Galaxy-Galaxy Strong Gravitational Lenses for Early-Type Galaxy Density Profile Measurements

Early type galaxies are typically pressure supported and lack bright kinematic tracers at large radii making the inference of their mass density profiles difficult if not impossible using conventional methods. One technique that overcomes these limitations is galaxy-galaxy strong gravitational lensing which enables the measurement of the mass-density properties of early type galaxies at cosmological distances. To date only a few hundred galaxy-galaxy strong gravitational lenses have been identified and examined limiting their utility for mass measurements to constrain galaxy evolution models.

Spectroscopic strong gravitational lens searches stand out for not only the volume of detections to date, but also for their ability to measure the red shifts of both the lens and source galaxies which are required for most lensing science cases. The Sloan Digital Sky Survey (SDSS) Baryon Oscillation Spectroscopic Survey (BOSS) has performed spectroscopy on over 1.5 million luminous galaxies out to a red shift of 0.7 providing an ample data set among which to find strong gravitational lenses. The incidence rate of galaxy-galaxy strong gravitational lensing has been estimated to be on the order of one percent, while current efforts to find these objects using SDSS spectroscopic data have had detection rates on the order of one in one thousand. This suggests that many more lens systems could be found in the BOSS data, but a major impediment to finding galaxy-galaxy lenses is due to the significant amount of sky noise in the long wavelength regions of the spectra.

Principal component analysis (PCA) has been implemented to create a basis by which to represent the sky subtraction residual errors, allowing for the post processing of the sky subtracted spectra minimizing subtraction systematics. Using this method on the BOSS spectra has resulted in the identification of nearly 500 multi line detections of galaxy-galaxy strong gravitational lenses where a multi-line detection is considered to be an [OII] doublet detection at a S/N of 3 or greater along with the detection of at least two other lines among [OIII], Hα, Hβ, Hγ, Hδ, [NII], or [SII] at a S/N of greater than 2.5 with all the detected lines occurring at the same red shift greater than the red shift of the lens. The early type lens galaxies found using this process have an average red shift of 0.45, with red shifts ranging from 0.18 to 0.65, and an average velocity dispersion of 231 ± 54 km/s with 123 having a velocity dispersion less than 200 km/s. This set of lenses provides an extensive mass selected sample from which to probe the mass evolution of early type galaxies.