Monday, 28 January 2013

Does the Sagittarius Stream constrain the Milky Way halo to be triaxial?

I am still well behind on posting papers that were accepted at the end of last year, so a brief post to help clear the backlog. Brevity is also forced upon me as it is grant writing time, and as well as writing a grant, I am mentoring others.

Today's paper was accepted in December, but it is based on work I've been involved with for more than 15 years. The lead author is my old colleague, Rodrigo Ibata, who you've met before, and the subject matter should be familiar, namely the Sagittarius Dwarf Galaxy.

Basically, Sagittarius is orbiting the Milky Way, and is being torn apart my the gravitational pull of the larger galaxy. It looks like this
It's ragged appearance is a sign of its ongoing demise. But that's not the cool part. The bit we're interested in is all of the material that has been torn off and now forms a complete stream around our Milky Way. That looks like this
The structure in the streams is rather complex and we don't really understand quite what is going on, but there are a few things that we do understand. Basically, the streams are orbiting the Milky Way, not just the stars you can see, but also the dark matter that you can't.

Can we use the details of the streams to tell us about the properties of the dark matter?

The answer is yes, and long, long ago, we wrote this paper which showed that if the dark matter is spherical, then the orbit of the Sagittarius Stream will stay in a single plane. If, however, the dark matter is flattened or triaxial (which is what you'd expect from what we know about dark matter), then the orbits will precess and the streams will be a bit of a mess over the sky. In that paper, we concluded that the Milky Way was spherical!

Since then, there have been arguments back and forth, and some have concluded that the dark matter halo is actually flattened (I can go into detail if you really want, but there's a lot more data and it's a complex story).

So, we recently revised this very question, and asked "Does the data that we have *really* need the dark matter halo to be flattened?"

How do we test this? It's simple. We know how the laws of physics work (and here, we need only the two great works of Newton, motion and gravity), so we throw lots of test dwarfs into test halos and try and find the one that most resembles reality (this, in reality, what science is all about).

But there is a problem. Namely, doing all of these test is computationally expensive. We need fast computers to try and model the demise of lots of dwarfs, and so a while ago we developed a new way to do this.

OK - Here's a summary of the results (sorry, time is of the essence)

Lovely! The messiness of science. The purple dots are the "best model" that has been produced, and the green (and blackness) our model, and the red dots are the stars that have been measured. You might be looking at this and thinking "Just how do you compare the model with the data?". In all honesty, that is what science really is all about, and we spend a lot of time understanding just how to do this. Funnily enough, I think we do a bad job of telling student how to do this.

Anyway, to cut to the case, we show that we show that the conclusion that the halo of the Milky Way is not spherical is not as robust as people had claimed. In fact, we find that a nice spherical halo can happily reproduce what we see on the sky.

The sting in the tail? Well, there has to be one. And it's in the rotation curve of the Milky Way. The rotation curve is a very simple thing, being basically how fast a star in the Milky Way is circling the centre of the galaxy. Basically, the Sun zips around at 220 km per - wait for it - second.

What do we say for the rotation curve of the Milky Way? Here it is
Hmm. If you've done some basic astronomy, you'll know that we expect that rotation curves of galaxies are flat. But we predict that, if we are right, then the rotation curve should keep increasing toward almost 300 km/s at large distance.

Ha ha - you shout, we know the rotation curve of the Milky Way! But, in reality, living inside the thing limits what we can actually understand about the place in which we find ourselves.

But don't worry. I am sure this is not the end of the story, and we'll be seeing Sagittarius and the shape of the dark matter halo very soon again.

Well done Rod!

Does the Sagittarius Stream constrain the Milky Way halo to be triaxial?


Recent analyses of the stellar stream of the Sagittarius dwarf galaxy have claimed that the kinematics and three-dimensional location of the M-giant stars in this structure constrain the dark matter halo of our Galaxy to possess a triaxial shape that is extremely flattened, being essentially an oblate ellipsoid oriented perpendicular to the Galactic disk. Using a new stream-fitting algorithm, based on a Markov Chain Monte Carlo procedure, we investigate whether this claim remains valid if we allow the density profile of the Milky Way halo greater freedom. We find stream solutions that fit the leading and trailing arms of this structure even in a spherical halo, although this would need a rising Galactic rotation curve at large Galactocentric radius. However, the required rotation curve is not ruled out by current constraints. It appears therefore that for the Milky Way, halo triaxiality, despite its strong theoretical motivation, is not required to explain the Sagittarius stream. This degeneracy between triaxiality and the halo density profile suggests that in future endeavors to model this structure, it will be advantageous to relax the strict analytic density profiles that have been used to date.

Friday, 25 January 2013

Just what is "The Big Bang"?

Notice that I didn't title this post "Just what was The Big Bang?".

I know I've written about this before, but there is this misconception that the Big Bang, the cosmological not the tv show, is something that happened in the deep distant past of the Universe. This leads to no end of arguments about the creation of the Universe, the mystery of what came "before", and the question of a creator (tip of the hat to the nice young man from the Church of Latter Day Saints who had a 10 minute chat with me at the train station first thing yesterday morning).

So, I am here to teach you what the Big Bang *is*. You may think that this might make this an extremely long post, with lots of jargon, equations, difficult concepts and bizzarre grants. Well, Dear Reader, it won't. You won't have time to even make a cup of tea.

Ready? OK, here it is
"The Big Bang tells us that the Universe was hotter and denser in the past"
Done! Now that was painless wasn't it.

Hang on, I hear you shouting. "I've been short changed". But you haven't. That's it.

Before you leave dejected, you need to look at this fantastic cartoon from xkcd
But replace "String Theory" with "Big Bang" and the string theory wibble-wobble with "Suppose that the Universe was hotter and denser in the past". The difference would be that instead of "I dunno" we would have a wealth of observational tests, all which has been passed with flying colours".

I don't have time to go through all of the evidence today, so I am going to focus on a single thing, something that I think is very cool.

Before continuing, you have to remember that the predictions we make are mathematical, made within the framework of Einstein's General Relativity. It's not like we sit around waving our hands and waffling predictions (unlike a certain area of human endeavour).

So, let's start with an observation (which was originally predicted back in the 1940s), namely the Cosmic Microwave Background. If we have microwave sensitive eyes, our sky would look like this;
Unfortunately, I don't have enough time to explain the bumps and wiggles, but I will one day.

Essentially, we sit in a bath of this radiation, which has a temperature of about 2.7K. This is chilly.

In the past, however, the Universe was hotter and denser (think of what happens when you pump up a bike tyre - when you squeeze the air in the pump, it gets denser and hotter). So this Cosmic Radiation was hotter in the past.

But how do we test this? Well, it turns out that we can measure the temperature. How, I hear you ask?

Well, as well as the radiation out there, we also have atoms and molecules, and the two interact. The radiation actually warms molecules, making them spin a little faster. This faster rotation changes the way that the molecules absorb light from more distant objects, and by working with the spectra of distant quasars and galaxies that pass though more nearby gas, we can measure the temperature of the sea of radiation in which they sit.

I now this sounds a little "out there", but it's all standard chemistry in action. In fact, what is often forgotten that the first measurement of the Cosmic Microwave Background was undertaken in 1940 when Canadian Andrew McKellar noticed that molecules consisting of just Carbon and Nitrogen in the Milky Way appeared to be sitting in a radiation bath with a temperature of 2.4K.

This was before the predictions of the existence of the left over radiation from the extremely hot stages of the very early epochs of the universe, so what it was doing there was a bit of a mystery. Nobel Prize winner, Gerhard Herzberg in his book Spectra of Diatomic Molecules a few years later commented that  this radiation bath has "only a very restricted meaning".

So, now we can do this in the distant universe, which bring us to a very recent press release which tells us that CSIRO Telescope Takes the Temperature of the Universe. Now, this is not the most distant that we have measured the temperature, but it is extremely accurate.

I'm going to let the data speak for itself, so here's a copy of a figure from the paper:
The red dot is the new measurement, the green are more nearby measurements, while the black and blue are in the distant universe. What we have here is the temperature of the Cosmic Microwave Background "out there" on the y-axis, whereas the x-axis is the redshift (which we measure directly, but is related to the expansion of the universe between then and now.

The main point, however, is the dotted-line. This is not a fit to the date, but is the prediction we make from our cosmological equations. Essentially, the temperature out there, T, at a redshift, z, is related to the temperature now, To, by
T = To ( 1 + z )
How simple is that? And it works! Extremely well.

So, just what is the Big Bang? It is the most accurate tool we have for making predictions about the Universe. Nothing else comes close.

Science is awesome (well, cosmology is :)
 

Friday, 18 January 2013

Visiting the AAT....

I haven't posted anything about the bushfires up in the Warrumbungle National Park, especially those that tore through Siding Spring Observatory, my old workplace. Part of the reason was that I was too stunned - I was chatting about this on a skype call this morning, talking about the tidal streams of the Sagittarius Dwarf Galaxy, and remembering that it was discovered at the Anglo-Australian Telescope. It was 46C in Sydney today, and living on the edge of the bush, the thoughts of fires tearing through the suburbs is very scary.

Anyway, a fellow welsh person visited the observatory today.
(pinched from Amanda Bauer's blog)
 Prime Minister Julia Gillard was born in Barry, not far from where my brother lives. But just behind her in this picture is the AAO Director's Cottage, a place where I have slept a few times after very long nights observing with the scary 2dF instrument, and instrument whose robot tried to attack me more than the one in Saturn 3.

But the picture that hurt is this one
This is the lodge, where I (and many, many others) dozed through the day after observing. Those split open rooms facing you are the bedrooms. I slept there so many times.

More importantly. in the top right, the fire is still burning, and many people have lost their homes.  While we should be pleased the telescopes have survived, we should feel for the people who now have to pick up the pieces.

In closing, on the left-hand side of the picture is the Wedding Cake, one of my fav walks at the telescopes. I am looking forward to going back and doing it again.

Monday, 14 January 2013

Cosmic dance challenges our understanding of the universe

A quick post today to follow-up my previous Andromeda post. I've published an article in The Conversation titled Cosmic dance challenges our understanding of the universe which describes the result. I've covered most of this in the previous post, but a slightly different spin. Enjoy.

Saturday, 12 January 2013

A Vast Thin Plane of Co-rotating Dwarf Galaxies Orbiting the Andromeda Galaxy

As promised, here's a post about our recent Nature paper. I think the title of the paper basically says it all, although this is a the result of a very large amount of work. It also turns out that I am writing some articles on this for public consumption, which I will also post here, so here's the summary version.

I written previously, here and here, about the sterling work by PhD student, Anthony Conn, on measuring the distances to the almost 30 dwarf galaxies in Andromeda, as part of the PAndAS program.

Well, we now have the distances. So, the question is, are the dwarfs just thrown about at random (which is what you would expect from our cosmological simulations of structure formation), or is there a pattern. Here's the sample, on the PAndAS footprint:
The red and blue circles are the locations of the galaxies orbiting Andromeda. Just looking at them on the sky doesn't seem to reveal any structure, but remember, with the distances, we have the full 3d distribution. But even in 3d, there doesn't appear to be any particular substructure.

So then lead author, Rodrigo Ibata, and the rest of us ask "what if there is substructure in a subsample of the galaxies?" As you can imagine, with a sample of 27 galaxies, there 27 configurations of 1 galaxy, and 1 configuration of 27 galaxies, but how many subsamples of say 15 galaxies are there?

This is given by the binomial coefficient, and the answer is "A lot!" (well, 17,383,860). And so we set out to test each subsample, comparing their distribution of random samples of 27 galaxies.

To cut a very computationally long story short, we found a significant plane of dwarfs! They are the ones in red in the figure above. The plane is narrow, only 14 kpc, but immense, being 400 kpc in diameters. Here's the 3d view:

Quite clearly, the edge of the plane appears to be pointing straight at us!! But what are those arrows doing in the picture? I've also written previously about our measurements of the velocities of all of the dwarfs (there are actually several teams doing this) and so we know how fast things are moving. And what we find is there is velocity structure in 13 of the 15 galaxies in the plane; all those north of Andromeda are coming towards us, and all of those south are moving away. It look like the plane is rotating!

As that sinks in, watch this movie.
As an aside, the music is being played by Neil Ibata, a coauthor on the paper; not bad for a 15 year old (as you might guess, he's the son of the lead author of the paper).

So what does this mean? Well, as I've noted above, such a structure is not expected in our standard models of galaxy formation and evolution. There have also been claims of a similar structure, known as the Vast Polar Structure (VPoS) around our own Milky Way. Here's what it looks like
And these authors have asked "Do the Milky Way's companions spell trouble for dark matter?", and you can read their take on our paper at Marcel S Pawlowski's blog (although some may claim these views are a little closer to the fringe than standard cosmology).

So, what's the answer? Is this a challenge to cosmology or not? I don't know, but it is going to be fun trying to find out :)

Well done Rod!

A Vast Thin Plane of Co-rotating Dwarf Galaxies Orbiting the Andromeda Galaxy

Rodrigo A. Ibata, Geraint F. Lewis, Anthony R. Conn, Michael J. Irwin, Alan W. McConnachie, Scott C. Chapman, Michelle L. Collins, Mark Fardal, Annette M. N. Ferguson, Neil G. Ibata, A. Dougal Mackey, Nicolas F. Martin, Julio Navarro, R. Michael Rich, David Valls-Gabaud, Lawrence M. Widrow
Dwarf satellite galaxies are thought to be the remnants of the population of primordial structures that coalesced to form giant galaxies like the Milky Way. An early analysis noted that dwarf galaxies may not be isotropically distributed around our Galaxy, as several are correlated with streams of HI emission, and possibly form co-planar groups. These suspicions are supported by recent analyses, and it has been claimed that the apparently planar distribution of satellites is not predicted within standard cosmology, and cannot simply represent a memory of past coherent accretion. However, other studies dispute this conclusion. Here we report the existence (99.998% significance) of a planar sub-group of satellites in the Andromeda galaxy, comprising approximately 50% of the population. The structure is vast: at least 400 kpc in diameter, but also extremely thin, with a perpendicular scatter <14.1 kpc (99% confidence). Radial velocity measurements reveal that the satellites in this structure have the same sense of rotation about their host. This finding shows conclusively that substantial numbers of dwarf satellite galaxies share the same dynamical orbital properties and direction of angular momentum, a new insight for our understanding of the origin of these most dark matter dominated of galaxies. Intriguingly, the plane we identify is approximately aligned with the pole of the Milky Way's disk and is co-planar with the Milky Way to Andromeda position vector. The existence of such extensive coherent kinematic structures within the halos of massive galaxies is a fact that must be explained within the framework of galaxy formation and cosmology.

Wednesday, 9 January 2013

The Lowest Highest Point....

The first week of 2013 has come to a close, and it has been quite a week, with an accepted cosmological paper, a Nature paper appearing (which will be blogged about before the weekend is through), and another achievement of which I am proud. It's all to do with this place.
Some of you may recognise this as the summit of Mount Kosciuszko, the tallest mountain in Australia, and hence the highest mountain on the continent. I climbed this with the family on Sunday under an extremely toasty Australian Sun and, as we didn't take the easy way back, but followed the far more up-and-downy Main Range loop, and so covered 22km, and were left with sore legs, and a golden brown colour.

I will admit that Kosciuszko is not the tallest of mountains, it's only 2,228 metres, and it is not the hardest to climb (the simply up and down path is doable by most people), but it is the highest mountain on a continent, and, while it is the lowest of the Seven Summits, it's the only one I am likely to achieve (the next highest, Vinson Massif, is in Antarctica, although Kilimanjaro is a possible!).

Before people start complaining, I do know that if we include the island of Papua as part of the Australian continent, there are taller mountains, but I would like to remind my British colleagues that Kosciuszko is almost 1km taller than Ben Nevis, and 2 1/4 times the height of England's highest mountain, Scarfell Pike.

But it was to be a week of extremes, as we tried to head home yesterday, the hottest day on record. We were on the South Coast, near Batemans Bay, and after a tasty lunch we hopped into the car to drive back to Sydney. This is what greeted us in the car
That's 43 degrees C. I've actually been hotter in Australia (44 degrees C), but 43 is hot. The South-East, where Sydney finds itself, has a lot of trees and grass-land  and, when the temperature gets this hot, then we bushfires bursting out. And when they start, you get to realise two things.

1) Bushfires burn fast and firefighters have a significant battle on their hands. Fire races towards towns and roads, people get evacuated and roads get closed.

2) Australia has a sparse road network.

We trundled up the main highway, found the road closed, back tracked hundreds of kms, tried to head over the mountains to Canberra and then onto Sydney only to find

 For those in the know, this is them closing the Kings Highway at the Goulburn turn. The smoke in the distance was moving quite quickly towards us. So, backtrack, and then 50 km of dirt road and highway into Queanbeyan eventually got us to Canberra and then to Sydney this morning (after a sleep in Canberra). Exhausting.

While Australia may not be extreme in its mountain-tops, it is extreme in oh-so-many other ways.


Friday, 4 January 2013

Phantom Energy and the Cosmic Horizon: Rh is still not a horizon!

Back to work, and a good start to the New Year. I will write more on this shortly, but our paper in Nature was published yesterday (the various astronomical societies I spoke to last year know about this result, but I've kept it quiet in professional circles - hopefully it will cause a few wave).

I know I keep saying it, but time is squeezed so a brief post today. And that's on another paper that I had accepted yesterday (I am still trying to catch up on the Christmas list... Ah well, I will get to those eventually).

I've written before about the work I have done on "The Cosmic Horizon". It's a long story, but there have been claims that this is a magical part of the Universe which has not been properly understood and modern cosmology has it all wrong etc. Simply put, these claims are wrong, but they keep appearing.

So, this new paper was in response to another paper which claimed our previous paper was wrong. As ever, the claims are wrong, as I describe.

As I've stated before, horizons are important things in cosmology, and there are some classic papers which describe their importance. There is another important quantity in cosmology, called the Hubble Sphere. Objects on the Hubble Sphere are moving away from us at the speed of light; to the rival papers, the Hubble Sphere is the mysterious "Cosmic Horizon".

There has been huge confusion over the years about what you can see in the Universe, and how can you observe things moving faster than the speed of light etc. If you find yourself in this state, I suggest you read this.

To cut to the chase, the claim is that the Hubble Sphere is a horizon, and that the light rays we receive are limited by the presence of the horizon (i.e. they can only go through it in one direction etc). In this paper, I showed that it is simple to make the Hubble Sphere increase and decrease by just mucking about with the constituents of the Universe, changing the spots of dark energy.

A picture speaks a thousand words:
The blue is the Hubble sphere, which initially moves away from us after the Big Bang, then, as phantom energy takes over, moves towards us, and then, as I allow the phantom energy decay into photons, it moves away from us again.

The red lines are light rays which, after the Big Bang, move away from us, then some turn around and arrive back as the observer. Notices that one light ray, the one that arrives at the observer at a time of 130 billion years after the Big Bang, crosses the blue line three times, in differing directions. This is not Horizon-like behaviour.

In fact, in mucking about with the make up of the Universe, I could make the blue line dance in and out, and play with a photon, making it cross the blue line as many times as I wanted before we observed it. Simply put, the blue line is well understood. It's the Hubble Sphere. It is not a magical Cosmic Horizon.

And just to make Prajwal smile, I did the calculations in Python.

Well done ... errrm .... me :)

Phantom Energy and the Cosmic Horizon: Rh is still not a horizon!

Geraint F. Lewis
There has been a recent spate of papers on the Cosmic Horizon, an apparently fundamental, although unrecognised, property of the universe. The misunderstanding of this horizon, it is claimed, demonstrates that our determination of the cosmological makeup of the universe is incorrect, although several papers have pointed out key flaws in these arguments. Here, we identify additional flaws in the most recent claims of the properties of the Cosmic Horizon in the presence of phantom energy, simply demonstrating that it does not act as a horizon, and that its limiting of our view of the universe is a trivial statement.