DARK-MATTER DISTRIBUTION AND THE H-IOTA-H-ALPHA CONNECTION IN IC-2574

A detailed study of the low surface brightness late-type dwarf spiral galaxy IC 2574 is presented. By combining data from broadband surface photometry, narrow-band H alpha imaging, and H I radio synthesis observations, it is possible to study the structure of the stellar disk and the properties of both the neutral and ionized gaseous components, as well as their interaction. From the B and R photometry, an extrapolated central surface brightness B(0)(c)=23.44 mag arcsec(-2), a scale length of alpha 2 kpc, and a color index (B-R)=0.96 are derived. From the radio synthesis observations, it is possible to trace the H I emission over similar to 1.8 D-25 and to derive a rotation curve out to similar to 8 kpc. This curve rises slowly, barely flattening at its end, with a maximum rotational velocity of 67 km s(-1). Noncircular motions are the main source of uncertainties for r<4'. The mass model that best fits the data suggests a (M/L(B))(double dagger) =0.4 for the luminous stellar disk and a core radius r(c)=8 kpc and a central density rho(0)=0.0064M(.)pc(-3) for the dark halo component. Contrary to more massive galaxies where the luminous disk dominates in the inner parts, the dark component dominates at all radii. This is not particular to IC 2574, but seems to be the case for many low surface brightness late-type spirals. In this respect, the properties of the dark matter distribution in IC 2574 are closer to what is seen in dwarf irregulars than in massive spirals. Comparison of HI column density and velocity maps suggests that HI holes correspond to expanding shells or rings. Most of the H II regions are found around the rims of HI holes. The giant northeastern HII complex is associated with a peak of HI emission; its properties are consistent with triggered star formation resulting from the collapse of swept-up matter in an expanding and decelerating ring.