Presentation #145.04 in the session Dwarf and Irregular Galaxies — iPoster Session.
We present deep optical imaging and photometry of a sample of four objects classified as “Almost-Dark” galaxies in the ALFALFA survey because of their gas-rich nature and extremely faint or missing optical emission in existing catalogs. They have HI masses and distances of 107 solar masses (at a distance of 9 Mpc) up to 109 solar masses (at 104 Mpc). Observations with the WIYN 3.5-m telescope and One Degree Imager reveal faint stellar components with central surface brightnesses of ~24-25 mag/sq.arcsec in the g band. Their stellar masses range from 106 to 108 solar masses, yielding neutral-gas-to-stellar mass ratios of up to 114. These four Almost-Dark galaxies have been identified as possible Tidal Dwarf Galaxies (TDGs) based on their proximity to one or more massive galaxies, some of which have tidally disturbed HI. We demonstrate that two of the four Almost-Dark objects, AGC229398 and AGC333576, have the low dark matter content representative of TDGs. They are located much farther from their progenitors than previously studied TDGs, suggesting they are older and more evolved. One of the Almost-Dark objects, AGC219369, is a regular (though faint) dark-matter-dominated dwarf, while AGC123216 has a dark matter content that is unusually high for a TDG, but low for a normal dwarf galaxy. We consider possible mechanisms for the formation of the TDGs such as a traditional major merger scenario and gas ejection from a high velocity fly-by. Blind HI surveys like ALFALFA enable the detection of gas-rich, optically faint Tidal Dwarf Galaxies that can be overlooked in more traditional surveys, thereby providing a more complete census of the low-mass galaxy population and an opportunity to study TDGs at a more advanced stage of their life cycle.