The Arecibo Legacy Fast ALFA Survey

HI1225+01b: A Dark Galaxy?

A truly "dark galaxy'" is an isolated halo consisting only of dark matter. In some scenarios, it is possible that some optically dark objects may contain enough HI that a blind HI survey would detect them. A good example of a "dark" object is the southwestern component of the binary system known as HI1225+01, discovered serendipitously at Arecibo by Giovanelli & Haynes in 1989. While the northeastern HI component hosts a small, star forming dwarf, the SW component has no detectable stellar counterpart. VLA observations reveal a velocity field implying rotation with a peak rotational velocity of 14 km/s, yielding a dynamical mass of 109 solar masses and thus a dynamical mass-to-light ratio of 200 (Chengalur, Giovanelli & Haynes 1995). However, it is not an isolated object, being part of an apparent binary system.

Find the original paper at: Chengalur, Giovanelli & Haynes, Astronomical Journal, 109, 2415.

HI map of the binary system HI1225+01 made with the Very Large Array. Click here for larger view.

Enlargement of the HI contours superposed on the optical images of the NE component HI1225+01a.
Click here for larger view.

Both images are from Chengalur, Giovanelli & Haynes 1995.

Most recently, Matsuoka et al. (2012) still find no associated starlight with HI1225+01b.

Last modified: Thu Aug 23 11:23:28 EDT 2007 by martha