# Centre (projected) of a galaxy cluster

A galaxy cluster consists of somewhere around a few tens to several hundred galaxies.

To measure the position of a galaxy cluster, i.e. it's RA and DEC, do astronomers always assume it to be its Brightest Cluster Galaxy BCG's position?

It has been shown by Johnston et al. 2007 that BCGs are not always found in the centre of a cluster/halo. If so, does the assumption of taking the position of a BCG to be the position of the galaxy cluster still hold true?

• Do you mean, what is the nominal ra/dec listed in certain catalogues? Note by the way that the barycenter of a 3D object, is by no means aligned with the 2D barycenter of the 2D "image" of the object as seen from a specific direction. (Indeed, note that the "center" of an extended 3D object is simply undefined. Picture anything ... say, the chair you're sitting on, or a tree - where is the "center"?) Oct 14 '16 at 13:08
• By centre, I only refer to the 2D centre, i.e. the RA and DEC. Yes you are right, not the 3D centre. Oct 14 '16 at 13:48

Longer answer, it's complicated. There are, in essence, five different measures for the centre of a galaxy cluster, based on different physical properties of the cluster, and occasionally not in agreement with each other.

• BCG, the brightest cluster galaxy. It should sink to the bottom of the gravitational well, but that's only the case if the cluster had enough time to evolve. Of course, galaxy clusters are, cosmologically speaking, quite young objects and still in the process of forming, so this is not a given.
• Mean position of the cluster galaxies. Works for spherically symmetrical clusters with lots of galaxies. Problems: Poisson noise due to limited number of galaxies, identification problems (is this galaxy really a part of the cluster), asymmetry.
• X-ray: the hot intra-cluster gas emits X-ray radiation that we can observe - more and hotter gas means more and harder X-rays and if the gas is in equilibrium, the highest intensity radiation comes from the centre. Unfortunately, the gas is typically not in equilibrium, there are cooling flows, streams, cold cores and other things that make the measurement less straight-forward.
• (Weak) gravitational lensing. Measures the total mass of the cluster and doesn't care about ugly gas physics. Problems: Shape noise (measurements are based on mean ellipticity of background galaxies), resolution / smoothing issues with the mass map created from averaging over lots of background galaxies.
• Sunyaev-Zel'dovich effect. Again the hot cluster gas, but this time we have inverse Compton scattering of photons in the cosmic microwave background on the electrons in the gas. The problems with gas physics should apply here, too.

So, there are other methods to find the centre of a galaxy cluster. This paper suggests that ususally the weak lensing centre is offset with respect to both X-ray and BCG. I am not aware of anyone using the mean position of the galaxies as a measure for the cluster centre.

This paper evaluates the cluster centre offset between measurements in X-ray and the SZ. This is interesting because they both trace the hot intra-cluster gas.

• Say, what is the "brightest cluster galaxy", BCG? Does that mean "the brightest galaxy, in, a given cluster" -? Or something else. Oct 14 '16 at 14:28
• Many clusters have a huge red elliptical galaxy in the approximate centre. It's (unless there's a merger between two clusters) quite obvious which one is the BCG, as it much larger than the other cluster galaxies. Here is what wikipedia has to say about it.
– Alex
Oct 14 '16 at 14:44
• OK, got it. My confusion was, "brightest cluster galaxy" is strange in English; it should be something more like "brightest galaxy in cluster". Anyway got it, thanks! Oct 14 '16 at 16:38
• Ok, now I understand your original question. You were thinking of "brightest galaxy in any cluster"? I'm afraid this is just one of the standing terms in astronomy. You can sort of guess from the name, but it has a precise meaning one just has to know. Could be a topic on meta - strange astronomy terms and names.
– Alex
Oct 14 '16 at 17:25
• yup totally got it now. great QA thanks! Oct 14 '16 at 18:11