Many of the results in modern astrophysics rest on the notion that the Initial Mass Function (IMF) is universal. Our observations of a sample of H I selected galaxies in the light of Hα and the far-ultraviolet (FUV) challenge this result. The extinction corrected flux ratio F Hα / f FUV from these two tracers of star formation shows strong correlations with the surface-brightness in Hα and the R band: Low Surface Brightness (LSB) galaxies have lower F Hα / f FUV ratios compared to High Surface Brightness (HSB) galaxies as well as compared to expectations from equilibrium models of constant star formation rate (SFR) using commonly favored IMF parameters. Weaker but significant correlations of F Hα / f FUV with luminosity, rotational velocity and dynamical mass are found as well as a systematic trend with morphology. The correlated variations of F Hα / f FUV with other global parameters are thus part of the larger family of galaxy scaling relations. The F Hα / f FUV correlations can not be due to residual extinction correction errors, while systematic variations in the star formation history can not explain the trends with both Hα and R surface brightness nor with other global properties. The possibility that LSB galaxies have a higher escape fraction of ionizing photons seems inconsistent with their high gas fraction, and observations of color-magnitude diagrams of a few systems which indicate a real deficit of O stars. The most plausible explanation for the correlations is the systematic variations of the upper mass limit M u and/or the slope γ which define the upper end of the IMF. We outline a scenario of pressure driving the correlations by setting the efficiency of the formation of the dense star clusters where the highest mass stars preferentially form. Our results imply that the star formation rate measured in a galaxy is highly sensitive to the tracer used in the measurement. A non-universal IMF would also call into question interpretation of metal abundance patterns in dwarf galaxies as well star formation histories derived from color magnitude diagrams.

We use the first data release from the SINGG H survey of H iYselected galaxies to study the quantitative behavior of the diffuse, warm ionized medium ( WIM ) across the range of properties represented by these 109 galaxies. The mean fraction f WIM of diffuse ionized gas in this sample is 0:59 AE 0:19, slightly higher than found in previous samples. Since lower surface brightness galaxies tend to have higher f WIM , we believe that most of this difference is due to selection effects favoring large, optically bright, nearby galaxies with high star formation rates. As found in previous studies, there is no appreciable correlation with Hubble type or total star formation rate. However, we find that starburst galaxies, defined here by an H surface brightness >2:5 ; 10 39 erg s À1 kpc À2 within the H half-light radius, do show much lower fractions of diffuse H emission. The cause apparently is not dominated by a lower fraction of field OB stars. However, it is qualitatively consistent with an expected escape of ionizing radiation above a threshold star formation rate, predicted from our model in which the ISM is shredded by pressure-driven supernova feedback. The H i gas fractions in the starburst galaxies are also lower, suggesting that the starbursts are consuming and ionizing all the gas, and thus promoting regions of density-bounded ionization. If true, these effects imply that some amount of Lyman continuum radiation is escaping from most starburst galaxies, and that WIM properties and outflows from mechanical feedback are likely to be pressure-driven. However, in view of previous studies showing that the escape fraction of ionizing radiation is generally low, it is likely that other factors also drive the low fractions of diffuse ionized gas in starbursts.

The Northern HIPASS catalogue (NHICAT) is the northern extension of the HIPASS catalogue, HICAT. This extension adds the sky area between the declination (Dec.) range of +2° < δ < +25°30′ to HICAT's Dec. range of −90° < δ < +2°. HIPASS is a blind H i survey using the Parkes Radio Telescope covering 71 per cent of the sky (including this northern extension) and a heliocentric velocity range of −1280 to 12 700 km s−1. The entire Virgo Cluster region has been observed in the Northern HIPASS. The galaxy catalogue, NHICAT, contains 1002 sources with vhel > 300 km s−1. Sources with −300 < vhel < 300 km s−1 were excluded to avoid contamination by Galactic emission. In total, the entire HIPASS survey has found 5317 galaxies identified purely by their HI content. The full galaxy catalogue is publicly available at http://hipass.aus-vo.org.

We introduce the Survey for Ionization in Neutral Gas Galaxies (SINGG), a census of star formation in H iselected galaxies. The survey consists of H and R-band imaging of a sample of 468 galaxies selected from the H i Parkes All Sky Survey ( HIPASS). The sample spans three decades in H i mass and is free of many of the biases that affect other star-forming galaxy samples. We present the criteria for sample selection, list the entire sample, discuss our observational techniques, and describe the data reduction and calibration methods. This paper focuses on 93 SINGG targets whose observations have been fully reduced and analyzed to date. The majority of these show a single emission line galaxy (ELG). We see multiple ELGs in 13 fields, with up to four ELGs in a single field. All of the targets in this sample are detected in H , indicating that dormant (non-star-forming) galaxies with M H i k 3 ; 10 7 M are very rare. A database of the measured global properties of the ELGs is presented. The ELG sample spans 4 orders of magnitude in luminosity ( H and R band), and H surface brightness, nearly 3 orders of magnitude in R surface brightness and nearly 2 orders of magnitude in H equivalent width ( EW ). The surface brightness distribution of our sample is broader than that of the Sloan Digital Sky Survey (SDSS) spectroscopic sample, the EW distribution is broader than prism-selected samples, and the morphologies found include all common types of star-forming galaxies (e.g., irregular, spiral, blue compact dwarf, starbursts, merging and colliding systems, and even residual star formation in S0 and Sa spirals). Thus, SINGG presents a superior census of star formation in the local universe suitable for further studies ranging from the analysis of H ii regions to determination of the local cosmic star formation rate density.

We present the largest catalogue to date of optical counterparts for H i radio‐selected galaxies, HOPCAT. Of the 4315 H i radio‐detected sources from the H i Parkes All Sky Survey (HIPASS) catalogue, we find optical counterparts for 3618 (84 per cent) galaxies. Of these, 1798 (42 per cent) have confirmed optical velocities and 848 (20 per cent) are single matches without confirmed velocities. Some galaxy matches are members of galaxy groups. From these multiple galaxy matches, 714 (16 per cent) have confirmed optical velocities and a further 258 (6 per cent) galaxies are without confirmed velocities. For 481 (11 per cent), multiple galaxies are present but no single optical counterpart can be chosen and 216 (5 per cent) have no obvious optical galaxy present. Most of these ‘blank fields’ are in crowded fields along the Galactic plane or have high extinctions. Isolated ‘dark galaxy’ candidates are investigated using an extinction cut of A italicB italicj < 1 mag and the blank‐fields category. Of the 3692 galaxies with an A B j extinction <1 mag, only 13 are also blank fields. Of these, 12 are eliminated either with follow‐up Parkes observations or are in crowded fields. The remaining one has a low surface brightness optical counterpart. Hence, no isolated optically dark galaxies have been found within the limits of the HIPASS survey.