The Nature and Origin of Tidal Dwarf Galaxies

Facilities: BIMA, FCRAO
Collaborator: Dr. C.Taylor
Status: We already have observations of three fields in the interacting Hickson Compact Group HCG 92 (a.k.a. Stephan's Quintet). We have proposed to seach for other CO bright candidates with FCRAO

Summary: By studying the molecular gas dynamics of star forming clumps in the tidal tails of merging galaxies, we can: a) determine if they are gravitationally bound or simply transient objects; b) determine the origin of the molecular gas in tidal dwarf galaxies; and c) confirm the alleged lack of dark matter in tidal dwarf galaxies. This study will also shed light on the origin of the overall dwarf galaxy population.

Tidal dwarf galaxies (TDGs) are actively star forming systems which appear to be currently forming from debris in the tidal arms of interacting and merging spirals. Except for higher average metal abundances, their global properties are almost identical to those of ``normal'' dwarf galaxies. To date only a handful of tidal dwarfs have been observed in CO, and these have been exclusively with single dish telescopes which lack the spatial resolution to resolve them. We have obtained the first ever high resolution CO observations of a tidal dwarf. We intend to enlarge this sample in the future, which will allow us to address the following questions.

a) Are tidal dwarfs are self gravitating systems? If they are permanent new galaxies and not temporary concentrations of gas and stars in tidal arms, then they will contribute to the overall dwarf galaxy population, especially in regions of high galaxy density.

b) What is the origin of the molecular gas in the tidal dwarfs. Molecular gas in spirals tends to be concentrated towards the center, implying that the molecular gas in tidal dwarfs has been formed in situ from atomic gas that used to reside in the outer regions of a spiral galaxy. If this is true, then the ratio of molecular gas to dust will be lower than is found in the inner disks of the parent galaxies, assuming a single global total gas to dust ratio. However, if the molecular gas in tidal dwarfs has been ejected from the parent galaxy already in molecular form, then it should retain its original gas to dust ratio. Spatially resolved CO observations are the necessary first step, followed by dust measurements obtained in the sub-millimeter regime.

c) Is there a lack of dark matter in tidal dwarf galaxies. Braine et al. (2001) estimated the masses of the molecular clouds in several TDGs by measuring their CO line widths. They tentatively conclude that TDGs are nearly free from dark matter, which suggests that i) most dwarf galaxies (which are known to contain dark matter) are not merely old TDGs, and ii) dark matter is not located in the disks of galaxies. Our high resolution observations of these systems will allow us to more accurately address this question.


Braine, J., et al., 1993, A&AS, 97, 887