ALMA Astrochemical Observations of the Infrared-Luminous Merger NGC 3256

TL;DR

This study uses ALMA observations to analyze molecular compositions in the galaxy NGC 3256, revealing shock-enhanced species and comparing chemical variations across different galactic environments and with other galaxies.

Contribution

First detailed multi-band molecular analysis of NGC 3256 revealing shock-related chemistry and comparisons with other galactic nuclei and spiral arms.

Findings

SiO and CH₃OH vary significantly across NGC 3256 regions.

Higher SiO and HC₃N abundances in Arp 220 compared to NGC 3256 and NGC 253.

Shock tracers are more abundant in NGC 3256 than in W 51.

Abstract

In external galaxies, molecular composition may be influenced by extreme environments such as starbursts and galaxy mergers. To study such molecular chemistry, we observed the luminous-infrared galaxy and merger NGC 3256 using the Atacama Large Millimeter/sub-millimeter Array. We covered most of the 3-mm and 1.3-mm bands for a multi-species, multi-transition analysis. We first analyzed intensity ratio maps of selected lines such as HCN/HCO$^+$, which shows no enhancement at an AGN. We then compared the chemical compositions within NGC 3256 at the two nuclei, tidal arms, and positions with influence from galactic outflows. We found the largest variation in SiO and CH$_3$OH, species that are likely to be enhanced by shocks. Next, we compared the chemical compositions in the nuclei of NGC 3256, NGC 253, and Arp 220; these galactic nuclei have varying star formation efficiencies. Arp 220 shows higher abundances of SiO and HC$_3$N than NGC 3256 and NGC 253. Abundances of most species do not show strong correlation with the star formation efficiencies, although the CH$_3$CCH abundance seems to have a weak positive correlation with the star formation efficiency. Lastly, the chemistry of spiral arm positions in NGC 3256 is compared with that of W 51, a Galactic molecular cloud complex in a spiral arm. We found higher fractional abundances of shock tracers, and possibly also higher dense gas fraction in NGC 3256 compared with W 51.