We analyze how passive galaxies at z \sim 1.5 populate the mass–size plane as a function of their stellar age , to understand if the observed size growth with time can be explained with the appearance of larger quenched galaxies at lower redshift .
We use a sample of 32 passive galaxies extracted from the Wide Field Camera 3 Infrared Spectroscopic Parallel ( WISP ) survey with spectroscopic redshift 1.3 \lesssim z \lesssim 2.05 , specific star–formation rates lower than 0.01 Gyr ^ { -1 } , and stellar masses above 4.5 \times 10 ^ { 10 } \mathrm { M _ { \odot } } .
All galaxies have spectrally determined stellar ages from fitting of their rest-frame optical spectra and photometry with stellar population models .
When dividing our sample into young ( age \leq 2.1 Gyr ) and old ( age > 2.1 Gyr ) galaxies we do not find a significant trend in the distributions of the difference between the observed radius and the one predicted by the mass–size relation .
This result indicates that the relation between the galaxy age and its distance from the mass–size relation , if it exists , is rather shallow , with a slope \alpha \gtrsim - 0.6 .
At face value , this finding suggests that multiple dry and/or wet minor mergers , rather than the appearance of newly quenched galaxies , are mainly responsible for the observed time evolution of the mass–size relation in passive galaxies .