Non-circular motions and the diversity of dwarf galaxy rotation curves

Abstract

We use mock interferometric H I measurements and a conventional tilted-ring modelling procedure to estimate circular velocity curves of dwarf galaxy discs from the APOS-TLE suite of ΛCDM cosmological hydrodynamical simulations. The modelling yields a large diversity of rotation curves for an individual galaxy at fixed inclination, depending on the line-of-sight orientation. The diversity is driven by non-circular motions in the gas; in particular, by strong bisymmetric fluctuations in the azimuthal velocities that the tilted-ring model is ill-suited to account for and that are difficult to detect in model residuals. Large misestimates of the circular velocity arise when the kinematic major axis coincides with the extrema of the fluctuation pattern, in some cases mimicking the presence of kiloparsec-scale density 'cores', when none are actually present. The thickness of APOSTLE discs compounds this effect: more slowly-rotating extra-planar gas systematically reduces the average line-of-sight speeds. The recovered rotation curves thus tend to underestimate the true circular velocity of APOSTLE galaxies in the inner regions. Non-circular motions provide an appealing explanation for the large apparent cores observed in galaxies such as DDO 47 and DDO 87, where the model residuals suggest that such motions might have affected estimates of the inner circular velocities. Although residuals from tilted ring models in the simulations appear larger than in observed galaxies, our results suggest that non-circular motions should be carefully taken into account when considering the evidence for dark matter cores in individual galaxies.

Slicing the cool circumgalactic medium along the major axis of a star-forming galaxy at z = 0.7

“…Secondly, we do not detect absorption at the same distance on the opposite side of G1 (i.e., NE positions #10 and #11). Third, velocities in simulated dwarfs fall down by only 20% at 20 kpc (Oman et al 2019), while here we see a decline of about 80%. Indeed, SW positions #10 and #11 have much less specific angular momentum than the rest.…”

Dark Matter Haloes and Subhaloes

“…This mechanism, first proposed in the 1990s [277], became fashionable again several years later [278][279][280][281][282][283][284][285] when tentative observational evidence for cores, particularly in dwarf galaxies, began to emerge [286]. This evidence, however, is controversial [287,288]. While the proof of concept in [277] was based on a single explosive event, recent simulations have shown that repeated outflows can create rapid fluctuations in the gravitational potential which efficiently transfer energy to the dark matter [281].…”

The H i velocity function: a test of cosmology or baryon physics?

“…The code uses the ANARCHY implementation Schaller et al (2015) of pressure-entropy smoothed particle hydrodynamics (Hopkins 2013), and includes prescriptions for radiative cooling (Wiersma et al 2009a), an ionizing background (Haardt & Madau 2001) The code martini 2 was used to produce neutral hydrogen (H i) emission line profiles for a selection of galaxies from the APOSTLE simulations. A detailed description of an earlier version is available in Oman et al (2019). The hydrogen ionization fraction of each simulation particle is estimated following Rahmati et al (2013); the neutral hydrogen is further partitioned into atomic and molecular gas following Blitz & Rosolowsky (2006).…”

“…§ 5 summarises our main results. In the Appendix A we compare our model for the Hi emission line of galaxies with the more complex Hi emission lines obtained from the cosmological hydrodynamical simulations apostle (Oman et al 2019).…”