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). Large format: GIF
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 excedes 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 two of the simulations. They show the development of the
corotation radius and the bar length with time. The wiggles are due to
the fact that bars develop buckling instabilities which make their
growth uneven. The green areas are those where the bar length would be
called "fast" in previous studies. Although the corotation radius
increases with time, in these simulations the bar length increases
faster, so that at the end of the simulation, in both cases the bar
would have been classfied as "fast" even though it has slowed down
considerably. Large format: JPEG
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.
More information:
"New evidence in favour of dark matter: the bars in galaxies are spinning more slowly than we thought", IAC press release, 7th February 2017.
J. Font, J. E. Beckman, I. Martínez-Valpuesta, A. S. Borlaff, P. A. James, S. Díaz-García, B. García-Lorenzo, A. Camps-Fariña, L. Gutiérrez, and P. Amram, 2017, "Kinematic Clues to Bar Evolution for Galaxies in the Local Universe: Why the Fastest Rotating Bars are Rotating Most Slowly", ApJ, 835, 279 [ Paper ].
GHaFaS web site.
Contact:
Javier Méndez
(Public Relations Officer)
More information:
"New evidence in favour of dark matter: the bars in galaxies are spinning more slowly than we thought", IAC press release, 7th February 2017.
J. Font, J. E. Beckman, I. Martínez-Valpuesta, A. S. Borlaff, P. A. James, S. Díaz-García, B. García-Lorenzo, A. Camps-Fariña, L. Gutiérrez, and P. Amram, 2017, "Kinematic Clues to Bar Evolution for Galaxies in the Local Universe: Why the Fastest Rotating Bars are Rotating Most Slowly", ApJ, 835, 279 [ Paper ].
GHaFaS web site.
Contact:
Javier Méndez
(Public Relations Officer)
Source: Isaac Newton Group of Telescopes