The long tails and bridges extending from interacting and merging galaxies are often the birthplace of stars. These tails often contain large quantities of atomic gas, as seen by the 21 cm line of atomic hydrogen. Stars are believed to form out of molecular rather than atomic gas clouds; do these tails and bridges also contain a lot of molecular gas?

To search for molecular gas in these features, we have been conducting a survey of the 2.6mm line of carbon monoxide in a sample of tails and bridges. This line is the standard tracer of molecular gas in external galaxies. In this survey, the first detection of CO in an extragalactic tail was made, in the eastern tail of the peculiar galaxy NGC 2782 (Arp 215).

Left: an optical (B) image of NGC 2782, showing the distribution of stars. Note the stellar tail extending to the east (left) (from (Jogee, Kenney, and Smith 1998, Astrophysical Journal, 494, L185). Right: the distribution of atomic hydrogen in NGC 2782, as seen in the 21 cm HI line. The radio data were obtained with the Very Large Array, part of the National Radio Astronomy Observatory. The elongated feature on the right (west) is a tidal tail extending 50 kiloparsecs (5 arcminutes) from the main body of this galaxy. To the east, a gaseous counterpart to the stellar tail is visible: the gas does not extend as far as the stars in this direction (from Smith 1994, Astronomical Journal, vol. 107, p. 1695).

• We searched for CO in these tails using the National Radio Astronomy Observatory (NRAO) 12m telescope on Kitt Peak in Arizona.

The locations of the NRAO 12m CO beams (crosses), on the optical (contours) and 21 cm HI map (color) of NGC 2782 (from Smith et al. 1999, Astronomical Journal, 117, 1237).

• CO was detected in the shorter eastern tail: the first time CO was ever found in an extragalactic tail!!!

• The velocities and widths of these CO lines agree with those of the atomic gas in this tail, showing that the molecular gas is associated with the tail rather than the disk.

• The gas in the tail is `counter-rotating' compared to the gas in the disk.

The 21 cm HI velocity map of NGC 2782. Redshifted gas is shown as red; blueshifted gas as blue. Note that the tails are `counter-rotating' with respect to the gas in the inner disk, probably because the tails are not in the same plane as the disk (from Smith 1994, Astronomical Journal, vol. 107, p. 1695).

This `counter-rotation' is also visible in this right ascension-velocity map of NGC 2782. The greyscale and contours are the 21 cm HI data; the crosses are the CO data (from Smith et al. 1999, Astronomical Journal, 117, 1237).

• Clues to how the peculiar structure of NGC 2782 was created are the prominent `ripples' in the inner disk, suggesting a collision between two galaxies.

An unsharp masked optical image of the inner disk of NGC 2782, showing the ripples (from Smith 1994, Astronomical Journal, vol. 107, p. 1695).

• The peculiar double-tailed structure of NGC 2782 may have formed by the collision of two disk galaxies of different masses. The smaller galaxy may have passed through the larger galaxy, creating the `ripples'. The shorter eastern tail may be the remnant of the smaller companion; the longer tail, material drawn from the outer part of the main disk.

A possible model of how the peculiar NGC 2782 structure formed. This simulation only includes stars (from Smith 1994, Astronomical Journal, vol. 107, p. 1695).

• The large gas-star offset in the eastern tail may have been caused by ram pressure stripping of the gas during the collision: the stars passed through unimpeded, but the gas clouds collided.

  To go back to my home page, click here.