The Star Formation History and Dynamics of the Ultra-diffuse Galaxy Dragonfly 44 in MOND and MOG

Abstract

The observed line-of-sight velocity dispersion σ los of the ultra diffuse galaxy Dragonfly 44 (DF44) requires a Newtonian dynamical mass-to-light ratio of M dyn /L I = 26 +7 −6 Solar units. This is well outside the acceptable limits of our stellar population synthesis (SPS) models, which we construct using the integrated galactic initial mass function (IGIMF) theory. Assuming DF44 is in isolation and using Jeans analysis, we calculate σ los profiles of DF44 in Milgromian dynamics (MOND) and modified gravity (MOG) theories without invoking dark matter. Comparing with the observed kinematics, the best-fitting MOND model has M dyn /L I = 3.6 +1.6 −1.2 and a constant orbital anisotropy of β = −0.5 +0.4 −1.6 . In MOG, we first fix its two theoretical parameters α and µ based on previous fits to the observed rotation curve data of The HI Nearby Galaxy Survey (THINGS). The DF44 σ los profile is best fit with M dyn /L I = 7.4 +1.5 −1.4 , larger than plausible SPS values. MOG produces a σ los profile for DF44 with acceptable M dyn /L I and isotropic orbits if α and µ are allowed to vary. MOND with the canonical a 0 can explain DF44 at the 2.40σ confidence level (1.66%) if considering both its observed kinematics and typical star formation histories in an IGIMF context. However, MOG is ruled out at 5.49σ (P -value of 4.07 × 10 −8 ) if its free parameters are fixed at the highest values consistent with THINGS data.

Chemical evolution of ultra-faint dwarf galaxies in the self-consistently calculated integrated galactic IMF theory

“…Our results are also in line with Webster et al (2015), showing that the shortest SFT required for the observed self-enrichment of the UFDs is about 0.1 Gyr. The gas of low-mass-satellite galaxies was removed by their interaction with the Milky May (Romano et al 2015(Romano et al , 2019 such that they shut off star formation on a short timescale, as also found for Dragonfly 44 by Haghi et al (2019).…”

“…The chemical abundance of the dwarf galaxy combined with the IGIMF theory (i.e. a computed top-light gwIMF) suggests a short SFT of about 0.1 Gyr (similar to the dwarf galaxy Dragonfly 44, Haghi et al 2019). This not only agrees well with the stellar synthesis study of Boötes I (Brown et al 2014), but is also self-consistent with the empirical gas-depletion timescale of galaxies when the IGIMF theory is assumed (Pflamm-Altenburg & Kroupa 2009).…”

A massive blow for ΛCDM – the high redshift, mass, and collision velocity of the interacting galaxy cluster El Gordo contradicts concordance cosmology

“…MOG is a covariant scalar-tensor-vector gravity theory which allows the gravitational constant G, a vector field coupling ω, and the vector field mass µ to vary with space and time. This model was recently ruled out at 5.49σ using the velocity dispersion profile of the ultra-diffuse galaxy Dragonfly 44 (Haghi et al 2019). While perhaps the basic ideas of MOG can still be saved by applying some corrections or modifications, its current formulation seems to be in conflict with observations.…”

Scale-invariant dynamics in the Solar system

“…However, including the Galactic external field did not much improve the agreement with observations (Derakhshani & Haghi 2014). MOND can also explain the velocity dispersion of the ultra-diffuse galaxy Dragonfly 2 (DF2, Kroupa et al 2018b), DF4 (Haghi et al 2019a), and DF44 (Bílek et al 2019), with the latter galaxy having a measured dispersion profile that is consistent with MOND at 2.40σ (Haghi et al 2019b). The calculations for DF2 and DF4 are significantly affected by the EFE.…”