Enterprise Merger: Mission Log 1

Stage 1: Code Generation

The Mission begins! And as I had expected much of what needs to be done, before the analysis of The Enterprise Merger can really begin, is the manipulation of the telescope images in order to obtain useful data.

Related image
The Hubble Space Telescope in orbit around the Earth. [Image Credit: NASA and StSci]
The VLT in action*
The Very Large Telescope. It is comprised of four 8.2m diameter telescopes and four smaller 1.8m diameter telescopes. [Image Credit: ESO]

For this mission I have images from the MUSE (or Multi Unit Spectroscopic Explorer) instrument on the VLT and the Hubble Space Telescope (HST). The MUSE instrument takes many images of a region of sky in different wavelengths, this creates a datacube of position (2D) in the sky and wavelength (the third Dimension of the datacube) from which the spectrum of light emitted by the Enterprise Merger can be obtained. The Hubble Space Telescope only produces images at one wavelength filter band, but unlike the VLT it is in orbit around the Earth and so takes much clearer images as it is outside of Earth’s atmosphere.

From the Hubble we have two images taken with two different optical filters (F110W and F160W). Using these images the three galaxy components of the Enterprise Merger can be identified and thus we are able to identify the sky coordinates (Right Ascension and Declination) of the centres of each component galaxy of this Merger.

HST images of the Enterprise merger with the three distinct components and the central point of the system indicated. On the left is the F110W filter image and on the right is the F160W filter image.

Now we have obtained the coordinates of our target we need to calculate the redshift (a measure of the distance to our galaxy merger) of the Enterprise and identify the emission lines that appear in the spectrum of light produced by the Enterprise. To do this we need to use the MUSE datacube and utilise our computational capabilities. Fire python torpedos!

Actually extracting the spectral data was fairly easy, just a little time consuming. First you have to cut the datacube to get a minicube of just the Enterprise Merger, with all wavelengths. We do this by keeping only the pixel values at the coordinates which lie within the edges of a square centred on the Enterprise Merger. We took cuts of the datacube of only a few pixels around the centres of each component in order to then extract both the whole system’s spectrum and the spectrum of each individual component galaxy. Then the spectra needed to be extracted from these minicubes. To extract the spectra we sum the value of all pixels in each wavelength slice of the minicube, this gives us the signal (or flux) of the Enterprise at each wavelength imaged by MUSE. Lastly, these wavelength value are actually the air wavelength values, as the atmosphere actually alters the wavelengths slightly from their true value in the vacuum. You canna change the laws of physics, so we have to convert the air wavelength values extracted to the vacuum wavelength values. Now we have our spectra!

Emission line continuum of the component Enterprise-alpha, with the key emission lines indicated.
Emission line continuum of the component Enterprise-beta, with the key emission lines indicated. Note this spectrum also contains more emission lines, including NII, indicating this component is more metal rich than the other two.
Emission line continuum of the component Enterprise-gamma, with the key emission lines indicated.

From these spectra we can see there are clear emission lines (peaks in the signal measured at certain wavelengths) which are due to particular species of ion. In the Enterprise spectrum we identify emission line corresponding to ionise Hydrogen, Oxygen, Nitrogen and Sulphur. Using the wavelengths of emissions from these same ions when measured in a laboratory we are able to calculate the redshift of the Enterprise, using the doppler equation.

The doppler equation for redshift (z), where lambda_observed is the observed wavelength of the emission line from Earth and lambda_emitted is the wavelength of the emission line when it was emitted by the galaxy.

Now we know how far away the Enterprise is and its emission signature, we can start looking at its properties, such as how quickly it is forming stars, its metal content (in astrophysics terms that is all elements above Hydrogen and Helium) and if it contains any AGN. This means we need to obtain images from the MUSE data which contain the total signal of each emission line, these are effectively narrow wavelength range filter images (a pseudo-narrowband image).

As you can see from the spectra above the emission lines cover a narrow range of wavelength values, it is this range we want to get the pseudo-narrowband image of. However as the MUSU image only has a wavelength slice every 1.25Å the true extent of each emission line is not known. Thus we need to fit gaussian curves to each emission line to find their true central wavelengths and the width of each line.

The Gaussian fits to the key emission lines required for further analysis of the Enterprise.
The scale of the gaussian fits of (left to right) Ha, NII, SII6718 and SII6732. As can be seen Ha and NII are very close, so we had to remove the Ha emission line from the spectral data before fitting the NII gaussian.

Now we know the width of each pseudo-narrowband image (we use 3x the standard deviation of the gaussian fit as this will contain ~95% of the emission line values) we take only the wavelength slices in our Enterprise minicube which fall within the range of wavelengths which would form our pseudo-narrowband image and sum the flux of each pixel across that section of minicube. This gives the total signal that would be measured if a narrowband image had been taken across that wavelength range.

This has taken some time to get the python codes written and working, and numerous instances of reaching odd functional impasses (I got, uh…stuck). However we have almost completed this stage of the mission, with the pseudo-narrowband emission images of the Enterprise we can begin investigating the true properties of the Enterprise Galaxy Triple Merger!

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