2018MNRAS.478...41S

Studying bright (M_B < -20), intermediate-redshift (0.4 < z< 1.0), disc-dominated (n_B < 2.5) galaxies from Hubble Space Telescope/Advanced Camera for Surveys and Wide Field Camera 3 in Chandra Deep Field-South, in rest-frame B and I band, we found a new class of bulges that is brighter and more compact than ellipticals. We refer to them as 'bright, compact bulges' (BCBs) - they resemble neither classical nor pseudo-bulges and constitute ∼12 per cent of the total bulge population at these redshifts. Examining free-bulge + disc decomposition sample and elliptical galaxy sample from Simard et al., we find that only ∼0.2 per cent of the bulges can be classified as BCBs in the local Universe. Bulge to total light ratio of disc galaxies with BCBs is (at ∼0.4) a factor of ∼2 and ∼4 larger than for those with classical and pseudo-bulges. BCBs are ∼2.5 and ∼6 times more massive than classical and pseudo-bulges. Although disc galaxies with BCBs host the most massive and dominant bulge type, their specific star formation rate is 1.5-2 times higher than other disc galaxies. This is contrary to the expectations that a massive compact bulge would lead to lower star formation rates. We speculate that our BCB host disc galaxies are descendant of massive, compact, and passive elliptical galaxies observed at higher redshifts. Those high-redshift ellipticals lack local counterparts and possibly evolved by acquiring a compact disc around them. The overall properties of BCBs support a picture of galaxy assembly in which younger discs are being accreted around massive pre-existing spheroids.