Tidal Debris Posing as Dark Galaxies

Abstract

Abstract. Debris sent into the intergalactic medium during tidal collisions has received much attention as it can tell us about several fundamental properties of galaxies, in particular their missing mass, both in the form of cosmological Dark Matter and so-called Lost Baryons.High velocity encounters, which are common in clusters of galaxies, are able to produce faint tidal debris that may appear as star-less, free floating HI clouds. These may be mistaken for Dark Galaxies, a putative class of gaseous, dark matter (DM) dominated, objects which for some reason never managed to form stars. VirgoHI21, in the Virgo Cluster, is by far the most spectacular and most discussed Dark Galaxy candidate so far detected in HI surveys. We show here that it is most likely made out of material expelled 750 Myr ago from the nearby spiral galaxy NGC 4254 during its fly-by at about 1000 km s −1 by a massive intruder. Our numerical model of the collision is able to reproduce the main characteristics of the system: in particular the absence of stars, and its prominent velocity gradient. Originally attributed to the gas being in rotation within a massive dark matter halo, we find it instead to be consistent with a combination of simple streaming motion plus projection effects .Based on our multi-wavelength and numerical studies of galaxy collisions, we discuss several ways to identify a tidal origin in a Dark Galaxy candidate such as optical and millimetrewave observations to reveal a high metallicity and CO lines, and more importantly, kinematics indicating the absence of a prominent Dark Matter halo. We illustrate the method using another HI system in Virgo, VCC 2062, which is most likely a Tidal Dwarf Galaxy . Now, whereas tidal debris should not contain any dark matter from the halo of their parent galaxies, it may exhibit missing mass in the form of dark baryons, unaccounted for by classical observations, as recently found in the collisional ring of NGC 5291 and probably in the TDG VCC 2062. These "Lost Baryons" must originally have been located in the disks of their parent galaxies.

The H I content of the advanced merger NGC 4441

“…1). From the kinematics there are no indications that these clumps are or will be separate gravitationally bound entities like those simulated by Duc & Bournaud (2007).…”

Computer Simulation, Experiment, and Novelty

“…The simulation pictures suggest that VirgoHI21 formed 750 millions of years ago after a collision between two spiral galaxies. VirgoHI21 is therefore not a galaxy but only collision debris (Duc, Bournaud, and Brinks 2007). Because of its high speed, the second galaxy-which is most likely Messier 98-has already gone very far away.…”

“…Such a discovery contradicts cosmological theory since VirgoHI21 is as massive as the Milky Way while theoretically there is no galaxy without a star as massive as VirgoHI21. Thus, in order to explain this prima facie contradiction, the formation of VirgoHI21 has been reproduced by simulation (Duc, Bournaud, and Brinks 2007). For that purpose, astrophysicists designed a possible scenario of the formation based on empirical observations.…”