Types of galaxies (Credit: NASA)
In the late 1970s, astronomers Vera Rubin and Albert Bosma independently found that spiral galaxies rotate at a nearly constant speed: the velocity of stars and gas inside a galaxy does not decrease with radius, as one would expect from Newton’s laws and the distribution of visible matter, but remains approximately constant. Such ‘flat rotation curves’ are generally attributed to invisible, dark matter surrounding galaxies and providing additional gravitational attraction. Now a team led by Case Western Reserve University researchers has found a significant new relationship in spiral and irregular galaxies: the acceleration observed in rotation curves tightly correlates with the gravitational acceleration expected from visible mass only.
“If you measure the distribution of star light, you know the rotation curve, and vice versa,” said Stacy McGaugh. The finding is consistent among 153 spiral and irregular galaxies, ranging from giant to dwarf, those with massive central bulges or none at all. It is also consistent among those galaxies comprised of mostly stars or mostly gas. McGaugh et al argue that the relation they’ve found is tantamount to a new natural law. An astrophysicist who reviewed the study said the findings may lead to a new understanding of internal dynamics of galaxies.
“Galaxy rotation curves have traditionally been explained via an ad hoc hypothesis: that galaxies are surrounded by dark matter,” said Prof. David Merritt. “The relation discovered by McGaugh et al. is a serious, and possibly fatal, challenge to this hypothesis, since it shows that rotation curves are precisely determined by the distribution of the normal matter alone. Nothing in the standard cosmological model predicts this, and it is almost impossible to imagine how that model could be modified to explain it, without discarding the dark matter hypothesis completely.”
McGaugh and Schombert have been working on this research for a decade and with Lelli the last three years. Near-infrared images collected by Spitzer Space Telescope during the last 5 years allowed them to establish the relation and that it persists for all 153 galaxies. The key is that near-infrared light emitted by stars is far more reliable than optical-light for converting light to mass.
The researchers plotted the radial acceleration observed in rotation curves published by a host of astronomers over the last 30 years against the acceleration predicted from the observed distribution of ordinary matter now in the Spitzer Photometry & Accurate Rotation Curves database McGaugh’s team created. The 2 measurements showed a single, extremely tight correlation, even when dark matter is supposed to dominate the gravity. “There is no intrinsic scatter, which is how far the data differ on average from the mean when plotted on a graph,” McGaugh said. “What little scatter is found is consistent with stellar mass-to-light ratios that vary a little from galaxy to galaxy.”
Lelli compared the relation to a long-used natural law. “It’s like Kepler’s third law for the solar system: if you measure the distance of each planet from the sun, you get the orbital period, or vice versa” he said. “Here we have something similar for galaxies, with about 3,000 data points.”
“In our case, we find a relation between what you see in normal matter in galaxies and what you get in their gravity,” McGaugh said. “This is important because it is telling us something fundamental about how galaxies work.”
“The natural inference is that this law stems from a universal force such as a modification of gravity like MOND, the hypothesis of Modified Newtonian Dynamics proposed by Israeli physicist Moti Milgrom. But it could also be something in the nature of dark matter like the superfluid dark matter proposed by Justin Khoury,” McGaugh said. “Most importantly, whatever theory you want to build has to reproduce this.”
http://phys.org/news/2016-09-spiral-irregular-galaxies-current-dark.htmljCp     https://arxiv.org/abs/1609.05917
Recent Comments