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Discovery of a slow bar in a spiral galaxy

» Wednesday 20 May 2009

Since more than a decade, kinematical observations of spiral galaxies have shown that the stellar bars seen in their centre are fast rotators. These measurements have been mostly obtained for bright early-type spirals, such as lenticular galaxies (S0). New observations perfomed by an international team, led by an astronomer from the Observatoire de Paris, report the discovery of a slow stellar bar in another type of galaxy, a late-type spiral galaxy (Sm). The observed target (UGC 628) has the particularity to be also a low surface brightness disc whose mass distribution is fully dominated by the dark matter.

Bars are density waves which rotate at constant angular speed in a disk galaxy, whereas the speed of stars varies as a function of the distance to the center: very fast in the center, and slower at the edge. The bar thus rotates more slowly that the stars at the same radius, and cannot extend beyond the radius of corotation, where it turns at the same speed as the stars. On the other hand, simulations predict that the bar could stop much before corotation, either because of a late galaxy morphology without bulge, or because of the braking of the dark matter. But rare are these cases of slow bars in the observations, and the theory could not yet be entirely confirmed.

Figure 1: Galaxy UGC 628
broad band image (left panel), Halpha emission map (central panel), Halpha velocity field (right panel). The bar is easily seen in the broad band image as a brighter structure in the centre of the galaxy disc.

UGC 628 (Figure 1) has been observed by means of optical Fabry-Perot interferometry in the Halpha emission line of the ionized gas with the instrument FaNTOmM, which was mounted on the Canada-France-Hawaii Telescope. The angular velocity of the bar (Omega-p) has been derived by applying the Tremaine-Weinberg method (TW), which is the only observational method that enables to directly measure a bar pattern speed, with no need of modeling the observations.

Figure 2: Pattern speed measurement of the stellar bar of UGC 628 with the TW method
The angular speed of the bar is the slope of the relationship (shown as a solid line) fitted to the kinematical points in the bar region (shown as full diamonds). Open symbols are for data outside the bar. They are fitted by the dashed line, the slope of which could correspond to the pattern speed of the spiral, outside the bar.

A bar is fast when the ratio of its corotation radius (Rc, where its speeds equals that of stars in the disk) to its semi-length (Rb, or bar radius) is within 1.0 and 1.4: 1.0 < Rc/Rb < 1.4. A bar with Rc/Rb > 1.4 is slow. Until now, the TW method has only identified pattern speeds consistent with bars being fast (in bright early-type galaxies). It is measured Omega-p = 11.3 km/s/kpc for UGC 628 (Figure 2). This amplitude is the lowest observed for a stellar bar up to now. It implies a ratio Rc/Rb = 2.0, and thus a slow bar in UGC 628.

Though it still remains an isolated case, the discovery of a slow bar in a Sm galaxy seems to point out a correlation between the bar pattern speed and the morphological type of galaxies: the angular velocity decreases from early types towards late types of galaxies. Moreover, the dominating mass of the dark matter over the mass distribution of UGC 628 likely contributes to slow down more efficiently the bar rotation. That discovery is in full agreement with results from many numerical simulations showing the evolution of barred galaxies.

Reference

  • A slow bar in a dark matter dominated galaxy - Laurent Chemin & Olivier Hernandez
  • A&A Letter, in press

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