Your ignorance is showing again.
First, the paper in question:
Big fish, small fish: Two New Ultra-Faint Satellites of the Milky Way
Belokurov,
et al., accepted for publication in Astrophysical Journal Letters
Abstract:
We report the discovery of two new Milky Way satellites in the neighboring constellations of Pisces and Pegasus identified in data from the Sloan Digital Sky Survey. Pisces II, an ultra-faint dwarf galaxy lies at the distance of ~180 kpc, some 15 degrees away from the recently detected Pisces I. Segue 3, an ultra-faint star cluster lies at the distance of 16 kpc. We use deep follow-up imaging obtained with the 4-m Mayall telescope at Kitt Peak National Observatory to derive their structural parameters. Pisces II has a half-light radius of ~60 pc, while Segue 3 is twenty times smaller at only 3pc.
Now, our Milky Way has a baryonic mass no more than 10
11 solar masses and a total
virial mass of about 10
12 solar masses, so the baryons constitute only about 10% of the total mass of the Galaxy, assuming of course that we are using the right law of gravity (
Xue, et al., 2008). So if the "dark matter" is made up of all these ultra faint systems, they have to cover 90% of the mass. But of course, the reason they are ultra faint is that they are ultra small, meaning ultra low in mass. The Belokurov,
et al., paper cited by Mozina compares Picsces II to other dwarf galaxies, Leo IV & Leo V. Those two show dynamical masses of 1.5x10
6 solar masses & 3x10
5 solar masses respectively (
Moretti, et al., 2009,
Walker, et al., 2009). But these masses could be to low because they are derived from stellar samples that may reveal only the central mass (the motions of stars can only reliably determine the dynamical mass at lesser, but not greater distance from the center of the system). It has been proposed that the combination of Leo IV + Leo V could be as high as 8x10
9 solar masses (
de Jong, et al., 2009), but this does require an abnormally high mass to light ratio. Segue 3, on the other hand, carries only a "few tens of stars", as the Belokurov,
et al., paper tells us, and so is insignificant.
It should come as no surprise to anyone that smaller, less massive systems are dimmer, and larger, more massive systems as brighter. So we face the dilemma of 10% of the stellar mass (i.e., the baryonic mass of the Galaxy) being hugely bright, while 90% of the stellar mass is in such ultra faint systems that they are barely visible at all. I take it the unbiased reader can understand that this is a hard thing to believe.
Meanwhile, I noted above that "
Your ignorance is showing again". In this case, specific ignorance of the "missing satellites problem":
Where Are the Missing Galactic Satellites?
Klypin,
et al., The Astrophysical Journal, Volume 522, Issue 1, pp. 82-92; September 1999
Abstract:
According to the hierarchical clustering scenario, galaxies are assembled by merging and accretion of numerous satellites of different sizes and masses. This ongoing process is not 100% efficient in destroying all of the accreted satellites, as evidenced by the satellites of our Galaxy and of M31. Using published data, we have compiled the circular velocity (Vcirc) distribution function (VDF) of galaxy satellites in the Local Group. We find that within the volumes of radius of 570 kpc (400 h-1 kpc assuming the Hubble constant h=0.7) centered on the Milky Way and Andromeda, the average VDF is roughly approximated as n(>Vcirc)~55+/-11(Vcirc/10 km s-1)-1.4+/-0.4 h3 Mpc-3 for Vcirc in the range ~10-70 km s-1. The observed VDF is compared with results of high-resolution cosmological simulations. We find that the VDF in models is very different from the observed one: n(>Vcirc)~1200(Vcirc10 km s-1)-2.75 h3 Mpc-3. Cosmological models thus predict that a halo the size of our Galaxy should have about 50 dark matter satellites with circular velocity greater than 20 km s-1 and mass greater than 3x108 Msolar within a 570 kpc radius. This number is significantly higher than the approximately dozen satellites actually observed around our Galaxy. The difference is even larger if we consider the abundance of satellites in simulated galaxy groups similar to the Local Group. The models predict ~300 satellites inside a 1.5 Mpc radius, while only ~40 satellites are observed in the Local Group. The observed and predicted VDFs cross at ~50 km s-1, indicating that the predicted abundance of satellites with Vcirc~50 km s-1 is in reasonably good agreement with observations. We conclude, therefore, that unless a large fraction of the Local Group satellites has been missed in observations, there is a dramatic discrepancy between observations and hierarchical models, regardless of the model parameters. We discuss several possible explanations for this discrepancy including identification of some satellites with the high-velocity clouds observed in the Local Group and the existence of dark satellites that failed to accrete gas and form stars either because of the expulsion of gas in the supernovae-driven winds or because of gas heating by the intergalactic ionizing background.
So we see that one of the problems that has faced the Lambda-CDM concordance cosmology has long been that we see far fewer dwarf satellite galaxies than we expected to see.
Fast forward to ...
The Kinematics of the Ultra-faint Milky Way Satellites: Solving the Missing Satellite Problem
Simon & Geha, The Astrophysical Journal, Volume 670, Issue 1, pp. 313-331; November, 2007
Abstract:
We present Keck DEIMOS spectroscopy of stars in eight of the newly discovered ultra-faint dwarf galaxies around the Milky Way. We measure the velocity dispersions of Canes Venatici I, Canes Venatici II, Coma Berenices, Hercules, Leo IV, Leo T, Ursa Major I, and Ursa Major II from the velocities of 18-214 stars in each galaxy and find dispersions ranging from 3.3 to 7.6 km s-1. The six galaxies with absolute magnitudes MV<-4 are highly dark matter dominated, with mass-to-light ratios approaching 1000 Msolar/Lsolar,v. For the fainter galaxies we find tentative evidence for tidal disruption. The measured velocity dispersions of the ultra-faint dwarfs are correlated with their luminosities, indicating that a minimum mass for luminous galactic systems may not yet have been reached. We also measure the metallicities of the observed stars and find that the new dwarfs have mean metallicities of [Fe/H]=-2.0 to -2.3 these galaxies represent some of the most metal-poor stellar systems known. The six brightest of the ultra-faint dwarfs extend the luminosity-metallicity relationship followed by more luminous dwarfs by a factor of ~30 in luminosity. We detect metallicity spreads of up to 0.5 dex in several objects, suggesting multiple star formation epochs. UMa II and Com, despite their exceptionally low luminosities, have higher metallicities that suggest they may once have been much more massive. Having established the masses of the ultra-faint dwarfs, we re-examine the missing satellite problem. After correcting for the sky coverage of the Sloan Digital Sky Survey, we find that the ultra-faint dwarfs substantially alleviate the discrepancy between the predicted and observed numbers of satellites around the Milky Way, but there are still a factor of ~4 too few dwarf galaxies over a significant range of masses. We show that if galaxy formation in low-mass dark matter halos is strongly suppressed after reionization, the simulated circular velocity function of CDM subhalos can be brought into approximate agreement with the observed circular velocity function of Milky Way satellite galaxies.
Note from the abstract above:
Having established the masses of the ultra-faint dwarfs, we re-examine the missing satellite problem. After correcting for the sky coverage of the Sloan Digital Sky Survey, we find that the ultra-faint dwarfs substantially alleviate the discrepancy between the predicted and observed numbers of satellites around the Milky Way, but there are still a factor of ~4 too few dwarf galaxies over a significant range of masses. Far from being a problem for dark matter, as Mozina says, the discovery of ultra faint dwarf galaxies like this is in fact a big deal in
confirming that dark matter cosmology is consistent with observations, by bringing the number of observed dwarf satellite galaxies up to the number expected from theory. As noted in the Simon & Geha paper, there remained a significant deficit of observed versus expected galaxies. But as we are able to see deeper & dimmer, we continue to discover more ultra faint dwarf galaxies, closing the gap between observation and theory.
Get used to that scenario. It's happens a lot and it's going to keep happening.
That's right Mozina, get used to it. The more of these ultra faint dwarf systems we find, the better dark matter cosmology looks and the worse you look.
