### Different aspects of Active Galactic Nuclei can aid or slow star formation. How they play together is not well known.
https://www.nasa.gov/sites/default/files/thumbnails/image/hubble_survey_black_holes_05282015.jpg
[source](https://www.nasa.gov/sites/default/files/thumbnails/image/hubble_survey_black_holes_05282015.jpg)
# Winds Both Compress and Blow Away Gas
Giant clouds of gas and dust float around in space. These clouds of gas eventually collapse and create stars or are dispersed.
### Active Galactic Nuclei
Active galactic nuclei, AGN for short, are black holes at the center of galaxies that are shooting out matter and energy. This happens when the black holes are feeding on gas clouds or stars. While AGN are eating gas they are often brighter than the rest of the galaxy they inhabit. Few gas clouds last beyond a million years.
Outshining the rest of the galaxy will have an effect on the rest of the galaxy. This energy heats up the gas clouds in the galaxy. Under normal conditions this would spread out and push the gas clouds away from the AGN. The outflow would normally stop star formation, but this may actually act as a catalyst for the compression of the gas.
### Simulation Setup
Physics, especially astrophysics, can no longer operate without simulations. Nobody has millions of years to wait and see what happens.
Simulations were run to calculate the likely effects of the AGN under multiple conditions. The simulation contained a range of AGN with different luminosities and different gas compositions.
Over time different particles turn into “sink particles”. Sink particles are the equivalent of compressed gas clouds with a higher probability of forming stars. These tend to absorb a lot more energy from the AGN. The gas cloud must hit a certain density and reach a temperature floor before it will form a star.
Before the AGN is turned on the gas is left to rest for about one million years.
https://www.nasa.gov/sites/default/files/images/156013main_image_feature_643_ys_full.jpg
[source](https://www.nasa.gov/sites/default/files/images/156013main_image_feature_643_ys_full.jpg)
### The Results of the Simulations
As soon as the AGN is turned on the gas becomes turbulent. The highest and lowest density parts vary by a factor of 30. About 7.6% of the gas is absorbed by the black hole and 0.15% of the gas is turned into sink particles.
The AGN compressed gas near the leading edge of the gas. This gas formed into pockets and bubbles that tended to break if the AGN lasted for long periods of time. Overall once the AGN was turned on the gas heated up and was blown away, reducing the amount of clumps that formed. After the AGN was switched off the fragmentation resumed and in some places increased. This increase was likely due to the prior compression of the gas due to the AGN pushing it together.
The luminosity of the AGN was measured in terms of L, which equaled: 1.3×10^46 erg s^−1. They tested a range of luminosities from L0.5 to L10. A luminosity of L0.5 to L1 had no major difference from the control while and L5 to L10 completely stopped star formation. The small difference from L0.5 to L2 was likely because the effect of the extra clumping was about equal to the effects of keeping most of the gas above the temperature floor. The problem with L4 to L10 was that it blew away too much gas and much of it never had a chance to clump together. The only luminosities to show an increase over the control were L2, L3, and L4. L2 increased star formation, but only by a small amount. L3 had the greatest increase in star formation over the baseline. The L3 had a period where all the gas flowed outward but then collapsed back in towards the black hole, promoting star formation. L4 had a non-zero amount of star formation, but most of the gas was blown away before it could clump together.
Next they decided to see where the fragmentation was occurring. The did this with the luminosities of 3 and 5. In the beginning the fragmentation occurs closer to the center edge of the gas cloud. During the time the AGN was active nothing really happened. Once the AGN is turned back off the gas then starts to clump back up. In the L3 fragmentation occurs once again not only near the center, but also near the outside edges. This happens even more with the L5, but with the L5 little gas fragments near the center.
Later they wanted to find the effect on individual gas clumps. They picked a random 5113 particles and only 182 turned into sink particles before the AGN is turned on. In both L3 and the control simulation outflow filaments are created. In L3 these filaments are more heavily compressed by the AGN and end up forming more stars within them. In L5 there is a greater outflow, but it is less dense. Only 76 sink particles are created during the L5 simulation. The L5 simulation does support many areas of high density but they are dispersed before the AGN turns off and do not end up fracturing and creating stars.
This leads to the conclusion that AGN can create starbursts, but can also prevent star formation. Stars require a cool and dense clump of gas to form, the AGN can create clumps and time can cool them.
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This is a summary of [this](https://arxiv.org/pdf/1703.10782.pdf) Arxiv paper.
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