Figure 2 .
DES J0221- 0646 . The spectra for source images A1 and A2 are shown on the left . The gray-scale DES g-band image in the upper right shows where we placed the GMOS slits as needed to optimize flux from the source images . In the color co-add image in the middle right panel , the Einstein radius is marked by a white arc .
redshifts provide a measure of distance , but they are relatively imprecise and much less reliable for the more remote galaxies likely to be lensed .
To measure redshifts , we look for specific features , like emission or absorption lines , in the spectra of source galaxies : the higher the redshift , the further these features shift from their rest wavelength . We use the R150 grating in conjunction with the GG455 filter to obtain spectra with a wavelength coverage of 4500-10000 angstroms ( Å ). For the sources that we expect to be late-type emission line galaxies , this would allow us , in many cases , to detect [ OII ] 3727 to z ~ 1.7 , H-beta to z ~ 1.0 , and Lyman-alpha in the range 2.7 < z < 7.2 . We use the B600 grating to obtain spectral coverage of 3250-6250 Å , which would allow us to detect sources with z > 2.0 that emit Lyman-alpha .
We acquired spectroscopic data for 21 of the 53 systems : 17 were observed at Gemini South ( taken through the Gemini Large and Long program ), and five were observed with the Inamori Magellan Areal Camera and Spectrograph at the Magellan / Baade telescope ( with an overlap of one system ). We confirmed six strong lensing candidates with Gemini South , rejected two , and the status of 13 remain inconclusive . For the rejected systems , we found that the putative source galaxies were actually foreground galaxies . The inconclusive systems may have spectral features at higher wavelengths , which would require the use of other instruments or telescopes .
Let ’ s look in detail at some of the confirmed systems . Figure 2 shows spectra of the source galaxy images A1 and A2 , along with images of the lensing system DES J0221- 0646 . Taking into account the absence of other spectral features , we assign the features at 4535 Å to be Lyman-alpha , which gives a redshift of ~ 2.752 , placing it behind the redshift 0.672 lensing galaxy .
The middle color image of Figure 2 shows an Einstein radius estimated by manually fitting a circle that passes through the spectroscopically confirmed source images , where the center is chosen to be the arcs ’ center of curvature . With these radii , we estimated the mass enclosed within that circle . This system ,
14 GeminiFocus January 2017 | 2016 Year in Review