A survey for OB associations in the Sculptor Group spiral galaxy NGC 7793

¹ Universidad de Concepción, Departamento de Fisica, Casilla 160–C, Concepción, Chile e-mail: pietrzyn@hubble.cfm.udec.cl; wgieren@astro-udec.cl
² Warsaw University Observatory, AL. Ujazdowskie 4, 00-478 Warsaw, Poland e-mail: kulaczyk@astrouw.edu.pl
³ Institute for Astronomy, 2680 Woodlawn Drive, Honolulu, HI 96822, Hawaii, USA e-mail: [bresolin; kud]@ifa.hawaii.edu

Received: 15 September 2004
Accepted: 13 April 2005

Abstract

We report on the results from application of an objective algorithm (PLC) to find OB associations, to B and V images of the Sculptor spiral galaxy NGC 7793, which were obtained with the ESO VLT and FORS instrument and basically cover the entire spatial extent of the galaxy. We detected 148 associations. Statistical tests show that less than 6 of these detections are caused by randomly concentrated blue stars. In the size distribution, a sharp peak is observed at a value of about 35 microradians, which corresponds to a linear diameter of 135 pc, assuming a distance of 3.91 Mpc to the galaxy. We also find 25 much larger objects. A second application of the PLC technique shows that 20 of them are stellar complexes consisting of multiple sub-associations with typical sizes on the order of 130 pc. A comparison of the size distribution of the detected OB associations in NGC 7793 with observed distributions in other galaxies suggests that the conditions in two Sculptor Group galaxies (NGC 300 and NGC 7793) favour the formation of large associations. We provide a catalog giving coordinates and physical parameters for all the associations and stellar complexes we have found in our survey.