Archive for dust

Galactic Loops as Sources of Polarized Emission

Posted in The Universe and Stuff with tags , , , , , on April 8, 2014 by telescoper

Since I seem to have established myself as an arch-sceptic concerning the cosmological interpretation of the the BICEP2 measurement of the polarization of the cosmic microwave background (CMB), I couldn’t resist posting a link to an interesting paper by Liu et al. that has just appeared on the arXiv.

The abstract is:

We investigate possible imprints of galactic foreground structures such as the `radio loops’ in the derived maps of the cosmic microwave background. Surprisingly there is evidence for these not only at radio frequencies through their synchrotron radiation, but also at microwave frequencies where emission by dust dominates. This suggests the mechanism is magnetic dipole radiation from dust grains enriched by metallic iron, or ferrimagnetic molecules. This new foreground we have identified is present at high galactic latitudes, and potentially dominates over the expected B-mode polarisation signal due to primordial gravitational waves from inflation.

The authors argue that foreground emission from our own Galaxy has not been fully subtracted from maps of the cosmic microwave background. This emission could result in significant contamination of the CMB polarization if it is associated with dust grains aligned with the Galaxy’s magnetic field.

I’m grateful to one of the authors of the paper, Philip Mertsch, for sending me this map of the Galactic Loops with the BICEP2 region superimposed on it, demonstrating that there is potential for a contribution…




This paper is likely to provoke quite a discussion, so I thought I’d suggest one possible way of testing it, namely by updating the analysis presented by myself and Patrick Dineen in 2003 with new data. Here’s the abstract of our old paper:

We present a diagnostic test of possible Galactic contamination of cosmic microwave background sky maps designed to provide an independent check on the methods used to compile these maps. The method involves a non-parametric measurement of cross-correlation between the Faraday rotation measure (RM) of extragalactic sources and the measured microwave signal at the same angular position. We argue that statistical properties of the observed distribution of rotation measures are consistent with a Galactic origin, an argument reinforced by a direct measurement of cross-correlation between dust, free-free and synchrotron foreground maps and RM values with the strongest correlation being for dust and free-free. We do not find any statistically compelling evidence for correlations between the RM values and the COBE DMR maps at any frequency, so there is no evidence of residual contamination in these CMB maps. On the other hand, there is a statistically significant correlation of RM with the preliminary WMAP individual frequency maps which remains significant in the Tegmark et al. Wiener-filtered map but not in the Internal Linear Combination map produced by the WMAP team.

The idea is that cross-correlating the CMB pattern with Faraday rotation measures should provide an independent diagnostic of the effect of magnetic fields. Our analysis was based on old CMB data, so there’s an interesting project to be done updating it with, e.g., Planck CMB data and a larger set of rotation measures. See the comment below for a reference to more recent work along these lines, but still not including Planck.

Anyway, this all goes to show that there’s one question you can always ask about an astrophysics result: have you considered the possible role of magnetic fields?

The Heat Death of Herschel

Posted in The Universe and Stuff with tags , , , , on March 13, 2013 by telescoper

Most of the astronomers who read this blog will have heard the news that the Herschel Space Observatory is running out of the Helium that it has been using to keep it cool enough (~1.4K) to be sensitive to the far-infra-red radiation emitted by very distant objects.

There’s a gallery of wonderful images obtained by Herschel since it was launched in 2009 at the news item linked to above, but my favourite is one of the least photogenic:


Many of these fuzzy blobs correspond to immensely distant galaxies; what we see is starlight from very young stars absorbed by vast amounts of cosmic dust and then re-radiated in the infra-red. Understanding these sources is decidedly non-trivial and it will take many years to get all the information out that is hidden in images like this.

Anyway, one thing worth pointing out here is that what is going on now with Herschel is not some kind of failure. Quite the contrary, in fact. The original mission lifetime was planned to be three years, and Herschel has now been operating for nine months longer than that. The supply of Helium was always going to be the limiting factor as the spacecraft operates at the second Lagrange point of the Earth-Sun system, which is almost a million miles away and thus too far to be replenished. When the Helium does run out, Herschel will rapidly heat up to the point where its detectors are swamped. It will then be blind.

I was at this point going to make a cheap joke to the effect that after years on its own in the dark preoccupied with images of heavenly bodies, it was entirely predictable that Herschel would go blind. But I decided not to. I’ll save that kind of off-colour remark for Twitter…

ps. Coincidentally, on this day (March 13th) in 1781,  William Herschel  discovered the planet Uranus. The telescope is named in Herschel’s honour because he was also the first person to demonstrate the existence of infra-red radiation.


Posted in Poetry, The Universe and Stuff with tags , , , on July 4, 2010 by telescoper

I was reading through a collection of poems by Rupert Brooke this lazy sunday afternoon and found this. I haven’t posted much poetry recently so thought I’d add it here. I’m sure my many friends who work on astrophysical dust will enjoy it, especially those involved with the European Space Agency’s  Herschel Space Observatory. Apparently they’re all “passionate about dust”. If that’s true I wonder if one of them might want to write a wikipedia entry on the subject, because for some reason there isn’t one…

When the white flame in us is gone,
And we that lost the world’s delight
Stiffen in darkness, left alone
To crumble in our separate night;

When your swift hair is quiet in death,
And through the lips corruption thrust
Has still’d the labour of my breath -
When we are dust, when we are dust !

Not dead, not undesirous yet,
Still sentient, still unsatisfied,
We’ll ride the air, and shine, and flit,
Around the places where we died,

And dance as dust before the sun,
And light of foot and unconfined,
Hurry from road to road, and run
About the errands of the wind.

And every mote, on earth or air,
Will speed and gleam, down later days,
And like a secret pilgrim fare
By eager and invisible ways,

Nor ever rest, nor ever lie,
Till, beyond thinking, out of view,
One mote of all the dust that’s I
Shall meet one atom that was you.

Then in some garden hush’d from wind,
Warm in a sunset’s afterglow,
The lovers in the flowers will find
A sweet and strange unquiet grow

Upon the peace; and, past desiring,
So high a beauty in the air,
And such a light, and such a quiring,
And such a radiant ecstasy there,

They’ll know not if it’s fire, or dew,
Or out of earth, or in the height,
Singing, or flame, or scent, or hue,
Or two that pass, in light, to light,

Out of the garden, higher, higher. . . .
But in that instant they shall learn
The shattering ecstasy of our fire,
And the weak passionless hearts will burn

And faint in that amazing glow,
Until the darkness close above;
And they will know – poor fools, they’ll know!
One moment, what it is to love.

Planck and the Cold Galaxy

Posted in The Universe and Stuff with tags , , , , , , on March 17, 2010 by telescoper

Just a quick post to show a cool result from Planck which has just been released by the European Space Agency (ESA). It will be a while before any real cosmological results are available, but in the meantime here are a couple of glimpses into the stuff we cosmologists think of as foreground contamination but which are of course of great interest in themselves to other kinds of astronomers.

The beautiful image above (courtesy of ESA and the HFI Consortium) covers a portion of the sky about 55 degrees across. It is a three-colour combination constructed from Planck’s two shortest wavelength channels (540 and 350 micrometres, corresponding to frequencies of 545 and 857 GHz respectively), and an image at 100 micrometres obtained with the Infrared Astronomical Satellite (IRAS). This combination effectively traces the dust temperature: reddish tones correspond to temperatures as cold as 12 degrees above absolute zero, and whitish tones to significantly warmer ones (a few tens of degrees above absolute zero) in regions where massive stars are currently forming. Overall, the image shows local dust structures within 500 light years of the Sun.

Our top man in the HFI Consortium,  Professor Peter Ade, is quoted as saying

..the HFI is living up to our most optimistic pre-flight expectations.  The wealth of the data is seen in these beautiful multicolour images exposing previously unseen detail in the cold dust components of our galaxy.  There is much to be learned from detailed interpretation of the data which will significantly enhance our understanding of the star formation processes and galactic morphology.

This Planck image was obtained during the first Planck all-sky survey which began in mid-August 2009. By mid-March 2010 more than 98% of the sky has been observed by Planck. Because of the way Planck scans the sky 100% sky coverage for the first survey will take until late-May 2010.

Other new results and a more detailed discussion of this one can be found here and here.


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