The Graben:. Dark Matter Galaxies

The current theory is that although on the initial collapse massive galaxies will most likely produce stars, some galaxies will only form halo stellar components, while their disk remains gaseous and quiescent if the surface density is low enough and if they do not undergo further mergers. These galaxies will be extreme cases of low-surface-brightness (LSB) galaxies also known as low-density galaxies (LDN) (Jimenez et al. 1997). Although these galaxies will not be completely dark, the small amount luminous material would make them extremely hard to detect at great distances. In some cases, models suggest that some galaxies may form without any star formation occurring at all. Another theory of dark galaxy formation is the result of dark matter halo interactions in clusters of galaxies. In some cases large masses of dark matter may be separated from the interacting galaxies, thus forming small dwarf dark matter galaxies. If dark galaxies formed in this way, then the frequency of galactic interactions would indicate that there should be many more small dark galaxies them luminous ones, Drs. Trentham, Möller, and Ramirez-Ruiz suggests that there are as many as 100 dark galaxies to every luminous galaxy. This dark galaxy hypothesis is one way to solve the problem of cold dark matter (CDM) theories overproducing large numbers of small galaxies (Trentham, Möller, & Ramirez-Ruiz 2001). If the theories are valid, detections will be extremely difficult and with current technology will be based on the predicted composition of dark matter.

Composition:

Dark galaxies will most likely be composed of, as the name suggests, dark matter. From the current understanding of dark matter it would be expected to find within dark galaxies CDM, baryonic matter, and other unknown matter. It has been shown that some of the smallest galaxies known are at least 99 percent dark matter by mass (Pryor & Kormendy 1990). Thus, there should be some percentage of CDM within that 99 percent, and it has been theorized that it will have a density profile close to a Navarro, Frenk & White (NFW) profile (Navarro, Frenk, & White 1996, 1997). The CDM will be hard to detect with current technology, but the baryonic component of dark matter would have important implications for detectability. One form of baryonic matter is the brown dwarf, a star with a mass lower than the burning limit of hydrogen. Brown dwarfs are not a major component of the dark mass in the outer parts of the Milky Way, as inferred by the MACHO (Alcock et al. 2000) and EROS (Afonso et al. 1999) microlensing experiments, so they will probably not be a major component of dark galaxies. Brown dwarfs will generate microlensing effects and will emit radiation at mid-infrared wavelengths, thus allowing for an unseen galaxy to be detected (Trentham et al. 2001). Another form of baryonic matter could be clouds of molecular hydrogen. It is possible that small clumps of molecular hydrogen may exist throughout some dark galaxies. The clouds would be thermally stable could be detected by their 21 cm radio emissions. Other possible objects that might be found are stellar remnants such as neutron stars, black holes, and cold white dwarfs. Although these are all extremely hard to detect, they all have identifiable properties that could allow for detection.

Detection Mechanisms:

As previously mentioned, an indication of a dark galaxy can be found by either studying strong gravitationally lensed objects where the lensing object is not detectable or by observing the gravitational effects dark galaxies might have on the luminous matter in standard galaxies. Neither of these would provide conclusive evidence, but they would point toward possible candidates that should be studied further. Once a candidate has been identified, infrared studies can be performed in the region. Brown dwarfs will emit radiation in the mid-infrared wavelengths (5-30 mm) and can be used to detect the dark galaxy (Trentham et al. 2001). Cold white dwarfs have a spectral energy distribution that peaks at 1 mm and can possibly be detected as a faint excess of the sky in the J-band (1.25mm) (Hodgkin et al. 2000). Such a population could be distinguished from other low surface brightness stellar systems on the basis of their optical plus near infrared colors (Trentham et al. 2001). Infrared surveys searching for these faint objects would require sensitive equipment and long integration times. One of the favored theories for the formation of gamma ray bursts is the merger of compact stellar remnants like neutron stars or black holes (Narayan, Paczyńsky, & Piran 1992), therefore if two of the remnants happen to merge then a gamma ray burst may result. The interaction between the gamma ray burst and the surrounding material will result in an afterglow at lower energies (Mészáros & Rees 1998). The properties of this afterglow will depend upon the density of the ambient medium which would be far less in a dark galaxy than in a normal star forming galaxy by several orders of magnitude. Thus, in principle a gamma ray burst can be used to identify a dark galaxy. (Trentham et al. 2001). The molecular clouds of hydrogen will emit radiation at a wavelength of 21 cm, thus probing the region for 21 cm emissions would indicate the existence of molecular hydrogen clouds. If the region of molecular hydrogen appears to be massive enough it may indicate the existence of a dark galaxy as well.

Candidates:

In 2001 with the release of their theory, Drs. Trentham, Möller, and Ramirez-Ruiz identified a galaxy that shows evidence of an interaction with a massive dark object. The galaxy UGC 10214, figure 1, also known as the Tadpole Galaxy was studied by the INT wide field survey. The survey revealed an isolated galaxy with material flowing out of the galaxy towards apparently nothing (Trentham et al. 2001). Drs. Trentham, Möller, and Ramirez-Ruiz theorize that the material is being gravitationally pulled out of the galaxy by a dark companion because it appears to be too thick to be a jet from an active galaxy and there is no evidence of another object nearby that might have caused the outflow. Currently, studies of the region are being conducted to locate the dark object responsible for the galactic distortion.