An international team of astronomers from Chile, Europe andNorth America  is announcing the most accurate distance yet measuredto a galaxy beyond our Milky Way's close neighbours. The distance wasdetermined using the brightness variation of a type of stars known as"Cepheid variables".
The team used the ISAAC near-infrared cameraand spectrometer on ESO's 8.2-m VLT Antu telescope to obtain deepimages in the near-infrared of three fields in the spiral galaxy NGC300. Together these fields contain 16 long-period Cepheids. These starshad previously been discovered by the team in a wide-field imagingsurvey of this galaxy conducted with the Wide Field Imager (WFI) cameraon the ESO/MPG 2.2-m telescope at La Silla.
The spiral galaxy NGC300 is a beautiful representative of its class, a Milky-Way-like memberof the prominent Sculptor group of galaxies in the southernconstellation of the same name.
The astronomers derive a distanceto NGC 300 of a little above 6 million light-years . "The VLT datahave led to accurate period-luminosity relations in the J- and K-bands, allowing us to determine the distance to NGC 300 with anunprecedented uncertainty of only three percent", says Wolfgang Gieren,of the University of Concepcion (Chile) and leader of the team. One ofthe reasons for this high accuracy was the opportunity to preciselycombine the new near-infrared ISAAC data with the previous optical WFIdata.
Cepheid variables constitute a key element in themeasurement of distances in the Universe. It has been known for manyyears that the pulsation period of a Cepheid-type star depends on itsintrinsic brightness (its "luminosity"). Thus, once its period has beenmeasured, the astronomers can calculate its luminosity. By comparingthis to the star's apparent brightness in the sky, they can obtain thedistance to the star. This fundamental method has allowed some of themost reliable measurements of distances in the Universe and has beenessential for all kinds of astrophysics, from the closest stars to theremotest galaxies.
This first Cepheid distance based onnear-infrared imaging with the Very Large Telescope is a milestone inthe team's Araucaria Project in which they seek to improve the localcalibration of the distance scale with stellar standard candles,including Cepheid variables, by determining precisely how thesestandard candles depend on a galaxy's properties, such as its contentin chemical elements and age.
: The team is composed of Wolfgang Gieren(Principal Investigator of the Araucaria Project), Grzegorz Pietrzynskiand Igor Soszynski (Universidad de Concepcion, Chile), Rolf-PeterKudritzki and Fabio Bresolin (Institute for Astronomy, Hawaii, USA),Dante Minniti (Pontificia Universidad Catolica, Chile), and JesperStorm (Astrophysikalisches Institut Potsdam, Germany).
: Thisdistance determination is tied to an assumed distance of 163,000light-years to the Large Magellanic Cloud (LMC). The accuratemeasurement of the LMC distance itself to which the extragalacticdistance scale is currently tied will still require years of hard work.
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