Paper Review: Does black-hole growth depend on the cosmic environment?

This paper (accessible from:, by Yang et al. (2018), asks whether the growth rate of supermassive black holes (SMBHs) at the centres of galaxies depends on its ‘cosmic environment’. A galaxy’s ‘cosmic environment’ is a fancy way of saying whether it is closely interacting with other galaxies (in a ‘cluster’ or ‘filament’), or fairly isolated (in the ‘field’). The paper gives much more information about all the techniques the authors used – it is very rigorous, and I won’t go into every test they do here.

Using the ‘COSMOS’ survey, the authors made a catalogue of 170,000 sources, and classified them according to their cosmic environment. This is done on small scales (<1Mpc), referred to as the ‘overdensity’, and larger scales (1-10Mpc), which they call the ‘cosmic web environment’. The authors determine they were isolated (in the ‘field’), in a cluster, or in a ‘filament’ (a string-like region connecting clusters). They did this by making looking at a circular region around each source, and comparing the density of galaxies around each source to observations of nearby galaxies.

It is known that a galaxy’s evolution depends strongly on its cosmic environment. In general, galaxies that are in a cluster have lower star formation rates than those in the field. To form stars, cold gas is needed (hot gas is too energetic to collapse to form stars). Galaxies in dense regions can have their cold gas depleted through tidal interactions and ‘ram pressure stripping’, for example. This is known as ‘feedback’.

Since the growth of SMBHs also depends on a supply of cold gas, it is reasonable to ask whether there is a similar effect on the black hole accretion rate (BHAR) due to environment. AGN are compact regions of extremely high luminosity. The energy source comes from SMBHs, so looking at AGN is a good way of finding a SMBH.

To find AGN (and so SMBHs), the authors looked for X-ray sources in the Chandra Observatory data, and matched them to sources in the COSMOS survey (they couldn’t classify environment based on the X-ray data, since normal galaxies do not radiate X-rays). To maximise the number of SMBHs, they look at redshifts from 1.5-2.5, when AGN activity peaked.

Screen Shot 2018-07-25 at 13.56.19.png
Example image of overdensity (top) and how this was used to classify galaxy environment (bottom)

At first it appears that SMBHs in denser regions have higher accretion rates (see first graph, below). However, these results are biased by the galaxy mass. More massive galaxies have more material, so SMBHs in massive galaxies can grow faster than those in smaller galaxies. These galaxies are also more likely to be found in clusters (since they have a stronger gravitational pull, and attract more galaxies towards themselves). So is this trend actually due to the cosmic environment, or just the mass of the galaxy?

To see if the trend is real, the authors consider subsets of high and low mass galaxies. After doing this, they find that the dependence of BHAR on environment disappears. Instead, the BHAR depends strongly on galaxy mass, regardless of environment.

Screen Shot 2018-07-25 at 14.06.12.png
At first there appears to be a correlation between accretion rate (BHAR) and overdensity (1+δ)…



Screen Shot 2018-07-25 at 14.05.05.png
…But once you correct for the dependence of BHAR on mass, the correlation disappears.

So what are the physical implications of this? It suggests that the growth of SMBHs is not correlated with the galaxy growth. This is in contrast to some previous studies, which did not control for galaxy mass as carefully as the authors of this paper.

tldr: Accretion rate of supermassive black holes depends on the mass of the host galaxy, not where the the host galaxy is.