High-resolution ALMA study of CO (2-1) line and dust continuum emissions in cluster galaxies at z = 1.46

TL;DR

This study uses high-resolution ALMA observations to analyze the spatial distribution of CO (2-1) line and dust continuum emissions in cluster galaxies at z=1.46, revealing extended molecular gas and insights into star formation efficiency and galaxy evolution.

Contribution

First spatially resolved measurements of CO and dust emissions in cluster galaxies at z=1.46, showing extended gas reservoirs and star formation characteristics.

Findings

CO emission is more extended than dust continuum by a factor of ~2.8.

Central regions have higher star formation efficiency and shorter gas depletion times.

Star formation activity is concentrated inside extended gas reservoirs, possibly leading to bulge formation.

Abstract

We present new Atacama Large Millimeter/submillimeter Array (ALMA) results obtained from spatially resolved CO $J$=2-1 line ($0.4''$ resolution) and 870 $\mu$m continuum ($0.2''$ resolution) observations of cluster galaxies in XMMXCS J2215.9-1738 at $z=1.46$. Our sample comprises 17 galaxies within $\sim0.5$ Mpc ($0.6R_{200}$) of the cluster center, all of which have previously been detected in the CO $J$=2-1 line at a lower resolution. The effective radii of both the CO $J$=2-1 line and 870 $\mu$m dust continuum emissions are robustly measured for nine galaxies by modeling the visibilities. We find that the CO $J$=2-1 line emission in all of the nine galaxies is more extended than the dust continuum emission by a factor of $2.8\pm1.4$. We investigate the spatially resolved Kennicutt-Schmidt (KS) relation in two regions within the interstellar medium of the galaxies. The relation for our sample reveals that the central region ($0<r<R_{e,{\rm 870\mu m}}$) of galaxies tends to have a shorter gas depletion timescale, i.e., a higher star formation efficiency, compared to the extended region ($R_{e,{\rm 870\mu m}}<r<R_{e,{\rm CO}}$). Overall, our result suggests that star formation activities are concentrated inside the extended gas reservoir, possibly resulting in the formation of a bulge structure. We find consistency between the ALMA 870 $\mu$m radii of star-forming members and the Hubble Space Telescope/1.6 $\mu$m radii of passive members in a mass-size distribution, which suggests a transition from star-forming to passive members within $\sim0.5$ Gyr. In addition, no clear differences in the KS relation nor in the sizes are found between galaxies with and without a close companion.