Cloudy in Belfast?

Participants at the Cloudy Summer School 2014 (QUB). Photo credit: Paul Woods

Participants at the Cloudy Summer School 2014 (QUB). Left to right, Nicole Reindl, Anne Fox, Patricia Bessiere, Larissa Takeda, Catherine McEvoy, Kingsley Gale-Sides, Megan Whewell, Tek Prasad Adhikari, Helen Meskhidze, Mattia Bulla, Catia Silva, Matt Nicholl, Andri Prozesky, Ted of School, Gary Ferland, Janet Chen, Tommy Nelson, Jake Turner, Richard Tunnard, Brianna Smart, Tom Finzell, Ting-Wen Lan, Joe Polshaw Photo credit: Paul Woods

In August (18th-22nd) I attended a Summer School at Queen’s University Belfast. The week was based around learning how to use a computer programme called Cloudy, which is an atomic physics code that simulates the spectrum you should expect to observe from many astrophysical objects and scenarios.

A spectrum (plural: spectra) is a way of collecting and analysing light. Spectroscopy, the study of spectra, is used a lot in astronomy because it can give you detailed information about the temperature, velocity and density (among other things) of astronomical objects.

The basis of spectroscopy is splitting light into different wavelengths (or energies). You can then measure how much light you receive at one wavelength separately from how much you receive at another wavelength. This is then plotted on a graph with wavelength across the x-axis and flux (a measure of how much light was received) on the y-axis (e.g. Fig 1).

Fig. 1. Spectrum of Mrk 509 taken using RGS onboard XMM Newton. From Detmers et al . 2011

Fig. 2. Spectrum of Mrk 509 taken using RGS onboard XMM-Newton. From Detmers et al . 2011

Figure 2 (right) is an example spectrum. You can see the red data points (the measurements of light at each wavelength), a blue line (the theoretical model to explain the positions of those data points) and six black labels (one on each feature of the spectrum, noting what caused that feature). You don’t need to know much about the labels here, although the first letter represents which element is involved (in this case all the features are caused by various forms (ions) of oxygen).

The blue line shows three large dips (labelled O VIIr (z=0), O VIIr and O VI), two small dips (O VI and O V), one quite sharp peak (O VIIf) and one broader rise (upwards of 22Å, unlabelled). The dips are absorption features, where the light from these wavelengths has been absorbed by something (gas in this case) before it reaches us. The sharp peak is an emission feature, where light is emitted due to one particular movement of electrons in one particular species.

Looking at a spectrum over a wide range of wavelengths is like looking at a barcode or fingerprint. If you know how to read it, the information encoded into a spectrum can tell you about the emitting and absorbing gases between the main source of light (that gives the continuum, the general level of light across the spectrum) and us.

Fig. 3. Inside the Botanic Gardens, Belfast

Fig. 3. Inside the Botanic Gardens, Belfast

So far in my PhD I have been looking at spectra and trying to work out the characteristics of the emitting gas causing the narrow emission lines I see (they look a lot like the sharp peak in Fig. 2). The Cloudy Summer School focussed on a different way of analysing spectra.

If you give Cloudy information about the cloud you want to analyse and the light hitting it from an astronomical object, Cloudy then computes all the particle physics inside that cloud and shows you what the spectrum would look like from your direction. This means you can analyse your data to draw one set of conclusions, look at the spectra produced by Cloudy in various different situations to draw another set of conclusions, and compare them (and hopefully the conclusions match!).

I have had a set of results from Cloudy for a while, because another member of the consortium I’m in had already run Cloudy to produce these results, but I have been struggling to match them to my data. My aim for the Summer School was to learn more about how Cloudy works and then I hoped that knowledge would help me develop ideas about how to solve my problem.

Overall I believe I achieved this aim; I have come home with new ideas, much more knowledge and confidence about using Cloudy and on top of that I really enjoyed my week in Belfast.

A pint glass at a pub we visited called "Fithy McNasty's"

Fig. 4. A pint glass at a pub we visited called “Fithy McNasty’s”

There were less than 25 people at the Summer School which meant I came away feeling like we got to know each other pretty well over the five days. We looked around Belfast a little when we had time (including the beautiful Botanic Gardens, see various photos in this blog). We had great discussions about things like the differences between ‘grad school’ and the education system in various countries around the world (including the USA, UK, Brazil, Germany and South Africa). We worked well during our group projects and all seemed to feel comfortable sharing ideas and explaining science concepts from our different specialisms. We even had a 50:50 female:male ratio for participants! But best of all, it was just a group of lovely people who I enjoyed interacting and working with for those five days.

Queen's University Belfast at night

Queen’s University Belfast at night



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