SKA South Africa Radio Astronomy Research Group

Park Park, South Africa

SKA South Africa Radio Astronomy Research Group

Park Park, South Africa
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Leisman L.,Cornell University | Haynes M.P.,Cornell University | Janowiecki S.,University of Western Australia | Hallenbeck G.,Union College at Schenectady | And 10 more authors.
Astrophysical Journal | Year: 2017

We present a sample of 115 very low optical surface brightness, highly extended, H i-rich galaxies carefully selected from the ALFALFA survey that have similar optical absolute magnitudes, surface brightnesses, and radii to recently discovered "ultra-diffuse" galaxies (UDGs). However, these systems are bluer and have more irregular morphologies than other UDGs, are isolated, and contain significant reservoirs of H i. We find that while these sources have normal star formation rates for H i-selected galaxies of similar stellar mass, they have very low star formation efficiencies. We further present deep optical and H i-synthesis follow-up imaging of three of these H i-bearing ultra-diffuse sources. We measure H i diameters extending to ∼40 kpc, but note that while all three sources have large H i diameters for their stellar mass, they are consistent with the H i mass-H i radius relation. We further analyze the H i velocity widths and rotation velocities for the unresolved and resolved sources, respectively, and find that the sources appear to inhabit halos of dwarf galaxies. We estimate spin parameters, and suggest that these sources may exist in high spin parameter halos, and as such may be potential H i-rich progenitors to the ultra-diffuse galaxies observed in cluster environments. © 2017. The American Astronomical Society. All rights reserved..


Vargas C.J.,New Mexico State University | Heald G.,CSIRO | Heald G.,Netherlands Institute for Radio Astronomy | Heald G.,University of Groningen | And 10 more authors.
Astrophysical Journal | Year: 2017

We use new deep 21 cm H i observations of the moderately inclined galaxy NGC 4559 in the HALOGAS survey to investigate the properties of extraplanar gas. We use TiRiFiC to construct simulated data cubes to match the H i observations. We find that a thick-disk component of scale height ∼2 kpc, characterized by a negative vertical gradient in its rotation velocity (lag) of ∼13 ±5 km s-1 kpc-1 is an adequate fit to extraplanar gas features. The tilted ring models also present evidence for a decrease in the magnitude of the lag outside R 25, and a radial inflow of ∼10 km s-1. We extracted lagging extraplanar gas through Gaussian velocity profile fitting. From both the 3D models and extraction analyses we conclude that ∼10%-20% of the total H i mass is extraplanar. Most of the extraplanar gas is spatially coincident with regions of star formation in spiral arms, as traced by Hα and GALEX FUV images, so it is likely due to star formation processes driving a galactic fountain. We also find the signature of a filament of a kinematically "forbidden" H i feature, containing ∼1.4 106 M o of H i, and discuss its potential relationship to a nearby H i hole. We discover a previously undetected dwarf galaxy in H i located ∼0.°4 (∼58 kpc) from the center of NGC 4559, containing ∼4 × 105 M o. This dwarf has counterpart sources in SDSS with spectra typical of H ii regions, and we conclude that it is two merging blue compact dwarf galaxies. © 2017. The American Astronomical Society. All rights reserved.


den Heijer M.,University of Bonn | den Heijer M.,Max Planck Institute for Radio Astronomy | Jozsa G.I.G.,University of Bonn | Jozsa G.I.G.,SKA South Africa Radio Astronomy Research Group | And 11 more authors.
Astronomische Nachrichten | Year: 2015

We examine the H I kinematics of the "Bluedisk" ensemble of 48 galaxies selected from the Sloan Digital Sky Survey and observed in H I with the Westerbork Synthesis Radio Telescope. The sample consists of 25 galaxies with a high H I mass fraction and a comparatively large control sample comprising 23 galaxies of comparable stellar mass, stellar mass surface density, redshift, and inclination. By studying the H I velocity fields of these galaxies, we investigate whether there are signatures of ongoing gas accretion: i.e. global asymmetries and indications for warping and kinematical lopsidedness. We find no enhanced kinematical asymmetries between the H I-rich sample and the control sample galaxies, indicating no significant difference in kinematical signatures such as warping and lopsidedness. Furthermore, we find no difference in position angle and systemic velocity offset with respect to the optical between both sub-samples. We therefore do not find compelling evidence for enhanced global asymmetry of the H I-excess galaxies ensemble properties in comparison to the control sample galaxies. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.


Wang J.,CSIRO | Serra P.,CSIRO | Jozsa G.I.G.,SKA South Africa Radio Astronomy Research Group | Jozsa G.I.G.,Rhodes University | And 10 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2015

Using data taken as part of the Bluedisk project, we study the connection between neutral hydrogen (HI) in the environment of spiral galaxies and that in the galaxies themselves. We measure the total HI mass present in the environment in a statistical way by studying the distribution of noise peaks in the HI data cubes obtained for 40 galaxies observed with Westerbork Synthesis Radio Telescope. We find that galaxies whose HI mass fraction is high relative to standard scaling relations have an excess HI mass in the surrounding environment as well. Gas in the environment consists of gas clumps which are individually below the detection limit of our HI data. These clumps may be hosted by small satellite galaxies and/or be the high-density peaks of a more diffuse gas distribution in the intergalactic medium. We interpret this result as an indication for a picture in which the HI-rich central galaxies accrete gas from an extended gas reservoir present in their environment. © 2015 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.


Den Heijer M.,University of Bonn | Den Heijer M.,Max Planck Institute for Radio Astronomy | Oosterloo T.A.,Netherlands Institute for Radio Astronomy | Oosterloo T.A.,University of Groningen | And 18 more authors.
Astronomy and Astrophysics | Year: 2015

We study the HiK-band Tully-Fisher relation and the baryonic Tully-Fisher relation for a sample of 16 early-Type galaxies, taken from the ATLAS3D sample, which all have very regular Hi disks extending well beyond the optical body (5 Reff). We use the kinematics of these disks to estimate the circular velocity at large radii for these galaxies. We find that the Tully-Fisher relation for our early-Type galaxies is offset by about 0.5-0.7 mag from the relation for spiral galaxies, in the sense that early-Type galaxies are dimmer for a given circular velocity. The residuals with respect to the spiral Tully-Fisher relation correlate with estimates of the stellar mass-To-light ratio, suggesting that the offset between the relations is mainly driven by differences in stellar populations. We also observe a small offset between our Tully-Fisher relation with the relation derived for the ATLAS3D sample based on CO data representing the galaxies' inner regions (1 Reff). This indicates that the circular velocities at large radii are systematically 10% lower than those near 0.5-1 Reff, in line with recent determinations of the shape of the mass profile of early-Type galaxies. The baryonic Tully-Fisher relation of our sample is distinctly tighter than the standard one, in particular when using mass-To-light ratios based on dynamical models of the stellar kinematics. We find that the early-Type galaxies fall on the spiral baryonic Tully-Fisher relation if one assumes M/LK = 0.54 M/Lfor the stellar populations of the spirals, a value similar to that found by recent studies of the dynamics of spiral galaxies. Such a mass-To-light ratio for spiral galaxies would imply that their disks are 60-70% of maximal. Our analysis increases the range of galaxy morphologies for which the baryonic Tully-Fisher relations holds, strengthening previous claims that it is a more fundamental scaling relation than the classical Tully-Fisher relation. © ESO, 2015.

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