The Correlation Dimension of Young Stars in Dwarf Galaxies

We present the correlation dimension of resolved young stars in four actively star-forming dwarf galaxies that are sufficiently resolved and transparent to be modeled as projections of three-dimensional point distributions. We use data in the Hubble Space Telescope archive; photometry for one of them, UGCA 292, is presented here for the first time. We find that there are statistically distinguishable differences in the nature of stellar clustering among the sample galaxies. The young stars of VII Zw 403, the brightest galaxy in the sample, have the highest value for the correlation dimension and also the most dramatic decrease with logarithmic scale, falling from $1.68\pm0.14$ to $0.10\pm0.05$ over less than a factor of ten in $r$. This decrease is consistent with the edge effect produced by a projected Poisson distribution within a 2:2:1 ellipsoid. The young stars in UGC 4483, the faintest galaxy in the sample, exhibit very different behavior, with a constant value of about 0.5 over this same range in $r$, extending nearly to the edge of the distribution. This behavior may indicate either a scale-free distribution with an unusually low correlation dimension, or a two-component (not scale-free) combination of cluster and field stars.Comment: 25 pages, 10 figures, accepted by A

The AMIGA sample of isolated galaxies. XIII. The HI content of an almost "nurture free" sample

We present the largest catalogue of HI single dish observations of isolated galaxies to date and the corresponding HI scaling relations, as part of the multi-wavelength project AMIGA (Analysis of the interstellar Medium in Isolated GAlaxies). Despite numerous studies of the HI content of galaxies, no revision has been made for the most isolated L* galaxies since 1984. In total we have measurements or constraints on the HI masses of 844 galaxies from the Catalogue of Isolated Galaxies (CIG), obtained with our own observations at Arecibo, Effelsberg, Nancay and GBT, and spectra from the literature. Cuts are made to this sample to ensure isolation and a high level of completeness. We then fit HI scaling relations based on luminosity, optical diameter and morphology. Our regression model incorporates all the data, including upper limits, and accounts for uncertainties in both variables, as well as distance uncertainties. The scaling relation of HI mass with optical diameter is in good agreement with that of Haynes & Giovanelli 1984, but our relation with luminosity is considerably steeper. This is attributed to the large uncertainties in the luminosities, which introduce a bias when using OLS regression (used previously), and the different morphology distributions of the samples. We find that the main effect of morphology on the relations is to increase the intercept and flatten the slope towards later types. These trends were not evident in previous works due to the small number of detected early-type galaxies. The HI scaling relations of the AMIGA sample define an up-to-date metric of the HI content of almost "nurture free" galaxies. These relations allow the expected HI mass, in the absence of interactions, of a galaxy to be predicted to within 0.25 dex, and are thus suitable for use as statistical measures of the impact of interactions on the neutral gas content of galaxies. (Abridged)Comment: 29 pages, 14 figures, 16 tables. Accepted for publication in A&A. Full data tables will be main available with the final publicatio