The Hα Galaxy Survey^*

VI. Star-forming companions of nearby field galaxies

¹ Astrophysics Research Institute, Liverpool John Moores University, Twelve Quays House, Egerton Wharf, Birkenhead CH41 1LD, UK e-mail: paj@astro.livjm.ac.uk
² Wirral Grammar School for Girls, Heath Road, Bebington, Wirral CH63 3AF, UK
³ Planetary Science Group, Mullard Space Science Laboratory, Holmbury St. Mary, Dorking, Surrey RH5 6NT, UK

Received: 20 December 2007
Accepted: 5 May 2008

Abstract

Aims. We searched for star-forming satellite galaxies that are close enough to their parent galaxies to be considered analogues of the Magellanic Clouds.

Methods. Our search technique relied on the detection of the satellites in continuum-subtracted narrow-band Hα imaging of the central galaxies, which removes most of the background and foreground line-of-sight companions, thus giving a high probability that we are detecting true satellites. The search was performed for 119 central galaxies at distances between 20 and 40 Mpc, although spatial incompleteness means that we have effectively searched 53 full satellite-containing volumes.

Results. We find only 9 “probable” star-forming satellites, around 9 different central galaxies, and 2 more “possible” satellites. After incompleteness correction, this is equivalent to 0.17/0.21 satellites per central galaxy. This frequency is unchanged whether we consider all central galaxy types or just those of Hubble types S0a–Sc, i.e. only the more luminous and massive spiral types. The satellites found are generally similar to the Magellanic Clouds and to field Sm and Im galaxies, in terms of their normalised star formation rates. However, this conclusion is somewhat circular as the similarity of properties to known Sm/Im galaxies was used as a classification criterion. The Small Magellanic Cloud is just below the median values of both star formation rate and R-band luminosity of the 9 probable satellites. The Large Magellanic Cloud, however, has a higher R-band luminosity than any of the 9 and is only exceeded in star formation rate by the one satellite that appears to be undergoing a tidally-induced starburst. Thus the Milky Way appears to be quite unusual, both in having two star-forming satellite galaxies and in the high luminosity of the Large Magellanic Cloud.