An HI survey of the Centaurus and Sculptor Groups - Constraints on the space density of low mass galaxies

We present results of two 21-cm HI surveys performed with the Australia Telescope Compact Array in the nearby Centaurus A and Sculptor galaxy groups. These surveys are sensitive to compact HI clouds and galaxies with HI masses as low as 3E+06 Msun, and are therefore among the most sensitive extragalactic HI surveys to date. The surveys consist of sparsely spaced pointings that sample approximately 2% of the groups' area on the sky. We detected previously known group members, but we found no new HI clouds or galaxies down to the sensitivity limit of the surveys. If the HI mass function had a faint end slope of alpha = 1.5 below M_{HI} = 10^{7.5} Msun in these groups, we would have expected ~3 new objects. Cold dark matter theories of galaxy formation predict the existence of a large number low mass DM sub-halos that might appear as tiny satellites in galaxy groups. Our results support and extend similar conclusions derived from previous HI surveys that a HI rich population of these satellites does not exist.Comment: Accepted for publication in A&

Exploring the causes of adverse events in hospitals and potential prevention strategies

Objectives To examine the causes of adverse events (AEs) and potential prevention strategies to minimise the occurrence of AEs in hospitalised patients. Methods For the 744 AEs identified in the patient record review study in 21 Dutch hospitals, trained reviewers were asked to select all causal factors that contributed to the AE. The results were analysed together with data on preventability and consequences of AEs. In addition, the reviewers selected one or more prevention strategies for each preventable AE. The recommended prevention strategies were analysed together with four general causal categories: technical, human, organisational and patient-related factors. Results Human causes were predominantly involved in the causation of AEs (in 61% of the AEs), 61% of those being preventable and 13% leading to permanent disability. In 39% of the AEs, patient-related factors were involved, in 14% organisational factors and in 4% technical factors. Organisational causes contributed relatively often to preventable AEs (93%) and AEs resulting in permanent disability (20%). Recommended strategies to prevent AEs were quality assurance/peer review, evaluation of safety behaviour, training and procedures. For the AEs with human and patient-related causes, reviewers predominantly recommended quality assurance/peer review. AEs caused by organisational factors were considered preventable by improving procedures. Discussion Healthcare interventions directed at human causes are recommended because these play a large role in AE causation. In addition, it seems worthwhile to direct interventions on organisational causes because the AEs they cause are nearly always believed to be preventable. Organisational factors are thus relatively easy to tackle. Future research designs should allow researchers to interview healthcare providers that were involved in the event, as an additional source of information on contributing factors.

Comparing Galaxies and Lyman Alpha Absorbers at Low Redshift

A scenario is explored in which Lyman alpha absorbers at low redshift arise from lines of sight through extended galaxy disks, including those of dwarf and low surface brightness galaxies. A population of galaxies is simulated based upon observed distributions of galaxy properties, and the gas disks are modeled using pressure and gravity confinement. Some parameter values are ruled out by comparing simulation results with the observed galaxy luminosity function, and constraints may be made on the absorbing cross sections of galaxies. Simulation results indicate that it is difficult to match absorbers with particular galaxies observationally since absorption typically occurs at high impact parameters (>200 kpc) from luminous galaxies. Low impact parameter absorption is dominated by low luminosity dwarfs. A large fraction of absorption lines is found to originate from low surface brightness galaxies, so that the absorbing galaxy is likely to be misidentified. Low redshift Lyman alpha absorber counts can easily be explained by moderately extended galaxy disks when low surface brightness galaxies are included, and it is easily possible to find a scenario which is consistent with observed the galaxy luminosity function, with low redshift Lyman limit absorber counts, and with standard nucleosynthesis predictions of the baryon density, Omega_Baryon.Comment: 17 pages, 8 figures, accepted to the Astrophysical Journa

Detection of chloronium and measurement of the 35Cl/37Cl isotopic ratio at z=0.89 toward PKS1830-211

We report the first extragalactic detection of chloronium (H2Cl+), in the z=0.89 absorber in front of the lensed blazar PKS1830-211. The ion is detected through its 1_11-0_00 line along two independent lines of sight toward the North-East and South-West images of the blazar. The relative abundance of H2Cl+ is significantly higher (by a factor ~7) in the NE line of sight, which has a lower H2/H fraction, indicating that H2Cl+ preferably traces the diffuse gas component. From the ratio of the H2^35Cl+ and H2^37Cl+ absorptions toward the SW image, we measure a 35Cl/37Cl isotopic ratio of 3.1 (-0.2; +0.3) at z=0.89, similar to that observed in the Galaxy and the solar system.Comment: Accepted for publication in A&A Lette

The Survey for Ionization in Neutral Gas Galaxies- II. The Star Formation Rate Density of the Local Universe

We derive observed Halpha and R band luminosity densities of an HI-selected sample of nearby galaxies using the SINGG sample to be l_Halpha' = (9.4 +/- 1.8)e38 h_70 erg s^-1 Mpc^-3 for Halpha and l_R' = (4.4 +/- 0.7)e37 h_70 erg s^-1 A^-1 Mpc^-3 in the R band. This R band luminosity density is approximately 70% of that found by the Sloan Digital Sky Survey. This leads to a local star formation rate density of log(SFRD) = -1.80 +0.13/-0.07(random) +/- 0.03(systematic) + log(h_70) after applying a mean internal extinction correction of 0.82 magnitudes. The gas cycling time of this sample is found to be t_gas = 7.5 +1.3/-2.1 Gyr, and the volume-averaged equivalent width of the SINGG galaxies is EW(Halpha) = 28.8 +7.2/-4.7 A (21.2 +4.2/-3.5 A without internal dust correction). As with similar surveys, these results imply that SFRD(z) decreases drastically from z ~ 1.5 to the present. A comparison of the dynamical masses of the SINGG galaxies evaluated at their optical limits with their stellar and HI masses shows significant evidence of downsizing: the most massive galaxies have a larger fraction of their mass locked up in stars compared with HI, while the opposite is true for less massive galaxies. We show that the application of the Kennicutt star formation law to a galaxy having the median orbital time at the optical limit of this sample results in a star formation rate decay with cosmic time similar to that given by the SFRD(z) evolution. This implies that the SFRD(z) evolution is primarily due to the secular evolution of galaxies, rather than interactions or mergers. This is consistent with the morphologies predominantly seen in the SINGG sample.Comment: 15 pages, 5 figures, ApJ in press. Data available at http://sungg.pha.jhu.edu/PubData/ Corrected: Minor typos and formatting issues fixe

Exploring Neutral Hydrogen and Galaxy Evolution with the SKA

One of the key science drivers for the development of the SKA is to observe the neutral hydrogen, HI, in galaxies as a means to probe galaxy evolution across a range of environments over cosmic time. Over the past decade, much progress has been made in theoretical simulations and observations of HI in galaxies. However, recent HI surveys on both single dish radio telescopes and interferometers, while providing detailed information on global HI properties, the dark matter distribution in galaxies, as well as insight into the relationship between star formation and the interstellar medium, have been limited to the local universe. Ongoing and upcoming HI surveys on SKA pathfinder instruments will extend these measurements beyond the local universe to intermediate redshifts with long observing programmes. We present here an overview of the HI science which will be possible with the increased capabilities of the SKA and which will build upon the expected increase in knowledge of HI in and around galaxies obtained with the SKA pathfinder surveys. With the SKA1 the greatest improvement over our current measurements is the capability to image galaxies at reasonable linear resolution and good column density sensitivity to much higher redshifts (0.2 < z < 1.7). So one will not only be able to increase the number of detections to study the evolution of the HI mass function, but also have the sensitivity and resolution to study inflows and outflows to and from galaxies and the kinematics of the gas within and around galaxies as a function of environment and cosmic time out to previously unexplored depths. The increased sensitivity of SKA2 will allow us to image Milky Way-size galaxies out to redshifts of z=1 and will provide the data required for a comprehensive picture of the HI content of galaxies back to z~2 when the cosmic star formation rate density was at its peak.Comment: 25 pages, 5 figures, 3 tables. Contribution to the conference 'Advancing Astrophysics with the Square Kilometre Array', June 8-13, 2014, Giardini Naxos, Ital

Simulation of a flux emergence event and comparison with observations by Hinode

We study the observational signature of flux emergence in the photosphere using synthetic data from a 3D MHD simulation of the emergence of a twisted flux tube. Several stages in the emergence process are considered. At every stage we compute synthetic Stokes spectra of the two iron lines Fe I 6301.5 {\AA} and Fe I 6302.5 {\AA} and degrade the data to the spatial and spectral resolution of Hinode's SOT/SP. Then, following observational practice, we apply Milne-Eddington-type inversions to the synthetic spectra in order to retrieve various atmospheric parameters and compare the results with recent Hinode observations. During the emergence sequence, the spectral lines sample different parts of the rising flux tube, revealing its twisted structure. The horizontal component of the magnetic field retrieved from the simulations is close to the observed values. The flattening of the flux tube in the photosphere is caused by radiative cooling, which slows down the ascent of the tube to the upper solar atmosphere. Consistent with the observations, the rising magnetized plasma produces a blue shift of the spectral lines during a large part of the emergence sequence.Comment: A&A Letter, 3 figure