Date post: | 22-Nov-2014 |
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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