A peculiar faint satellite in the remote outer halo of M31

The Pan-Andromeda Survey (PAndAS) continues to be a gold-mine for science. We're squeezing it hard to get out key results, but next year, the data will become public and everyone can have a looksie and write their own paper.

Here we have another paper by ANU astronomer, Dougal Mackey. Dougal's expertise is understanding the globular clusters orbiting the Andromeda galaxy, especially the distant clusters. He published a really nice piece of work recently which showed that these distant globulars are not just scattered randomly about Andromeda, but are more likely to be sitting on the stellar substructure we see. This substructure is the tidal debris from smaller galaxies that have fallen in and been shredded, meaning that the globulars are immigrants, having been born outside Andromeda, but joining the halo when their parent galaxy is destroyed; this is galactic cannibalism in action.

This new paper is about a particular cluster of stars orbiting Andromeda, named PAndAS-48 (who says astronomers aren't imaginative when it comes to naming things!). While this cluster was initially observed with the Canada-France-Hawaii Telescope (CFHT) as part of PAndAS, this paper presents new observations with the Hubble Space Telescope.

While the CFHT, at 3.6m, is larger than Hubble (2.5m), the lack of an atmosphere means we get much sharper images, and hence can see a lot fainter. Here's images from CFHT (left) compared to Hubble (right).

Nice! We actually observed the cluster in a couple of photometric bands with Hubble, which allowed us to make a colour-magnitude diagram; as you know, stars are not randomly scattered in such a picture, but sit on sequences that are driven by stellar evolution. What do we see?

For those in the know, yes, the faintest stars in there are around 28^th magnitude!

In there, we can see the Red Giant Branch and Horizontal Branch, and that allows us to understand lots of things about the globular, such as how far away it is and what stage it is at in terms of its evolution.

We can also measure the distribution of stars, and measure the shape of the clusters.

So, what is this cluster of stars? Is it a dwarf galaxy, dominated by dark matter? or a globular cluster, which are thought not to contain dark matter? It's actually very hard to tell. This piccy illustrates the issue.

The picture is pretty self-explanatory; size is along the bottom in parsecs, and brightness is up the side. The dots are colour-coded in terms of how elliptical they are.  The squares on the right are dwarf galaxies; they tend to be big and elliptical. The dots on the left are globular clusters, which tend to be small and circular (but notice that they can be of the same brightness as the dwarfs).

Where's PAndAS-48? It's the point with a circle around it, stubbornly right between the two populations! In fact, the ultimate conclusion is that we don't know what it is. If it is one or the other, then there are problems. But that's cool too!

 It is worth noting that PAndAS-48 appears to sit on the vast thin plane of satellites orbiting Andromeda, which makes it even more intriguing, but we haven't got it's velocity so can't confirm if it is orbiting in the same sense. But if it is, it will be extra cool.

As ever, the more we learn, the more questions we have. Yay!!

Well done Dougal!

A peculiar faint satellite in the remote outer halo of M31

Dougal Mackey, Avon Huxor, Nicolas Martin, Annette Ferguson, Aaron Dotter, Alan McConnachie,Rodrigo Ibata, Mike Irwin, Geraint Lewis, Charli Sakari, Nial Tanvir, Kim Venn

(Submitted on 30 Apr 2013)

We present Hubble Space Telescope imaging of a newly-discovered faint stellar system, PAndAS-48, in the outskirts of the M31 halo. Our photometry reveals this object to be comprised of an ancient and very metal-poor stellar population with age > 10 Gyr and [Fe/H] < -2.3. Our inferred distance modulus of 24.57 +/- 0.11 confirms that PAndAS-48 is most likely a remote M31 satellite with a 3D galactocentric radius of 149 (+19 -8) kpc. We observe an apparent spread in color on the upper red giant branch that is larger than the photometric uncertainties should allow, and briefly explore the implications of this. Structurally, PAndAS-48 is diffuse, faint, and moderately flattened, with a half-light radius rh = 26 (+4 -3) pc, integrated luminosity Mv = -4.8 +/- 0.5, and ellipticity = 0.30 (+0.08 -0.15). On the size-luminosity plane it falls between the extended globular clusters seen in several nearby galaxies, and the recently-discovered faint dwarf satellites of the Milky Way; however, its characteristics do not allow us to unambiguously class it as either type of system. If PAndAS-48 is a globular cluster then it is the among the most elliptical, isolated, and metal-poor of any seen in the Local Group, extended or otherwise. Conversely, while its properties are generally consistent with those observed for the faint Milky Way dwarfs, it would be a factor ~2-3 smaller in spatial extent than any known counterpart of comparable luminosity.

Bad Physics: Midsomer Murders

I've lived in Australia for thirteen years, but in the way that Sting was an English Man in New York, I have never quite felt "Australian", rather, I am a Welsh Man in Sydney. Anyway, I still feel very British, and am a fan of British TV (apart from a few highlights, Australian TV is generally bilge).

Anyway, I've always loved a good murder mystery, and I like Midsomer Murders, even though they have changed the lead character (and the new chief inspector was actually a criminal in a previous episode). The premise of Midsomer's is simple; a cop in the quite fictional county of Midsomer solves murders. However, the show has been running for 15 years, and there seems to have been an awful lot of murders (although the murder rate is considerably lower than Honduras!). To keep the stories going, murders are set in, quite often, bizzarre circumstances.

A recent episode, Written in the Stars, focused on the intrigue and mystery at a research observatory at Midsomer University (up until this point, I don't think there had been mention of a university in the county). With usual stereotypical fashion, we have a mean professor, who is ready to steam-roller anybody to build his reputation, and a young genius who is writing her thesis (on the Heisenberg uncertainty principle) and threatens to dethrone the evil professor.

As part of her research, she needs to look at an eclipse (go figure) and the murder mayhem ensues. That's not the bad physics (but doesn't help).

Here's the young genius at work, presenting her work in the dome of a telescope (not sure why she is not in an office or lecture room).

Someone has gone to great effort to fill the board with lots of scientific squiggles. It's not, however, gibberish. I'm not sure if they used a text book, or wikipedia, but there are some correct things there.

However,  something annoyed me. Zooming in on the board, what do we see?

Plank's constant! Argh!! You'd think that our young genius who has written a thesis on quantum mechanics and is presenting her research to evil and nasty professor could spell Planck's name correctly. But there is more! Whoever wrote the squiggles got the symbol, h, correct, and even the value, 1.054 x 10^-27, correct, but they completely screwed up the units (that's too painful to go into) and what this number actually is is ħ ,which is Planck's constant divided by 2π.

Why would they bother going to the effort of writing something semi-correct, but pay so little attention that they make a mess of it? Why not just do it right? Don't they realise that professors of astrophysics might be watching?

One other thing that annoyed me is that they did the "astronomers only do their work inside telescope domes" thing

We don't. We have offices like everyone else. And even when we are at the telescope, we are in the control room, not freezing our bottoms off in the dome.

Before finishing, I think it's worth noting that the observatory actually used in the show is actually a university observatory. It is the University of London Observatory at Mill Hill

Even though I was a student at the University of London, I never used this observatory, although I did visit there when I was looking for a PhD position. However, the observatory is not in the picturesque county of Midsomer, but is next to the A1 in North West London.

Like a lot of observatories around the world, it was build outside of a city, but the cities have grown around them.

Anyway, the murderer was not the evil astrophysicist..... It was actually the friendly professor of Quantum Physics! I'm sure his knowledge of the uncertainty principle will help him in prison.

Nature doesn't care how smart you are

Random Monday Morning Thought:

Becoming a science professor sorta snuck up on me. Not getting the title, as that happened at a distinct point in time (namely the first of January 2009), but the 'separation' from being a student and then postdoctoral researcher grows somewhat slowly. A colleague of mine recently expressed surprise when he discovered his students were somewhat daunted when speaking with him (this is partly as there is the perennial fear of "looking stupid" that students have), and I'm pretty sure my fellow faculty member does not feel that different to the students he talks to.

The important point, I think, is that students should realise that you don't get smarter with age; in fact, it's probably the opposite. What you do gain is experience. When a professor speaks from authority, it is not necessarily that they are "smart", but they have gathered significant experience over the years. But it's important to realise that there is a limit to experience, and just because a particular professor makes a pronouncement, it doesn't necessarily mean it's correct. Over at Letters to Nature, Luke Barnes has a nice article on appealing to authority.

Anyway, I just wanted to add to this a marvellous quote

In high school, my two idols were Einstein and Feynman. While Einstein felt that QM must be wrong, Feynman felt it was the ultimate truth of the universe. This discrepancy bothered me, and I wasn't sure who to believe. So, about six weeks into physics X, I screwed up my courage and asked Feynman about the "dice" and Einstein.  "Dr. Feynman", I asked, "Einstein was one of the greatest geniuses of physics, and certainly a lot smarter than me. He knew more physics that I ever hope to. But, he didn't believe in quantum mechanics--so why should I?" 

Feynman paused -- which surprised all of us -- and smiled. He looked at me and said, in that wonderful Far Rockaway accent, "Nature doesn't care how smart you are. You can still be wrong." He went on to explain some background on Einstein's view of physics, and why he might feel that way. 

(from here).

"Nature doesn't care how smart you are"; I think that's an important lesson that all of us should remember.

Bad Physics: No Energy Radio Waves

Just a little bit of Bad Physics, this time from the L. A. Times. The story was "Mysterious hydrogen clouds detected in space, puzzling scientists", a nice story.

I quote

"The clouds don't emit light or energy, but the neutral atomic hydrogen that they are made of gives off a distinct radio signal that astronomers were able to pick up using the National Science Foundation's Green Bank Telescope which measures radio waves."

Apparently, these clouds do not emit light or energy, but they do emit radio waves that we can detect with out telescopes. Aghhh!!

I know that you already know this, but radio waves are one form of electromagnetic radiation, and (in terms of quantum mechanics) are packets of energy called photons. The energy of a photon is given by

where λ is the wavelength of the radiation.

Light has a wavelength of around 550nm, and so each photon carries some energy. Radio waves, however, have wavelengths from millimetres to many hundreds of kilometres. Such long wavelengths mean each photon carries a much smaller amount of energy, but they still carry energy!

Real warp drives – can distributed computing help?

Some how, I blinked and discovered we're in mid-May. This is scary as I have some travel coming up, and need get some things out of the way. It's been a busy week (but heck, when isn't it) and part of the load is that I am on the Australian Research Council College and am ranking the current round of Future Fellowships; this is a lot of work, but is incredibly interesting, especially looking at the sheer excellence of research being undertaken in Australia. However, it's also completely confidential, and so I'm not going to talk about any details :)

However, I will say that this is the last round of Future Fellowships, removing the opportunity for mid-career researchers to focus on, well, research. This will leave a hole in research efforts in Australia.

Anyway, some stories never die, especially of it's about warp drives. A couple of years ago, my new PhD student, Brendan McMonigal, and I wrote a cool paper called "The Alcubierre warp drive: On the matter of matter". I've written about this before, but we showed that as the warp drive accelerates, it starts to gather up particles and radiation, and this is released in a burst as it decelerates, frying all your relatives who have come to meet you.

If you think you have seen Brendan recently, you're probably correct.

Anyway, we receive a steady stream of requests for comments on the paper, especially for inclusion in news stories. And here's another.

(picture taken from the article). The crux of the story is that we effectively did a 2 dimensional analysis of the warp drive, one in time and one in space, What we really want to do is do a full 4-dimensional analysis, one in time and three in space, to really understand what happens to light rays, especially those that come in obliquely. But this is computationally expensive, and we need to use big computers to do this.

The computational problem is that we need to follow the path of a particle in 4-D space-time, and to do this we use the Geodesic Equation. This looks like this

For those in the know, to integrate this, we have 8 coupled one-dimensional equations, and we have to follow the path of each particle and each bit of light. But this is a "stupidly parallel" problem - each particle and light beam is independent and so we can calculate on on one machine, and one on another machine, and pull all the results together at the end.

Anyway, you can read more about this in the article. But the point I want to make is the catch-cry of rating modern science is impact. One form of impact is easy to measure - it is citations to your work by other scientists. The harder one to quantify is "societal impact", and that is the broader impact your results have (things like science appearing in the press, or leading to a technological spin-off etc).

This is why I am happy that this story happily keeps bumbling along, with each new article highlighting our work and the science being done at the University of Sydney. It might not change the world, but at it is, at least, blooming interesting :)

Bad Physics: ``Ballistic'' does not mean Evil!

A very quick rant about some bad physics that has been annoying me.

The tension between North Korea and the rest of the world is not a laughing matter, but has been all over the news recently. But when reporting, we often hear the that North Koreans are on the verge of producing a missile capable of carrying a nuclear warhead. But not only a missile! A ballistic missile.

Maybe I am reading too much into it, but when a reporter says ballistic, it's often said as if it is synonymous with evil! I'm sure that it's because we like to say something is "going ballistic" when something crazy happens.

But all ballistic means is that the missile is unpowered (after launch) and is moving only under the power of gravity. That's it.

There are powered missiles. Modern anti-tank missiles are usually powered and guided onto target. Here's one -

Here is a ballistic missile.

See - not so scary!

Another week gone, and no time for too much deep thinking (although progress is being made, so hopefully will have some interesting things to report). Luckily, the smart students out there are squirreling away, and so today I can present the latest in the study of the galactic bulge by ANU student
Melissa Ness.

I've written before about this really cool study, using the AAOmega Spectrograph on the might Anglo-Australian Telescope to measure the speeds and chemical make-up of stars in the Galactic Bulge, the centre of our Milky Way galaxy. This is hard work, as there are a lot of stars spread over a large amount of sky, and so to get lots of spectra, you need to use the multi-fibres and large field of view of AAOmega.

This has been a mammoth task over the last few years, with us taking 28,000 spectra, and in this study almost 17,5000 stellar velocities were used. But what is it we want to know?

Well, the present day shape, velocities and chemistry of the Milky Way is a consequence of its birth and growth, dependent on lots of different factors, such as what dark matter collapses to form the seed of the galaxy in the early universe, to the way that gas flows in and cool, and the way that stellar populations have evolved. So, if we can pick-apart the present day structure, then that will tell us about the formation history of the Galaxy, a topic known as galactic archaeology.

On the face of it, the bulge of the Milky Way looks quite simple, with stars basically buzzing about randomly, but in fact, the way the stars are distributed and the formation history means that the underlying dynamics can be quite complex. So, what did we find?

Firstly, here's the fields we observed

Even though the AAOmega spectrograph is mighty, you can see that the coverage is still relatively sparse, and so we are trying to work out the dynamics from a series of key-hole views through the Bulge. We, of course, would like to cover the entire Bulge, but that would take years and years of observing (and they won't let us have complete control of the telescope!).

Here is the rotation curve for the bule - yes, it rotates, and also the velocity dispersion. So, the motions of the stars are not orderly orbits, like the planets in the Solar System, but a mixture of orderly and random buzzing about.

Essentially, we can see that the velocity depends upon the location on the Bulge, and the latitude of the observations. Things get really interesting when we start to slice the stars up in terms of metallicity (which means chemical make-up).

Strongly negative numbers means the stars are chemically poor, while the values close to 0 are chemically richer (it's a logarithmic scale); now, all of these stars are almost entirely hydrogen and helium in their atmospheres (the bits we get the light from), but the higher metallicity stars are have more heavier elements than the metallicity poor stars. These extra elements come from the pollution by supernovae of gas clouds from which the stars were formed.

How do we start to understand these curves. Remember, we are looking through some key-hole views into the Bulge, but things are a little more complicated than that. The problem is that the Bulge contains structure, firstly large scale structure in the form of a Galactic Bar. Here's a model of the distribution of stars in the galaxy

We're located at 0 on the x-axis, and -8.5 kpc on the y-axis, and you can see that the Bulge (the red bit) is elongated into a bar, and it's almost along the line-of-sight. In fact, this complicated geometry has really only been known in detail for the last two decades.

The fact that we see stars with differing metallicities having different velocity signatures is telling us something interesting. It is telling us that the formation of the Bulge was a complicated thing. I could not have formed just as a single mass of stars, from a single gas cloud (if it did, we would expect the stars to be very similar metallicities) but something else happened, something more complicated.

It can be hard to unravel just what the detailed processes were, but what we can do is try and compare various models to the observed data. Here's one such comparison;

Pretty good! I don't know about you, but I am always amazed when you take ideas (and equations) formed in peoples heads, and make predictions for what you should see in a mass of stars located many light years away, and it works!

The fit is not perfect, however - there are places where the data and model differ significantly - and it is clear that we cannot have the whole story yet. But the general picture of a Bulge composed of multiple populations and a bar, which transitions into the disk populations that we are part of, is correct. Now we need to workout the complicated structure hidden away in the Bulge, which will provide us with even more clues to its hidden history is underway. Can't wait to unravel the secrets!

Well done Melissa!

ARGOS IV: The Kinematics of the Milky Way Bulge

M. Ness, K. Freeman, E. Athanassoula, E. Wylie-de-Boer, J. Bland-Hawthorn, M. Asplund, G.F Lewis, D. Yong, R.R. Lane, L.L Kiss, R. Ibata

(Submitted on 26 Mar 2013)

We present the kinematic results from our ARGOS spectroscopic survey of the Galactic bulge of the Milky Way. Our aim is to understand the formation of the Galactic bulge. We examine the kinematics of about 17,400 stars in the bulge located within 3.5 kpc of the Galactic centre, identified from the 28,000 star ARGOS survey. We aim to determine if the formation of the bulge has been internally driven from disk instabilities as suggested by its boxy shape, or if mergers have played a significant role as expected from Lambda CDM simulations. From our velocity measurements across latitudes b = -5 deg, -7.5 deg and -10 deg we find the bulge to be a cylindrically rotating system that transitions smoothly out into the disk. Within the bulge, we find a kinematically distinct metal-poor population ([Fe/H] < -1.0) that is not rotating cylindrically. The 5% of our stars with [Fe/H] < -1.0 are a slowly rotating spheroidal population, which we believe are stars of the metal weak thick disk and halo which presently lie in the inner Galaxy. The kinematics of the two bulge components that we identified in ARGOS paper III (mean [Fe/H] = -0.25 and [Fe/H] = +0.15, respectively) demonstrate that they are likely to share a common formation origin and are distinct from the more metal poor populations of the thick disk and halo which are colocated inside the bulge. We do not exclude an underlying merger generated bulge component but our results favour bulge formation from instabilities in the early thin disk.