Galaxy Hunting: Level 4.2

Will and I spent the last couple of weeks writing a programme to automatically detect sources in the cube. Ultimately, this was incredibly frustrating and rather fruitless, but it did finally allow us to finally detect the source around which our whole cube was centred, the galaxy VR7.

We used a programme called SExtractor, which is able to automatically detect sources within a 2D image. This meant we first had to bin our cube into smaller slices and run the programme on each one. A lot of time was spent carefully choosing the parameters used by the SExtractor so that when applied to a white light image, it would detect all the sources with minimal inclusion of noise. Sadly any choice of parameters seemed to give us an unreasonable amount of noise round the edge of the cube, so ultimately the decision was made to simply cut away the edges these edges. Eventually we outputted 286 catalogues (one for each 8 wavelength value slice) with everything SExtractor detected at a sigma level of 2 for 7 or more pixels.

A very noisy source extraction

Obviously we didn’t want to sort through each of these catalogues individually so we wrote another programme which in its early stages was so inefficient, we were blamed for a city-wide power cut. This point of this programme was to compile the individual catalogues, such that each distinct (defined as within one arcsecond) source only appeared once. This outputted a catalogue of one hundred and something blindly found sources, with the number of times they had been detected and the wavelength at which they were first detected. Unfortunately, most of which turned out to be noise once their spectra were taken.

However, here were five survivors of the noise cull, which showed strong asymmetric emission lines. These are hypothesised to be Lyman-alpha emitters, which are high redshift galaxies offering a glimpse into the history of the universe. However, the limited range over which our datacube runs means that it is very difficult to confirm this, as if we see the Lyman-alpha peak in a spectrum, we are likely to only see that peak so have no further evidence with which to solid our redshift.

One such of these Lyman-alpha emitters is, of course, VR7, right at the centre of the cube. As one would expect, this showed a very clear spectrum with a strong Lyman-alpha peak, giving us a redshift of 6.5345, right at the edge of our ‘PIG galaxy’. For context this corresponds to a luminosity distance of over 65,000 Mpc. For further context that is 1.34*10^17 times the distance from earth to the sun or more than 80,000 times the distance from us to our nearest neighbour galaxy, Andromeda.

VR7’s Lyman alpha peak

The incredible distance at which we managed to find a source almost makes all the frustration worth it and nevertheless the experience taught me valuable lessons about the patience needed in programming and in research in general.


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