
 We present early science results from Deep Investigation of Neutral Gas Origins (DINGO), an $\rm H$ i survey using the Australian Square Kilometre Array Pathfinder (ASKAP). Using ASKAP sub-arrays available during its commissioning phase, DINGO early science data were taken over ∼ 60 deg2 of the Galaxy And Mass Assembly (GAMA) 23 h region with 35.5 hr integration time. We make direct detections of six known and one new sources at z < 0.01. Using $\rm H$ i spectral stacking, we investigate the $\rm H$ i gas content of galaxies at 0.04 < z < 0.09 for different galaxy colours. The results show that galaxy morphology based on optical colour is strongly linked to $\rm H$ i gas properties. To examine environmental impacts on the $\rm H$ i gas content of galaxies, three sub-samples are made based on the GAMA group catalogue. The average $\rm H$ i mass of group central galaxies is larger than those of satellite and isolated galaxies, but with a lower $\rm H$ i gas fraction. We derive a variety of $\rm H$ i scaling relations for physical properties of our sample, including stellar mass, stellar mass surface density, NUV − r colour, specific star formation rate, and halo mass. We find that the derived $\rm H$ i scaling relations are comparable to other published results, with consistent trends also observed to ∼0.5 dex lower limits in stellar mass and stellar surface density. The cosmic $\rm H$ i densities derived from our data are consistent with other published values at similar redshifts. DINGO early science highlights the power of $\rm H$ i spectral stacking techniques with ASKAP.

/pdf/deep-investigation-of-neutral-gas-origins-dingo-h-i-stacking-rkdmc1t4.pdf

Deep Investigation of Neutral Gas Origins (DINGO): H i stacking experiments with early science data

We present the Australian Square Kilometre Array Pathfinder (ASKAP) WALLABY pre-pilot observations of two `dark' HI sources (with HI masses of a few times 10^8 Msol and no known stellar counterpart) that reside within 363 kpc of NGC 1395, the most massive early-type galaxy in the Eridanus group of galaxies. We investigate whether these `dark' HI sources have resulted from past tidal interactions or whether they are an extreme class of low surface brightness galaxies. Our results suggest that both scenarios are possible, and not mutually exclusive. The two `dark' HI sources are compact, reside in relative isolation and are more than 159 kpc away from their nearest HI-rich galaxy neighbour. Regardless of origin, the HI sizes and masses of both `dark' HI sources are consistent with the HI size-mass relationship that is found in nearby low-mass galaxies, supporting the possibility that these HI sources are an extreme class of low surface brightness galaxies. We identified three analogues of candidate primordial `dark' HI galaxies within the TNG100 cosmological, hydrodynamic simulation. All three model analogues are dark matter-dominated, have assembled most of their mass 12-13 Gyr ago, and have not experienced much evolution until cluster infall 1-2 Gyr ago. Our WALLABY pre-pilot science results suggest that the upcoming large area HI surveys will have a significant impact on our understanding of low surface brightness galaxies and the physical processes that shape them.

/pdf/wallaby-pre-pilot-survey-two-dark-clouds-in-the-vicinity-of-1lsp3wz7pf.pdf

WALLABY pre-pilot survey: two dark clouds in the vicinity of NGC 1395

We present results from a search for the HI 21-cm line in absorption towards 16 bright radio sources with the 6-antenna commissioning array of the Australian Square Kilometre Array Pathfinder (ASKAP). Our targets were selected from the 2-Jy sample, a flux-limited survey of the southern radio sky with extensive multi-wavelength follow-up. Two sources were detected in HI absorption including a new detection towards the bright FRII radio galaxy PKS 0409-75 at a redshift of z=0.674. The HI absorption line is blueshifted by ~3300 km/s compared to the optical redshift of the host galaxy of PKS 0409-75 at z=0.693. Deep optical imaging and spectroscopic follow-up with the GMOS instrument on the Gemini-South telescope reveal that the HI absorption is associated with a galaxy in front of the southern radio lobe with a stellar mass of $3.2 - 6.8 \times 10^{11}M_\odot$, a star-formation rate of $\sim 1.24 M_\odot$ yr$^{-1}$ and an estimated HI column density of $2.16\times10^{21}$ cm$^{-2}$, assuming a spin temperature of $T_{\rm spin}=500$ K and source covering factor of $C_{\rm f}=0.3$. Using polarisation measurements of PKS 0409-75 from the literature we estimate the magnetic field of the absorbing galaxy to be ~14.5$\mu$G, consistent with field strengths observed in nearby spiral galaxies, but larger than expected for an elliptical galaxy. Results from this pilot study can inform future surveys as new wide-field telescopes allow us to search for 21-cm HI absorption towards all bright radio sources as opposed to smaller targeted samples.

H i absorption at z ∼ 0.7 against the lobe of the powerful radio galaxy PKS 0409−75


 The transformation and evolution of a galaxy is strongly influenced by interactions with its environment. Neutral hydrogen (H i) is an excellent way to trace these interactions. Here, we present H i observations of the spiral galaxy NGC 895, which was previously thought to be isolated. High-sensitivity H i observations from the MeerKAT large survey project MIGHTEE reveal possible interaction features, such as extended spiral arms, and the two newly discovered H i companions, that drive us to change the narrative that it is an isolated galaxy. We combine these observations with deep optical images from the Hyper Suprime Camera to show an absence of tidal debris between NGC 895 and its companions. We do find an excess of light in the outer parts of the companion galaxy MGTH$\_$J022138.1-052631 which could be an indication of external perturbation and thus possible sign of interactions. Our analysis shows that NGC 895 is an actively star-forming galaxy with a SFR of 1.75 ± 0.09[M⊙/yr], a value typical for high stellar mass galaxies on the star forming main sequence. It is reasonable to state that different mechanisms may have contributed to the observed features in NGC 895 and this emphasizes the need to revisit the target with more detailed observations. Our work shows the high potential and synergy of using state-of-the-art data in both H i and optical to reveal a more complete picture of galaxy environments.

MIGHTEE-H i: Possible interactions with the galaxy NGC 895

We present the discovery of a new H i structure in the NGC 7194 group from the observations using the Karl G. Jansky Very Large Array. NGC 7194 group is a nearby (z ∼ 0.027) small galaxy group with five quiescent members. The observations reveal a 200 kpc long H i plume that spans the entire group with a total mass of M H I = 3.4 × 1010 M ⊙. The line-of-sight velocity of the H i gas gradually increases from south (7200 km s−1) to north (8200 km s−1), and the local velocity dispersion is up to 70 km s−1. The structure is not spatially coincident with any member galaxies but it shows close associations with a number of blue star-forming knots. Intragroup H i gas is not rare, but this particular structure is still one of the unusual cases in the sense that it does not show any clear connection with sizable galaxies in the group. We discuss the potential origins of this large-scale H i gas in the NGC 7194 group and its relation with the intergalactic star-forming knots. We propose that this H i feature could have originated from tidal interactions among group members or the infall of a late-type galaxy into the group. Alternatively, it might be leftover gas from flyby intruders.

Discovery of a Large-scale H i Plume in the NGC 7194 Group

Full list of authors: Namumba, B; Koribalski, B S; Jozsa, G I G; Lee-Waddell, K; Jones, M G; Carignan, C; Verdes-Montenegro, L; Ianjamasimanana, R; de Blok, W J G; Cluver, M; Garrido, J; Sanchez-Exposito, S; Ramaila, A J T; Thorat, K; Andati, L A L; Hugo, B V; Kleiner, D; Kamphuis, P; Serra, P; Smirnov, O M; Maccagni, F M; Makhathini, S; Molnar, D Cs; Perkins, S; Ramatsoku, M; White, S V; Loi, F

/pdf/meerkat-64-discovers-wide-spread-tidal-debris-in-the-nearby-9srrynv9bd.pdf

MeerKAT-64 discovers wide-spread tidal debris in the nearby NGC 7232 galaxy group

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MeerKAT-64 discovers wide-spread tidal debris in the nearby NGC 7232 galaxy group