SHREDS: Week 4

During the fourth week of our project the group continued working on the three areas of the project, investigating the Star Formation Rates, Sersic profiles and activity of galaxies. In the Active Galactic Nuclei (AGN) section of the project the main area of investigation is the activity of the supermassive black holes in active galaxies.

AGN are highly active supermassive blackholes of over 106 times the mass of our sun, at the centre of galaxies known as active galaxies. Dust and stars in the accretion disc of the AGN is heated as it falls towards the supermassive blackhole, which causes high energy radiation to be emitted in the form of X-rays. Using telescopes such as the Chandra space telescope we can observe the X-rays from the accretion disc of AGN and from the brightness (measured as the luminosity) of these galaxies we can find the accretion rate (used as a measure of the activity) of the AGN. The Black hole accretion rate is calculated using the equation shown below (equation 1) then converted into units of solar masses per year.

Where BHAR is the black hole accretion rate, epsilon is the accretion rate set to 0.1 for our calculations, c is the speed of light and LbolAGN is the bolometric luminosity of the AGN.

Due to the energy of the matter falling into the black hole some particles are launched into space from the edge black hole forming huge jets that extend out beyond the active galaxy. These jets of high energy particles emit light of both X-ray and Radio wavelengths and in some AGN form lobe structures as the particles collect in a region towards the end of the jets.

The first step in this part of the project was to identify all AGN in our catalogue, which was done using images from the Chandra Telescope and the VLA (Very Large Array), to then find the brightness of these active galaxies in our catalogue in both X-ray and Radio wavelengths of light. First the background noise was removed from the images and the signal of the active galaxies in our catalogue, which appear in the X-ray and Radio regions, was measured using a program called SExtractor. From the measured signal values the luminosity (a measure of brightness) of all galaxies was calculated, and the blackhole accretion rate found from the X-ray luminosity. Plotting the fraction of the galaxies in our catalogue which emit X-rays or Radio Waves at different redshifts we can see that the fraction of these galaxies which are AGN (or emit Radio or X-rays) peaks at around a redshift of 3 and is more level at higher redshifts, showing there was a peak in the number of AGN during this period of cosmological time (see figure 1). This plot is also the result of the week for this week!

Figure 1: Result of the Week! A plot showing the fraction of galaxies in our catalogue which emit X-rays or Radio Waves or are AGN (emit in both X-rays and Radio waves) at different redshifts.
Figure 2: Radio and X-ray image of SC4K-IA427-47810 showing distinct radio jets.

We then wrote a program using python to extract images of all our AGN in the catalogue from the Chandra and VLA images. This allowed us to obtain some interesting images of the AGN in multiple wavelengths. The result of the week from week 3 was one of the clearer AGN found and the images showed clear radio jets emanating from the supermassive black hole of the galaxy SC4K-IA427-47810 (see figure 2). The aim with these Radio and X-ray images is to create contour plots of the AGN using another python program which will hopefully show the structure of these galaxies more clearly than the noisy images we have at the moment.

With the accretion rate of our AGN calculated we where able to start investigating the relation between the activity of the supermassive black hole in AGN with the Star Formation Rate and Morphology of the galaxy using the data obtained by the other two sub-groups. Collecting the morphologies data and the AGN data we were able to make plots which show the trends in the shape of the galaxy about AGN. Plotting the fraction of AGN in our catalogue with different mean visual morphologies we found visual that there is a peak at a morphology of between 0 and 1, this shows that most AGN appear as point-like and elliptical when viewed in optical wavelengths (see figure 2).

Figure 2: Plot showing the fraction of our catalogue which are AGN, with different visual morphologies. 0 is point-like, 1 is elliptical, 2 is disky, 3 is irregular and -1 means the galaxy did not appear in Hubble images thus could not be visually classified.

The Morphologies sub-project also calculated the size of our galaxies at which different fractions of the total light is emitted, using two different methods. The radii from which 20%, 50% and 80% of the light of our AGN are emitted plotted against the Black Hole Accretion Rate shows there is a slight increase in the light radius with the accretion rate, indicating larger galaxies are likely to have more active AGN (see figure 3). Plotting the Sersic profile of the active galaxies shows that most AGN have a low Sersic profile, which suggests most of the light produced by them is from the central region of the galaxy, where the Supermassive Black hole is located, hence the emission of an active galaxy is dominated by the black hole at it’s centre (see figure 4).

Figure 3: Plot of the AGN activity (measured as the accretion rate) against the 20%, 50% and 80% light radii of the AGN in our catalogue.
Figure 4: Plot of the AGN (measured as the accretion rate) against the Sersic profile (a measure of the morphology) of the galaxies.

As AGN emit strongly in Radio and X-ray wavelengths the Star Formation Rates of AGN cannot be found using their brightness in UV or Radio and IR will underestimate the SFR. So one of our next tasks is to plot the Lyman-α  star formation rate against the black hole accretion rate of our AGN. During the next week we will also begin linking the three sub-projects back together in order to show how galaxies evolve over cosmic time, and how our own galaxy the Milky-Way may have looked earlier in its life.

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