We study the late-time evolution of the central regions of two Milky Way-like simulations of galaxies formed in a cosmological context , one hosting a fast bar and the other a slow one .
We find that bar length , R _ { b } , measurements fluctuate on a dynamical timescale by up to 100 % , depending on the spiral structure strength and measurement threshold .
The bar amplitude oscillates by about 15 % , correlating with R _ { b } .
The Tremaine-Weinberg-method estimates of the bars ’ instantaneous pattern speeds show variations around the mean of up to \sim 20 \% , typically anti-correlating with the bar length and strength .
Through power spectrum analyses , we establish that these bar pulsations , with a period in the range \sim 60 - 200 Myr , result from its interaction with multiple spiral modes , which are coupled with the bar .
Because of the presence of odd spiral modes , the two bar halves typically do not connect at exactly the same time to a spiral arm , and their individual lengths can be significantly offset .
We estimated that in about 50 % of bar measurements in Milky Way-mass external galaxies , the bar lengths of SBab type galaxies are overestimated by \sim 15 \% and those of SBbc types by \sim 55 \% .
Consequently , bars longer than their corotation radius reported in the literature , dubbed ‘ ‘ ultra-fast bars '' , may simply correspond to the largest biases .
Given that the Scutum-Centaurus arm is likely connected to the near half of the Milky Way bar , recent direct measurements may be overestimating its length by 1 - 1.5 kpc , while its present pattern speed may be 5 - 10 km s ^ { -1 } kpc ^ { -1 } smaller than its time-averaged value .