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

Abstract

We present new Atacama Large Millimeter/submillimeter Array (ALMA) results obtained from spatially resolved CO (2-1) line (0. ′′ 4 resolution) and 870 µm continuum (0. ′′ 2 resolution) observations of cluster galaxies in XMMXCS J2215.9-1738 at z = 1.46. Our sample comprises 17 galaxies within ∼ 0.5 Mpc (0.6R 200 ) from the cluster center, all of which have been detected in CO (2-1) line previously in a lower resolution. The effective radii of both CO (2-1) line and 870 µm dust continuum emissions are robustly measured for nine galaxies by modeling the visibilities. We find that the CO (2-1) line emission in all of the nine galaxies is more extended than the dust continuum emission by a factor of 2.8 ± 1.4. We investigate the spatially resolved Kennicutt-Schmidt (KS) relation in two regions within the interstellar medium (ISM) of the galaxies. The relation for our sample reveals that the central region (0 < r < R e,870µm ) of galaxies tends to have a shorter gas depletion timescale, i.e. a higher star formation efficiency (SFE), compared to the extended region (R e,870µm < r < R e,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 µm radii of star-forming members and the HST/1.6 µm radii of passive members in a mass-size distribution, which suggest transition from star-forming to passive members within ∼ 0.5 Gyr. In addition, no clear differences in the KS relation and in the sizes are found between galaxies with and without a close companion.