The Significance of Thermally Pulsing Asymptotic Giant Branch Stars in Post-starburst Galaxies

We measure the significance of thermally pulsing asymptotic giant branch(TP-AGB)stars via the spectral energy distributions(SEDs)of a sample of post-starburst(PSB)galaxies at z=0.2-0.7.Using ground-and space-based photometry from the 3D-HST catalog,as well as associated near-infrared(NIR)Hubble Space Telescope(HST)slitless grism spectroscopy,we evaluate the importance of TP-AGB stars in the SEDs of 177 PSB galaxies by fitting simple stellar populations with different levels of TP-AGB contributions.The grism spectra,despite their low resolution of R~100,enable the detection of molecular features specific to TP-AGB stars and thus improve constraints on their contribution.A majority(~70%)of galaxies in the PSB sample show features indicative of TPAGB stars,while the remainder does not and they are well fit by Bruzual&Charlot TP-AGB light models.Stacked spectra of sources classified to be the best fit by TP-AGB heavy/mild models reveal strong detections of NIR molecular features associated with TP-AGB stars.Additionally,we observe a tentative trend with redshift where more TP-AGB heavy galaxies are observed in the higher redshift PSB galaxy population.Finally,neglecting the contribution of TP-AGB stars can yield an over-prediction of stellar masses measured in the K-band ranging from 0.13-0.23 dex.