Monday, 31 December 2012

Young accreted globular clusters in the outer halo of M31

Due to the Twilight Zone between Christmas and New Year, I am losing the catch-up game, but have just spent a rather wonderful couple of days up at Barrington Tops, camping in the wilderness and being "off the grid"; while I love camping, being off the grid is a rare situation for me. But that's not the story.

It's time to look at something I have written about before, and that is where did the globular clusters we see orbiting the Andromeda Galaxy come from? And as you might guess, this is another result drawn from the Pan-Andromeda Archaeological Survey (PAndAS).

This work is led by Dougal Mackey who is now at The Mount Stromlo Observatory in Canberra. Now, there are lots of globulars in Andromeda, most of them found close to the galaxy that is plain to see. What we have been doing in PAndAS is looking for the distant globulars, those far out in the faint stellar halo of the galaxy. Here's a picture from the paper
The underlying grey-scale is the density of stars we see in PAndAS, with Andromeda itself located in the middle of the mush on the left-hand side of the picture. You can also see lots of streams and shreds and bits and pieces, the detritus left over from cannibalised dwarf galaxies that have ventured too close.

The red dots are the globulars. As you can see, there are lots close to the actual Andromeda galaxy, but there are also lots scattered about at much larger distances. So, the question we have is are these globulars randomly scattered through the halo, or are they associated with the stellar substructure we see? If it is the latter, then that is exciting because it means that these globular were brought in with the dwarfs that were ripped apart, and they are in fact interlopers, immigrant globulars from other galaxies.

The focus of this paper was just two of the globulars, labelled PA-7 and PA-8 (see, astronomers are not boring at naming things). We used the Gemini telescope to firstly image the globulars, and here;s what they looked like.
Cute eh? The red lines indicate where the slit for the Gemini Multi-Object Spectrographs (GMOS) to collect the light and disperse it to get the spectra. Now, remember that these objects are faint, and even with the 8-metre Gemini mirrors, we still need to integrate (i.e. expose to collect light) for an hour and a half to get a decent spectrum.

Now, you could write a book on what you can learn from the photometry and spectra of stars (and there are many books that exist doing just this), but clearly the abundance of chemicals is a key thing. A lot of work over the years has gone into understanding stellar evolution, and what we know is that a population of stars (i.e. a mix of stars with a range of masses, but, in this case, all of the same age) will have a particular Colour-Mangitude Diagram based on the age of the population. There are subtleties based upon the chemical mix in the stars, but we know the chemical make-up from the spectra.

So, what do we find? Here's the key figure
The pairs of lines are for pairs of ages for the stellar populations in the globulars, the pinky-purple being young (less than 4 billion years old) and the red being old (roughly 12 billion years old). As you can see, the actual density of stars in there is quite sparse, but from where they lie, it does look like this is a pair of young clusters, with ages less than 6 billion years (and probably closer to a few billion years).

As we saw in the opening picture, these two globulars are quite close on the sky, and they are both young, so are they related? Remember, one other thing we can measure from the spectra is the velocities of the globulars. If the velocities are hugely different, then these globulars are like ships passing in the night, and we just catch them at the right moment.

What we see is that PA-7 is moving at -433 km/s, and PA-8 is moving at -411 km/s (with uncertainties of less than 8 km/s); for reference, the Andromeda galaxy is moving at -300 km/s. So, these globulars have almost the same velocity! It looks like they are moving together through the halo of Andromeda.

But there's more! Remember at the start we saw that these two globulars are sitting on a smush of stars, the remnant of a disrupted dwarf. We haven't got the velocities of the stars in this blob (yet!) but will (eventually!), but the overlap is more than coincidence. The stars and the globulars are part of the same system, something that has fallen in and is being disrupted. PA-7 and PA-8 are newly arrived immigrants to the globular cluster system of Andromeda. How cool is that?

Well done Dougal!!

Young accreted globular clusters in the outer halo of M31

A.D. Mackey, A.P. Huxor, A.M.N. Ferguson, M.J. Irwin, J. Veljanoski, A.W. McConnachie, R.A. Ibata, G.F. Lewis, N.R. Tanvir
We report on Gemini/GMOS observations of two newly discovered globular clusters in the outskirts of M31. These objects, PAndAS-7 and PAndAS-8, lie at a galactocentric radius of ~87 kpc and are projected, with separation ~19 kpc, onto a field halo substructure known as the South-West Cloud. We measure radial velocities for the two clusters which confirm that they are almost certainly physically associated with this feature. Colour-magnitude diagrams reveal strikingly short, exclusively red horizontal branches in both PA-7 and PA-8; both also have photometric [Fe/H] = -1.35 +/- 0.15. At this metallicity, the morphology of the horizontal branch is maximally sensitive to age, and we use the distinctive configurations seen in PA-7 and PA-8 to demonstrate that both objects are very likely to be at least 2 Gyr younger than the oldest Milky Way globular clusters. Our observations provide strong evidence for young globular clusters being accreted into the remote outer regions of M31 in a manner entirely consistent with the established picture for the Milky Way, and add credence to the idea that similar processes play a central role in determining the composition of globular cluster systems in large spiral galaxies in general.

Thursday, 27 December 2012

Star talk slows chat

More alternative publishing success. This letter appeared in the Sydney Morning Herald just before Christmas
Being an astronomer and lecturer in relativity, I had to respond, and my letter was published today. Here it is
I wonder if this counts as the elusive "impact" that scientists will be judged on in the future? Anyway, the keen-eyed will see that a typo has crept into the text, as Tau Ceti is almost 12 light years away, not 120. I am sure there will be some smart-alec follow-up letters :)

Wednesday, 26 December 2012

Decelerating the Universe

It's Christmas day here in Sydney, and the rain has fallen incessantly; it feels like when I was growing up in Wales.

Anyway, I thought a short post. Over at In The Dark, Peter Coles was talking about the new results from the WMAP satellite, the latest of the probes of the Cosmic Microwave Background; I've mentioned before that I am old enough to remember the results from COBE, especially this front page story.
Basically, Peter's article points out that these observations have radically changed your understanding of the Universe. Mind you, looking at the above newspaper, and seeing "Bosnia ceasefire crumbles", tells me that in the same period, humans haven't advanced very far in the same period.

Anyway, in the comments, there was an interesting issue raised by Phillip Helbig (hi Phil!). Phillip and I are of the same vintage, and were raised on a diet of VMS before Unix became all pervasive in astronomy. Essentially, it is to do with the coincidence problem.

This is a simple problem to understand. Over cosmic history, as the universe has expanded, it was previously dominated by matter, but the density of matter has continued to thin out as the Universe got larger. However, the energy density of the other major component in the Universe, namely dark energy, remains constant even though the Universe expands.

So, in the past, the Universe was "matter-dominated", and in the future it will be "dark energy-dominated". Here's a piccy to illustrate this
The solid-line is the density of matter, and the dotted-line is the density of dark energy (normalized to the universal "Critical Density"). It's important to understand the time-scale on the x-axis, as it is logarithmic (rather than linear), so it includes everything from the tiny-time-scales of inflation, to the extensive time scale into the distant Universe. Look how rapidly (on this time axis) the change from matter dominated to dark energy dominated is.

The weird thing is that we currently find ourselves at a time where roughly a quarter of the energy density of the Universe is in matter and the other three-quarters is dark energy - so we find ourselves in the very narrow change over. We have a coincidence!!

So, where is my part in this story? Well, it's in a paper called "Through the Looking Glass: Why the "Cosmic Horizon" is not a horizon", but then MSc student, Pim van Oirschot, and then PhD student, Juliana Kwan (both of whom have moved on to better and brighter things).

The focus of the paper was not on cosmic coincidences, but we found one. We were looking at something called the deceleration parameter (normally called q).

How is q defined? It a combination of things that characterise the expansion of the Universe, its size, velocity and acceleration (for those in the know, these are the zeroth, first and second derivatives of the expansion factor with respect to cosmic time).

This is easy to understand - if the expansion of the universe is constant, then q is zero. If, however, the universe is decelerating, then q is positive. And vice-versa for acceleration.

Now, we know that during the time that dark matter dominated, then the Universe was decelerating, but now the Universe is becoming dominated by dark energy, it is accelerating. So, the question is, what is the average of the deceleration and then acceleration over cosmic history?

This is one of the things we calculated in the paper. Basically, we took the cosmic parameters as we have currently measured them and calculated the time averaged value of q. Here's what we got
So, these have the same matter and dark energy densities noted above. Along the bottom is time, where zero is today, positive is into the future, and negative is into the past. The different coloured lines are for different equations of state of dark energy (the cosmological constant is the red line with w=-1).

Let's look first at the dashed lines, as these are the normalized densities of matter in each of these cosmologies (right-hand y-axis). In the past, in all of them, the Universe was more matter dominated, and into the future the matter density continues to diminish. But notice how on this linear scale, the "coincidence" of the matter density being not to dissimilar to one is no longer so spectacular.

Now check out the solid curves (left-hand y-axis) which presented the time averaged value of q. In the past, where matter was dominating, q was positive and so the time averaged value of q was positive.

As the Universe expands and dark energy comes to dominate, then the expansion starts to accelerate, and so q becomes smaller, and (for some of the models) becomes negative. For these, there is a switch in the time averaged value of q from positive to negative. For the black model, it was in the past. For the green model it appears to be in the future. But for the red model, the cosmological constant model, it appears to be - eerrrm - right now!

If we plug in the values of the cosmological parameters as we have now measured them, and included their uncertainties, how big do we expect the time averaged value of q to be? Well -
To within the uncertainty, it is zero. The switch over is right now!

What's the significance of this? I don't know. Some think that it's a fundamental property of the Universe, telling us something deep about the cosmos that we currently don't really understand. Others think that essentially its numerology, and if you play around with enough numbers, some combinations will surprisingly be close to zero, or pi, or one, or ten etc.

But, we now have the new numbers from WMAP, telling us the parameters of the Cosmos. These have changed slightly from the last numbers, and the errors have become tighter. The question is, has the time averaged value of q changed significantly with the new numbers, and has it become even closer to zero? If it has, it is interesting.

I will get to this, but being Christmas it may not be until the New Year. Something to think about during the Twilight Zone that this week is.

Have a Merry Christmas and Happy New Year :)



Thursday, 20 December 2012

First Science with SAMI: A Serendipitously Discovered Galactic Wind in ESO 185-G031

Still playing catch up with papers, research, and just about everything at the moment. Even though my university has closed down for the Christmas break, work has not stopped (or even really slowed down that much), but I will try and fire off a few quick posts to at least get the papers out of the way. I also don't have too much time this morning, as I have to head to The Opera House to see this
Just for the kids of course!

I've written about the Sydney AAO Multi-Object Integral FIeld Spectrograph (SAMI) previously. Here's a bit on it that I pinched from James Allen's webpage on it.
The concept is simple (but the implementation is not). In the old days, we would do single-slit spectroscopy, where we would collect light from a distant galaxy through a narrow slit, and pass that through a disperser to get a spectrum. This would give us the details of what was going on along that slit (so we could measure rotation curves).

But this is slow. You can only look at one galaxy at a time. So, there was a move to multi-fibre spectroscopy, where you plonk a fibre on the galaxy and collect the light from that. You get a spectrum of the galaxy, but there is no longer any spatial information. But you can still do lots of things. One of the most successful multi-fibre spectrographs is 2dF/AAOmega and the resulting 2dF Galaxy Redshift Survey.

SAMI overcomes the limitation of simple multi-fibres by using integral-field units to collect light from different parts of a galaxy. This is effectively a bundle of fibres which gives you a spectrum at a large number of points over a galaxy, allowing you to measure the velocities, chemistry, star formation etc at each of the points.

The second paper using SAMI data was recently accepted, with lead author Lisa Fogarty. It was targeting a specific galaxy, the spiffily named ESO 185-G031. Here's that it looks like

It's quite clearly a close-to edge on spiral galaxy. The red circle shows how big the SAMI fibre bundle is.  So, we can collect the spectra at the points within the red circle, and measure the emission we get for a range of elements in there. Here's maps of some of them.
Look at how uneven some of that emission is! Remember, single fibres don't see all of the details of this, but just collect a single measurement of the strength of each of the elements.

What we do with these is a little complex, but generally you plot the strength on one element against another. Here as some of these (lovingly known as diagnostic plots).

 So, with a single fibre, this galaxy would be a single point on each of those diagnostic plots. One dot. And based upon where it sits on there, it would be classified as star forming, dominated by an active nucleus (an accreting black hole) or shock powered (thats the green, red and blue colour-coding in the picture). What we find is that the ratio of the elements in each of the fibres is different, and that different parts of the galaxy are dominated by differing physical processes, with the disk parts dominated by star formation, but with shocks and AGN-like activity. How cool is that!!

Basically, what we are seeing is a wind being driven off the disk, a wind driven by the star formation activity inside the disk. The Universe - you've got to love it!!!

I will say it again. Single fibres don't see any of this detail, they just see an integrated smoosh of light. Thousands of galaxies have been classified based upon where their integrated light plops them on the diagnostic plots, but they are probably much more complex, messy objects. With SAMI, we are seeing a real shift in how we will understand the working of galaxies. Cool!

Well done Lisa :)


We present the first scientific results from the Sydney-AAO Multi-Object IFS (SAMI) at the Anglo-Australian Telescope. This unique instrument deploys 13 fused fibre bundles (hexabundles) across a one-degree field of view allowing simultaneous spatially-resolved spectroscopy of 13 galaxies. During the first SAMI commissioning run, targeting a single galaxy field, one object (ESO 185-G031) was found to have extended minor axis emission with ionisation and kinematic properties consistent with a large-scale galactic wind. The importance of this result is two-fold: (i) fibre bundle spectrographs are able to identify low-surface brightness emission arising from extranuclear activity; (ii) such activity may be more common than presently assumed because conventional multi-object spectrographs use single-aperture fibres and spectra from these are nearly always dominated by nuclear emission. These early results demonstrate the extraordinary potential of multi-object hexabundle spectroscopy in future galaxy surveys.

Thursday, 13 December 2012

Paratrooper Speakers

Given that the Christmas period is upon us, but the rate of things to do has not declined, and people keep asking me if I have completed my Christmas shopping (no! and in truth, I haven't even started) time for a Scrooge-like post.

The following may upset my colleagues, so I will lay my card on the table. I am as guilty as the next person, but I am trying very hard not to be. I no longer want to be a Paratrooper Speaker. And I think my colleague should do the same.
Conferences, meetings and workshops have long been seen as an important aspect in astronomy (and a lot of academia). The chance to get together, hear some great talks discuss some good science, meet people and network. A chance for Early Career Researchers to meet some of those names on papers, and see just what is going on.

I am going to reminisce about the good old days, about conferences I went to when I was a student. Once, I got invited, after the conference dinner, to sit at the table with Geoffrey BurbidgeMartin Rees, and (I think - although this was a long time ago) Jerry Ostriker. The names do not matter, but I was sitting at a table with the greats in the field, who were having a roving, robust discussion. It was terrifying (for me as a student), but great.

But I've noticed something happening over the years, something that has been bothering me more and more. I know we are all getting busier and busier, and our time conferences are squeezed by committees, administration and just other things.

So, some (but not all!!) senior researchers have been spending less and less time at meetings. Don't get me wrong, they go to more and more actual meetings, and spend less and less time at each meeting.

Some might be too busy to spend an entire week at a meeting, and so they come in for a couple of days (ensuring they get their talk in), and then they are off again.
Some are so busy, they will come in for a day (the day of their talk!), and then be gone.

But now, what I see more and more, is people arriving in town only for their session, a few hours, and then they are gone.

Well, to be blunt, "Don't be so bloody rude!"

While you may think that the message you are putting out is "I'm a busy person, so be pleased that I can spend some time with you to share my brilliance", what you are really saying is "What I talk about is important and I don't really need to listen to what you have to say - information flows one way, from me to you, now I am out of here - Bazzinga".

I pointed out at the start that I have gotten into this mode, and am trying hard to change it, and properly take part in meetings. When I sign-up for a conference or meeting, I try to spend the entire time at the meeting, network, whinge at colleagues, chat with Early Career Researchers, and hopefully become one of the terrifying old guard that I sat with when I was a student.

I know those experiences had a profound impact on me. I hope the best and brightest are not making themselves so aloof and fly-by-night that the younger generation do not really get a chance to interact with them.

I hope some of my fellow senior researchers will think about their approach to attending meetings, and stop being paratrooper speakers.

Gump over. Have a Merry Christmas!

Sunday, 9 December 2012

PAndAS in the mist: The stellar and gaseous mass within the halos of M31 and M33

Time for another PAndAS papers (and yes, there are more to come!), but this one is from me :)

As you know, the Pan-Andromeda Archaeological Survey (PAndAS) is a big program we've been doing on the Canada-France-Hawaii Telescope, mapping out the locations of all the stars in the halo of Andromeda (M31) and Triangulum (M33) galaxies.

It's taken a lot of work, but with all the data now in, it's time to squeeze out the science. There is actually a bit of a hurry about this, as in the next year, we are making all of the data public, and even you can write PAndAS papers.

What's this current paper about? Well, let's start with the first picture in the paper.
So, the dense bits are M31 in the topish-right, and M33 in the bottom-left. The grey-scale is the density of stars in the map, and the regions outlined are the various bits of substructure orbiting around. Basically, there is a lot of stuff there, lots of streams, chunks, bits and pieces. Clearly, there has been a lot going on.

The question we were asking was "Where's the gas?" What I mean by that is that if we assume that the infalling little galaxies, that end up as all the stellar shreds, contain gas as well as stars, then where does the gas end up?

Now, you might simply say "Isn't it in the same place as the stars?" It would be, but the forces acting on the gas are different. Gas when it hits gas results in Ram Pressure Stripping, and so the different forces can drive the gas into different locations.

So, what do we see? Here's the stars with the gas on top (you'll have to read the paper on how we got the gas densities, but a lot of work by radio astronomer extraordinaire Robert Braun).

The numbers on there are the velocities of the gas. We do have some velocities of stars, but in very sparse points as (we've noted before) we need to use telescopes like Keck to get the spectra of individual stars.

While you can see there is lots of gas on the galaxies themselves (both M31 and M33 are spirals and have gas zipping about), but what about the streams and bits and blobs? Hmmm - nothing too obvious.

We can zoom in on the individual galaxies. Here's M31
There are two images as the radio in each panel was obtained with different telescopes, and so has a different resolution. Again, gas in the main disk, bits and pieces around, but not a lot of gas where the stars are outside of the main disk. There is an intriguing bunch of clouds moving very fast (around -500 km/s) where the giant stellar stream is crashing into the disk. Well, roughly in the same place.

What about M33? That's even more interesting.

Clearly, M33 has a whip of stars coming off it, probably due to an interaction with M31. There's also gas whipped off as well, but notice it's not in the same place as the stars.

So, what do we conclude? Well, the stars and gas are in different place, and that there must be different forces at work on the stars as compared to the gas. There must be a halo of gas surrounding the big galaxies which is driving the gas in a different direction to the stars. Exactly how much gas there is, and how much is it interacting with the material pulled from infalling galaxies is still an open question, but we're working on it. It's going to take some cool simulations on big computers, but we're working on it.

In the meantime, well done .... errrm ... me :)

PAndAS in the mist: The stellar and gaseous mass within the halos of M31 and M33


Large scale surveys of the prominent members of the Local Group have provided compelling evidence for the hierarchical formation of massive galaxies, revealing a wealth of substructure that is thought to be the debris from ancient and on-going accretion events. In this paper, we compare two extant surveys of the M31-M33 subgroup of galaxies; the Pan-Andromeda Archaeological Survey (PAndAS) of the stellar structure, and a combination of observations of the HI gaseous content, detected at 21cm. Our key finding is a marked lack of spatial correlation between these two components on all scales, with only a few potential overlaps between stars and gas.The paucity of spatial correlation significantly restricts the analysis of kinematic correlations, although there does appear to the HI kinematically associated with the Giant Stellar Stream where it passes the disk of M31. These results demonstrate that that different processes must significantly influence the dynamical evolution of the stellar and HI components of substructures, such as ram pressure driving gas away from a purely gravitational path. Detailed modelling of the offset between the stellar and gaseous substructure will provide a determination of the properties of the gaseous halo of M31 and M33.
PS - in closing, the Hume Highway, the main road from Sydney to Canberra (well, it by-passes Canberra and heads to Melbourne), is being renamed to Motorway 31 - or M31! Excellent.

Thursday, 6 December 2012

Alert: you may be living in a simulated universe

Today is a busy day, with the allocation of supercomputer time though ASTAC and the announcement of the 2012 Excellence in Research for Australia, which always leads to robust "discussion", and board of examiners meetings. So, only time for a quick post.

A new article in The Conversation on the controversial topic of "Are we living in a Synthetic Universe?". It's called Alert: you may be living in a simulated universe.

Given the comments, I could have alternatively called it "Baiting philosophers is easy and fun", but have a read yourself and let me know what you think.

Sunday, 2 December 2012

Misunderstanding Peer Review

Sunday morning. I've got a busy week coming up, having to head to Canberra tomorrow for my induction into the Australian Research Council (ARC) College, then a few board of examiners meetings, the allocation of supercomputer time, and then Christmas drinkies on the Botany Lawn.


So a quick post on peer review.

There has been an awful lot written on this, and I will focus on academic peer review, especially when it comes to publications. I read this interesting article on peer review, which points out that the press does not seem to understand the difference between a published article, and things written in popular science magazines, on blogs, or simply chatted about down the pub.

But before I start, let me remind you how a paper gets published in a journal. 

It starts by you having your idea, doing your work, and deciding that the results are significant enough to be interesting to everyone else. 

You need to decide where you are going to publish it. Journals are tiered. There are top tier journals (for astronomers, these are Nature and Science), then there are the middle tier, which are the main journals (again, for astronomers, these are The Astrophysical JournalAstronomical JournalMonthly Notices of the Royal Astronomical Society and Astronomy & Astrophysics). There there are other journals (but I won't name them :) I'll tell you what sets the journal tiers a little later.

You bundle you paper into the appropriate format for your journal of choice and send it off. The journal receives the paper and an editor takes a quick look. The editor will then make a choice. Is the science of enough importance to publish? Is it appropriate for the journal? If not, it gets rejected.

If it is appropriate, then it goes out to the referees. This is the part that people typically see as peer review.

The referees are generally experts in the field, and typically review the paper for free. Many find this a bit weird, why would anyone do the work for the journal for free?
Honestly, I don't really know. It's just one of things expected of academics. I got my first paper to referee as a student, and was chuffed that someone thought I was enough of an expert to examine someone elses work. And so it be began, and I now referee around 8-10 papers per year.

The referee may not be as impressed as you about the importance of your work, or might find a flaw in your reasoning or approach. They could recommend that you paper be rejected on these grounds.

But they might like it, and recommend publication. Then your paper is officially "accepted" and will appear in the journal. Yay!

This is the part that people have a problem with. They argue that as only a few eyes have scanned the paper before it was accepted, it could still be flawed or wrong. Peer review is a failure, they cry.

Here's my take on the situation, and I will put it in bold as I think it is very important: Peer Review Does Not Stop Once You Paper Has Been Accepted. In Fact, It Has Only Just Begun!

You might say "Huh?" to this. To professionals, it is not the fact that your paper has been accepted that is important, it is how it is received my the community. What you want is for your work to me noticed, and more importantly, cited. This is why there is a growing obsession with bibliometrics, not only with individual researchers, but also with governments. 

The new buzz word is impact. Not only does your science have to be excellent, it has to be noticed, it has to be cited, it has to be used, it has to have impact.

And impact is also related to where you paper is published, and it is generally thought that if you can get your article into journals with a high impact factor, it clearly must be good science (this is what sets the tiers of journals). This is not a universally held view.

So, the referee process is nothing part of the first step of the journey of your paper. You might publish it and it may sink like a stone, unnoticed by the community. These sunken papers may reappear 10, 50 or 100 years in the future as a new direction in science, unrealised during your life, comes to fruition. 

But what scientists want is for their research to be noticed, and this is nothing but a continuous review by your peers. 


Saturday, 1 December 2012

Young accreted globular clusters in the outer halo of M31

I am still playing catch up on the papers I've recently had accepted, and after this one there are two more that I have to write up.

Today's paper is lead by Dougal Mackey at Mount Stromlo Observatory in Canberra (officially, its name is now The Research School of Astronomy and Astrophysics (RSAA), but it will always be Mount Stromlo to me).

The paper is yet another from the Pan-Andromeda Archaeological Survey (PAndAS), and so you know will be a cool result :). Dougal's expertise is globular clusters, balls of a few million stars that orbit large galaxies.

If we look at a nice globular in our own galaxy, it looks like this.
How beautiful is that?

However, the ones in the PAndAS images are 2 million light years away, and look something like this
Yes, those little balls of stars in the middle. Maybe a little less spectacular, but still extremely interesting. Why?

Well, the question we have is "Where do globular clusters come from?" Unlike other groups of stars we see out there, globulars don't appear to reside in a big halo of dark matter, and so it is a case of "What you see is what you get". As I've written about before, this makes them an ideal place to to test theories of gravity.

We do know that there are hundreds of globulars orbiting the Andromeda galaxy, more than twice as many as our own Milky Way galaxy. Why?

One of the things that PAndAS has really allowed us to do is take a panoramic view of where all the globulars are, especially in relation to all shreds of disrupted galaxies that we are seeing in the halo of Andromeda.  Here's what we see:
The red dots are the globular clusters surrounding the Andromeda galaxy (which lies at at the centre of the green circle). You can see that the where there are stars outside of the galaxy (the grey-scale underneath) there are generally more globular clusters. This suggests that the globulars were brought in with the little galaxies that were torn apart to make the substructure. Cool.

For two of these globulars, PA-7 and PA-8 (ain't astronomers boring when it comes to naming things) we got long-slit spectroscopy with the GMOS on the Gemini Telescope (the red lines on the figures above are the orientation of the slits), and, using the Doppler effect we are able to measure their velocities (if you want to know, -433 and -411 km/s respectively), showing that they are moving together (and reinforcing the possibility that they are associated with the underlying smudge of stars).

The really cool thing though is the ages of the globulars. Generally, they are old, many as old as the Universe itself. But these guys are a few billion years younger than the other globular we know orbiting Andromeda. Why? We don't know. But maybe they were born over late in the Universe due to the environment they found themselves in? Maybe they were born in the interaction with Andromeda itself? Either way, these are strange puppies, and I am sure there is more to come!

Well done, Dougal.

Young accreted globular clusters in the outer halo of M31


A.D. Mackey, A.P. Huxor, A.M.N. Ferguson, M.J. Irwin, J. Veljanoski, A.W. McConnachie, R.A. Ibata, G.F. Lewis, N.R. Tanvir
We report on Gemini/GMOS observations of two newly discovered globular clusters in the outskirts of M31. These objects, PAndAS-7 and PAndAS-8, lie at a galactocentric radius of ~87 kpc and are projected, with separation ~19 kpc, onto a field halo substructure known as the South-West Cloud. We measure radial velocities for the two clusters which confirm that they are almost certainly physically associated with this feature. Colour-magnitude diagrams reveal strikingly short, exclusively red horizontal branches in both PA-7 and PA-8; both also have photometric [Fe/H] = -1.35 +/- 0.15. At this metallicity, the morphology of the horizontal branch is maximally sensitive to age, and we use the distinctive configurations seen in PA-7 and PA-8 to demonstrate that both objects are very likely to be at least 2 Gyr younger than the oldest Milky Way globular clusters. Our observations provide strong evidence for young globular clusters being accreted into the remote outer regions of M31 in a manner entirely consistent with the established picture for the Milky Way, and add credence to the idea that similar processes play a central role in determining the composition of globular cluster systems in large spiral galaxies in general.