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Abstract

Gravitational lensing is a powerful tool to probe invisible objects. General Relativity stated a massive foreground object deflects background light. Such deflection of light can magnify and deform the background light to create lens images. By observing lens images, astronomers can detect foreground invisible object, such as super massive black hole (SMBH) and dark matter (DM).

In my MPhil study, my 3 main scientific questions are:
1. How well can gravitational lensing probe SMBH?
2. Does the brightest cluster galaxy (BCG) have its own galactic DM halo?
3. What is the nature of DM?

For question 1, the galaxy cluster MS 2137 has been found the curvature of the radial arc cannot be reproduced by lens model predictions (Gavazzi, et al. 2003; Donnarumma, et al. 2009). This disagreement gave a clue that something is missing in the lens modelling, maybe a supermassive black hole (SMBH). Since their models were simple, I construct a robust lens model for this cluster by including all cluster members which they did not. My result finds the lens modelling do not require any supermassive black hole. MS 2137 cannot be used to probe SMBH.

For question 2, simulations of galaxy clusters (e.g. Borgani & Kravtsov 2011) show the only a cluster scale dark matter (DM) halo exists at the brightest cluster galaxy (BCG). It is not clear that does the BCG has its own galactic DM halo like the other cluster members. Both relaxed MS 2137 and merging MACS J1206 have lensed radial arcs at the BCG region. I construct lens models to infer the existence of BCG galactic DM can or cannot improve the lens images prediction. I can then see how does the state of cluster related to BCG galactic DM. My result from MS 2137 shows it cannot be told whether the BCG galactic exists or not.

For question 3, the most accepted DM model is the Cold Dark Matter (CDM). CDM states DM is weak interacting massive particle which interact with other particles by gravity only. CDM agrees very well in the Cosmological scale. However, CDM cannot reproduce the observed flux of lens images, named as flux ratio anomalies. A competitive model Wavelike Dark Matter (ΨDM) states DM is wavelike which undergoes quantum interference to form under and over dense granulations in galactic scale. It gives the same Cosmology as CDM does. It predicts such ΨDM granulation can locally magnify or demagnify the lens images. So that solve the flux ratio anomalies. The cluster-galaxy lens MACS J1206 and the galaxy-galaxy lens iPTF16geu are shown to have flux ratio anomalies in Chen, et al. (2021) and Mörstell, et al. (2020). I construct CDM model for these lenses. Then further investigate how likely can ΨDM improve the model.