Are There Outlier Ultra Diffuse Galaxies?

Most ultra-diffuse galaxies don't deviate significantly from the baryonic Tully-Fisher scaling relation which is a good fit to all individual galaxies (there is a similar but different relationship that exists in galaxy clusters). 

But a new paper with the largest sample size that can be compared to the baryonic Tully-Fisher scaling relation does identify some hydrogen gas rich outlier ultra-diffuse galaxies, but that the outlier galaxies tend to obey narrower original Tully-Fisher scaling relation. 

In the MOND paradigm, the baryonic Tully-Fischer relation flows trivially from the gravity modifications that MOND contains, and outliers from the baryonic Tully-Fisher relation can arise due to an external field effect, or because a galaxy is not in equilibrium. 

In Deur's paradigm, the gravitational effects of spherically symmetric interstellar gas can be parsed from the effects of non-spherically symmetric clusters of stars, as illustrated by his analysis of the Bullet Cluster. This kind of analysis could be relevant to gas rich galaxies.

In the dark matter particle paradigm, outliers could be explained by tidal stripping, or because of random variation in dark matter halo sizes associated with a galaxy that may be muted for larger galaxies due to the law of averaged over multiple galaxy mergers involving in assembling larger galaxies.

The paper conjectures that it may be related to the hydrogen fraction of these galaxies but doesn't reach a firm conclusion regarding the cause of these outliers (which is sensible given that there isn't a firm consensus on the source of the baryonic Tully-Fisher or the original Tully-Fischer scaling relationships).

The baryonic Tully-Fisher relation (BTFR), which connects the baryonic mass of galaxies with their circular velocities, has been validated across a wide range of galaxies, from dwarf galaxies to massive galaxies. Recent studies have found that several ultra-diffuse galaxies (UDGs) deviate significantly from the BTFR, indicating a galaxy population with abnormal dynamical properties. However, such studies were still confined within a small sample size. In this study, we used the 100% complete Arecibo Legacy Fast Arecibo L-band Feed Array (ALFALFA) to investigate the BTFR of 88 HI-rich UDGs (HUDGs), which is the largest UDG sample with dynamical information. We found that the HUDGs form a continuous distribution in the BTFR diagram, with high-velocity galaxies consistent with normal dwarf galaxies at 1 σ level, and low-velocity galaxies deviating from the BTFR, in line with that reported in the literature. We point out that the observed deviation may be subject to various selection effects or systemic biases. Nevertheless, we found that the significance of the deviation of HUDGs from the BTFR and TFR are different, i.e., they either deviate from the BTFR or from the TFR. Our result indicates that a high-gas fraction may play an important role in explaining the deviation of HUDGs from BTFR.

Hui-Jie Hu, Qi Guo, Zheng Zheng, Hang Yang, Chao-Wei Tsai, Hong-Xin Zhang, Zhi-Yu Zhang, "Global dynamic scaling relations of HI-rich ultra-diffuse galaxies" arXiv:2303.16232 (March 28, 2023) (accepted for publication in ApJL).

Another experimental test of galaxy properties is this preprint:

In this paper, we use SPARC's HSBs, and LSBs galaxies to verify two issues. 

The first one is related to one claim of Donato D09, namely: is the DM surface density (DMsd) a constant universal quantity, equal to log(Σ/M⊙pc−2)=2.15±0.2, or does it depend on the baryon surface density of the system? 

The second one, is based on a MOND prediction that for HSBs the DMsd is constant, and equal to log(Σ/M⊙pc−2)=2.14, while for LSBs the surface density is not constant and takes values that are smaller than for HSBs and D09 prediction Milgrom 2009. 

We find that HSBs shows a constant DMsd vs magnitude as in D09, and a constant DMsd vs Σeff as in MOND prediction, for HSBs with Σeff>200L⊙/pc2, and Σeff>300L⊙/pc2. However, the value of the DMsd is larger, Σ≃2.61 (in the case of the DMsd-magnitude with Σeff>300L⊙/pc2), and Σ≃2.54 (in the case of the surface DMsd-surface brightness with Σeff>200L⊙/pc2). This value slightly depends on the threshold to determine wheter a galaxy is HSB. 

In the case of LSBs, for Σeff<100L⊙/pc2, and Σeff<25L⊙/pc2, the surface density vs magnitude, for lower magnitudes, is approximately equal to that predicted by D09, but several galaxies, for magnitude M>−17, have smaller values than those predicted by D09. The DMsd vs Σeff shows a similar behavior in qualitative, but not quantitative, agreement with MOND predictions. 

In summary, in the case of HSBs both D09 and MOND are in qualitative, but not quantitative, agreement with the data. In the case of LSBs D09 is mainly in disagreement with the data, and MOND only in qualitative agreement with them.

Antonino Del Popolo, "SPARC HSBs, and LSBs, the surface density of dark matter haloes, and MOND" arXiv:2303.16658 (March 29, 2023) (Physics of the Dark Universe Volume 40, May 2023, 101203).