Gravitational Lensing in Clusters of Galaxies

Stefanie A. Gallegos

Dr. Michael Gladders

What are gravitational lenses?

• A gravitational lens is the result of the bending of light from a distant source around a massive object

• Giant arcs are the result of strong lensing• Deviation angle is a function of the mass of the lens (more mass=larger angle) and the observer-lens-source geometry

Credit: GeminiCredit: Gemini Credit: SDSS

Research Motivation

• Lenses are very rare events

• Can be predicted in detail from simulations

• Great way to see background universe (objects that are too faint)

• Help further define cosmological parameters and understand the nature of dark matter

Credit: SDSSCredit: Gemini

• Theory suggests that lensing clusters are a biased (concentrated, aligned etc.) subset of all clusters.

QUESTION: Do we see any evidence in the galaxy population of lensing clusters to suggest that this is true?

Research Goals

Credit: Gemini Credit: Gemini

• Sloan Digital Sky Survey (SDSS) and the Gemini Observatory for follow-up images

Data Sources

Credit: SDSS Credit: Gemini Observatory

• The Sloan Digital Sky Survey (SDSS) is a multi-filter imaging and spectroscopic redshift survey using a 2.5-m wide-angle optical telescope at Apache Point Observatory in New Mexico

Research Approach• Do the same thing with a comparison sample, which consists of clusters of the same richness and at the same redshift

• Measure galaxy properties from both samples

Credit: Gemini

• Begin by looking through a catalog of follow-up images and identifying cluster centers and positions

Findings -- Luminosities• Comparison sample

-1.26 magnitudes brighter than M*

• Lensing sample

-1.78

• Large Einstein Radii

-2.05

• Lenses with Einstein radii < 7.5 arcsec

-1.60

Note: Uncertainty <<0.1

Findings -- Offsets• Positional offset of

central galaxy to weighted mean position of the other cluster members

• Comparison sample

31±1.6 arcsec

• Lensing sample

24±3.5 arcsec

• Large Einstein Radii

21±2.2 arcsec

• Lenses with Einstein radii < 7.5 arcsec

32±2.7

In the absence of central galaxy, cluster median is found from moment distributions

Conclusions• Magnitude differences between central galaxies of lensing sample vs. comparison sample

Lensing clusters have much more luminous central galaxies!

• Positional offsets are smaller in lensing clusters

Lensing clusters are more relaxed

• Hypothesis 1: Lenses are seen typically in old, relaxed clusters of extreme density

• Hypothesis 2: Lenses are the result of temporarily intense moments of density due to mergers

What we found: Hypothesis 2 appears

unlikely This is because the central galaxies are so luminous and well-centered that the clusters must be old and dynamically relaxed

Theoretical Expectations for Lensing

Credit: Gemini

Future Prospects• Shapes, concentration and substructure

• Cluster mass estimates

• With this data supporting common characteristics of lensing clusters, it will be easier to identify them in future surveys.

• Mass to light ratios dark matter halos

Credit: KICP Giant Arcs Collaboration Effort