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Huge Population of “Ultra-Dark Galaxies” Discovered

posted by janrinok on Tuesday July 14 2015, @11:49PM   
from the they're-dark-Jim,-but-not-as-we-know-it dept.

Phoenix666 writes:

About 321 million light-years away from us is the Coma Cluster, a massive grouping of more than 1,000 galaxies. Some of its galaxies are a little unusual, however: they're incredibly dim. So dim, in fact, that they have earned the title of "Ultra-Dark Galaxies" (UDGs). (The term is actually "Ultra-Diffuse Galaxies", as their visible matter is thinly spread, though "ultra-dark" has been used by some sources and, let's face it, sounds a lot better). This was discovered earlier this year in a study that identified 47 such galaxies.

Dimness isn't necessarily unusual in a galaxy. Most of a galaxy's light comes from its stars, so the smaller a galaxy is (and thus the fewer stars it has), the dimmer it will be. We've found many dwarf galaxies that are significantly dimmer than their larger cousins.

What was so unusual about these 47 is that they're not small enough to account for their dimness. In fact, many of them are roughly the size of our own Milky Way (ranging in diameter from 1.5 to 4.6 kiloparsecs, compared with the Milky Way's roughly 3.6) but have only roughly one thousandth of the Milky Way's stars. The authors of the recent study interpret this to mean that these galaxies must be even more dominated by dark matter than are ordinary galaxies.

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Original Submission

• Re:Dark matter DOES NOT EXIST (Score: 2, Interesting) by Anonymous Coward on Wednesday July 15 2015, @11:27AM

  by Anonymous Coward on Wednesday July 15 2015, @11:27AM (#209307)

  Different AC. It is not my area of expertise, but I know the consensus model of cosmology is now built up on a huge number of ad hoc explanations:

  (1) 1980: Curvature and homogeneity: The BB would imply the universe to be highly inhomogeneous and curved in disagreement with observations. This is solved by introducing inflation (Guth & Tye 1980).

  (2) 1981: The super-Keplerian galactic rotation curve: Rotation curves of disk galaxies are observed to remain quite flat (Rubin & Ford 1970; Bosma 1981). This is solved by introducing cold or warm DM (Blumenthal et al. 1984).

  (3) 1991: Angular momentum: Disk galaxies forming in the C/WDM cosmological model dissipate too much angular momentum by virtue of the baryons falling into the DM potential wells, ending up being too compact with too little angular momentum in comparison with observed disk galaxies (Navarro & Benz 1991, see also Piontek & Steinmetz 2011; Martig et al. 2012; Dutton & van den Bosch 2012; Scannapieco et al. 2012).

  (4) 1991: The cusp/core: CDM haloes have cusps whereas the observationally deduced DM halo profiles have substantial core radii similar to the dimension of the luminous galaxy (Dubinski & Carlberg 1991, see also Gilmore et al. 2007a,b; de Blok 2010; Chen & McGaugh 2010; Jardel & Gebhardt 2012). A possible solution has been suggested by the simulations of Governato et al. (2012) but relies among other assumptions on a bursty star-formation rate (SFR) required to repeatedly blow out gas and a steep Kennicutt- Schmidt exponent (n = 1.5) in SFR ∝ �n gas, where SFR is the star formation rate and �gas is the local gas density. However, in reality it is not clear if the dSph and UFD satellites experienced bursty SFRs, n = 1 (Pflamm-Altenburg & Kroupa 2008, 2009), the IMF would have had a lack of massive stars at the low SFRs of the MW satellites, as is inferred by Tsujimoto (2011), and the threshold for SF is lower in reality than assumed in the simulations (see further below). Repeated gas blow-out which is required to evolve the cusps to cores is thus not likely to be possible. WDM models tuned to account for the observed large cores in dwarf galaxies have such long DMparticle streaming lengths that the dwarf galaxies cannot form in the first place (Maccio’ et al. 2012).

  (5) 1998: Dark energy: The fluxes and redshifts of observed type Ia supernovae (SNIa) do not match the cosmological model (Riess et al. 1998; Schmidt et al. 1998; Perlmutter et al. 1999) unless the universe is assumed to expand at an ever larger rate. To account for the implied accelerated expansion dark energy (DE) is introduced. As with inflation, while mathematically allowed, it remains unclear if DE constitutes physics (see e.g. the discussion in Afshordi 2012). The SNIa flux–redshift data may at least partially be explained with an inhomogeneous universe (Wiltshire 2009; Smale & Wiltshire 2011; Marra et al. 2012) rather than with DE, whereby systematics in SNIa light curve fitting remain an issue (Smale &Wiltshire 2011). Bull & Clifton (2012) find that the “appearance of acceleration in observations made over large scales does not necessarily imply or require the expansion of space to be accelerating, nor does it require local observables to indicate acceleration.” In fact, it might perhaps be surprising that a homogeneous SMoC should lead to a perfect agreement with the observed SNIa data. In other words, the SNIa data that stem from the real inhomogeneous universe (Karachentsev 2012) should show some deviations from a homogeneous SMoC. If none are seen then this may imply an over-constrained model.17

  (6) 1999: Missing satellites: Computations with more powerful computers showed that many more DM sub-structures form than observed galaxies have satellites (Klypin et al. 1999; Moore et al. 1999; the problem is somewhat reduced with WDM: Menci et al. 2012).

  (7) 2002: Hierachical structure formation: As moremassive galaxies are build-up hierarchically from smaller building blocks in the SMoC, their [�/Fe] ratios ought to reflect the sub-solar [�/Fe] ratios of the building blocks (e.g., dE galaxies have low [�/Fe] ratios). In conflict with this expectation, observed massive E galaxies show high near-solar [�/Fe] values (Thomas et al. 2002). This may be partially alleviated by a prescription
  for AGN quenching of star formation in massive haloes but not so in the intermediategalaxy- mass regime (Pipino et al. 2009, see also Nagashima et al. 2005; Recchi et al. 2009).

  (8) 2005: The Disk of Satellites (DoS/VPOS): The observed satellite galaxies of the MW are arranged in a vast polar structure (Kroupa et al. 2005; Metz et al. 2007, 2008, 2009a; Kroupa et al. 2010; Pawlowski et al. 2012b). Of all objects at Galactocentric distances larger than 10 kpc, only 4 per cent are not associated with the VPOS (Sec. 10.1.7). Extragalactic anisotropic satellite systems are common, and Andromeda appears to have a flattened satellite system seen edge-on (Sec. 13.6).

  (9) 2007: The TDG mass-deficit: Unexpectedly, observed young TDGs show evidence for DMwhich however is not possible if the SMoC were true (Barnes & Hernquist 1992) unless they contain undetectable gas (Bournaud et al. 2007, see also Gentile et al. 2007).

  (10) 2008: Invariant disk galaxies: Observed disk galaxies are too similar following a simple oneparameter scaling law over many orders of magnitude in mass in conflict with the expected variation in the SMoC due to the chaotic formation history of each DM host halo (Disney et al. 2008, see also Hammer et al. 2007; Kroupa et al. 2010).

  (11) 2008: The common mass-scale: In the SMoC, DM sub-haloes are distributed according to a power-law mass function. But observed satellite galaxies have too similar DM masses (Mateo et al. 1993; Strigari et al. 2008, see also Kroupa et al. 2010 and for Andromeda Tollerud et al. 2012).

  (12) 2009: Constant surface density: Considering the matter distribution in observed galaxies within one DM-halo scale radius, Gentile et al. (2009) find “This means that the gravitational acceleration generated by the luminous component in galaxies is always the same at this radius. Although the total luminous-to-dark matter ratio is not constant, within one halo scale-length it is constant”. In the SMoC there is no physical principle according to which the DM and baryonic densities ought to be invariant within this radius.

  (13) 2010: The luminous sub-halo mass function: The mass function of observed satellite galaxies disagrees with the predicted mass function of luminous sub-haloes (Kroupa et al. 2010).

  (14) 2010: Bulge-less disk galaxies: That the bulgeto- disc flux ratios are smaller than those produced by LCDM simulations is pointed out by Graham & Worley (2008). 58-74 per cent of all observed disk galaxies are claimed to not have a classical bulge (Kormendy et al. 2010). This is in conflict with the heavy merging history expected for bright galaxies if the SMoC were true (Hammer et al. 2007). For attempts to produce bulgeless disk galaxies see text below.

  (15) 2010: Isolated massive galaxies: In the observed Local Volume of galaxies there are three massive disk galaxies that are too far off the matter filament (Peebles & Nusser 2010).

  (16) 2010: The void: The Local Void is observed to be too empty in comparison to the SMoC expectation (Tikhonov et al. 2009; Peebles & Nusser 2010).

  (17) 2010: The Bullet Cluster: The observed large relative velocity of the two interacting galaxy clusters is not accountable for in the SMoC (Lee & Komatsu 2010; Thompson & Nagamine 2012).

  (18) 2011: The missing bright satellites: The predicted mass function of DM sub-haloes implies that a significant number of bright satellite galaxies is missing (Bovill & Ricotti 2011; Boylan- Kolchin et al. 2011a). Vera-Ciro et al. (2012) suggest this problem does not occur if the DM sub-haloes have Einasto rather than NFW density profiles. Wang et al. (2012) suggest this problem does not occur if the MW DM halo is less massive than about 2×1012 M⊙. But this is unlikely as the large proper motion of the LMC implies the MW to be more massive than about 2×1012 M⊙ and the LMC to be a recent acquisition and on its first passage (Boylan-Kolchin et al. 2011b). Furthermore, if this were the case then the question would need to be posed as to how likely the LMC happens to pass the MW within the VPOS.

  (19) 2011: The thin old disk: The MW has a thin disk which has stars as old as 10 Gyr. Such old thin disks have still not been produced in the SMoC (House et al. 2011).

  (20) 2012: The Train-Wreck Cluster: The galaxy cluster A520 has been shown to contain what appears to be a DM core with too few galaxies as well as evidence for a cluster–cluster encounter. The C/WDM paradigm cannot account for this separation of DM from the luminous matter, which is the opposite behaviour to the Bullet Cluster (Failure 17 above, Jee et al. 2012, see also Mahdavi et al. 2007).

  (21) 2012: Missing Dark Matter: Over spatial scales of 100 Mpc extend the density of matter fluctuates by 10 per cent if the SMoC were valid. By counting up all matter within the local sphere with a radius of 50 Mpc, Karachentsev (2012) demonstrates the actual density to be too low by a factor of 3–4. Most of the missing mass is DM.

  (22) 2012 Massive Galaxy Clusters: The most massive most distant galaxy clusters are important constraints on cosmological theory because the rapidity with which mass assembles to galaxy clusters depends on dark matter and/or on modified gravitation (Sec. 16.5). Gonzalez et al. (2012) discover a giant lensed arc near the cluster IDCS J1426.53508 and deduce “For standard LCDM structure formation and observed background field galaxy counts this lens system should not exist.”

  http://arxiv.org/pdf/1204.2546v2.pdf [arxiv.org]

  Parent

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