NGC 3433 (image from SDSS). Superposed: MUSE field (red square);
GHaFaS field (yellow square); projected corotation circle for the bar
(solid green line); and the other measured corotation radii (dashed
green lines). Large format: [ JPEG ].
Astronomers from the Instituto de Astrofísica de Canarias (IAC) have produced a complex velocity analysis of
the spiral galaxy NGC 3433 with surprisingly precise results. They
compared observations using the 2D Fabry-Perot spectrometer GHaFaS on the
William Herschel Telescope (WHT) with those of the same object taken
with the IFU spectrograph MUSE on the Very Large Telescope (VLT) in
Chile.
Font and Beckman (IAC) have developed a technique (FB) for finding the
corotation radii in galaxies, i.e. the radii at which the spiral
density waves propagate at the same angular speed as the stars and the
gas. Simple theory suggests that the bar of a galaxy should stimulate a
density wave outside it, which in turn stimulates and maintains the
spiral arms. Using FB, the IAC researchers had shown previously in a
large sample of galaxies that normally more than a single corotation is
found in a galaxy, but that only one of them is related to the bar,
while the others are found in the spiral arms, at increasing radii, or
associated with a smaller, nuclear bar or oval distortion at small
radii.
While MUSE has a 1 arcmin × 1 arcmin field and offers spectral
coverage of both stellar and gas components with a resolution of some 50
km/s in velocity, GHaFaS has a larger field of view, 3.4 arcmin × 3.4
arcmin field, gives a resolution in velocity of 6 km/sec, but it can
observe only the ionised interstellar gas.
Now with NGC 3433 they have put all of their previous work on a
firmer basis by comparing it with a classical, and quite different,
technique for finding corotation radii, developed by Tremaine and
Weinberg (TW). They applied both methods to the velocity fields of both
the stars and the insterstellar gas, using the observations of both
GHaFaS and MUSE. They found four corotation radii.
The innermost one, in the circumnuclear zone, could be detected only
using FB, but was found clearly in both MUSE and GHaFaS data for the gas
with a difference of 7% in the values. The second one, corresponding to
the main bar, and the most intense, was measured in six different ways:
using FB on the gas with GHaFaS, FB on the gas with MUSE, FB on the
stars with MUSE, TW on the gas with GHaFaS, TW on the gas with MUSE, and
TW on the stars with MUSE. The uncertainty in the corotation found by
using all 6 values was only 4%.
A third corotation was found using FB on gas for both GHaFaS and
MUSE, and a fourth corotation, beyond the limits of the field of MUSE,
was measured using FB and TW on the gas with GHaFaS. The values for the
corotation radii in both cases, gave excellent agreement between the two
methods used. Measured this way the corotation radii are among the most
accurately determined parameters of the galaxy, compared with, for
example, the bar length.
Panel (a): velocity map of NGC 3433 using the first moment map of Hα
emission in the FP data cube from GHaFaS. The box in black shows the
size of the MUSE data. Panel (b): velocity map of Hα emission from the
central square arcmin from the MUSE data cube. Panel (c): velocity map
of the stellar component from the MUSE data. Large format: [ JPEG ].
Although this study deals with only a single galaxy, its results are
powerful because they verify FB as a method, and it is considerably
easier than TW to apply to large numbers of objects, demanding less
observing time. Measuring the principal corotation radius allows us to
measure the pattern speed of the bar, and this allows us to perform a
whole range of tests on the evolution of galaxies, including measuring
the braking effects of dark matter halos. "For the time being we are
confined to low redshifts, but as our techniques advance we have hopes
of reaching intermediate redshift objects in the fairly near future",
said John Beckman. Isaac Newton Group of Telescopes
More Information
Beckman, John E.; Font, Joan; Borlaff, Alejandro; García-Lorenzo,
Begoña, 2018, "Precision Determination of Corotation Radii in Galaxy
Disks: Tremaine-Weinberg versus Font-Beckman for NGC 3433", ApJ, 854, 182 [ ADS ].
"New Light on Dark Matter Halos", ING web news release, 13th February 2017.
Contact:
Javier Méndez
(Public Relations Officer)
