Bulgeless galaxies in the COSMOS field: environment and star formation evolution at $z<1$

Abstract

Combining the catalogue of galaxy morphologies in the COSMOS field and the sample of H$\hat{I}\pm$ emitters at redshifts $z=0.4$ and $z=0.84$ of the HiZELS survey, we selected $\sim$ 220 star-forming bulgeless systems (Sérsic index $n\le 1.5$) at both epochs. We present their star formation properties and we investigate their contribution to the star formation rate function (SFRF) and global star formation rate density (SFRD) at $z<1$. For comparison, we also analyse H$\hat{I}\pm$ emitters with more structurally evolved morphologies that we split into two classes according to their Sérsic index $n$: intermediate ($1.5<n\le 3$) and bulge-dominated ($n>3$). At both redshifts the SFRF is dominated by the contribution of bulgeless galaxies and we show that they account for more than 60% of the cosmic SFRD at $z<1$. The decrease of the SFRD with redshift is common to the three morphological types but it is stronger for bulge-dominated systems. Star-forming bulgeless systems are mostly located in regions of low to intermediate galaxy densities ($\hat{I}\pounds \sim 1-4$ Mpc${}^{-2}$) typical of field-like and filament-like environments and their specific star formation rates (sSFRs) do not appear to vary strongly with local galaxy density. Only few bulgeless galaxies in our sample have high (sSFR $>$ 10${}^{-9}$ yr${}^{-1}$) and these are mainly low-mass systems. Above $M_{\ast }\sim 10^{10}$ M${}_{\odot }$ bulgeless are evolving at a "normal" rate (10${}^{-9}$ yr${}^{-1}<$ sSFR $<$10${}^{-10}$ yr${}^{-1}$) and in the absence of an external trigger (i.e. mergers/strong interactions) they might not be able to develop a central classical bulge.

14 page, 9 figures, accepted for publication in MNRAS