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

TL;DR

This study investigates the properties and evolution of bulgeless star-forming galaxies in the COSMOS field at redshifts below 1, highlighting their dominant role in cosmic star formation and their environmental dependence.

Contribution

It provides the first detailed analysis of bulgeless galaxy contribution to star formation rate density and their environmental distribution at z<1.

Findings

Bulgeless galaxies account for over 60% of the cosmic SFRD at z<1.

The SFRF is dominated by bulgeless galaxies at both studied epochs.

Bulgeless galaxies are mostly found in low to intermediate density environments.

Abstract

Combining the catalogue of galaxy morphologies in the COSMOS field and the sample of H$\alpha$ emitters at redshifts $z=0.4$ and $z=0.84$ of the HiZELS survey, we selected $\sim$ 220 star-forming bulgeless systems (S\'ersic index $n \leq 1.5$) 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$\alpha$ emitters with more structurally evolved morphologies that we split into two classes according to their S\'ersic index $n$: intermediate ($ 1.5 < n \leq 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 ($\Sigma \sim 1 - 4$ 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 $M_* \sim 10^{10}$ 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.