Too Much Dark Matter In CEG Mrk 1216

[ohwilleke]

New observations of Compact Elliptical Galaxy Mrk 1216 show a very high apparently dark matter fraction based upon the intragalaxy dynamics of stars in the galaxy, relative to the baryonic mass of the galaxy inferred from typical mass to light ratios for galaxies of that type.

The inferred amount of dark matter in the galaxy is 3-6 sigma above the ΛCDM model based upon seemingly reasonable assumptions about the galaxy.

Similarly, so much of the galaxy has a gravitational field strength in excess of the MOND constant, a₀ that MOND also predicts that there should b few observable dark matter phenomena there.

So, the mystery is why there is some much apparent dark matter there where it doesn't belong.

The paper is:

The Extremely High Dark Matter Halo Concentration of the Relic Compact Elliptical Galaxy Mrk 1216
David A. Buote, Aaron J. Barth (UC Irvine)
(Submitted on 8 Feb 2019 (v1), last revised 12 Feb 2019 (this version, v2))

Compact elliptical galaxies (CEGs) are candidates for local analogs of the high-redshift "red nuggets" thought to represent the progenitors of today's early-type galaxies (ETGs). To address whether the structure of the dark matter (DM) halo in a CEG also reflects the extremely quiescent and isolated evolution of its stars, we use a new ≈122 ks Chandra observation together with a shallow ≈13 ks archival observation of the CEG Mrk 1216 to perform a hydrostatic equilibrium analysis of the luminous and relaxed X-ray plasma emission extending out to a radius 0.85r2500. We examine several DM model profiles and in every case obtain a halo concentration (c200) that is a large positive outlier in the theoretical ΛCDM c200−M200 relation; i.e., ranging from 3.4σ−6.3σabove the median ΛCDM relation in terms of the intrinsic scatter. The high value of c200 we measure implies an unusually early formation time that firmly establishes the relic nature of the DM halo in Mrk 1216. The highly concentrated DM halo leads to a higher DM fraction and smaller total mass slope at 1Re compared to nearby normal ETGs. In addition, the highly concentrated total mass profile of Mrk 1216 cannot be described by MOND without adding DM, and it deviates substantially from the Radial Acceleration Relation. Mrk 1216 contains ≈80% of the cosmic baryon fraction within r200. The radial profile of the ratio of cooling time to free-fall time varies within a narrow range (tc/tff≈14−19) over a large central region (r≤10 kpc) suggesting "precipitation-regulated AGN feedback" for a multiphase plasma, though presently there is little evidence for cool gas in Mrk 1216. The properties of Mrk 1216 are remarkably similar to those of the nearby fossil group NGC 6482.

Comments: 28 pages, 12 figures, submitted to The Astrophysical Journal
Subjects: Astrophysics of Galaxies (astro-ph.GA)
Cite as: arXiv:1902.02938 [astro-ph.GA]
(or arXiv:1902.02938v2 [astro-ph.GA] for this version)

I haven't been able to discern what changes were made in version two of the paper, released yesterday, but whatever changes were made, they don't seem to disturb the main thrust of the scientific conclusions of the original version of the article.

If true, this would be the most paradigm upsetting discovery in many years or even more than one decade. But, there are multiple ways that this could plausibly arise from various kinds of systemic error in the measurements done.

 

[phyzguy]

I may be wrong, but I think these analyses assume that the galaxy is in some sort of dynamic equilibrium. If that is not the case because of a recent "event", such as a merger, SMBH coalescence or something else, then won't the analyses based on stellar dynamics come to the wrong conclusions?

 

[ohwilleke]

A "recent" event could explain it, but in other respects the system looks very old and static.