## Dark Matter: Dearth Evaded

Posted in Astrohype, The Universe and Stuff with tags , , , , , , on May 23, 2012 by telescoper

While I’m catching up on developments over the last week or so I thought I’d post an update on a story I blogged about a few weeks ago. This concerns the the topic of dark matter in the Solar Neighbourhood and in particular a paper on the arXiv by Moni Bidin et al. with the following abstract:

We measured the surface mass density of the Galactic disk at the solar position, up to 4 kpc from the plane, by means of the kinematics of ~400 thick disk stars. The results match the expectations for the visible mass only, and no dark matter is detected in the volume under analysis. The current models of dark matter halo are excluded with a significance higher than 5sigma, unless a highly prolate halo is assumed, very atypical in cold dark matter simulations. The resulting lack of dark matter at the solar position challenges the current models.

In my earlier post I remarked that this  study   makes a number of questionable assumptions about the shape of the Milky Way halo – they take it to be smooth and spherical – and the distribution of velocities within it is taken to have a very simple form.

Well, only last week a rebuttal paper by Bovy & Tremaine appeared on the arXiv. Here is its abstract:

An analysis of the kinematics of 412 stars at 1-4 kpc from the Galactic mid-plane by Moni Bidin et al. (2012) has claimed to derive a local density of dark matter that is an order of magnitude below standard expectations. We show that this result is incorrect and that it arises from the invalid assumption that the mean azimuthal velocity of the stellar tracers is independent of Galactocentric radius at all heights; the correct assumption—that is, the one supported by data—is that the circular speed is independent of radius in the mid-plane. We demonstrate that the assumption of constant mean azimuthal velocity is physically implausible by showing that it requires the circular velocity to drop more steeply than allowed by any plausible mass model, with or without dark matter, at large heights above the mid-plane. Using the correct approximation that the circular velocity curve is flat in the mid-plane, we find that the data imply a local dark-matter density of 0.008 +/- 0.002 Msun/pc^3= 0.3 +/- 0.1 Gev/cm^3, fully consistent with standard estimates of this quantity. This is the most robust direct measurement of the local dark-matter density to date.

So it seems reports of the dearth were greatly exaggerated..

Having read the paper I think this is a pretty solid refutation, and if you don’t want to take my word for it I’ll also add that Scott Tremaine is one of the undisputed world experts in the field of Galactic Dynamics. It will be interesting to see how Moni Bidin et al. respond.

This little episode raises the question that, if there was a problem with the assumed velocity distribution in the original paper (as many of us suspected), why wasn’t this spotted by the referee?

Of course to a scientist there’s nothing unusual about scientific results being subjected to independent scrutiny and analysis. That’s how science advances. There is a danger in all this, however, with regard to the public perception of science. The original claim – which will probably turn out to be wrong – was accompanied by a fanfare of publicity. The later analysis arrives at a much less spectacular conclusion,  so will probably attract much less attention. In the long run, though, it probably isn’t important if this is regarded as a disappointingly boring outcome. I hope what really matters for scientific progress is people doing things properly. Even if it  don’t make the headlines, good science will win out in the end. Maybe.

## On the Dearth of Dark Matter in the Solar Neighbourhood

Posted in Astrohype, The Universe and Stuff with tags , , , , , , , , on April 22, 2012 by telescoper

I’m a bit late getting onto the topic of dark matter in the Solar Neighbourhood, but it has been generating quite a lot of news, blogposts and other discussion recently so I thought I’d have a bash this morning. The result in question is a paper on the arXiv by Moni Bidin et al. which has the following abstract:

We measured the surface mass density of the Galactic disk at the solar position, up to 4 kpc from the plane, by means of the kinematics of ~400 thick disk stars. The results match the expectations for the visible mass only, and no dark matter is detected in the volume under analysis. The current models of dark matter halo are excluded with a significance higher than 5sigma, unless a highly prolate halo is assumed, very atypical in cold dark matter simulations. The resulting lack of dark matter at the solar position challenges the current models.

As far as I’m aware, Oort (1932, 1960) was the first to perform an analysis of the vertical equilibrium of the stellar distribution in the solar neighbourhood. He argued that there is more mass in the galactic disk than can be accounted for by star counts. A reanalysis of this problem by Bahcall (1984) argued for the presence of a dark “disk” of a scale height of about 700 pc. This was called into question by Bienaymé et al. (1987), and by Kuijken & Gilmore in 1989. In a later analysis based on a sample of stars with HIPPARCOS distances and Coravel radial velocities, within 125 pc of the Sun. Crézé et al. (1998) found that there is no evidence for dark matter in the disk of the Milky Way, claiming that all the matter is accounted for by adding up the contributions of gas, young stars and old stars.

The lack of evidence for dark matter in the Solar Neighbourhood is not therefore a particularly new finding; there’s never been any strong evidence that it is present in significant quantities out in the suburbs of the Milky Way where we reside. Indeed, I remember a big bust-up about this at a Royal Society meeting I attended in 1985 as a fledgling graduate student. Interesting that it’s still so controversial 27 years later.

Of course the result doesn’t mean that the dark matter isn’t there. It just means that its effect is too small compared to that of the luminous matter, i.e. stars, for it to be detected. We know that the luminous matter has to be concentrated more centrally than the dark matter, so it’s possible that the dark component is there, but does not have a significant effect on stellar motions near the Sun.

The latest, and probably most accurate, study has again found no evidence for dark matter in the vicinity of the Sun. If true, this may mean that attempts to detect dark matter particles using experiments on Earth are unlikely to be successful.

The team in question used the MPG/ESO 2.2-metre telescope at ESO’s La Silla Observatory, along with other telescopes, to map the positions and motions of more than 400 stars with distances up to 13000 light-years from the Sun. From these new data they have estimated the mass of material in a volume four times larger than ever considered before but found that everything is well explained by the gravitational effects of stars, dust and gas with no need for a dark matter component.

The reason for postulating the existence of large quantities of dark matter in spiral galaxies like the Milky Way is the motion of material in the outer parts, far from the Solar Neighbourhood (which is a mere 30,000 light years from Galactic Centre). These measurements are clearly inconsistent with the distribution of visible matter if our understanding of gravity is correct. So either there’s some invisible matter that gravitates or we need to reconsider our theories of gravitation. The dark matter explanation also fits with circumstantial evidence from other contexts (e.g. galaxy clusters), so is favoured by most astronomers. In the standard theory the Milky Way is surrounded by am extended halo of dark matter which is much less concentrated than the luminous material by virtue of it not being able to dissipate energy because it consists of particles that only interact weakly and can’t radiate. Luminous matter therefore outweighs dark matter in the cores of galaxies, but the situation is reversed in the outskirts. In between there should be some contribution from dark matter, but since it could be relatively modest it is difficult to estimate.

The study by Moni Bidin et al. makes a number of questionable assumptions about the shape of the Milky Way halo – they take it to be smooth and spherical – and the distribution of velocities within it is taken to have a very simple form. These may well turn out to be untrue. In any case the measurements they needed are extremely difficult to make, so they’ll need to be checked by other teams. It’s quite possible that this controversy won’t be actually resolved until the European Space Agency’s forthcoming GAIA mission.

So my take on this is that it’s a very interesting challenge to the orthodox theory, but the dark matter interpretation is far from dead because it’s not obvious to me that these observations would have uncovered it even if it is there. Moreover, there are alternative analyses (e.g. this one) which find a significant amount of dark matter using an alternative modelling method which seems to be more robust. (I’m grateful to Andrew Pontzen for pointing that out to me.)

Anyway, this all just goes to show that absence of evidence is not necessarily evidence of absence…

## VISTA on Video

Posted in The Universe and Stuff with tags , , , , , on March 23, 2012 by telescoper

A chance tweet brought to my attention this video that fits well with a news story that’s been doing the rounds for a few days.   This concerns a very deep and wide survey called UltraVISTA, that has been made using the VISTA telescope at the European Southern Observatory’s Paranal Observatory in Chile. You can find the full press release from ESO that started the media interest here, where some lovely images can also be found.

VISTA is the world’s largest infra-red survey telescope, and is unusual among telescopes for having only one instrument on it, an Infra-red camera.  Technically, therefore,  it should really be called ISTA; owing to cost constraints the Visible camera that was initially proposed to accompany the Infra-red one and supply the V in its acronym,  was never built. Anyway, VISTA was designed explicitly to do survey work involving very distant and faint objects; its forte is to allow very deep images to be made with a very wide field of view, as demonstrated on the video…

Since I’m using the handle “telescoper” on this blog, I suppose I really should post about telescopes a bit more often than I do but I hope this will do for now!

## Galaxies con Alma

Posted in The Universe and Stuff with tags , , , , on October 3, 2011 by telescoper

It’s back to School with a vengeance today, so not much time for the blog. However, I couldn’t resist mentioning the fact that the European Southern Observatory’s Atacama Large Millimetre Array, known to its friends as ALMA, has at last opened its eyes. Or at least some of them. ALMA in fact is an interferometer which eventually will comprise 66 dishes,   working together to with baselines as long 16km to synthesize a single huge aperture. The preliminary results that have just been released were obtained using just 16 dishes so they only offer a taste of what the full ALMA will do when it’s completed in 2013.

ALMA works in the millimetre wave region of the spectrum, operating at wavelengths between 0.3 and 9.6 mm. The overlap with the  wavelength range probed by the Herschel Space Observatory together with its much higher resolution than Herschel, which is a single telescope of only 3.5m diameter, makes the two very complementary: Herschel is good for surveying large parts of the sky, because it has a large field of view, whereas ALMA can do high-resolution follow-up of selected regions.

Anyway, here is ALMA’s view of the Antennae Galaxies (left) shown next to an optical image taken with the Very Large Telescope (VLT).

The system consists of two galaxies so close together that they interact strongly with each other via enormous tidal forces, hence the disturbed structure. The coloured regions in the ALMA image show radiation emanating from carbon monoxide present in huge clouds both in and between the galaxies. Altogether these clouds contain several billion solar masses worth of gas which has never been viewed before.

## Bright and Early

Posted in The Universe and Stuff with tags , , , , , , on June 29, 2011 by telescoper

Some interesting astronomy news emerged this evening relating to a paper published in 30th June issue of the journal Nature. The press release from the European Southern Observatory (ESO) is quite detailed, so I’ll refer you there for the minutiae, but in a nutshell:

A team of European astronomers has used ESO’s Very Large Telescope and a host of other telescopes to discover and study the most distant quasar found to date. This brilliant beacon, powered by a black hole with a mass two billion times that of the Sun, is by far the brightest object yet discovered in the early Universe.

and the interesting numbers are given here (with links from the press release):

The quasar that has just been found, named ULAS J1120+0641 [2], is seen as it was only 770 million years after the Big Bang (redshift 7.1, [3]). It took 12.9 billion years for its light to reach us.

Although more distant objects have been confirmed (such as a gamma-ray burst at redshift 8.2, eso0917, and a galaxy at redshift 8.6, eso1041), the newly discovered quasar is hundreds of times brighter than these. Amongst objects bright enough to be studied in detail, this is the most distant by a large margin.

When I was a lad, or at least a postdoc, the most distant objects known were quasars, although in those days the record holders had redshifts just over half that of the newly discovered one. Nowadays technology has improved so much that astronomers can detect “normal” galaxies at even higher redshifts but quasars remain interesting because of their extraordinary luminosity. The standard model for how a quasar can generate so much power involves a central black hole onto which matter falls, liberating vast amounts of gravitational energy.

You can understand how efficient this is by imagining a mass $m$ falling onto a black hole of Mass $M$ from a large distance to the horizon of the black hole, which is at the Schwarzschild radius $R=2GM/c^2$. Since the gravitational potential energy at a radius $R$ is $-GMm/R$ the energy involved in bringing a mass $m$ from infinity to the horizon is a staggering $\frac{1}{2} mc^2$, i.e. half the rest mass energy of the infalling material. This is an overestimate  for various reasons but it gives you an idea of how much energy is available if you can get gravity to do the work; doing the calculation properly still gives an answer much higher than the amount of energy that can be released by, e.g., nuclear reactions.

The point is, though, that black holes aren’t built in a day, so if you see one so far away that its light has taken most of the age of the Universe to reach us then it tells us that its  black hole must have grown very quickly. This one seems to be a particularly massive one, which means it must have grown very quickly indeed. Through observations like this  we learn something potentially very interesting about the relationship between galaxies and their central black holes, and how they both form and evolve.

On the lighter side, ESO have also produced the following animation which I suppose is quite illustrative, but what are the sound effects all about?

## From Time to Time

Posted in Biographical, The Universe and Stuff with tags , , on June 5, 2011 by telescoper

It having been my birthday yesterday, and very nice it was too, thank you for asking, I’m filled this morning with thoughts about the passage of time. It’s strange how working in education imposes a cycle on your life: admissions, teaching, exams, graduation ceremonies, summer recess, and so on. The main thing that breaks this pattern of recurrence is when students finish their finals and leave for the big wide world. Since I moved to Cardiff in 2007, the 4th year students who have just finished their examinations are the first cohort that I’ve seen through their whole degree programme at Cardiff University, and it will be great to see them all get their degrees next month in St David’s Hall, but it will be yet another reminder of the passage of the years.

Not that I’m one to get depressed about such things. I’ve taken surprisingly well to middle age and gracefully (?) surrendered the things of youth some time ago. However, time is such a mysterious thing it’s hard not to think about its passing every now and then.

This time last year I was in Copenhagen for a small cosmology workshop. There’ll be a repeat performance next week too, so I’ll be off to Denmark for a few days. In fact I bought my ticket some time ago, but realised only on Friday that it was next week, and not the week after, so have had to rearrange a few things rather hastily. The advancing years have obviously addled my brain.

Anyway, all this talk about time and cycles gives me some sort of excuse to post the following video from the ESO Very Large Telescope in Chile. The photography is wonderful. Pity about the music, though. Spoils it a bit if you ask me…

## Das Kapital

Posted in Science Politics with tags , , on October 29, 2010 by telescoper

After a short-lived burst of optimism following the announcement of the better-than-expected results of the Comprehensive Spending Review for science funding, it seems levels of nervousness are again increasing about what might lie in store for the Science and Technology Facilities Council (STFC).

It appears the “near-cash” funding for RCUK, the umbrella organisation that sits above the seven Research Councils will be fixed over the period of the CSR but, within that overall pot, the Medical Research Council (MRC) will have its funding stream protected in real terms, meaning the others will be have to be reduced in real terms. How the pie will be divided up remains to be seen, but I believe there is some pretty tough negotiating going on behind the scenes right now. RCUK chief Adrian Smith has apparently been given detailed instructions by the Treasury on how to carry out the budget allocations, but I haven’t seen the manual…

However, this is only part of the story. The other part, as the e-astronomer has already pointed out, is rather scary. As well as “near-cash” funding, the Research Councils also receive funds marked “capital”. I’m not an accounting expert, but I’m told this is all pretty normal practice for large organisations. What has emerged recently is that the capital part of the RCUK budget is to be cut by a whopping 44%. A rumour is spreading that STFC has been told to expect at least a 30% cut in capital funding and instructed to budget accordingly.

This could herald a return of the nightmare scenario I blogged about not long ago. The point is that, owing to the nature of STFC, a very large part of its budget is tied up in the capital stream. In fact it’s even worse than you might imagine because large international subscriptions, including CERN and ESO, are currently paid for, at least in part, out of STFC’s capital budget. According to the journal Nature,

That money pays for everything from radio telescopes to Antarctic research stations. In particular, the cuts will hit the Science & Technology Facilities Council (STFC), which funds particle physics and astronomy. The council, which has struggled financially for years, has been told to prepare for its capital funding to fall by a third, according to documents seen by Nature. That could jeopardize Britain’s participation in organizations such as the European Southern Observatory.

Another factor that adds to the vulnerability of our ESO membership is the longrunning saga of the VISTA telescope. When the UK joined ESO (in 2002), this telescope – which hadn’t been built then – was put up as an “in kind” contribution to the UK’s joining fee. However, VISTA wasn’t actually handed over to ESO until December 2009, significantly behind schedule and it is still undergoing performance verification. In principle, ESO could require the UK to pay a substantial fine for the late delivery and it is STFC that would have to find the funds. I don’t know what the precise fine would be, but I’ve heard figures from £16M to about twice that.

Although Science Minister David Willetts is on record as saying that he was not planning to withdraw from any European collaborations, including ESO, the VISTA debacle may force his hand if it means the cost of continuing membership exceeds the penalties for withdrawal.

Withdrawing from ESO would destroy a huge part of the UK’s ground-based astronomy activity, as we have already withdrawn from (or are planning to withdraw from) most other facilities we used to have access to. It would also damage our credibility as international scientific partners in a more general sense. But if it isn’t ESO that gets chopped it will be something else. It’s difficult to see how STFC can cope with this cut without something going to the wall.

The Royal Astronomical Society has written to Fellows requesting that they write to their MPs to point out the consequences of drastic cuts to STFC’s budget, citing withdrawal from ESO as one possible outcome.

There’s no doubt about it, it’s brown trousers time.