Saturday, 28 June 2014

The Nature and Origin of Substructure in the Outskirts of M31 -- II. Detailed Star Formation Histories

I am still playing catch up on papers, and I've just woken up early here in San Francisco and have a small amount of time before I have to prepare for my talk today. So, this will be quick.

The topic again is our nearest largest companion, the Andromeda Galaxy, especially working out the history of how stars have formed in the (relatively) inner regions of the galaxy. It might seem a little strange that we can work that out, because all we can see is stars, but with the magic of science, it is possible. That's the topic of this new paper by postdoctoral researcher, Edouard Bernard.

This beautiful science is done with the Hubble Space Telescope. The first thing you need to do is decide where to look. So, here's the fields we looked at
One of the sad things is that the area Hubble can image (its field of view) is tiny compared to the extent of Andromeda, and so we are doing key-hole surgery in select areas on interesting bits of Andromeda, especially prominent bits of substructure scattered about.

We image each of the fields in two colour bands, a blue(ish) one and a red(ish) one, and once you have this you can construct a colour-magnitude diagram. But how do you work out the star formation history?

Well, every new star that is born lives initially on the main sequence, but massive stars live on there for a relatively short time, and little stars sit on there for a long time. So, if you create a bunch of stars at the same time, they are all on the main sequence, but as you wait, the massive ones move off first, and then the lesser massive ones etc. In fact, you can tell the age by looking at the mass of stars now moving off the main sequence, something called the main sequence turn-off (rather imaginatively). Here's a nice picture from the Lick Observatory


If you want to have a look at evolutionary tracks in details, have a look at the Padova isochrones.

So, every burst of stars gives you a new population on the main sequence, and then after time they start to move off, and if you imagine this happening over and over again, you get a complex mess in the colour-magnitude diagram. And, with hard work, you can invert this. Here's a picture from the paper
The colour magnitude diagram is in the upper right; if you are an amateur astronomer and understand the magnitude scale, check out the numbers on the side. This tells you about the power of Hubble!

The other panels give the star formation rate (SFR) in the upper left, and metallicity (chemical enhancement) in the bottom right.

So, what do we find? Fields were identified as being disk-like, being part of the main body of Andromeda, stream-like, so they look like they are associated with the giant tidal stream in Andromeda, and composite, which are, well, more complicated.

Here's the cumulative star formation history for the various fields, but it should be clear that stars in the disk-like fields (blue) formed more recently than those in the other fields. But why?
Here's the actual histories, which shows how much mass in stars is formed as a function of time.
Now, they look similar, but the disk-like distribution is skewed to lower times, again showing that more stars formed more recently. 

Argh! I'm running out of time, but could go on for ages, but we are scratching the surface. Essentially, but clearly we have on going star formation in the galaxy disk, making a lot recently, where as the stars in the giant stream (which was formed a while ago and then fell into Andromeda) are somewhat older.

But the star formation histories are not smooth, all with a broad peak in their earlier history, but rather curiously possessing a spike in star formation around two billion years ago. What caused this? Well, it looks like it occurred when M33 crashed the party, exciting a new burst of stars, and scattering others to large distance.

I'm out of time, but this is all cool stuff. Well done Eduoard!

The Nature and Origin of Substructure in the Outskirts of M31 -- II. Detailed Star Formation Histories

While wide-field surveys of M31 have revealed much substructure at large radii, understanding the nature and origin of this material is not straightforward from morphology alone. Using deep HST/ACS data, we have derived further constraints in the form of quantitative star formation histories (SFHs) for 14 fields which sample diverse substructures. In agreement with our previous analysis of colour-magnitude diagram morphologies, we find the resultant behaviours can be broadly separated into two categories. The SFHs of 'disc-like' fields indicate that most of their mass has formed since z~1, with one quarter of the mass formed in the last 5 Gyr. We find 'stream-like' fields to be on average 1.5 Gyr older, with <10 percent of their stellar mass formed within the last 5 Gyr. These fields are also characterised by an age--metallicity relation showing rapid chemical enrichment to solar metallicity by z=1, suggestive of an early-type progenitor. We confirm a significant burst of star formation 2 Gyr ago, discovered in our previous work, in all the fields studied here. The presence of these young stars in our most remote fields suggests that they have not formed in situ but have been kicked-out from through disc heating in the recent past.

Sunday, 22 June 2014

The outer halo globular cluster system of M31 - II. Kinematics

Well, I don't know where that month vanished, but I now find myself sitting in the very nice Swan's Hotel in Victoria, Canada, after doing some nice new work with Alan McConnachie at the Herzberg Institute of Astrophysics. The week has gone fast, and I've given two talks, and have four more to give in California next week.

But I realised that I have neglected the blog, and there has been quite a few papers of mine put on the arxiv I should talk about. I have quite a bit of catching up to do, so here's a first post, and I will try and post some more over the next couple of weeks.

A few weeks ago, I posted an article about the fact that we now have the final catalog of the globular clusters found in the PAndAS survey. This has been a major undertaking, seeing out these balls of a few million stars in the outer reaches of Andromeda. But it's been very successful, and we can now start to ask the question "what have we learnt?" This is the topic of this post, a new paper by recently- minded postdoc, Jovan Veljanoski.

Jovan went beyond what we can do with PAndAS data and used spectroscopy to measure the speeds of the globular clusters orbiting Andromeda. As globular clusters are significantly brighter than individual stars, we can use smaller telescopes, such as the William Herschel Telescope, to do this, it is still not a simple measurement.

But we can cut to the chase and see a nice graphical view of what we found:

So, here the globular clusters have been colour-coded by velocity, relative to Andromeda (which is moving at 300 km/s towards us). It doesn't take much staring to reveal a couple of very interesting things.

Firstly, on the South-West Cloud (the blob on the right side, just below the middle) there are three globulars with a very similar shade of blue, meaning they are moving at about the same speed. Are these associated with the globulars directly associated with the underlying stars, suggesting that they were brought in on a system now disrupting? More on that in the near future!

On the left, on the Eastern Cloud, there are two yellowy-orangey clusters, again moving at the same speed. Are these associated with the Eastern Cloud? That's something we'll have to come back to!

But there are more groups of globulars moving at similar velocity, and here we high-light a few of them.

This is cool as it's what you would expect from our models of galaxy formation and evolution, where galaxies grow over time through the accretion of smaller systems. We are really seeing galaxy evolution in action.

But there is more! It's also quite simple to see that the clusters on the left side of the picture are more orange and red, meaning they are moving away from us relative to Andromeda, whereas those on the right are blue, meaning they are coming towards us; the globular cluster population in Andromeda is rotating! Wow!

This is quite puzzling! If we are seeing a snapshot of galaxy evolution, where Andromeda has eaten lots of little dwarfs that fell in higgledy-piggledy, where does the coherent rotation come from? Well, we have a couple of options. Either what we are seeing is actually the result of one big accretion, which came in and deposited all of this material on one go, which strikes me as unlikely given the mess that we see, or the accretion wasn't random and dwarfs have been coming with a preferred direction.

Both options are head-scratchingly weird. But as we have said before, there are lots of weird motions in the halo of Andromeda, especially with the existence of the Vast Thin Plane of Dwarf Galaxies (and, I promise more on this interesting thing in the very near future), and, at least to me, all of this is pointing towards something we really don't understand. And I think that's fantastic!

Well done Jovan!

The outer halo globular cluster system of M31 - II. Kinematics

We present a detailed kinematic analysis of the outer halo globular cluster (GC) system of M31. Our basis for this is a set of new spectroscopic observations for 78 clusters lying at projected distances between Rproj ~20-140 kpc from the M31 centre. These are largely drawn from the recent PAndAS globular cluster catalogue; 63 of our targets have no previous velocity data. Via a Bayesian maximum likelihood analysis we find that GCs with Rproj > 30 kpc exhibit coherent rotation around the minor optical axis of M31, in the same direction as more centrally- located GCs, but with a smaller amplitude of 86+/-17 km s-1. There is also evidence that the velocity dispersion of the outer halo GC system decreases as a function of projected distance from the M31 centre, and that this relation can be well described by a power law of index ~ -0.5. The velocity dispersion profile of the outer halo GCs is quite similar to that of the halo stars, at least out to the radius up to which there is available information on the stellar kinematics. We detect and discuss various velocity correlations amongst subgroups of GCs that lie on stellar debris streams in the M31 halo. Many of these subgroups are dynamically cold, exhibiting internal velocity dispersions consistent with zero. Simple Monte Carlo experiments imply that such configurations are unlikely to form by chance, adding weight to the notion that a significant fraction of the outer halo GCs in M31 have been accreted alongside their parent dwarf galaxies. We also estimate the M31 mass within 200 kpc via the Tracer Mass Estimator, finding (1.2 - 1.6) +/- 0.2 10^{12}M_sun. This quantity is subject to additional systematic effects due to various limitations of the data, and assumptions built in into the TME. Finally, we discuss our results in the context of formation scenarios for the M31 halo.