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.