Galaxy Hunting: Level 4.3 – The Enterprise Merger

Within the data cube, not much above the central star, is a bright object, PIG-7351-1417. This was the PIGEON of the week for week one, and is a galactic triple merger.
A galactic merger is an event where two or more galaxies move so close to each other they collide, becoming one much larger galaxy, and are one of the most violent interactions between galaxies which occurs within our universe. The strong gravitational tidal forces draw out gas and dust from the galaxies involved and the friction between these gases results in an increase of Star Formation within the merging galaxies.
As the galaxies are so close together there was quite a lot of contamination in the minicubes I took of each galaxy in the merger, so to reduce this contamination I had to take some very small minicubes of only about four pixels. This seemed a little odd at first, but produced almost the same spectra for each galaxy, minus the contamination, as the previous minicubes. It is worth noting however that Matthew found later that the smaller minicubes cannot be used to find the metallicity or star formation rate (SFR) of the galaxies with any accuracy as the flux of the signal is inaccurate with such small minicubes.
As the wavelengths in the datacube have intervals of 1.25Å the centre of the peaks in the spectra collected are not as obviously different in their positions, so the redshifts calculated from them are not as accurate. To smooth these peaks out I wrote a program to fit Gaussians to each important and identifiable peak in the spectra. This program also proved to be very easy to expand to analysis much of the spectra collected, so I had a lot of fun one day just expanding the program to work with the whole catalogue and find some additional properties from the spectra.
With Gaussian curves fit to the spectra it was far more obvious that there is a slight difference in the redshifts of the three components, as you can see in the figure below (Red is the object designated PIG-7351-1417-α, green is PIG-7351-1417-β and blue is PIG-7351-1417-γ) . Having fit Gaussians to the spectra of each of the component galaxies I then used the central wavelength of each peak to calculate the redshifts and average it for each galaxy.
GaussMergerEmissions.pngScreen Shot 2018-07-13 at 14.31.51.png
Having values for the central wavelength of each emission peak in the three spectra I was then able to calculate the corresponding velocities, which I did using the equation v=((λobservedintrinsic)/λintrinsic)c, where v is the velocity, λobserved is the observed wavelength, λintrinsic is the original, emitted wavelength and c is the speed of light. In order to show how the galaxies in the triple merger are moving in relation to each other I calculated the velocities with respect to one of the galaxies in the merger, taking the central wavelength of each emission peak of that galaxy as the value of the original wavelength, λintrinsic. Plotting the relative positions of the galaxies and indicating the magnitude and direction of their relative velocities shows that the galaxies are indeed moving towards each other, thus PIG-7351-1417 is a merger of three galaxies!
VelocityGauss(Disc Centred).pngRelativeVelocityPlotPIG-7351-1417(1 centred).png
In order to better show the motion of the galaxies I used python to collect data and then plot contours of the velocity of the Merger elements to show how the different parts of the merger are moving in relation to each other (See the figure below)Figure_SUMMARY.png.
It is worth noting the velocities I was able to calculate from the spectra only give the velocity of each component along the normal to the plane of the sky, in other words only the velocity towards/away from us. So the true motion of the galaxies cannot be found from the spectra collected by MUSU.
It would be interesting in the future to look into estimating the mass and obtaining better calculations for the metallicity and SFR of the merging galaxies. Having the masses of the galaxies would mean I could estimate the position of their centre of mass and the 3D velocities of the galaxies within the merger, and the metallicity and SFR would give an indication of the current state of the merger, as the metallicity apparently decreases during merging and the SFR increases during merging.
The Enterprise merger has been really fascinating to investigate and there is still much about this triple merger I would like to explore!


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