ALBERT

Description: We present an analytic model to predict the H i mass contributed by confused sources to a stacked spectrum in a generic H i survey. Based on the ALFALFA (Arecibo Legacy Fast ALFA) correlation function, this model is in agreement with the estimates of confusion present in stacked Parkes telescope data, and was used to predict how confusion will limit stacking in the deepest Square Kilometre Array precursor H i surveys. Stacking with LADUMA (Looking At the Distant Universe with MeerKAT) and DINGO UDEEP (Deep Investigation of Neutral Gas Origins – Ultra Deep) data will only be mildly impacted by confusion if their target synthesized beam size of 10 arcsec can be achieved. Any beam size significantly above this will result in stacks that contain a mass in confused sources that is comparable to (or greater than) that which is detectable via stacking, at all redshifts. CHILES (COSMOS H i Large Extragalactic Survey) 5 arcsec resolution is more than adequate to prevent confusion influencing stacking of its data, throughout its bandpass range. FAST (Five hundred metre Aperture Spherical Telescope) will be the most impeded by confusion, with H i surveys likely becoming heavily confused much beyond z  = 0.1. The largest uncertainties in our model are the redshift evolution of the H i density of the Universe and the H i correlation function. However, we argue that the two idealized cases we adopt should bracket the true evolution, and the qualitative conclusions are unchanged regardless of the model choice. The profile shape of the signal due to confusion (in the absence of any detection) was also modelled, revealing that it can take the form of a double Gaussian with a narrow and wide component.

Description: We search for environmental dependence of the H  i mass function in the Arecibo Legacy Fast ALFA survey (ALFALFA) 70 per cent catalogue. The catalogue is split into quartiles of environment density based on the projected neighbour density of neighbours found in both Sloan Digital Sky Survey (SDSS) and 2MASS Redshift Survey (2MRS) volume-limited reference catalogues. We find the Schechter function ‘knee’ mass to be dependent on environment, with the value of log ( M * /M ) shifting from 9.81 ± 0.02 to 10.00 ± 0.03 between the lowest and highest density quartiles. However, this dependence was only observed when defining environment based on the SDSS reference catalogue, not 2MRS. We interpret these results as meaning that the local environment is the dominant cause of the shift in M * , and that the larger scales that 2MRS probes (compared to SDSS) are almost irrelevant. In addition, we also use a fixed aperture method to probe environment, and find tentative evidence that H  i -deficiency depresses the value of M * in the highest density regions. We find no significant dependence of the low-mass slope on environment in any test, using either method. Tensions between these results and those from the literature, are discussed and alternative explanations are explored.

Description: We present a comprehensive model to predict the rate of spectroscopic confusion in H  i surveys, and demonstrate good agreement with the observable confusion in existing surveys. Generically the action of confusion on the H  i mass function was found to be a suppression of the number count of sources below the ‘knee’, and an enhancement above it. This results in a bias, whereby the ‘knee’ mass is increased and the faint end slope is steepened. For ALFALFA and HIPASS, we find that the maximum impact this bias can have on the Schechter fit parameters is similar in magnitude to the published random errors. On the other hand, the impact of confusion on the H  i mass functions of upcoming medium depth interferometric surveys, will be below the level of the random errors. In addition, we find that previous estimates of the number of detections for upcoming surveys with Square Kilometre Array-precursor telescopes may have been too optimistic, as the framework implemented here results in number counts between 60 and 75 per cent of those previously predicted, while accurately reproducing the counts of existing surveys. Finally, we argue that any future single dish, wide area surveys of H  i galaxies would be best suited to focus on deep observations of the local Universe ( z  〈 0.05), as confusion may prevent them from being competitive with interferometric surveys at higher redshift, while their lower angular resolution allows their completeness to be more easily calibrated for nearby extended sources.

Description: We search for environmental dependence of the H  i mass function in the Arecibo Legacy Fast ALFA survey (ALFALFA) 70 per cent catalogue. The catalogue is split into quartiles of environment density based on the projected neighbour density of neighbours found in both Sloan Digital Sky Survey (SDSS) and 2MASS Redshift Survey (2MRS) volume-limited reference catalogues. We find the Schechter function ‘knee’ mass to be dependent on environment, with the value of log ( M * /M ) shifting from 9.81 ± 0.02 to 10.00 ± 0.03 between the lowest and highest density quartiles. However, this dependence was only observed when defining environment based on the SDSS reference catalogue, not 2MRS. We interpret these results as meaning that the local environment is the dominant cause of the shift in M * , and that the larger scales that 2MRS probes (compared to SDSS) are almost irrelevant. In addition, we also use a fixed aperture method to probe environment, and find tentative evidence that H  i -deficiency depresses the value of M * in the highest density regions. We find no significant dependence of the low-mass slope on environment in any test, using either method. Tensions between these results and those from the literature, are discussed and alternative explanations are explored.

Description: Using mid-infrared star formation rate and stellar mass indicators in WISE (Wide-field Infrared Survey Explorer), we construct and contrast the relation between star formation rate and stellar mass for isolated and paired galaxies. Our samples comprise a selection of AMIGA (Analysis of the interstellar Medium in Isolated GAlaxies; isolated galaxies) and pairs of ALFALFA (Arecibo Legacy Fast ALFA) galaxies with H i detections such that we can examine the relationship between H i content (gas fraction, H i deficiency) and galaxy location on the main sequence (MS) in these two contrasting environments. We derive for the first time an H i scaling relation for isolated galaxies using WISE stellar masses, and thereby establish a baseline predictor of H i content that can be used to assess the impact of environment on H i content when compared with samples of galaxies in different environments. We use this updated relation to determine the H i deficiency of both our paired and isolated galaxies. Across all the quantities examined as a function of environment in this work (MS location, gas fraction, and H i deficiency), the AMIGA sample of isolated galaxies is found to have the lower dispersion: σAMIGA = 0.37 versus σPAIRS = 0.55 on the MS, σAMIGA = 0.44 versus σPAIRS = 0.54 in gas fraction, and σAMIGA = 0.28 versus σPAIRS = 0.34 in H i deficiency. We also note fewer isolated quiescent galaxies, 3 (0.6${{ m per cent}}$), compared to 12 (2.3${{ m per cent}}$) quiescent pair members. Our results suggest the differences in scatter measured between our samples are environment driven. Galaxies in isolation behave relatively predictably, and galaxies in more densely populated environments adopt a more stochastic behaviour, across a broad range of quantities.

Description: We report the discovery of large amounts of previously undetected cold neutral atomic hydrogen (H i) around the core triplet galaxies in the nearby NGC 7232 galaxy group with MeerKAT. With a physical resolution of ∼1 kpc, we detect a complex web of low-surface-brightness H i emission down to a 4σ column density level of ∼1 × 1019 cm−2 (over 44  km s−1). The newly discovered H i streams extend over ∼20 arcmin corresponding to 140 kpc in projection. This is approximately three times the H i extent of the galaxy triplet (52 kpc). The H i debris has an H i mass of ∼6.6 × 109 M⊙, more than 50 per cent of the total H i mass of the triplet. Within the galaxy triplet, NGC 7233 and NGC 7232 have lost a significant amount of H i while NGC 7232B appears to have an excess of H i. The H i deficiency in NGC 7232 and NGC 7233 indicates that galaxy–galaxy interaction in the group concentrates on this galaxy pair while the other disc galaxies have visited them over time. In comparison to the AMIGA sample of isolated galaxies, we find that with regards to its total H i mass the NGC 7232/3 galaxy triplet is not H i-deficient. Despite the many interactions associated to the triplet galaxies, no H i seems to have been lost from the group (yet).