Monday, February 13, 2017

New Light on Dark Matter Halos

NGC 1300 is a classic barred spiral galaxy, similar to those observed in the present study. It is 17 Mpc away and relatively face-on, with an inclination of about 35 degrees. Shown here is a colour composition from B, V and I-band CCD images obtained using the Prime-Focus Camera on the Isaac Newton Telescope (INT) in 1996. Credit: J. A. López Aguerri, M. Prieto, C. Muñoz-Tuñón, and A. M. Varela (IAC).

For the past twenty years observers have been trying to test the effects of the predicted dark matter halos on the bars in barred galaxies. The basic idea is that according to simulation models which include the halos, these should have acted as a gravitational brake and slowed down the rotation of the bars during the lifetimes of galaxy discs.

This could be tested by measuring the corotation radius corresponding to the bar, which is the radius at which the angular pattern speed of the bar is equal to the angular speed of the stars in the disc. Classical models as early as the 1980's showed that the corotation radius should be just outside the tip of the bar, while the simulations made just under 20 years ago suggested that the ratio of the corotation radius to the bar length should indeed be just bigger than unity, unless the bar has been slowed down by interaction with the halo.

In that case corotation would move progressively outwards in the disc. The simulators set a (somewhat arbitrary) criterion that if the ratio exceeds 1.4 this is satisfactory evidence of braking by the halo, and this criterion has been used by observers as a test for the presence of halos.

The problem has been the difficulty of measuring the corotation radius. Until recently this had been done, by a few different methods, for only a few tens of galaxies, and the results were somewhat surprising. The ratio of corotation radius to bar length was, in almost all cases, below 1.4. The conclusion seemed to be that dark halos do not have the braking effect predicted.

However a group at the Instituto de Astrofísica de Canarias (IAC), including Joan Font and John Beckman, devised a new method for corotation, using data from high resolution two-dimensional spectra taken with Fabry-Perot spectrographs, the most accurate among them GHaFaS on the William Herschel Telescope (WHT). They previously published corotation measurements on over 100 galaxies, and then applied their measurements to find the corotation-bar length ratios, using Spitzer satellite infrared images for the bar lengths, to avoid problems of dust absorption.

In their results, they found ratios in the range 1 to 1.4, but that was not all. They also computed the ratio of the bar angular rotation velocity to those of the discs, and showed that many bars, notably long, massive bars, have small values for this ratio, suggesting that braking must have occurred.

Puzzled by this, they asked themselves how these two opposite conclusions could be reconciled, and the only answer seemed to be that the bars, as well as slowing down, must be growing in length as the discs evolved, thus keeping the ratio of corotation radius to bar length below 1.4. They enlisted the help of Inma Martínez, a theorist at the IAC, who simulates bar evolution in galaxies, and she showed in her simulations that this is indeed what tends to occur, and had not been well taken into account in previous work. 

The results of their joint study were published in the February 1st issue of the Astrophysical Journal. The overall conclusion is that dark matter halos are no longer threatened by observations of rotating bars.


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