Diffuse neutral hydrogen in the H i Parkes All Sky Survey

Abstract

Context. Observations of neutral hydrogen can provide a wealth of information about the distribution and kinematics of galaxies.To learn more about large scale structures and accretion processes, the extended environment of galaxies must also be observed. Numerical simulations predict a cosmic web of extended structures and gaseous filaments. Aims. To detect H i beyond the ionisation edge of galaxy disks, column density sensitivities have to be achieved that probe the regime of Lyman limit systems. Typically H i observations are limited to a brightness sensitivity of N HI ∼ 10 19 cm −2 but this has to be improved by at least an order of magnitude. Methods. In this paper, reprocessed data is presented that was originally observed for the H i Parkes All Sky Survey (HIPASS). HIPASS provides complete coverage of the region that has been observed for the Westerbork Virgo Filament H i Survey (WVFS), presented in accompanying papers, and thus is an excellent product for data comparison. The region of interest extends from 8 to 17 hours in right ascension and from −1 to 10 degrees in declination. Although the original HIPASS product already has good flux sensitivity, the sensitivity and noise characteristics can be significantly improved with a different processing method. Results. The newly processed data has an 1σ rms flux sensitivity of ∼10 mJy beam −1 over 26 km s −1 , corresponding to a column density sensitivity of ∼3 × 10 17 cm −2 . While the rms sensitivity is improved by only a modest 20%, the more substantial benefit is in the reduction of spectral artefacts near bright sources by more than an order of magnitude. In the reprocessed region we confirm all previously catalogued HIPASS sources and have identified 29 additional sources of which 14 are completely new H i detections. We derived spectra and moment maps for all detections together with total fluxes determined both by integrating the spectrum and by integrating the flux in the moment maps within the source radius. Extended emission or companions were sought in the nearby environment of each discrete detection. Ten extra-galactic filaments are marginally detected within the moment maps. Conclusions. With the improved sensitivity after reprocessing and its large sky coverage, the HIPASS data is a valuable resource for detection of faint H i emission. This faint emission can correspond to extended halos, dwarf galaxies, tidal remnant and potentially diffuse filaments that represent the trace neutral fraction of the Cosmic Web.

An empirical determination of the dust mass absorption coefficient, κd, using the Herschel Reference Survey

“…However, it is unclear if James et al intentionally chose an Hi value that only extends out to some fraction of the optical radius of M 61, as most of the Hi masses employed by James et al are those compiled by the Scuba Local Universe Galaxy Survey (SLUGS, Dunne et al, 2000), whose 21 cm values are point-source integrated measurements taken from the literature. The Hi mass we take from Boselli et al (2014) comes from the 84.71 Jy flux measured by the ALFALFA survey (Arecibo Legacy Fast ALFA, Giovanelli et al, 2005;Haynes et al, 2011), which is in excellent agreement with the 85.2 Jy flux measured at the Westerbork Synthesis Radio Telescope by Popping & Braun (2011b), and the 85.1 Jy flux measured at the Parkes Radio Telescope by Popping & Braun (2011a). As such, it seems that the measurement we use is accurate; possibly simply a benefit of more modern observations.…”

Star formation sustained by gas accretion

“…HI filaments of this kind have been discovered in blind HI surveys. For example, Popping and Braun (2011a) analyze data from the Parkes All Sky Survey to find ten faint extragalactic filaments that can correspond to extended haloes, tidal remnants, or potentially diffuse filaments tracing the neutral fraction of the cosmic web. Popping and Braun (2011b) map the galaxy filament connecting the Virgo cluster with the local group, finding 20 new detections of neutral hydrogen with no obvious sign of stellar emission.…”

Evidence for Hi replenishment in massive galaxies through gas accretion from the cosmic web

“…HIPASS has played a significant part in understanding the role of H i in galaxies (e.g. Meyer et al 2004;Zwaan et al 2004;Koribalski et al 2004;Zwaan et al 2005;Doyle et al 2005;Wong et al 2006;Meyer et al 2008;Wong et al 2009;Popping & Braun 2011;Allison et al 2014) but is limited to only the brightest H i galaxies. Since the H i 21-cm emission line is weak, observing H i in galaxies with low column densities requires long integration times to reach the desired sensitivity that is often impractical.…”