GeminiFocus January 2017 | Page 6

Figure 1 . False-color image of
SPT-CL J0546 – 5345 at z = 1.067 . Gemini
GeMS / GSAOI K s
( this work ) = red , HST ACS F814W = green , HST
ACS F606W = blue . The red polygon shows the approximate sky coverage of the GSAOI pointings . PSF stars are indicated by white circles . The star near the top of the image with two circles is a binary star . The inset at lower left is a zoom-in of the cluster core . The scale bars in the inset and full image show the angular and physical projected distances at the cluster redshift . The red , green , and blue spots on the scale bar in the inset show the PSF FWHM for each band . North is up and east is left .
One important open question remains . It concerns how these galaxies grew in size and what main physical mechanism ( s ) are involved . Mergers of galaxies with similar stellar masses ( major mergers ), accretion of small satellite galaxies ( minor mergers ), and rapid mass loss caused by active galactic nuclei ( AGN ) or supernova winds ( adiabatic expansion ) could all contribute to the dramatic growth in the size of these galaxies . We can distinguish between these physical mechanisms by comparing the slope of the stellar mass – size relation of the massive high-redshift galaxies with the local sample .
Accurate determination of the stellar mass – size relation depends most strongly on the resolution of the images , the rest-frame wavelength of the observations , and the number of galaxies observed . Superb resolution is required to accurately measure the effective radius for the most compact galaxies at z > 1 . We need rest-frame observations at wavelengths longer than the 400 nanometer spectral break to restrict the observations to galaxies dominated by the light of the underlying old stellar population . The accuracy of the stellar mass – size relation is also improved as the number of galaxies observed increases . Previous high-angular-resolution images obtained from the ground are limited by the small effective field-of-view of the instruments used for the observations . Given the large field-of-view provided by the Gemini Multi-conjugate adaptive optics System , combined with the Gemini South Adaptive Optics Imager ( GeMS / GSAOI ), we avoid this problem by simultaneously observing several galaxies in a single field .
A High-angular-resolution View of Cluster Galaxies at z ~ 1
We used GeMS / GSAOI , the world ’ s most advanced adaptive optics system , to image galaxies in the cluster SPT-CL J0546-5345 at redshift z = 1.067 . This cluster , detected as part of the 2,500 square degree South Pole Telescope Sunyaev-Zel ’ dovich survey ( Brodwin , Mark , et al ., ApJ , 721 : 90 , 2011 ), is a massive cluster with a virial mass of 10 15 M Sun and several compact massive spheroid-like galaxies at its center . We imaged the galaxies using the K s filter at 2.2 microns . The final combined image presents a variation of the Point Spread Function ( PSF ) Full-Width Half-Maximum ( FWHM ) between 80-130 milliarcseconds ( mas ) within the GSAOI 85 ” x 85 ” field-ofview ( Figure 1 ) — translating to a physical size between 0.66-1.07 kiloparsecs ( kpc ) within a field-of-view of 697 kpc 2 ( 1 ” = 8.2 kpc at the cluster ’ s rest frame ). The PSF FWHM achieved with GeMS / GSAOI is , on average , a factor of 1.5 better than the angular resolution provided by the H 160 filter
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