"Three bright pulsating stars on the outskirts of the Milky Way galaxy could be beacons from an invisible dwarf galaxy that astronomers predicted was there based on its effects of galactic quakes in our galaxy. These galactic quakes, ripples in gas at the outer disk of our galaxy, have puzzled astronomers since they were first revealed by radio observations a decade ago. Now, astronomers believe these stars mark the location of a dark matter-dominated dwarf galaxy far beyond the edge of the Milky Way disk, which terminates at 60,000 light-years.
In 2009, Chakrabarti and Blitz used these techniques to predict the existence of a dwarf satellite galaxy in the direction of the constellation Norma, and last year she and her team used the Gemini South Telescope in Chile and Magellan telescopes to search for stars in that region that might be part of the galaxy. They found three pulsating stars called Cepheid variables, typically used as yardsticks to measure distance, that are at approximately the same distance from the sun: 300,000 light-years.
It's intriguing, but three stars do not a galaxy make. How many stray Cepheids are there in locations where the model didn't predict a galaxy ?
Astronomers discovered the first evidence of mysterious dark galaxies with no starlight in 2005 - VirgoHI 21 - a cloud of hydrogen in the Virgo Cluster 50 million light-years from the Earth was found to be colliding with our galaxy. Virgohi 21 revealed its existence from radio waves from neutral hydrogen coming from a rotating cloud containing enough hydrogen gas to spawn 100 million stars like the sun and fill a small galaxy.
Ouch. VIRGOHI21 wasn't the first dark galaxy candidate and it certainly isn't colliding with our own galaxy - it's 50 million light years (a.k.a. bloody miles) way ! It's apparently interacting with another spiral galaxy, NGC 4254. The hydrogen mass is more like 20 million solar masses, not 100 million, but the size is comparable to a fairly large galaxy.
The rotation of VirgoHI21 is far too fast to be consistent with the gravity of the detected hydrogen. Rather, it implies the presence of a dark matter halo with tens of billions of solar masses. Given the very small number of stars detected, this implies a mass-to-light ratio of about 500, far greater than that of a normal galaxy (which would be around 50). The large gravity of the dark matter halo in this interpretation explains the perturbed nature of the nearby spiral galaxy NGC 4254 and the bridge of neutral hydrogen extending between the two entities.
Implies is the operative word. Also, there were no stars detected at all, just upper limits based on the sensitivity of the data. It's entirely possible that the mass to light ratio is infinite because there's no light. But more importantly, a dark galaxy isn't the only explanation for VIRGOHI21 - it's possible, but it could also just be a weird tidal tail.
VirgoHI21 proved to be the first discovery of the dark galaxies anticipated by simulations of dark-matter theories.
No it didn't. Actually very few people were ever convinced by it, even when the evidence seemed pretty good. An alternative model of the gas being a tidal tail seemed to do a decent job of explaining it without a dark galaxy involved. This may or may not be the case, you'll have to wait for my next paper...
Although other dark-galaxy candidates have previously been observed, follow-up observations indicated that these were either very faint ordinary galaxies or tidal tails.
Oh, don't even get me started on that one.
The models Blitz developed in 2014 predict that the universe should contain far more dwarf galaxies than the tiny fraction that astronomers can identify.
Such models have in fact been around since the 1990's, this isn't a new problem.
http://www.dailygalaxy.com/my_weblog/2016/08/phantoms-of-the-milky-way-galactic-quakes-reveal-hundreds-of-invisible-starless-galaxies-orbiting-ou.html
“Of course we aren’t talking about a pond, but our galaxy, which is tens of thousands of light years across, and made of stars and gas, but the result is the same – ripples!”
ReplyDelete...tens of thousands?
Well, about 100,000, more or less.
ReplyDeleteI was just wondering when you switch scales.. could have just as easily said, "...which is light years across.." i mean it's only 100,000 or so, as opposed to tens of tens of thousands.. lots of people who don't read this kind of stuff much would probably answer someone who asked them how bug our galaxy is with "well, probably 20 or 30 thousand light years across." or our universe, "well the part we can see is tens of millions of light years across"
ReplyDeleteI'd say anything up to ~200,000 could be called "tens of thousands". It should be a multiple of an integer greater than one to be in the next scale. Worst case, people will come away thinking the galaxy is a few times smaller than it actually is.
ReplyDeleteRoger that, thanks Rhys Taylor.
ReplyDeleteHow about this sentence at the end: "The image at the top of the page shows contours of HI column density obtained from the ALFALFA observations of the field around VirgoHI21 and NGC 4254 superposed on an optical image."
ReplyDeleteNope - that's definitely the WSRT image they're showing there!