I’m really excited to be able to post that galaxies selected with the help of Galaxy Zoo classifications are being observed at the VLA (Very Large Array) in New Mexico, possibly right now.
The funny thing about observing at the VLA is that you do all of the work for the actual observations in advance.
The VLA runs in queue mode – as an observer you have to submit very (very) detailed information about what you want the telescope to do during your session (called a “scheduling block”) and a set of constraints about when it’s OK to run that (for example you tell them when the galaxy is actually up in the sky above the telescope!). Then the telescope operators pick from the available pool of scheduling blocks at any time to make best use of the array.
This means after you submit the scheduling blocks you just have to sit and wait until you start getting notifications from VLA that your galaxies have been observed. The observing semester for the B-array configuration started on 4th October (had a pause for the US shutdown) and runs until the 13th January 2014. I’m happy to report that we started getting notifications in late November of the first of our 2 hour scheduling blocks having been observed. At the time of writing four of our galaxies have each been observed at least once (we need six repeat visits to each one to get the depth of data we’d like) for a total of 16 hours of VLA time. I’ve been getting notifications every couple of days – which means that as I write this the VLA could be observing one of our galaxies!
Since making these very detailed observation files is the observing prodecure at the VLA – it takes the length of time you’d expect given that…..
So, in September in-between a crazy travel schedule, and with a lot of help from our collaborator Kelley Hess at Cape Town, I spent a lot of time scheduling VLA observations of some very interesting very gas rich and very strongly barred galaxies we identified in the Galaxy Zoo 2 sample (the bit which overlaps with the ALFALFA survey which measures total HI gas in each galaxy).
We have been granted time to observe up to 7 of these fascinating objects (depending on scheduling constraints at the VLA) which I think may reveal some really interesting physics about how bars drive gas around in the discs of galaxies.
You might notice from the picture (and the name) that the VLA is not a “normal telescope”. It’s what astronomers call a radio interferometer. Signals are collected from 27 separate antennas and combined in a computer. This means that as well as observing sources for flux calibration (so we can link how bright our target is through the telescope with physical units) we also have to observe, roughly every 20 minutes or so a “phase calibrator” to be able to know how to correctly add the signals together from each of the antennae (to add them “in phase”).
So a single scheduling block lasting 2 hours for one of our sources comprises:
1. Information to tell the VLA where to slew initially and what instrumentation to use (how to “tune” it to the frequency we know the HI in the galaxy will emit at).
2. A short observation of a known bright source for flux calibration.
Then there’s a loop of
a. Phase calibration
b. Source observation
c. Phase calibration
d. Source observation
and so on – ending with a Phase calibration (on Kelley’s advice we’ll do 5 source observations, and 6 phase calibrations). We have a total of 6 of these blocks for each galaxy, that makes 12 hours of telescope resulting in about 10 hours of collecting 21cm photons per galaxy.
We have to check which times all these sources are visible to the VLA, and set durations for each part which give enough slew time and on source time wherever the sources are on the sky. And this all has to add up exactly to 2 hours to fit the scheduling block.
The benefit of this though is a telescope which acts like it’s much larger than you could ever physically build. We’re trying to detect emission from atomic hydrogen in these galaxies which emits at 21cm. So we need a really large telescope to get a sharp picture.
And just to end, because they’re lovely, here are the four galaxies the VLA has observed so far in the Sloan Digital Sky Survey visible light images.
Thanks again for your help finding these rare and interesting galaxies. They’re rare, because they’re so gas rich and strongly barred – we have previously posted about how we showed strong bars are rare in galaxies with lots of atomic hydrogen. Hopefully we’ll have some exciting results to share once we’ve analysed these data.
(PS. That takes a lot of time too – it’ll be almost 1TB of data to process in total!).
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).
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.
Last September I blogged about a proposal that had just been accepted at the Giant Metrewave Radio Telescope (GMRT) to follow up on the Peas with radio observations. Now the observations are in, and we have successfully detected the Peas at radio wavelengths!
The Peas, which have very high star formation rates, are expected to host a large number of supernova, which are created when the most massive stars die. These supernova create shocks that accelerate electrons in galaxy to relativistic energies. These relativistic electrons emit a type of emission, synchrotron radiation, that is visible in radio wavelengths. Therefore, the radio emission can tell us about the stars that live (or lived) in the galaxy.
Three of the Peas from our paper (Cardamone et al. 2009), were followed up with deep observations using the GMRT. It turns out that the Peas have comparable, but systematically lower flux when compared to local starbursts.
Using the observed radio emission, the magnetic field of the galaxy can be derived. These new observations suggest a magnetic field in the peas similar to that of the Milky Way. Because galaxies are thought to build up their magnetic fields over time, it is surprising to see such a large magnetic field in such a young galaxy. (Estimates of the age of the stars in the Peas are roughly 1/100th that of the age of the stars in the Milky Way).
One of the reasons that the Peas are so fascinating is their similarities to vigorously star forming galaxies found in the early universe (known as Lyman Break Galaxies). These Lyman Break Galaxies are so far away, they haven’t yet been directly detected in radio emission. However, estimates of their radio flux (from a technique called ‘stacking’) also suggest consistent radio fluxes with those observed for the Peas.
These observations suggest that galaxies like the Peas (and the Lyman Break Galaxies), may start out early in their life with very large magnetic fields. These observations challenge the assumption that galaxies build up their magnetic fields slowly over time and it is another piece of the puzzel in understanding of how galaxies are formed.
The article will be coming soon to astro-ph and I will post it here to let you all know.
Working with scientists in India, we have been awarded time on the Giant Metrewave Radio Telescope (GMRT) to study the radio properties of the Green Pea galaxies discovered by Galaxy Zoo users. We hope to use this telescope to detect the first signs of radio emission from the Peas, establishing them as a new class of radio sources.
Why do we want to search for radio signals from the Peas? The radio emission comes from remnant supernovae which can accelerate relativistic electrons that emit synchrotron radiation. So when we are detecting star forming galaxies in radio emission, we are finding signatures from these supernovae, which tell us about the stars that live (or lived) in the galaxy. Therefore, using the radio emission we can trace recent star formation activity in the galaxy.
We are particularly interested in these Green Peas, because they are the closest analogues to a class of vigorously star forming galaxies found in the early universe (known as Lyman Break Galaxies). These galaxies behaved very differently from star forming galaxies in the present day universe, and can help us to understand how galaxies formed in the early universe. Because Lyman Break Galaxies are so far away, Astronomers have not yet been able to detect radio emission from any of these galaxies individually. In contrast, the Peas are much closer and we have a good chance of being able to directly detect them in radio emission. Detecting this radio emission, and determining whether or not the radio emission from the Peas is like that in nearby star forming galaxies will help us to understand the nature of star formation in the youngest galaxies.