  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 z=0.8-1.3. The sample is selected based on the presence
of a noticeable 4000A break in their GRAPES spectra, and by visual inspection
of the HUDF images. The HUDF images are used to measure bulge color and Sersic
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~50) spectral energy distribution (SED) around the 4000A 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 ~1.3 Gyr with stellar masses in the range log(M)=6.5-10 solar.
(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=1, bulges form through
secular evolution and disks via an inside-out process.

Astrophysics