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A 25 deg 2 region, including the M81 complex (M81, M82, NGC 3077), NGC 2976 and IC2574, was mapped during ∼ 3000 hours with the DRAO synthesis telescope. With a physical resolution of ∼ 1 kpc, these observations allow us to probe a large region down to column density levels of ∼ 1 × 10 18 cm −2 over 16 km s −1 , mapping the extent of the Hi arm connecting the system and NGC 2976, and resolving the Hi clouds adjacent to the arm. The observations also reveal a few clouds located between the system and IC 2574, probably tidally stripped from a past interaction between the two systems. Given the regular velocity distribution in the Hi envelope of the system, we attempt and derive an idealised large-scale rotation curve of the system. We observe a flat trend for the rotation velocity of the overall system from 20 kpc out to 80 kpc, well beyond the outskirts of the M81 disk, although with asymmetries like a wiggle at the vicinity of M82. This supports the assumption that intergalactic gas and galaxies in the system participate to a large-scale ordered rotation motion which is dominated by M81. Also, our Hi analysis of the group further supports the hypothesis that the galaxies forming the system moved closer from afar, in agreement with numerical simulations.

We present early observations of 20 galaxies in the MHONGOOSE survey galaxies using KAT-7, the GBT, and MeerKAT. We present the best calibrators for five of the MHONGOOSE galaxies observed with the KAT-7, and search for signs of gas accretion in the GBT data, down to 3σ column density levels of 2.2 × 10 18 cm −2 over a 20 km s −1 line width, but identify none. Using the KAT-7 and MeerKAT data, we have derived rotation curves and mass models for NGC 3621 and NGC 7424 out to an unprecedented extent. As a precursor to the SKA, the MeerKAT telescope combines both a high spatial resolution and a large field of view, necessary to map the extended neutral hydrogen in local galaxies. The mass models of the two galaxies were constructed for both the Dark Matter (DM) models (the pseudo-isothermal model and the Navarro-Frenk-White model) and MOND. Overall, we find that the DM models provide a better fit than MOND to the galaxies' rotation curves. Furthermore, the pseudo-isothermal model is found to be the most consistent with the observations.

We present the mass models of 31 spiral and irregular nearby galaxies obtained using hybrid rotation curves (RCs) combining high-resolution GHASP Fabry–Perot H α RCs and extended WHISP H i ones together with 3.4 $\mu$m WISE photometry. The aim is to compare the dark matter (DM) halo properties within the optical radius using only H α RCs with the effect of including and excluding the mass contribution of the neutral gas component, and when using H i or hybrid RCs. Pseudo-isothermal (ISO) core and Navarro–Frenk–White (NFW) cuspy DM halo profiles are used with various fiducial fitting procedures. Mass models using H α RCs including or excluding the H i gas component provide compatible disc M/L. The correlations between DM halo and baryon parameters do not strongly depend on the RC. Clearly, the differences between the fitting procedures are larger than between the different data sets. Hybrid and H i RCs lead to higher M/L values for both ISO and NFW best-fitting models but lower central densities for ISO haloes and higher concentration for NFW haloes than when using H α RCs only. The agreement with the mass model parameters deduced using hybrid RCs, considered as a reference, is better for H i than for H α RCs. ISO density profiles better fit the RCs than the NFW ones, especially when using H α or hybrid RCs. Halo masses at the optical radius determined using the various data sets are compatible even if they tend to be overestimated with H α RCs. Hybrid RCs are thus ideal to study the mass distribution within the optical radius.

We analyse the eastern region of a 5° × 5° deep H i survey of the M81 group containing the dwarf galaxy IC 2574 and the H i complex HIJASS J1021+68, located between the dwarf and the M81 system. The data show that IC 2574 has an extended H i envelope that connects to HIJASS J1021+68 in the form of a collection of small clouds, but no evident connection has been found between IC 2574 and the central members of the M81 group. We argue, based on the morphology of the clouds forming HIJASS J1021+68 and its velocity distribution, that the complex is not a dark galaxy as previously suggested, but is instead a complex of clouds either stripped from, or falling on to the primordial H i envelope of IC 2574. We also use the deep H i observations to map the extended H i envelope around IC 2574 and, using a 3D tilted-ring model, we derive the rotation curve of the galaxy to a larger extent than has been done before. Combining the obtained rotation curve to higher resolution curves from the literature, we constrain the galaxy’s dark matter halo parameters.

We present the analysis of the diffuse, low column density H i environment of 18 MHONGOOSE galaxies. We obtained deep observations with the Robert C. Byrd Green Bank Telescope and reached down to a 3σ column density detection limit of N HI = 6.3 × 1017 cm−2 over a 20 km s−1 line width. We analyze the environment around these galaxies, with a focus on H i gas that reaches column densities below N HI = 1019 cm−2. We calculate the total amount of H i gas in and around the galaxies, revealing that nearly all of these galaxies contained excess H i outside of their disks. We quantify the amount of diffuse gas in the maps of each galaxy, defined by H i gas with column densities below 1019 cm−2, and find a large spread in percentages of diffuse gas. However, by binning the percentage of diffuse H i into quarters, we find that the bin with the largest number of galaxies is the lowest quartile (0%–25% diffuse H i). We identified several galaxies that may be undergoing gas accretion onto the galaxy disk using multiple methods of analysis, including azimuthally averaging column densities beyond the disk, and identifying structure within our integrated intensity (moment 0) maps. We measured H i mass outside the disks of most of our galaxies, with rising cumulative flux even at large radii. We also find a strong correlation between the fraction of diffuse gas in a galaxy and its baryonic mass, and we test this correlation using both Spearman and Pearson correlation coefficients. We see evidence of a dark matter halo mass threshold of M halo ∼ 1011.1 M ☉ in which galaxies with high fractions of diffuse H i all reside below. It is in this regime that cold-mode accretion should dominate. Finally, we suggest a rotation velocity of v rot ∼ 80 km s−1 as an upper threshold to find diffuse-gas-dominated galaxies.

The MHONGOOSE (MeerKAT H i Observations of Nearby Galactic Objects: Observing Southern Emitters) survey maps the distribution and kinematics of the neutral atomic hydrogen (H i) gas in and around 30 nearby star-forming spiral and dwarf galaxies to extremely low H i column densities. The H i column density sensitivity (3σ over 16 km s −1 ) ranges from ∼ 5 • 10 17 cm −2 at 90 ′′ resolution to ∼ 4 • 10 19 cm −2 at the highest resolution of 7 ′′ . The H i mass sensitivity (3σ over 50 km s −1 ) is ∼ 5.5 • 10 5 M ⊙ at a distance of 10 Mpc (the median distance of the sample galaxies). The velocity resolution of the data is 1.4 km s −1 . One of the main science goals of the survey is the detection of cold, accreting gas in the outskirts of the sample galaxies. The sample was selected to cover a range in H i masses, from 10 7 M ⊙ to almost 10 11 M ⊙ , to optimally sample possible accretion scenarios and environments. The distance to the sample galaxies ranges from 3 to 23 Mpc. In this paper, we present the sample selection, survey design, and observation and reduction procedures. We compare the integrated H i fluxes based on the MeerKAT data with those derived from single-dish measurement and find good agreement, indicating that our MeerKAT observations are recovering all flux. We present H i moment maps of the entire sample based on the first ten percent of the survey data, and find that a comparison of the zeroth-and second-moment values shows a clear separation between the physical properties of the H i in areas with star formation and areas without, related to the formation of a cold neutral medium. Finally, we give an overview of the H i-detected companion and satellite galaxies in the 30 fields, five of which have not previously been catalogued. We find a clear relation between the number of companion galaxies and the mass of the main target galaxy.

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