We combine the exceptional depth of the Hubble Ultra Deep Field ( HUDF ) images and the deep GRism ACS Program for Extragalactic Science ( GRAPES ) grism spectroscopy to explore the stellar populations of 34 bulges belonging to late-type galaxies at 0.8 \leq z \leq 1.3 .
The sample is selected based on the presence of a noticeable 4000 Å break in their GRAPES spectra , and by visual inspection of the HUDF images .
The HUDF images are used to measure bulge color and Sérsic index .
The narrow extraction of the GRAPES data around the galaxy center enables us to study the spectrum of the bulges in these late-type galaxies , minimizing the contamination from the disk of the galaxy .
We use the low resolution ( R \simeq 50 ) spectral energy distribution ( SED ) around the 4000 Å break to estimate redshifts and stellar ages .
The SEDs are compared with models of galactic chemical evolution to determine the stellar mass , and to characterize the age distribution .
We find that , ( 1 ) the average age of late-type bulges in our sample is \sim 1.3 Gyr with stellar masses in the range 10 ^ { 6.5 } –10 ^ { 10 } M _ { \odot } .
( 2 ) Late-type bulges are younger than early-type galaxies at similar redshifts and lack a trend of age with respect to redshift , suggesting a more extended period of star formation .
( 3 ) Bulges and inner disks in these late-type galaxies show similar stellar populations , and ( 4 ) late-type bulges are better fitted by exponential surface brightness profiles .
The overall picture emerging from the GRAPES data is that , in late-type galaxies at z \simeq 1 , bulges form through secular evolution and disks via an inside-out process .