Bright Compact Bulges (BCBs) at intermediate redshifts

Abstract

Studying bright ($M_B<-20$), intermediate-redshift ($0.4<z<1.0$), disc dominated ($n_B<2.5$) galaxies from HST/ACS and WFC3 in Chandra Deep Field South, in rest-frame B and I-band, we found a new class of bulges which 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 $\sim 12$% of the total bulge population at these redshifts. Examining free-bulge $+$ disc decomposition sample and elliptical galaxy sample from Simard et al. (2011), we find that only $\sim 0.2$% of the bulges can be classified as BCBs in the local Universe. Bulge to total ratio $(B/T)$ of disc galaxies with BCBs is (at $\sim 0.4$) a factor of $\sim 2$ and $\sim 4$ larger than for those with classical and pseudo bulges. BCBs are $\sim 2.5$ and $\sim 6$ 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 supports a picture of galaxy assembly in which younger discs are being accreted around massive pre-existing spheroids.

arXiv comment10 pages, 7 figures, accepted for publication in MNRAS

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