CO($J$=1-0) mapping survey of 64 galaxies in the Fornax cluster with the ALMA Morita array

TL;DR

This study maps CO emission in 64 Fornax cluster galaxies using ALMA, revealing that reduced cold-gas fractions, especially atomic gas, primarily cause low star formation activity, influenced by environmental stripping processes.

Contribution

First comprehensive CO mapping survey of Fornax cluster galaxies with ALMA, linking cold-gas properties to environmental effects and star formation suppression.

Findings

Low star formation is due to decreased cold-gas fraction, not efficiency.

Atomic gas is more heavily stripped than molecular gas in low-SF galaxies.

Environmental processes like tidal and ram pressure stripping reduce cold-gas reservoirs.

Abstract

We conduct a $^{12}$C$^{16}$O($J$=1-0) (hereafter CO) mapping survey of 64 galaxies in the Fornax cluster using the ALMA Morita array in cycle 5. CO emission is detected from 23 out of the 64 galaxies. Our sample includes dwarf, spiral and elliptical galaxies with stellar masses of $M_{\rm star}\sim10^{6.3-11.6}$~M$_\odot$. The achieved beam size and sensitivity are $15''\times8''$ and $\sim12$~mJy~beam$^{-1}$ at the velocity resolution of $\sim10$~km~s$^{-1}$, respectively. We study the cold-gas (molecular- and atomic-gas) properties of 38 subsamples with $M_{\rm star}>10^9$~M$_\odot$ combined with literature HI data. We find that: (1) the low star-formation (SF) activity in the Fornax galaxies is caused by the decrease in the cold-gas mass fraction with respect to stellar mass (hereafter, gas fraction) rather than the decrease of the SF efficiency from the cold gas; (2) the atomic-gas fraction is more heavily reduced than the molecular-gas fraction of such galaxies with low SF activity. A comparison between the cold-gas properties of the Fornax galaxies and their environmental properties suggests that the atomic gas is stripped tidally and by the ram pressure, which leads to the molecular gas depletion with an aid of the strangulation and consequently SF quenching. Pre-processes in the group environment would also play a role in reducing cold-gas reservoirs in some Fornax galaxies.