Internal extinction of spiral galaxies in the NIR

Paper: astro-ph/0304398, appeared in July 2003 AJ.

The question of how much extinction there is present in external galaxies at first glance does not appear to be the most interesting of topics. It's not particularly fundamental and it doesn't seem to teach us any new Physics. The question however must be answered before we can get a reliable answer to some of the most pressing questions in Astronomy. Examples range from understanding the structure of spiral galaxies to using them as tracers of the distribution of mass in the universe. Even in the history of Astronomy it has played an important role - for example the first estimation of the size of our own Galaxy was seriously underestimated by Kapetyn because he did not take into account the role of interstellar extinction. The EGG group at Cornell, have used a significant amount of photometric data on edge-on spiral galaxies with the aim of measuring their distances. Since edge-on galaxies are the ones in which extinction effects will be the largest, this means that determining an accurate extinction law is very important. Edge-on galaxy NGC 891

Almost face-on galaxy M83, "The Southern Pinwheel" For this work we used data from the Two Micron All-Sky Survey (2MASS) which is an all-sky survey performed by two identical 1.3m telescopes in the NIR bands of J (1.25um), H (1.65um) and K (2.17um). We constructed a large sample of spiral galaxies (more than 15,000) with previously measured redshifts. Using this sample we searched for statistical correlations of various quanities with the inclination -- something which points to the effects of internal extinction, since the more inclined a galaxy is the further the light must pass through the disk to get out an reach us. Shown are two examples of spiral galaxies as seen by 2MASS where blue=J, green=H, red=K. These images are taken from the very nice poster of the 30 largest infrared galaxies. They illustrate the two extreme viewing angles for galaxies. M83 is seen almost face-on, so we expect little extinction, while NGC 891 is an edge-on galaxy and you can see the effects of extinction in this galaxy in the dark dust lane running through the centre. Also note that NGC 891 is redder than M83, which may also be due to extinction.

For more pretty pictures from 2MASS and much more information on the catalogue we used see Tom Jarrett's XSC page