Stellar Populations of Late-Type Bulges at z=1 in the HUDF

Abstract

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.