- Title: Finding dark galaxies from their tidal imprints
- Author: Sukanya Chakrabarti, Frank Bigiel, Philip Chang & Leo Blitz
- Institution: UC Berkeley, Berkeley, CA
The current hierarchical theory for galaxy formation — in which larger galaxies are built up from smaller building blocks and dark matter dominates — is well studied and reasonably well accepted. The book is still far from closed, however. The topic of galactic satellites has received particular attention in recent years: in simulations of the Milky Way based on hierarchical formation, a larger number of satellites are created than are observed in the real Universe (this is called the missing satellite problem). This has led astronomers to wonder whether the current paradigm applies at smaller-than-galaxy sizes.
The problem is that it’s not always easy to identify satellite galaxies, even around the Milky Way: they are smaller and fainter than the host galaxy and in addition many may be dark-matter dominated. The authors are working on a method in which one identifies a satellite based on its gravitational interaction with the host galaxy. Because this method does not rely on observing the satellites directly, in theory any satellite could be detected, even one comprised solely of dark matter. One benefit of this method over many others is that one does not need to make assumptions about the (entirely uncertain) nature of dark matter.
Tidal Analysis — as the authors call their method — relies on observing and simulating the tidal effects of galactic satellites on the galaxy. This effect is most pronounced in cold gas, so the authors look for disturbances in galactic HI disks (HI is neutral hydrogen; HII is ionized hydrogen; and H2 is molecular/diatomic hydrogen).
In this paper, the authors perform multiple simulations of the tidal interaction between two galaxies (M51 and NGC 1512) using GADGET-2. GADGET uses an N-body algorithm to model collisionless particles that interact gravitationally and smooth particle hydrodynamics (SPH) to model a gaseous component. Between simulations, they vary the ratio of the satellite to the galaxy mass and the distance between the satellite and galaxy at closest approach. For M51, they also vary initial conditions and orbits, keeping the mass ratio fixed at 1:3 and the minimal separation fixed at 7 kpc. The outcome of the simulation is only weakly dependent on the initial conditions and orbits, but strongly dependent on the mass ratio and minimal separation. Thus, one can constrain the best-fit mass ratio and minimal separation, although the dependence on the other parameters means that they can only be determined to a certain accuracy.
The figures show the best-fit simulation of M51 and the observations which the authors are trying to match. The observed image of M51 is from VLA and shows the distribution of HI in the galaxy; the results from the simulations show the gas density. The best-fit time is at t ~ 0.3 Gyr (the second panel in the top row in the simulations). From a comparison of the simulations to observations, the authors determine the mass ratio (and with the galaxy mass, the satellite mass is also known), the separation between the galaxy and satellite, and the azimuthal location of the satellite. This is done for both M51 and NGC 1512.
In this work the authors apply Tidal Analysis to galaxies with a known companion and are able to recover the satellite parameters. However, they hope to use the method with a large sample of galaxies, which would allow them to characterize dark matter substructure and galactic companions. Their goals include finding the local dwarf galaxy luminosity function and looking at the evolution of substructure with redshift.