Comment on “Missing dark matter in dwarf galaxies?” by Oman et al

Recent observations have brought into question a basic tenet of ΛCDM cosmology relating the expected decline in formation efficiency for lower mass galaxies (Oman 2016). This trend is analogous to a reduced baryonic fraction (fb) for dwarfs compared to spirals. Within the ΛCDM paradigm, this trend smoothly links (or reconciles) spiral and dwarf formation efficiencies with their galactic physical and dynamical properties. However, latest data does not fit this model very well with weak correspondence of the formation efficiency (i.e., circular rotation velocity) especially for lower mass halos. The data suggests a better fit can be obtained by keeping formation efficiency/baryonic fraction constant (fb=0.11) over the entire range of galactic mass as depicted in Figure 1 below.

 

Figure 1: Galactic Formation Efficiency: APOSTLE ΛCDM Simulations vs. observations illustrating the weak relationship between “feff” and circular velocity (i.e., galactic mass) – from Fig.2 (Oman 2016).

In the above figure, the ΛCDM model describes a significant drop in formation efficiency (feff fb) for galaxies having velocities <50 km/s. Additionally, the data tends to scatter in a downward direction below fb=0.11. This downward scatter represents a systematic understating of galactic baryons, especially for the lower mass ‘dark’ dwarf galaxy population. When all baryons are accounted for, the scatter in the data should diminish and move closer to the ‘universal’ constant fraction.

A more comprehensive view of this space, including the ΛCDM reconciling link between dwarfs and spirals is depicted in Figure 2 (Y.Sofue 2015). Notice this complementary data also does not support reconciliation between the two galactic populations (shown in mauve). Rather than this reconciling transitional link, the data can be reinterpreted as shown below, with dwarfs and spirals both complying with the constant baryonic fraction fb=0.11. However, the dwarf observations are displaced downward due to incomplete inventories of galactic baryons in similar fashion to Figure 1. By including all baryons (represented by the blue arrows) the dwarf population aligns with the spirals. On other words, it is not dark matter that is missing, only a portion of the galactic baryons.

 

Figure 2: GADGET2-SPH ΛCDM Simulation (Schaller 2015) and RC-SP Results Comparison. Original Figure from (Y.Sofue 2015) as adapted from Fig 12 (La Fortune 2015).

 

Figure 2 includes an interesting data point; the exceedingly dark irregular dwarf galaxy Andromeda IV (And IV) represented by the labeled triangle to the left side of the plot (Karachentsev 2015). From a ΛCDM perspective, this isolated galaxy should be teeming with dark matter (low formation efficiency/baryonic fraction). Surprisingly, this expectation was not confirmed, with data indicating And IV having a baryonic fraction consistent with the brighter spirals. From this perspective, And IV, MW and M31 seem to share a common formation history. These results support the contention of an early-era all-encompassing formation process for all disk-type galaxies (La Fortune 2015). In the context of ɅCDM, the authors interpreted this odd finding as a signature of a cored dark matter halo which also happens to describe dynamical phenomena associated with HI gas rich galaxies with high angular momentum!

With currently available data, the ɅCDM community is starting to question basic tenets of the dark matter paradigm. As more accurate baryonic accounting is made available, the ɅCDM model of galactic formation will find even stiffer challenges. Thanks to the Winnower as a vehicle to publish new ideas and have them available for review in an open setting.

Bibliography

Karachentsev, I.D., Chengalur, J.N., Tully, R.B., Makarova, L.N., Sharina, M.E., Begum, A., Rizzi, L. 2015. "Andromeda IV, a solitary gas-rich dwarf galaxy." arXiv. Dec 18. http://arxiv.org/pdf/1512.5907v2.pdf.

La Fortune, J.M. 2015. "Energy Density Driven Cosmology – a ΛCDM Alternative." The Winnower. Dec 2. Accessed 2015. https://thewinnower.com/papers/3036-energy-density-driven-cosmology-a-cdm-alternative.

Oman, K.A., Navarro, J.F., Sales, L.V., Fattahi, A., Frenk, C.S., Sawala, T., Shcaller, M., White, S.D.M. 2016. "Missing dark matter in dwarf galaxies?" arXiv. Jan 6. http://arxiv.org/pdf/1601.01026v1.pdf.

Schaller, M., Dalla Vecchia, C., Schaye, J., Bower, R. G., Theuns, T., Crain, R. A., Furlong, M., McCarthy, I. G. 2015. "The EAGLE simulations of galaxy formation: the importance of the hydrodynamics scheme." arXiv. Sep 16. http://arxiv.org/abs/1509.05056.

Y.Sofue. 2015. "Rotation Curve Decompositon for Size-Mass Relations of Bulge, Disk, and Dark Matter Halo." arXiv. Oct 20. http://arxiv.org/pdf/1510.05752v1.pdf.

 

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