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.

Thursday, 29 November 2012

‘Overmassive’ black hole holds the mass of 17 billion suns

A very quick post tonight. I was interviewed to comment on the discovery of a very massive black hole, much more massive than we would expect from the galaxy in which it is found.

This is a big black hole. Say it slowly... 17 billion times more massive than the Sun. That is a lot of mass in a very small volume.

The article is presented in The Conversation. You can read it here, and, as ever, I am happy to address any questions in the comments spot below.

Black holes. You have to love them!

Friday, 23 November 2012

Dynamics in the satellite system of Triangulum: Is AndXXII a dwarf satellite of M33?

I am playing serious catch up here, as we've had a wee flurry of papers accepted in the last few weeks, and I want to make sure they all get a mention. The good news is that I should have plenty to post over the Twilight Zone of the Christmas/New Year Period.

Today's paper takes us back to M33, and a bit of a curly question. As we've seen over recent papers, we've seen that the larger Andromeda Galaxy has a large population of dwarf galaxy companions, at least about 30 of them buzzing around (although not randomly, something I will come back to in the New Year).

M33 is about a tenth the size of Andromeda, and so we have the question "Does M33 have any dwarf galaxies of its very own?" This is actually a little complicated because M33 is in orbit about Andromeda, and passed near the larger galaxy a few billion years ago. It will do so again in a few more billion years.

Here's a map of the dwarf galaxies that we have found in the PAndAS survey, taken from the excellent paper by Jenny Richardson.
We can see the circled black blobs - these are the dwarf galaxies. But look down in the bottom-left-hand corner, just below M33 there is a little dwarf galaxy, And XXII (kids, a piece of advice. When you start a new survey, using roman numerals might make you look a little smart, but once you have more than a handful of objects, they are a pain!).

Now, And XXII looks very close to M33, so perhaps it is actually orbiting M33 (which, in turn, is orbiting M31). But how would be know?

It all comes down to the relative distances and speeds of the various objects of interest. Clearly, if they are separated by a huge distance, then they are likely to have nothing to do with each other, and we have nothing but a chance projected alignment.

Even if they are physically close, if the dwarf galaxy is moving relatively slowly with respect to M33, it could well be in orbit, but if the speed is to fast, then And XXII could be whizzing along on its own orbit of Andromeda, and happens to be just passing M33.

So this is the subject of this new paper, Dynamics in the satellite system of Triangulum: Is AndXXII a dwarf satellite of M33?, by collaborator-of-mine, Scott Chapman. The paper is quite involved, but includes the measurements of the velocities of stars in And XXII, using the DEIMOS spectrograph on the mighty Keck Telescope.

Why do we need such a big telescope? Because these stars are faint! Between 21st and 23rd magnitude (and if you are not sure what that means, take a look at this, and remember that we are not just taking an image, but are dispersing the light to get a spectrum!).

So, what do we find? Well, And XXII's radial velocity is -130 km/s, quite close to the speed of of M33 itself, but is it a satellite?

As ever, the question is never as straight-forward as it seems, as it depends on how much dark matter is swirling around M33. But we know that M33 has been violently shaken in the past due to its close pass of M31, and its dark matter will have also been shaken and some stripped off.
Basically, yes And XXII is a candidate for the first dwarf galaxy of M33, but as M33 loses its dark matter halo, it is going to become less and less bound to the little galaxy and will end up orbiting M31! Maybe this is how Andromeda got lots of its dwarfs? This is a subject we will return to soon!

Well done Scott!

Dynamics in the satellite system of Triangulum: Is AndXXII a dwarf satellite of M33?

S. C. Chapman, L. Widrow, M. L. M. Collins, J. Dubinski, R. A. Ibata, J. Penarrubia, M. Rich, A. M. N. Ferguson, M. J. Irwin, G. F. Lewis, N. Martin, A. McConnachie, N. Tanvir
We present results from a spectroscopic survey of the dwarf spheroidal And XXII and the two extended clusters EC1 and EC2. These three objects are candidate satellites of the Triangulum galaxy, M33, which itself is likely a satellite of M31. We use the DEep Imaging Multi-Object Spectrograph mounted on the Keck-II telescope to derive radial velocities for candidate member stars of these objects and thereby identify the stars that are most likely actual members. Eleven most probable stellar members (of 13 candidates) are found for AndXXII. We obtain an upper limit of sigma_v < 6.0 km s-1 for the velocity dispersion of AndXXII, [Fe/H] ~ -1.6 for its metallicity, and 255pc for the Plummer radius of its projected density profile. We construct a colour magnitude diagram for AndXXII and identify both the red giant branch and the horizontal branch. The position of the latter is used to derive a heliocentric distance to And XXII of 853 pm 26 kpc. The combination of the radial velocity, distance, and angular position of AndXXII indicates that it is a strong candidate for being the first known satellite of M33 and one of the very few examples of a galactic satellite of a satellite. N-body simulations imply that this conclusion is unchanged even if M31 and M33 had a strong encounter in the past few Gyr. We test the hypothesis that the extended clusters highlight tidally stripped galaxies by searching for an excess cloud of halo-like stars in their vicinity. We find such a cloud for the case of EC1 but not EC2. The three objects imply a dynamical mass for M33 that is consistent with previous estimates.

Friday, 16 November 2012

Kinematics of the stellar halo and the mass distribution of the Milky Way using BHB stars

I've been back for about a week, but it has been busy with a number of things.

Firstly, success in the current round Australian Research Council Discovery Projects. I'll write about this in a little more detail soon, but here's the summary.
There has been some uncertainty in the current round of funding, but it has all come out in the wash.

But there is more news. When I was traveling, PhD student, Parjwal Kafle, had his paper on measuring the mass distribution in the Milky Way.

This might strike you as a little odd. We live in the Milky Way, and it is the most studied galaxy in the entire Universe. Surely we know the mass of our own home galaxy? Don't we just add up all the stars we can see?

Well, if the stars were all there is, that would be correct. But we know there is more, much much more! There is dark matter, this stuff that dominates the gravitational attraction of the Universe. Our Sun is kept in its orbit by the halo of dark matter that surrounds the Galaxy.
Now we can see the problem. As well as there being a lot of dark matter, it is also more distributed than the stars we can see.

Now, normally we use "kinematic tracers" (fancy words for the speeds of stars) to measure the amount of mass present (including dark matter), but out there in the halo, where there is a lot of mass, there doesn't appear to be any stars.

But, there are a smattering of stars out there, in what is known as the stellar halo, although they are far away, and so we can really see only the brightest ones.

There are still a couple of problems. The density of these halo stars on the sky is generally quite low, and there are a lot of "contaminants" - annoying stars nearby in the Galaxy that look like they are far away in the halo.

The secret is to look at a large amount of the sky, and in recent years we've gotten sensitive telescopes to do this. with one of the best being the Sloan Digital Sky Survey. I've not got a enough time to go through the details, but this program identified distant halo stars, namely stars on the Blue Horizontal Branch.

But this is just the data. How do you measure the mass? This is a tricky problem as we don't have that many tracers, and while we know the positions on the sky well, but the distances can be a uncertain, and while we might have the velocity of the star along the line of sight, we don't know its full 3d velocity.

The answer is not straight-forward, but essentially to have to try and generate a model of mass distribution of mass and stars and see if it matches the properties of stars on the sky.  In summary, this takes a lot of calculations (lots of Bayesian modeling). But what's the result? The mass of the Galaxy is found to be
This is a cool result! If you are not in the field, you may look at the uncertainties (the +0.5 and -0.4 in there) and say "You don't know this better than about 30%". But this is the state of play. This is how well we understand the amount of mass in the Milky Way.

To illustrate this, here's a picture of the rotation curve of the that Prajwal works out.
The red line is Prajwal's fit, but look at the data! Look at how noisy it is. Clearly, we still have a lot to learn about our Milky Way galaxy. We're working on it :)

Well done Prajwal!

Kinematics of the stellar halo and the mass distribution of the Milky Way using BHB stars

Prajwal R. Kafle, Sanjib Sharma, Geraint F. Lewis, Joss Bland-Hawthorn
Here we present a kinematic study of the Galactic halo out to a radius of $\sim$ 60 kpc, using 4664 blue horizontal branch (BHB) stars selected from the SDSS/SEGUE survey, to determine key dynamical properties. Using a maximum likelihood analysis, we determine the velocity dispersion profiles in spherical coordinates ($\sigma_{r}$, $\sigma_{\theta}$, $\sigma_{\phi}$) and the anisotropy profile ($\beta$). The radial velocity dispersion profile ($\sigma_{r}$) is measured out to a galactocentric radius of $r \sim 60$ kpc, but due to the lack of proper-motion information, $\sigma_{\theta}$, $\sigma_{\phi}$ and $\beta$ could only be derived directly out to $r \sim25$ kpc. From a starting value of $\beta\approx 0.5$ in the inner parts ($9<r/\kpc<12$), the profile falls sharply in the range $r \approx 13-18$ kpc, with a minimum value of $\beta=-1.2$ at $r=17$ kpc, rising sharply at larger radius. In the outer parts, in the range $25<r/\kpc<56$, we predict the profile to be roughly constant with a value of $\beta\approx 0.5$. The newly discovered kinematic anomalies are shown not to arise from halo substructures. We also studied the anisotropy profile of simulated stellar halos formed purely by accretion and found that they cannot reproduce the sharp dip seen in the data. From the Jeans equation, we compute the stellar rotation curve ($v_{\rm circ}$) of the Galaxy out to $r \sim 25$ kpc. The mass of the Galaxy within $r \lesssim 25$ kpc is determined to be $2.1 \times 10^{11}$ $M_{\sun}$, and with a 3-component fit to $v_{\rm circ}(r)$, we determine the virial mass of the Milky Way dark matter halo to be $M_{\rm vir} = 0.9 ^{+0.4}_{-0.3} \times 10^{12}$ $M_{\sun}$ ($R_{\rm vir} = 249^{+34}_{-31}$ kpc).

Sunday, 11 November 2012

On the good ship Volendam

I'm back! But where have I been? I've spent on 10 days on a cruise ship, the m/s Volendam. Here's what she looks like.
I wasn't just enjoying myself, I was there to lecture astronomy. The trip trundled from Darwin to Perth, calling in at the islands of Komodo and Lombok in Indonesia.

To pay my way, I had to give five lectures. I wasn't sure who the audience was (it turned out to be mainly people over 60, but from a range of nations), so I talked on
  • The Secret Lives of Galaxies
  • The Big and Small of Stars
  • How to fall into a Black Hole
  • Just what happened at the Start of the Universe
  • Dark Energy and the Long Term Future of the Universe
As well as me, there was also Victor Gostin of the University of Adelaide, who spoke on the geology and geophysics of South East Asia and Australia - did you know there was a now drowned continent called Sundaland (not to be confused with Sunderland!)? I didn't, and I think it was cool. Here's a map from wikipedia.
Anyway, I think that the lectures were well received. It was good fun, and people were very interested (it was hard to get more than a few feet at times without people asking questions). I would happily do it again.

There were a bunch who were not interested in talking tho, and they lived on the island of Komodo. No matter what you said, they never cracked a smile, and when they want to go for a walk, you just get out of their way. Here they are
They looked pretty big in the zoo in Sydney - they looked larger in the wild!

Anyway, astronomy has moved anon while I was traveling, so I have a few posts to catch up on. More soon.

Monday, 29 October 2012

Publishing Success!!!!

A very short post this week, as I am swamped - I will be away for almost 2 weeks and will explain in detail when I get back - but today was a good day in terms of my publishing.

Firstly, I got a letter accepted in the Sydney Morning Herald on science funding. Here's the letter.
But the bestest bit is that not only did I get the letter published (does this go on my CV), but I also got the letters' page cartoon dedicated to my letter.
I'm going to order a copy of the original cartoon to hang on my wall - I think it is excellent.

More when I have some more time.

Saturday, 20 October 2012

Unearthing Foundations of a Cosmic Cathedral: Searching the Stars for M33's Halo

The Stellar Halo of a galaxy is a tenuous population of stars and other things surrounding the the bright spiral disk that characterises galaxies like our own Milky Way. Some of the oldest, most pristine, stars that we know can be found in the halo of our own galaxy, and so they provide clues to the processes that bought large galaxies into existence.

The problem is that they are, well, tenuous. When we look for halo stars in our own Milky Way, we have to sift through a sea of nearby fainter stars, to pick out the giant stars that are far away.

I've mentioned before about my work with the Pan-Andromeda Archaeological Survey, which has been mapping out the stars of our nearest large neighbours, the Andromeda and Triangulum galaxies.

Now that all the data is in, we're dissecting the various stellar populations in the vicinity of these tow galaxies, especially with regards to the large amount of substructure (the left over remnants of disrupted systems). But also we want to measure the shape and extent of the stellar halos.

But it is hard work. These things are still faint, and we still have to trawl through the contamination from our galaxy. Although it's important to remember that we are not hear to do the easy things!

And this is precisely what Rob Cockcroft of McMaster University has done. For the first time, we have measured the density of stars in the halo!

So here is an example of what we have to deal with. We see all the stars towards Triangulum (M33), but in the lots are in our galaxy (Disk and Halo), as well as there being misclassified galaxies in there as well (when something is small and faint, without a spectrum, it is hard to tell things apart).

Once we have cleaned all the contamination out of the way, we can actually try to measure the density drop off of what is the stellar halo. And this is it!
The important curves are the dashed ones with the data points on there. You can see that there are large error-bars, but this is because the measurement is hard. If we could do this and get really small error-bars, then someone would have done it years ago with crappier data and gotten large error bars :)

So, M33 has a faint halo of old stars, and now we need to think a little about just what it is telling us about the formation and evolution of this little galaxy (it is about 10th the mass of the Milky Way).

Just to give you an idea of what we are dealing with, now we have measured it, we can calculate that the total brightness of the halo represents less than 1% of the light emitted by the entire galaxy!

A stirling result! Well done Rob!

Unearthing Foundations of a Cosmic Cathedral: Searching the Stars for M33's Halo

Robert Cockcroft, Alan W. McConnachie, William E. Harris, Rodrigo Ibata, Mike J. Irwin, Annette M. N. Ferguson, Mark A. Fardal, Arif Babul, Scott C. Chapman, Geraint F. Lewis, Nicolas F. Martin, Thomas H. Puzia
We use data from the Pan-Andromeda Archaeological Survey (PAndAS) to search for evidence of an extended halo component belonging to M33 (the Triangulum Galaxy). We identify a population of red giant branch (RGB) stars at large radii from M33's disk whose connection to the recently discovered extended "disk substructure" is ambiguous, and which may represent a "bona-fide" halo component. After first correcting for contamination from the Milky Way foreground population and misidentified background galaxies, we average the radial density of RGB candidate stars over circular annuli centered on the galaxy and away from the disk substructure. We find evidence of a low-luminosity, centrally concentrated component that is everywhere in our data fainter than mu_V ~ 33 mag arcsec^(-2). The scale length of this feature is not well constrained by our data, but it appears to be of order r_exp ~ 20 kpc; there is weak evidence to suggest it is not azimuthally symmetric. Inspection of the overall CMD for this region that specifically clips out the disk substructure reveals that this residual RGB population is consistent with an old population with a photometric metallicity of around [Fe/H] ~ -2 dex, but some residual contamination from the disk substructure appears to remain. We discuss the likelihood that our findings represent a bona-fide halo in M33, rather than extended emission from the disk substructure. We interpret our findings in terms of an upper limit to M33's halo that is a few percent of its total luminosity, although its actual luminosity is likely much less.

Saturday, 13 October 2012

Catching the bus.....

OK, it's been a very good week (for reasons that will become clearer in the near future) and so I am going to take a breather for half an hour for a little recreational mathematics. The question is all about catching the bus.

One good thing about living in Sydney, which I've noted before, is that it easy to get to see international rugby at the Olympic Park. An excellent free bus service is provided to bring people in from the far-reaches of Sydney, and then take them home again.
It is quite impressive that it works, with tens of thousands of people pouring out of the grounds and onto buses quite efficiently.

So, I've been thinking - If people turn up at a bus stop at a certain rate, and buses arrive at a certain rate, then what do we expect the number of people on each bus to be?
OK, the question is easy if the people arrive at a fixed regular intervals, as do the buses. But we are not here to do the easy things.

Being a physicist, we start by simplifying the problem. Let's assume that the average number of people who turn-up per hour is λ1, and the average number of buses per hour that turn up is λ2. Let's also assume that the bus instantaneously picks up all the passengers who are waiting, and then heads off.

Imagine you are on the gate of the park, watching the buses leave. What's the distribution for the number of passengers on each bus?

The problem requires two parts, both based on the Poisson distribution. I mentioned this in the recent discussion on the German tank problem, but it is an extremely powerful description of random events, from the drops coming from a tap, the number of photons arriving at a detector, and, quite famously, the number of men kicked to death by horses in the Prussian cavalry.

The other side of the coin is that you can use this distribution to calculate the time between events, which is what we want to use to describe the time between buses arriving. It turns out that this is an exponential distribution, and the probability of a bus arriving between t and t+dt after the last one is

This means that there are lots of short gaps, and fewer long gaps between buses arriving. Now, real buses don't follow such a distribution in detail, but let's stick with this because the maths gets more funnerer.

Right, the next question is how many people accumulate at the bus stop between buses? It's over to the Poisson distribution again. In a time, t, the distribution for the number of people at the bus stop is given by

Why is there a distribution? Well, people are dribbling in at random and in the same time interval there might be one person, or two, or ten or even none.

OK, we cans stick the two equations together and ask the question "what is the distribution of number of people leaving on the buses?" What we end up with is an expression that looks like this.

Why is there an integral in there? Well, multiplying the two probabilities gives us the distribution of the number of people on a bus, after a waiting time between t and t+dt. A little algebra, this becomes

What is that? The final probability distribution depends upon a rather mysterious quantity called the Gamma function. I could write a long post on the gamma function, but seeing that we are only interested in integer values (because we can only have whole numbers of passengers on the bus), then we know that

and so the distribution for the number of passengers on the bus becomes
Isn't that lovely? Well, I think so. So, let's take a simple case. Let's assume that the average number of buses arriving per hour is the same as the average number of passengers arriving. As you can imagine, the number of people on the bus will fluctuate, with some buses having 1, or 2, and we should expect a number of buses should have no people on there at all.

The distribution of the number of passengers becomes
Cool huh? Let's ask the question, what is the probability that a bus has no passengers? Popping n=0 into the above, the chance of this is 1/2. And the chance that a bus has one person on it is 1/4, and 2 people is 1/8.

In fact, we have a pretty interesting progression here, and it has been known from antiquity that 1/2+1/4+1/8+.... = 1, which is what you want all of the probabilities to add up to.

Let's consider a more realistic scenario, with 1000 people per hour arriving at the bus stop, and 20 buses per hour. What does the distribution of bus occupancy look like? You might think that it is most likely that there are about 50 people on each bus. Here's the distribution
The most likely number of people on a bus is zero! And lots of the buses have only a hand full of people of them, while some are jam-packed with more than 150 on the bus - Standing room only!

You might think that chucking more buses per hour is a solution to the problem, but we always have this shape. The most likely number of people on a bus is zero. Throwing less buses doesn't help either, the most probable number of passengers is zero. Luckily this is not how real buses at Sydney Olympic Park operate, or we would never get home!

One last question then, as I am cooking pizzas tonight and need to get started. Let's imagine that you are not the gate guards, but a passenger, just a random passenger. What's the most likely number of passengers on the bus (including you)?

While the most probable number of passengers on a bus is zero, there is no one there to see that. And while there are lots of buses with 1 person on, it is actually unlikely to be you (I know this might sound a little paradoxical, but think about it).

So, we effectively have to weight each number of passengers probability but the number of passengers there to see it. What does that do?
This is the case that we looked at above, with 1000 passengers per hour, and 20 corresponding buses. Most people will report that they were comfortably on a bus with a total of 50 passengers, while only a few will report they were on really empty or really full buses, even though the guards on the gate say that many buses were leaving with virtually no one on them, and a few very very crowded.

And while you are chewing that over, I'm off to do some cooking.