Generation of galactic disc warps due to intergalactic accretion flows onto the disc

Abstract

Abstract.A new method is developed to calculate the amplitude of the galactic warps generated by a torque due to external forces. This takes into account that the warp is produced as a reorientation of the different rings which constitute the disc in order to compensate the differential precession generated by the external force, yielding a uniform asymptotic precession for all rings. Application of this method to gravitational tidal forces in the Milky Way due to the Magellanic Clouds leads to a very low amplitude of the warp, as has been inferred in previous studies; so, tidal forces are unlikely to generate warps, at least in the Milky Way. If the force were due to an extragalactic magnetic field, its intensity would have to be very high, greater than 1 µG, to generate the observed warps. An alternative hypothesis is explored: the accretion of the intergalactic medium over the disk. A cup-shaped distortion is expected, due to the transmission of the linear momentum; but, this effect is small and the predominant effect turns out to be the transmission of angular momentum, i.e. a torque giving an integral-sign shape warp. The torque produced by a flow of velocity ∼100 km s −1 and baryon density ∼10 −25 kg/m 3 is enough to generate the observed warps and this mechanism offers quite a plausible explanation. First, because this order of accretion rate is inferred from other processes observed in the Galaxy, notably its chemical evolution. The inferred rate of infall of matter, ∼1 M /yr, to the Galactic disc that this theory predicts agrees with the quantitative predictions of this chemical evolution resolving key issues, notably the G-dwarf problem. Second, the required density of the intergalactic medium is within the range of values compatible with observation. By this mechanism, we can explain the warp phenomenon in terms of intergalactic accretion flows onto the disk of the galaxy.

Warps and correlations with intrinsic parameters of galaxies in the visible and radio

“…We believe that this is to be expected for any mechanism that produces a warp as a reaction to an external torque, whatever it its origin (gravitational torque, magnetic torque, accretion torque; see López-Corredoira et al 2002). More massive (i.e.…”

“…A generic misalignment between halo and the disc (Debattista & Sellwood 1999) might answer the question, but then we need to think about the reason for the misalignment. -The accretion of the intergalactic medium directly on to the disc is another possibility (Revaz & Pfenninger 2001;López-Corredoira et al 2002). The torque produced in the different rings of the disc by an intergalactic flow of velocity ∼100 km s −1 and baryon density ∼10 −25 kg m −3 is enough to generate the observed warps and also predicts the existence of U-warps (cupshaped), which are less frequent than S-(integral-shaped) warps.…”

Star formation sustained by gas accretion

“…For example, spirals are known to have extended warped HI distributions, a pattern that cannot be sustained for long. Among other explanations, gas infall has been proposed as the origin of warped disks (e.g., López-Corredoira et al 2002;Bournaud et al 2005;Kamphuis et al 2013). Figure 5 shows an extreme case of distorted HI morphology.…”

Kinematic modelling of disk galaxies

“…Recent studies of Sánchez-Salcedo (2004 indicate that the intergalactic medium in the form of magnetic fields (Battaner et al 1990b,a;Battaner & Jimenez-Vicente 1998) or intergalactic gas (Kahn & Woltjer 1959;López-Corredoira et al 2002) can be ruled out as an interaction partner that enables warp formation. The possibility of the growth of self-inflicted bending instabilities is discussed by Revaz & Pfenniger (2004).…”