SHREDS: Week 3

Our third week of labs was setup mostly the same as last week, with each sub-project already split into teams and each team having a solid plan to work with. In the morphologies sub-project, analysis was done by fitting a surface brightness profile to a stacked image of Lyman α emitters. This is useful as most galaxies follow a certain function depending on their Sersic index number which means general insights into these galaxies can be made. The Sersic index was calculated last week as part of the Sersic data which was found using Galfit. The Sersic profile follows the equation: 

Where I(r) is the intensity at a radius r, I0 is the intensity at the centre of the galaxy, α is the scale length (radius at which the intensity has dropped by e1) and n is the Sersic index. 

Another profile, which is a special case of the Sersic profile, is De Vaucouleurs profile which is when n=4 and follows the equation:

Where Iis the surface brightness at Re, which is the radius that contains half the total luminosity. 
Figure 1 shows an image with different sized masks that were used to find the surface brightness profile of the image. The number of pixels was counted inside the area of each mask.

We then ran a Python script on the stacked image which counted the number of pixels at radii away from the centre of the stacked image. For example, this is the original stacked image and examples of different sized masks in which the number of pixels were counted.

The surface brightness of the stacked image against the radius can now be plot with the new data.

Figure 2 shows a plot of the surface brightness within the mask against the radius aways from the centre of the image.

Clearly, the data mostly fits the exponential fit, which is when n is 1 in equation 1. This suggests that Lyman α emitters tend to be disky galaxies, however elliptical are still common. However, when r>=4 the exponential fit no longer agrees with the data but a combination of the De Vaucouleurs fit and the exponential fit does. This could suggest that there is more than one object that is being detected in the surface brightness profile. 

Further evidence of more than one object being detected can be found by plotting the residuals from a sample fit of the surface brightness against the radius in arcseconds. 

Figure 3 shows a plot of the residuals from a sample fit of the surface brightness against the radius in arcseconds.

When the masks reached the edge of the galaxy it would be expected that the residual noise would be random since they are measuring background noise. However, since a pattern can clearly be seen, this could suggest that more than one physical region is being detected by the surface brightness profile. Possible candidates for these regions are the galaxy itself and a galactic halo. 

Result of the Week:

The result of the week for this week is the combined Radio and X-ray image of one of the galaxies, SC4K-IA427-47810, in our catalogue which was found to have an active galaxy, with a supermassive black hole at its centre. The image shows the jets of high energy particles thrown off the active galactic nucleus (AGN) and the bright region of the accretion disk of the supermassive black hole.

Figure 4 shows this week’s result of the week, a Radio and X-ray image of the AGN SC4K-IA427-47810.

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