How far the galactic neighbours?

Determining distances to individual galaxies using a variety of methods is a fundamental part of mapping out the distribution of galaxies in the Universe. Research work at Tuorla Observatory, part of a PhD thesis by Rami Rekola has recently focused on the issue of distances to nearby galaxies. Three very different techniques with which to measure distances: the first based on the brightness of small, ionised spheres of gas in the galaxies, called planetary nebula; the second, a classical technique, of using the brightness of pulsating, luminous stars called Cepheids; and the third, a method based on the variations in brightness from place to place within a high quality image of a galaxy.

Distances have been obtained for dwarf elliptical and irregular galaxies of the extended local group using the NOT, in collaboration with Helmut Jerjen at Mount Stromlo Observatory. A Cepheid based distance has been obtained for IC 342, a large, starburst spiral galaxy, for which few (and contradictory) distance estimates exist in the literature, using observations with the NOT over a long baseline (5 years). Planetary nebulae have been used to make a distance determination to NGC 253 (using imaging data from the ESO 3.6 metre).

Rami Rekola's work is part of a longer term programme to assemble reliable masses and distances for the extended local group galaxies with a view to simulating their dynamics. Many of these galaxies are at distances where the effects of "dark energy" on the Hubble flow are first noticable; accurate distances to the galaxies are therefore of particular interest. The surface brightness fluctuation method has been shown to be a particularly efficient means of determining good distances, especially for dwarf galaxies for which other methods cannot be easily applied. We are planning to use the method to measure distances for large numbers of nearby dwarfs in an ongoing effort to understand better the local Hubble flow.

Part of the giant spiral galaxy IC 342. It lies just beyond our own Local Group of galaxies in another group called Maffei group or IC342/Maffei group. The image above is a combination of images taken with BVRI filters (roughly blue, visual = green, red, and infrared) by Rami Rekola and Kari Nilsson at the Nordic Optical Telescope.