The Molecule-Rich Tail of the Peculiar Galaxy NGC 2782 (Arp 215)

TL;DR

This study reports the first detection of significant molecular gas in the extended tail of galaxy NGC 2782, revealing insights into gas distribution and star formation in galaxy interaction tails.

Contribution

First detection and mapping of molecular gas in the tail of NGC 2782, showing its mass, kinematics, and low star formation activity compared to molecular gas content.

Findings

Detected CO (1-0) at five locations in the tail

Estimated molecular gas mass of 6 x 10^8 solar masses

Star formation rate in the tail is very low despite abundant molecular gas

Abstract

We present the first detection of a large quantity of molecular gas in the extended tail of an interacting galaxy. Using the NRAO 12m telescope, we have detected CO (1 - 0) at five locations in the eastern tail of the peculiar starburst galaxy NGC 2782. The CO velocities and narrow (FWHM = 50 km/s) line widths in these positions agree with those seen in HI, confirming that the molecular gas is indeed associated with the tail rather than the main disk. As noted previously, the gas in this tail has an apparent `counter-rotation' compared to gas in the core of the galaxy, probably because the tails do not lie in the same plane as the disk. Assuming the standard Galactic conversion N(H2)/I(CO) factor, these observations indicate a total molecular gas mass of 6 X 10**8 M(sun) in this tail. This may be an underestimate of the total H2 mass if the gas is metal-poor. This molecular gas mass, and the implied H2/HI mass ratio of 0.6, are higher than that found in many dwarf irregular galaxies. Comparison with an available H-alpha map of this galaxy, however, shows that the rate of star formation in this feature is extremely low relative to the available molecular gas, compared to L(H-alpha)/M(H2) values for both spiral and irregular galaxies. Thus the timescale for depletion of the gas in this feature is very long.