Gravitational Lensing and Stuff
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Gravitational Lensing in Clusters of Galaxies
Stefanie A. Gallegos
Dr. Michael Gladders
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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
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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
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• 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
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• 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
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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
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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
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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
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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
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• 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
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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