Through the Looking Galaxy

May 5, 2015

A KIPAC researcher uses images of very distant galaxies to learn about somewhat nearer galaxies, through the phenomenon of gravitational lensing.

The right panel shows a background galaxy with the image of the lens galaxy (in the center) removed. A proper model of the mass distribution of the lens galaxy results in the reconstructed shape for the background galaxy in the left panel.

One of Einstein's most stunning predictions was that matter on large scales would bend light, acting as a lens to magnify and distort the images of objects behind some massive object. This has been observed on scales from the Solar System to the distant Universe, and has played a role in discoveries ranging from planets around other stars to dark energy.
 
With so-called 'weak' lensing, where large scale mass distributions subtly distort the shapes of background galaxies, astronomers can untangle how structure has grown in the history of the Universe, and through that probe crucial parameters of dark energy. This will be one of the major ways in which the LSST and Dark Energy Survey optical telescope surveys, with strong participation by KIPAC, will study dark energy in the Universe. A rarer phenomenon is 'strong' lensing, where a foreground and background object align just right so that the background one's light is bent into multiple images on the way to us, in the manner of light through the bottom of a wine glass. Because strong lensing is dependent on the total mass of the lens, it is a way to directly measure the dark matter in a system, even though that matter is unseen.
 
The shapes we see in strong lensing result from the combined effects of the distances involved, the inherent shape of the background object, and the mass distribution of the foreground object. When the foreground object is a galaxy, the lensing phenomenon offers a chance to learn about the matter distribution of that foreground galaxy that would otherwise be too distant to determine in detail. KIPAC postdoc Matteo Barnabe, along with four colleagues from the US, Austria, and Netherlands, set out to do just that, in order to understand the detailed mass structure of elliptical galaxies that are farther away than we could otherwise resolve well with telescope images, but happily happen to be lenses.
 
Barnabe and colleagues identified candidate lenses with data from the Sloan Digital Sky Survey, and took optical images and spectra of their targets with the Hubble Space Telescope. A data analysis routine - given the name "Cauldron" - pioneered by Barnabe and Leon Koopmans of the University of Groningen in Netherlands, was used to put together the lensing image information along with the spectral data which show how stars in different regions are moving in the galaxy. In combination these observations can extract the maximum information about the galaxies' mass distributions and kinematics.
 
They find that these elliptical galaxies do not differ significantly from the very nearby ones which are well studied. The inferred mass distribution indicates that the fraction of dark matter in the central part of the galaxies is modest but larger than was thought in the past - between 30% and 70% - but dark matter dominates the total mass on larger scales, which is consistent with observations of local galaxies, including our own. Barnabe and colleagues also find that the total fraction of dark matter increases with increasing mass, so that the largest galaxies are also the ones most dominated by dark matter. Finally, these elliptical galaxies break into two sets such that in only the smaller systems are the galaxies rotating in an ordered fashion where the stars' orbits about the center dominate over more random motions, whereas the largest mass systems do not show this ordered rotation. These details could only be reconstructed by combining the information from lensing with that of star motions from the spectra, and this work represents the most precise and detailed study of the mass structure and distribution of individual galaxies ever done for distant systems .  Learning more about these farther away galaxies and comparing them to more local galaxies is important for understanding the evolution of galaxies in the Universe.
 
This work is based on a paper to be published in Monthly Notices of the Royal Astronomical Society and available from astro-ph at arXiv:1102.2261.
 
Science Contact:
Matteo Barnabe
KIPAC
mbarnabe@stanford.edu