A sample of 35 SNe Ia with good to excellent photometry in B and V, minimum internal absorption, and 1200 < v < ∼ 30 000 km s −1 is compiled from the literature. As far as their spectra are known they are all Branch-normal. For 29 of the SNe Ia also peak magnitudes in I are known. The SNe Ia have uniform colors at maximum, i.e. =−0 m . 012 (σ=0.051) and =−0 m . 276 (σ=0.078). In the Hubble diagram they define a Hubble line with a scatter of σ M =0 m . 21-0 m . 16, decreasing with wavelength. The scatter is further reduced if the SNe Ia are corrected for differences in decline rate ∆m 15 or color (B−V ). A combined correction reduces the scatter to σ < ∼ 0 m . 13. After the correction no significant dependence remains on Hubble type or galactocentric distance. The Hubble line suggests some curvature which can be differently interpreted. A consistent solution is obtained for a cosmological model with Ω M =0.3, Ω Λ =0.7, which is indicated also by much more distant SNe Ia. Absolute magnitudes are available for eight equally blue (Branch-normal) SNe Ia in spirals, whose Cepheid distances are known. If their well defined mean values of M B , M V , and M I are used to fit the Hubble line to the above sample of SNe Ia one obtains H 0 =58.3 km s −1 Mpc −1 , or, after adjusting all SNe Ia to the average values of ∆m 15 and (B−V ), H 0 =60.9 km s −1 Mpc −1 .Various systematic errors are discussed whose elimination tends to decrease H 0 . The finally adopted value at the 90-percent level, including random and systematic errors, is H 0 =58.5±6.3 km s −1 Mpc −1 . Several higher values of H 0 from SNe Ia, as suggested in the literature, are found to depend on large corrections for variations of the light curve parameter and/or on an unwarranted reduction of the Cepheid distances of the calibrating SNe Ia.

Abstract. We present B and R band surface photometry of 25 Southern field dwarf galaxies within a distance of 10 Mpc. For each galaxy we give the essential model-free photometric parameters and, by fitting exponentials to the surface brightness profiles, the central extrapolated surface brightness and the exponential scale length, in both colour bands. Surface brightness and colour profiles are shown. One of the objects, a very faint dwarf elliptical in the vicinity of NGC 2784, has been discovered in the course of this work. Drawing on the data from this and all previous papers of this series, we construct a complete sample of 72 late-type ("irregular") dwarf galaxies in nearby groups and the field within the 10 Mpc volume, to study the exponentialdisk parameter relations of these galaxies with respect to galaxy environment. We confirm our previous finding of statistically lower scale lengths/higher central surface brightnesses for field and group galaxies as compared to cluster galaxies. However, using a clear-cut definition of "group" versus "field" environment, we find no significant difference in the photometric structure of group and field irregulars. A difference in the star formation history may partly account for this structure-environment relation: for a given luminosity cluster dwarfs are on average redder than field and group galaxies. We also report evidence for the colour gradients of dwarf irregulars being roughly inversely proportional to the disk scale lengths. Supplementing our photometric data with kinematic data from the literature, we study possible relations with kinematic properties of the inner disk. Applying the dark matter scaling relations for a Burkert halo we show that for field and group galaxies of a given luminosity faster-than-mean disk rotational velocities at a radius of about two scale lengths are correlated with larger-than-mean disk scale lengths.

Abstract. We study the distribution of bright star-forming complexes in a homogeneous sample of 72 late-type ("irregular") dwarf galaxies located within the 10 Mpc volume. Star-forming complexes are identified as bright lumps in B-band galaxy images and isolated by means of the unsharp-masking method. For the sample as a whole the radial number distribution of bright lumps largely traces the underlying exponential-disk light profiles, but peaks at a 10 percent smaller scale length. Moreover, the presence of a tail of star forming regions out to at least six optical scale lengths provides evidence against a systematic star formation truncation within that galaxy extension. Considering these findings, we apply a scale length-independent concentration index, taking into account the implied non-uniform random spread of star formation regions throughout the disk. The number profiles frequently manifest a second, minor peak at about two scale lengths. Relying on a two-dimensional stochastic self-propagating star formation model, we show these secondary peaks to be consistent with triggered star formation; for a few of the brighter galaxies a peculiar peak distribution is observed that is conceivably due to the onset of shear provided by differential rotation. On scales between 100 and 1000 pc, and by taking into account exponential-disk structure, bright lumps reveal cluster dimensions between 1.3 and 2, with a weak trend to higher dimensions for brighter galaxies. Cluster dimension weakly anticorrelates with the lumpiness index (the fraction of the total galaxy light due to the light contributed by the lumps), the latter index showing no dependence on luminosity. Lump spreading within the disk, as measured by the concentration index, and lump clustering, as given by the cluster dimension, are not linked to each other. Interpreting cluster dimension in terms of porosity of a self-similar intragalactic medium, we derive a relation between current star formation rate, scale length, and porosity.

Cepheids are the pillar of the extragalactic distance scale, but their reach in distance is not sufficient to calibrate Ho-Yet HST has provided Cepheid distances to eight galaxies which have produced SNe la. The latter are used as nearly perfect standard candles to carry the distance scale to 30 000 km s _ 1 , giving HQ = 58 ± 5. This determination finds support in other methods, but its range in distance and its weight is unparalleled.

Scaling relations among structural and kinematical features of 79 late-type spiral and dwarf irregular galaxies of the SPARC sample are revisited or newly established. The mean central surface brightness µ 0 <µ 0,[3.6] >= 19.63 ± 0.11 mag arcsec −2 allows for a clear-cut distinction between low and high surface brightness galaxies. At a given luminosity, LSB galaxies are more extended than HSB galaxies and the rotation curves have smaller inner circular velocity gradients dv(R d )/dr at one disk scale length R d . Irrespective of luminosity, the geometry of rotation curves is characterized by the relation dv(R d )/dr ≈ v max /R max , with v max being the maximum circular velocity reached at R max . For the rotation curve decompositions disk mass-tolight ratios are restricted to have constant, but semi-free best-fit values 0.2, 0.5, or 0.8 M /L at [3.6]; they exhibit an asymmetric bimodal distribution with the dominant peak located at the median value of 0.2 (minimum disks) and with the subdominant peak at 0.8 (maximum disks). Assuming dark matter halos of Burkert and of pseudo-isothermal (PITS) type, the former provide better fits for about two thirds of all galaxies. While the halo core densities ρ 0 are about equal, the core radii r 0 of PITS halos are systematically lower by a factor of about 0.6 as compared with those of the Burkert type. Focussing on the Burkert halo, the baryonic mass fraction at intermediate radii is included to address both an adjusted baryonic Tully-Fisher relation and the significance of deviations from the mean radial acceleration relation. The average radial decrease of the baryonic mass fraction within galaxies is quantified. The Burkert halo parameters obey ρ 0 ∝ r −1.5±0.1 0 with considerable scatter, but allowing v max as a third variable we find ρ 0 ∝ r −1.84±0.07 0 v 2.00±0.11 max with small scatter. The halo central surface density ρ 0 r 0 , with a sample median <ρ 0 r 0 > ≈ 121 M pc −2 (σ = 112), weakly correlates with µ 0 and with compactness C and strongly correlates with the observed radial acceleration g obs = v 2 obs (r)/r at different galactocentric radii. Consequently, because R max ∝ r 0 , we have a tight central halo column density versus maximum circular velocity relation v 2 max ∝ ρ 0 r 2 0 . Halo cores barely extend over the luminous disk, but their sizes do not correlate with the optical radii. We introduce an alternative to a prominent conventional universal rotation curve; it is based on the non-singular total matter density profile ρ total (r) = (v 2 max /4πGr 2 ) 1 − (1 − r/r c ) exp(−r/r c ) 2 , with the scaling parameter r c correlating with the halo core size r 0 . Fitting the synthetic URC to a selection of galaxies, the co-added doubly-normalized rotation curves exhibit a high degree of similarity. A couple of analytic URC decompositions into a baryonic disk and a dark matter component is accomplished.