# ALMA Observations of the Molecular Gas in the Elliptical Galaxy NGC3557

## TL;DR

This study uses ALMA to observe molecular gas in the elliptical galaxy NGC3557, revealing organized rotation, high excitation regions, and a stable molecular gas disk close to the galaxy's center.

## Contribution

First detailed ALMA interferometric observations of molecular gas in NGC3557, showing organized rotation and high excitation features in an elliptical galaxy.

## Key findings

- Molecular gas concentrated within 250 pc of the center.
- Detected high CO(2-1)/CO(1-0) line ratio of 0.7, higher than typical ellipticals.
- Identified a high excitation peak aligned with the radio jet.

## Abstract

We present the results of CO interferometric observations of the southern elliptical galaxy NGC3557 with ALMA. We have detected both the CO(1-0) emission line and a relatively strong continuum at 3mm. The continuum shows a flat-spectrum central unresolved source (at our angular resolution of 0.7arcsec) and two jets, associated with the larger scale emission observed at lower frequencies. The molecular gas in NGC3557 appears to be concentrated within 250 pc of the center, and shows evidence of organized rotation along the same axis as the stellar component and the symmetry axis of the nuclear dust absorption reported in the literature. We obtained M$_{H_2}$=(9.0$\pm$2.0)x10$^7$ M$_\odot$ of molecular gas, which has an average CO(2-1) to CO(1-0) line ratio of 0.7, which is relatively high when compared with the values reported in the literature for bona-fide ellipticals observed with single-dish telescopes. NGC3557 shows further a high excitation peak (i.e., CO(2-1)/CO(1-0) ~ 1.1$\pm$0.3 offset 0.7 arcsec from the center, which appears to be associated with a region of higher velocity dispersion that does not share the overall rotation pattern of the molecular gas, but aligned with the radio jet. The molecular gas disk in this object appears to be stable to local gravitational instabilities.