Archive for ROSAT

Missing Mass Hysteria

Posted in Astrohype, The Universe and Stuff with tags , , , on May 30, 2011 by telescoper

It’s usually very satisfying to see science get covered in the popular media. Even if the story gets a little simplified or, more likely, garbled, press coverage often succeeds in getting at least a bit of the truth across. My own field of astrophysics has more popular appeal than many other branches of physics, but can nevertheless involve complex theoretical ideas and difficult observations that can be difficult to disseminate in a form suitable for public consumption. For the most part, the press do a good job for astronomy but occasionally news stories emerge that are simply ridiculous.

Take this one, for example, which begins:

A Monash student has made a breakthrough in the field of astrophysics, discovering what has until now been described as the Universe’s ‘missing mass’. Amelia Fraser-McKelvie, working within a team at the Monash School of Physics, conducted a targeted X-ray search for the matter and within just three months found it – or at least some of it.

What makes the discovery all the more noteworthy is the fact that Ms Fraser-McKelvie is not a career researcher, or even studying at a postgraduate level. She is a 22-year-old undergraduate Aerospace Engineering/Science student who pinpointed the missing mass during a summer scholarship, working with two astrophysicists at the School of Physics, Dr Kevin Pimbblet and Dr Jasmina Lazendic-Galloway.

On the face of it, this sounds an extremely interesting story not only because it apparently involves a major scientific breakthrough, but also because the result was achieved by an undergraduate student working on a summer programme. Unfortunately, however, a little digging reveals that there is much less to it than meets the eye. I know of many astronomers around the world who think the Press Office at Monash University is guilty of shameless exaggeration in the press release that initiated this bubble. This sort of deliberately misleading distortion is very bad for science, as it almost inevitably ends up splattered in unrecognisable form all over the media, especially the downmarket end.

Here is the abstract of the actual paper which this story is supposed to be about:

Most of the baryons in the Universe are thought to be contained within filaments of galaxies, but as yet, no single study has published the observed properties of a large sample of known filaments to determine typical physical characteristics such as temperature and electron density. This paper presents a comprehensive large-scale search conducted for X-ray emission from a population of 41 bona fide filaments of galaxies to determine their X-ray flux and electron density. The sample is generated from Pimbblet et al.’s (2004) filament catalogue, which is in turn sourced from the 2 degree Field Galaxy Redshift Survey (2dFGRS). Since the filaments are expected to be very faint and of very low density, we used stacked ROSAT All-Sky Survey data. We detect a net surface brightness from our sample of filaments of (1.6 +/- 0.1) x 10^{-14} erg cm^{-2} s^{-1} arcmin^{-2} in the 0.9-1.3 keV energy band for 1 keV plasma, which implies an electron density of n_{e} = (4.7 +/- 0.2) x 10^{-4} h_{100}^{1/2} cm^{-3}. Finally, we examine if a filament’s membership to a supercluster leads to an enhanced electron density as reported by Kull & Bohringer (1999). We suggest it remains unclear if supercluster membership causes such an enhancement.

You won’t find anything in there about finding the “missing mass” of the Universe, nor will you find it anywhere else in the paper, because they haven’t. The “targeted X-ray search” involved stacking old ROSAT observations of filaments that were discovered and catalogued in previous papers; this study merely matched them to existing X-ray data. ROSAT ceased operations in 1999. The results do give some evidence for a higher electron density than previously thought in some of the filaments, so it’s a fairly interesting “incremental” paper, not by any stretch of the imagination revolutionary.

I’ve got nothing against Amelia Fraser-McKelvie, who seems to have done some solid scientific work during her summer internship, and who may not have played any role in spinnng the shameless press release that led to this story getting into the world’s media. However, the more senior scientists involved in this work should not have let the story come out in this form.

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