Final Instalment – Results

We split our LAEs into Active Galactic Nuclei (AGN) sources and star forming only sources. The scale-length of the LAEs in this case are measured in the same way as when measuring the stack of all LAEs at each redshift slice. We only have AGN sources that we can measure the halo for at low redshifts. Other redshift slices either contain no AGNs in the catalogue or we are unable to measure the halo for these. 

We measure the scale-lengths using the two different methods mentioned in the previous instalment. One is to perform the fit to the surface brightness profiles from the radius corresponding to 96% of the PSF in that filter to a radius of 5 arc-seconds and then secondly, is to perform our fit to the PSF subtracted surface brightness profile measurements across the whole radius range of detections. 

Figure 1 – Redshift evolution of the scale-lengths measured from the AGN only stacks (circle marker) and star forming only stacks (star marker) separately. The scale-lengths are shown measured from the two different approaches of fitting to the halo only (green) and from 96\% of the PSF (blue) with the median values plotted with dashed lines for AGN and dotted lines for star forming. AGN only points are shifted by 0.1 in redshift in order to make the points more visible.

The scale-lengths produced by the fit from 96% of the PSF are on average lower than those given by the fit to the halo only (PSF subtracted profile). We trust the PSF subtracted measurements over the others, as we can be sure these are not measuring the size and extension of the central bright regions of the galaxies and is only probing the halo. However, for the AGN only sources there are only two measurements of scale-length at z ~ 3 and z ~ 3.33, the PSF subtracted method, as the rest of the redshifts are unable to perform a fit due to not enough detections. This is due to the errors on the PSF subtracted measurements being a combination of the PSF errors and the SB profile measurement errors. 

The two methods are also in disagreement over whether the AGN or star forming sources produce LAEs with larger scale-lengths. The halo only (PSF subtracted) suggest that the star forming sources are on average producing greater scale-lengths than the AGN sources since the median is higher, however it is not a significant different. The fit from 96% of the PSF suggests the opposite, that the AGN driven LAEs produce greater scale-lengths than star forming driven LAEs, since the median in this case is higher. Again, the difference is not significant suggesting that in fact AGN and star-forming driven LAEs produce similar scale-lengths for the LAH. 

Figure 2 – Scale-lengths over redshfit for the AGN only stacks and sub-stacks of radio (square) and X-ray AGN (triangle). The median of Wisotzki et al (2018) results with MUSE are included in the plot for reference, dashed line. All AGN points are plotted at the median redshift, radio AGN are displaced by 0.5 in redshift and X-ray AGN by -0.5 in redshift, in order to aid visualisation of the points.

Next, we used new stacks that divided the AGN category further into radio AGN and X-ray AGN. The AGN and sub stacks of radio and X-ray only are plotted in Figure 2, with the scale-lengths determined from the two different fitting methods. The point at z=2.5 seems to suggest that X-Ray AGN produce greater scale-lengths than the radio sources, however the error bars on this point are considerably larger than any other. We cannot draw conclusions from the small subset of measurements we have, as to whether there is a difference in the haloes produced by the two different types of AGN. 

The next step to be able to do this would be to combine our stacks of LAEs across all redshifts to improve the data and the number of AGNs in a stack and thus improve the measurement of the scale-length for this stack. This way we would be able to see if overall the scale-lengths differ between the two samples of AGN. We are able to do this since we observe that there appears to be no evolution in the scale-length over redshift in the range that we investigate. In order to stack across redshift, we must first PSF match the images. The data taken in one filter is of different quality of that in the next filter due to different atmospheric seeing and the different effects this has on observations in different filters. We can worsen our images such that they all have the same PSF – the PSF of the ‘worst’ image. Then we will have more LAEs for the radio and X-ray stacks making the stacks more reliable and the possibility of measuring an accurate scale-length more likely. We can then compare the two measurements to see if the radio or X-ray AGN are, on average, producing different size LAHs. My supervisor, David Sobral, will carry out this work to further investigate the LAE stacks. 

Figure 3 – Scale-length vs luminosity, with the scale-lengths measured from the PSF subtracted surface brightness profile for the luminosity constrained stacks over the entire redshift sample.

Using new sub stacks we also investigated if the luminosity of the LAE correlates with its scale-length. The LAEs at each redshift were split into four luminosity bins; log10(LLya) = 42.5 – 42.8, 42.8 – 43.0, 43.0 – 43.3 and >43.3. Stacks were created for each luminosity bin for each redshift and the best fit scale-lengths derived from fitting an exponential profile to the measured SB profiles of each stack, as was done previously for the other stacks. Figure 3 shows the results of these luminosity sub stacks and it can be noted that no clear trend is obvious for each redshift or overall and it is suggestive that all luminosity LAEs produce the same size LAHs. 

Figure 4 – The redshift evolution of scale-lengths for this work and past literature. The three different approaches for fitting to the surface brightness profile to measure the scale-lengths are shown on the plot with the medians plotted with a dashed line in the corresponding colour.

Figure 4 shows the final results plot of the scale-length evolution over redshift as presented in the last instalment. I have included it again in this blog post as it presents all of my final results plots. The Wisotzki 2018 measurements were taken with MUSE and are the best measurements of the Lyman alpha haloes to date. The median of these values is in agreement with our results from fitting to the halo only. This sample of LAEs does not overlap with the SC4K sample at all and measures very different LAEs. The SC4K catalogue spans a large area of the sky where as Wisotzki image a small area of sky, going really deep and detect faint LAEs, which we do not have in our work. However, the fact that the results match up so well suggests that the scale-length is definitely invariant of luminosity of the LAE and that all LAEs are producing similar size LAHs. 

Figure 5 – Surface brightness profile for the narrow band LAE stacks with the PSF plotted in black and any respective fits that were successful in deriving a scale-length. The SB was measured using anuuli photometry.

Figures 5 and 6 present the surface brightness profiles for the narrow and medium bands respectively constructed in this study in order to measure the halo of the LAEs. The SB profiles were measured using annuli photometry starting from r=0.05 to 10.05 arc seconds in steps of 0.25 arc seconds. A measurement was only considered a detection if the S/N was greater than 3 and this is shown as a star marker in the plots, any non detections were re-set to 3 times the noise and presented as an upper limit using a triangle. The black edge measurements are of the SB profile measured from the image stacks and the coloured edge points are of the PSF subtracted SB profile results. The error on these points are a combination of the SB profile and PSF error such that some detections in the SB profile became non detections in the subtracted profile as the noise had changed such that the measurement was no longer above the S/N cut. Again the non-detections were reset to 3 times the error and presented as an upper limit. The SB profile and PSF subtracted profile measurements were then fit with an exponential profile, to determine the scale-length, if there were at least 3 consecutive detections.

Figure 6 – Surface brightness profile for the medium band LAE stacks with the PSF plotted in black and any respective fits that were successful in deriving a scale-length. The SB was measured using anuuli photometry.

In the first instalment, I mentioned about how studying the evolution of LAHs across redshift can help to constrain the time in which the epoch of reionisation occurred, as the size of the halo should increase during this epoch. However, our data is not deep enough to be able to detect LAHs at the redshifts required (z~ 6 and above) to aid the understanding of this epoch, but the methods could be in future applied to deeper, higher redshift data to aid this investigation.

The work I have completed this summer has led to me being able to write a scientific paper, with David, to present our methods and findings. I am really proud of the work I have managed to complete this summer, none of which would have been possible without the funding I received from the Ogden Trust. I am extremely grateful to have been given this wonderful opportunity, and for all the support I have received from David, not only this summer but throughout my time at University. I am extremely grateful for all of the support and guidance I have received from my supervisor and also the other XGAL interns. Although we were all working on our own individual projects this summer, there has been a real sense of team spirit in the astro lab, and I have really enjoyed being a part of this and getting to know such great people!


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