Large, Strangely Dim Galaxy Found Lurking On Far Side of Milky Way (sciencemag.org)

Iwastheone shares a reprot from Science Magazine: Circling our galaxy is a stealthy giant. Astronomers have discovered a dwarf galaxy, called Antlia 2, that is one-third the size of the Milky Way itself. As big as the Large Magellanic Cloud, the galaxy's largest companion, Antlia 2 eluded detection until now because it is 10,000 times fainter. Such a strange beast challenges models of galaxy formation and dark matter, the unseen stuff that helps pull galaxies together. The galaxy was discovered with data from the European Space Agency's Gaia satellite, a space telescope measuring the motions and properties of more than 1 billion stars in and around the Milky Way. Gabriel Torrealba, an astronomy postdoc at the Academia Sinica in Taipei, decided to sift the data for RR Lyrae stars. These old stars, often found in dwarf galaxies, shine with a throbbing blue light that pulses at a rate signaling their inherent brightness, allowing researchers to pin down their distance.

Gaia data helped the team see past the foreground stars. Objects in the Milky Way's disk are close enough for Gaia to measure their parallax: a shift in their apparent position as Earth moves around the sun. More distant stars appear fixed in one spot. After removing the parallax-bearing stars, the researchers homed in on more than 100 red giant stars moving together in the constellation Antlia, they report in a paper posted to the preprint server arXiv this week. The giants mark out a sprawling companion galaxy 100 times less massive than anything of similar size, with far fewer stars.

Comments

Re:Strike one problem of our physics list:

[Sique]

Actually no. Dark Matter is not simply "missing matter" in general, it's missing matter in very well defined scenarios. A dark galaxy out there would not solve other problems which are connected to Dark Matter. It wouldn't explain the rotation of galaxies we actually see. It wouldn't explain the behavior of the gas clouds at NGC 604. It wouldn't explain the amount of light deflection we see at galaxies which bend the light of the galaxies behind them and cause us to see Einstein rings.

Re:But it's a start

[Anonymous Coward]

Besides, wouldn't a really dim galaxy kind of 'be' dark matter since we could not see it before?

If by "dim" you mean the galaxy is far far away, or has lots of dust or small stars, then no, most physicists would not say it is dark matter. Most physicists use the phrase "dark matter" to refer to a non-baryonic mass-producing phenomenon, not protons and neutrons that are poorly illuminated.

Re:But it's a start

[Sique]

Currently, Dark Matter is the term for stuff that causes gravitational effects, but does not interact with electromagnetic fields. Ordinary matter does interact with electromagnetic fields and is often referred to as baryonic matter, but "Hubble pictures are too crisp." We can see light in pictures taken by the Hubble space telescope which has traveled more than 12 billion light years (or is more than 12 billion years old). We don't see any effects of that light to have interacted with baryonic matter during that time (e.g. slight phase differences between different frequencies, degrading sharpness or similar). Thus we can calculate an upper limit of the average amount of ordinary matter in the interstellar space. We can't account for all that baryonic matter yet, but the unaccounted for amount is shrinking, and dark galaxies like the one in the article will make the account more complete. There were other breakthroughs recently, but still, this is all baryonic (and thus ordinary and not Dark) matter.

No, Dark Matter is not simply dark matter, matter we haven't seen yet. Dark Matter is matter we absolutely can not see, e.g. detect by its direct interactions with electromagnetic waves. We only can tell that there is Dark Matter out there because it changes Spacetime according to General Relativity. There are models where Dark Matter does interact with the Weak Force (e.g. WIMPs, Weakly Interacting Massive Particles), but so far, none of those models has been proven by an experiment or an observation.