies in different parts of the sky somehow have spins that align, as has been claimed by earlier work. Kate and Anze have commented on the motivation for this work and blogged about how we did find an effect, were startled by it and so started the bias test to understand it. Kate and Anze used the bias test data to show conclusively that in the case of Galaxy Zoo it was an effect with the observers and that the universe isn’t mad.

This is one of the amazing and unique things about science. Good scientists spend most of their time arguing against the effects they see in their own data, to avoid falling into traps of seeing only what they expect to see. To see how unique and amazing this is, try to imagine a politician arguing against a piece of legislation s/he is sponsoring! This process of double, triple, and quadruple-checking one’s own work is a very important part of science.

Once we were convinced that we really understood what is going on, we could then write up our conclusions in the form of a scientific paper. Steven wrote here about the process of writing a paper; Kate went through the same process Steven described. Over the past few weeks, she passed her paper around to the rest of the Galaxy Zoo team for comments. Kate’s paper has thus passed through the first check — her own examination of her results — and the second — amongst the team itself.

The next step in scientific research is to submit the paper to a journal. This has now happened, and the paper Land et al. (2008) (where “et al.” means “and the rest,” including YOU!!) has been submitted to the top UK journal Monthly Notices of the Royal Astronomical Society (MNRAS).

The editor of this journal will now select an anonymous referee who can comment on the scientific and technical merits of the paper. The referee is another astronomer or cosmologist whom the editor can ask for an expert assessment of the work. He or she will have a few weeks to read it, think about it, and then make a number of recommendations to the editor of the journal. There are three options. The referee can reject the paper outright. This generally happens very rarely, except in highly competitive top journals like Nature and Science. They can support publication of the paper, asking for only a few minor modifications. This also happens quite rarely, though! The most common outcome is for her to write a “referee report,” suggesting a number of modifications and ask for clarifications. The referee might have questions about some part of the analysis, suggest some alternative thoughts and ideas, or criticise the methodology. Sometimes referees can be hostile to a paper; but often, they are genuinely helpful and constructive.

After receiving the report, we get a few weeks to digest it and modify the paper according to the referee’s comments, and argue against the points raised that we disagree with. This process may repeat itself a number of times if the referee isn’t happy with our modifications, and so it can often take weeks and months for a paper to get to a decision by the editor (acceptance or rejection). If a referee is being particularly unreasonable, we can write to the editor requesting a new referee. In extreme circumstances, we could even choose to submit the paper to a different journal and hope for a more reasonable referee.

The whole process is generally known as peer review since the referee is a peer — a fellow scientist and expert in the field. If the paper is accepted, it will appear both in the online and print version of the journal after another few weeks or months. A paper accepted in such a journal is then considered peer-reviewed.

So, if Kate’s paper hasn’t yet been peer-reviewed why is the paper already “public”? It’s general practice in astrophysics to post papers as preprints on a web server called astro-ph. Astro-ph is updated daily to make all papers publicly accessible for anyone. Most people post their papers there when they submit them to journals so they are available immediately. Some wait till the paper is accepted. Thus, not everything on astro-ph is peer-reviewed! In fact, in cosmology, some like to submit preprints to astro-ph before submitting so to allow the community to comment before the draft is submitted to a journal.

It’s important to note that something said in a “peer-reviewed” paper isn’t necessarily true. The point of peer-review is to weed out obviously flawed paper whose logic has holes or whose data don’t support the conclusion. Knowing that a paper has been peer-reviewed should give you extra confidence that its results are believable – that means that an expert in the field has read through the paper and thinks its conclusions are believable.It’s really just the first step of proper “peer-review,” because the process continues. As the community of astrophysicists digests the paper, they too pass judgement on whetherthey consider the paper important and whether they believe the conclusion. Thus, in the years after publication, other astrophysicists might deem Land et al. (2008) a key paper and cite it in the future, commenting on it positively. Or they might disagree with it, but that would still be a sign that it was important enough to comment on. Or it might just fade into obscurity if astronomers don’t consider it important. That’s the historical legacy of a paper – and that’s the ultimate peer-review.