Four new satellite galaxies

[Blob]

Four new satellite galaxies of the Milky Way have been discovered, bringing the total known to about 20. The pace of new discoveries suggests that many more such satellites remain unknown, which would present a serious challenge to models of dark matter as "warm", fast-moving particles.

The satellites are dwarf galaxies a few hundred to a few thousand light years across. The tiny galaxies are thought to be the building blocks of large galaxies, such as our own Milky Way – which is about 100,000 light years wide.
The four new discoveries were made by a team led by Vasily Belokurov of the University of Cambridge, UK. Named after the constellations in which they were found – Coma Berenices, Canes Venatici II, Hercules, and Leo IV, all of them lie between roughly 100,000 and 500,000 light years from Earth.
The largest and smallest are Hercules and Coma Berenices, which are about 1000 and 200 light years across, respectively. Like most of the other dwarfs discovered by SDSS, the new finds are much smaller and fainter than the 10 dwarfs that were known previously.

Source

[Blob]

Previously known Companions to the Galaxy:

Name Type RA Dec D(Gal) Mv (Fe/H)

N 5139 GC   13 26 46 -47 28 37   06    -10.3 -1.6
Sgr dSph     18 55 03 -30 28 42  19    -15.0 -0.5
LMC Ir        05 23 35 -69 45 22  50    -18.5 -0.6
SMC Ir       00 52 49 -72 49 43   63    -17.1 -1.2
UMi dSph     15 09 10 +67 12 52  69    -8.9 -1.9
Dra dSph     17 20 12 +57 54 55  79    -9.4 -2.0
Sex dSph    10 13 03 -01 36 53   86    -9.5 -1.9
Scl dSph     01 00 09 -33 42 33   88    -9.8 -1.5
N 2419 GC   07 38 08 +38 52 55   92     -9.6 -2.1
Car dSph     06 41 37 -50 57 58  94     -9.4 -1.8
UMa dSph    10 34 53 +51 55 12 105     -6.8  -2
For dSph     02 39 59 -34 26 57 138     -13.1 -1.2
Leo II dSph 11 13 29 +22 9 17   205     -10.1 -1.6
Leo I dSph  10 08 27 +12 18 27  270    -11.9 -1.4
Phe dIr/dSph 01 51 06 -44 26 41 405    -9.8 -1.9
NGC 6822 Ir 19 44 56 -14 52 11  500    -16.0 -1.2

[antoniseb]

Where are these in relation to the plane of the galaxy?
People discuss the existence of the dwarf galaxies as evidence for or against Dark Matter, as though these are leftovers from the galaxy formation process 13 billion years ago, but I think there is a significant likelyhood that these are leftovers from one or more previous interactions between the Milky Way and M31.

[Kullat Nunu]

The original article discuss how these new dwarf galaxies are problematic to hot dark matter theories; it doesn't mention that some models predicts even hundreds of such dwarf galaxies around the Milky Way. If there haven't been gas available, those galaxies would be totally dark.

[Blob]

Title: The Baryon Content of Extremely Low Mass Dwarf Galaxies
Authors: M. Geha, M. R. Blanton, M. Masjedi, A. A. West

We investigate the gas content and baryonic Tully-Fisher relationship for extremely low luminosity dwarf galaxies in the absolute magnitude range -13.5 > Mr > -16. The sample is selected from the Sloan Digital Sky Survey and consists of 101 galaxies for which we have obtained follow-up HI observations using the Arecibo Observatory and Green Bank Telescope. This represents the largest homogeneous sample of dwarfs at low luminosities with well-measured HI and optical properties. The sample spans a range of environments, from dense groups to truly isolated galaxies. The average neutral gas fraction is fgas=0.6, significantly exceeding that of typical gas-rich galaxies at higher luminosities. Dwarf galaxies are therefore less efficient at turning gas into stars over their lifetimes. The strong environmental dependence of the gas fraction distribution demonstrates that while internal processes can reduce the gas fractions to roughly fgas=0.4, external processes are required to fully remove gas from a dwarf galaxy. The average rotational velocity of our sample is vrot=50 km/s. Including more massive galaxies from the literature, we fit a baryonic Tully-Fisher slope of Mbaryon \propto vrot^(3.70 ± 0.15). This slope compares well with CDM models that assume an equal baryon to dark matter ratio at all masses. While gas stripping or other processes may modify the baryon to dark matter ratio for dwarfs in the densest environments, the majority of dwarf galaxies in our sample have not preferentially lost significant baryonic mass relative to more massive galaxies.

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Download dwarf galaxy data

[Blob]

Where are these in relation to the plane of the galaxy?
...I think there is a significant likelyhood that these are leftovers from one or more previous interactions between the Milky Way and M31.

There is some more information here:

Title: Cats and Dogs, Hair and A Hero: A Quintet of New Milky Way Companions
Authors: V. Belokurov, D. B. Zucker, N. W. Evans, J. T. Kleyna, S. Koposov, S. T. Hodgkin, M. J. Irwin, G. Gilmore, M. I. Wilkinson, M. Fellhauer, D. M. Bramich, P. C. Hewett, S. Vidrih, J. T. A. De Jong, J. A. Smith, H.-W. Rix, E. F. Bell, R. F. G. Wyse, H. J. Newberg, P. A. Mayeur, B. Yanny, C. M. Rockosi, O. Y. Gnedin, D. P. Schneider, T. C. Beers, J. C. Barentine, H. Brewington, J. Brinkmann, M. Harvanek, S. J. Kleinman, J. Krzesinski, D. Long, A. Nitta, S. A. Snedden

We present five new satellites of the Milky Way discovered in Sloan Digital Sky Survey (SDSS) imaging data, four of which were followed-up with either the Subaru or the Isaac Newton Telescopes. They include four probable new dwarf galaxies -- one each in the constellations of Coma Berenices, Canes Venatici, Leo and Hercules -- together with one unusually extended globular cluster, Segue 1. We provide distances, absolute magnitudes, half-light radii and colour-magnitude diagrams for all five satellites. The morphological features of the colour-magnitude diagrams are generally well described by the ridge line of the old, metal-poor globular cluster M92. In the last two years, a total of ten new Milky Way satellites with effective surface brightness muv >~ 28 mag/sq. arcsec have been discovered in SDSS data. They are less luminous, more irregular and appear to be more metal-poor than the previously-known nine Milky Way dwarf spheroidals. The relationship between these objects and other populations is discussed. We note that there is a paucity of objects with half-light radii between ~40 pc and ~ 100 pc. We conjecture that this may represent the division between star clusters and dwarf galaxies.

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