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Research Fellows Directory

Philip Marshall

Dr Philip Marshall

Research Fellow


University of Oxford

Research summary

The gravitational pull of massive galaxies deflects passing light rays, and

produces a distorted image of the background source. If the alignment is good

enough, multiple images form in characteristic patterns around the deflector

as the light is focused towards us. By measuring these patterns, we can weigh

these "gravitational lens" galaxies: part of my research is to develop new

ways of doing this, while another part is to try and find more instances of

strong lensing in very large surveys of the night sky.

Why is it important to measure mass? Galaxies form from smaller galaxies

merging together under gravity; knowing a galaxy's mass is key to

understanding its history. In the inner parts of massive galaxies, where the

lensed images appear, the stars and dark matter in the galaxy each make a

significant contribution to the mass. However, we can measure mass so

accurately that we can separate the two. For example, we found recently that

elliptical galaxies must have formed a certain mix of stars (some heavy, some

light), while disk galaxies formed a different one. This is important

information, because how stars form out of gas clouds is still not


Being able to accurately measure the mass distribution in strong gravitational

lenses means that we can use them in a high precision optics experiment. If

the source behind the lens is variable, we can measure the time delays

between each image's flickerings, and infer a characteristic distance in the

system. Using high resolution deep imaging of the Einstein ring, we have now

made a second distance measurement in this way, and used it to infer the

values of the cosmological parameters that govern our model for how the

Universe is expanding. The largest uncertainty in this measurement is due to

the unknown mass that could be lying not in the lens galaxy but along the line

of the sight; we have begun to try and model this in detail, a challenging


Interests and expertise (Subject groups)

Grants awarded

Dark Matter and Dark Energy with 2020 Gravitational Lenses

Scheme: University Research Fellowship

Dates: Oct 2010 - May 2013

Value: £415,293.60