The dark matter halos of the bluest low surface brightness galaxies

¹ Department of Astronomy and Space Physics, Box 515, 75120 Uppsala, Sweden e-mail: ez@astro.uu.se
² Tuorla Observatory, Väisäläntie 20, 21500 Piikkiö, Finland
³ Stockholm Observatory, AlbaNova University Center, 106 91 Stockholm, Sweden

Received: 13 September 2005
Accepted: 6 March 2006

Abstract

We present BVI photometry and long-slit Hα rotation curve data obtained with ESO VLT/FORS2 for six low surface brightness galaxies with extremely blue colours and very faint central regions. We find no evidence for a steep central density cusp of the type predicted by many N-body simulations of cold dark matter (CDM) halos. Our observations are instead consistent with dark matter halos characterized by cores of roughly constant density, in agreement with previous investigations. While unremarkable in terms of the central density slope, these galaxies appear unusually challenging for existing CDM halo models in terms of average central halo density, as measured by the parameter. Since most of our target galaxies are bulgeless disks, our observations also disfavour a recently suggested mechanism for lowering the central mass concentration of the halo by means of a fast collapse phase, as this scenario predicts that the original CDM profile should still be detectable in bulgeless galaxies. Other potential ways of reconciling the CDM predictions with these observations are discussed.