PSF FWHM of 180 mas , and is comparable to that of the HST Advanced Camera for Surveys ’ F814W ( I-band ) filter ( Figure 2 ).
The Stellar Mass – Size Relation at z = 1
An accurate calculation of the stellar mass – size relation requires a robust determination of the stellar mass and physical size of the galaxies . We estimated the stellar mass for 49 of the SPT-CL J0546 – 5345 cluster member galaxies using public codes . We then fit publicly available stellar population synthesis models ( used as tools for interpreting the integrated light of galaxies ) to the derived photometry assuming a certain Initial Mass Function ( which describes how mass is initially distributed within a stellar population ).
We selected galaxies with ages older than 10 giga-years and younger than the age of the Universe ( details can be found in the article accepted for publication ; see end of article ). We used the publicly available code GAL- FIT ( Peng , C . Y ., et al ., AJ , 139 : 2097 , 2010 ) to measure the circularized effective radius , defined as r e
= ab , where a and b are the effective semimajor and semiminor axis , respectively , as a measurement of the physical size of the galaxies . The K s
-band stellar mass – size relation for the SPT-CL J0546-05345 cluster member galaxies at z = 1 and SDSS galaxies at z = 0.1 is shown in the top left panel of Figure 3 . The stellar mass – size relation at z = 1 is offset from the local relation by ~ 0.21 dex , with a slope consistent with the slope seen for galaxies with same stellar masses in the local Universe .
The results , obtained here for the first time for galaxies in a cluster environment , are consistent with previous findings for field galaxies , indicating that the primary mechanism for galaxy growth in size since z ~ 1 in clusters is either minor mergers or adiabatic expansion due to AGN mass loss winds , or both . If major
January 2017 | 2016 Year in Review mergers were a dominant source of the evolution in the stellar mass – size relation , then the most massive galaxies would experience the most rapid growth ; the slope of the stellar mass – size relation would then be steeper in the local Universe than at z = 1 — an effect we do not see in our results .
Figure 3 also shows the stellar mass-size relation for the cluster SPT-CL J0546-5345 compared with other work in the literature . Our results emphasize the importance of highresolution observations at the rest-frame wavelength of the cluster galaxies . Observations at shorter wavelengths — i . e ., at the B- band and U-band rest frame ( top-right plot in Figure 3 ) — show a shallower slope and larger scatter in radius , due to ultravioletbright star-forming knots . In addition , the effect of resolution ( bottom right plot in Figure 3 ) can lead to misinterpretation of the true physical mechanism involved in the extraordinary growth in size of the massive galaxies over time .
With this research we have demonstrated that wide-field , near-infrared adaptive optics observations , such as those we obtained with GeMS / GSAOI , are critical in order to characterize the galaxy population at high redshifts . GeMS is limited by the magnitudes of the natural guide stars ( NGS ) used to compensate for tip-tilt and plate-scale mode variation ( currently R < 15.5 magnitude ). Highredshift clusters are located in regions where bright stars are absent or have magnitudes fainter than the current limit . The new NGS
GeminiFocus
Figure 2 . The core of the SPT-CL J0546-5345 galaxy cluster at z = 1.067 . Left : GeMS GSAOI K s image . Right : HST ACS F814W image . The scale bar at the lower left shows the angular and physical projected distance at the cluster redshift . North is up and east is left .
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