arXiv subjects

E. C. Elson

Publications and source records attributed to E. C. Elson.

At least 19 recordsLinked to original sources

arXiv2024

MHONGOOSE -- A MeerKAT Nearby Galaxy HI Survey

The MHONGOOSE (MeerKAT HI Observations of Nearby Galactic Objects: Observing Southern Emitters) survey maps the distribution and kinematics of the neutral atomic hydrogen (HI) gas in and around 30 nearby star-forming spiral and dwarf galaxies to extremely low HI column densities. The HI column density sensitivity (3 sigma over 16 km/s) ranges from ~ 5 x 10^{17} cm^{-2} at 90'' resolution to ~4 x 10^{19} cm^{-2} at the highest resolution of 7''. The HI mass sensitivity (3 sigma over 50 km/s) is ~5.5 X 10^5 M_sun at a distance of 10 Mpc (the median distance of the sample galaxies). The velocity resolution of the data is 1.4 km/s. 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 HI masses, from 10^7 M_sun to almost 10^{11} M_sun, 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 HI 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 HI 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 HI in areas with star formation and areas without, related to the formation of a cold neutral medium. Finally, we give an overview of the HI-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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arXiv2019

On the Uncertainties of Results Derived from HI Spectral Line Stacking Experiments

We present the results of a set of mock experiments aimed at quantifying the accuracy of results derived from HI spectral line stacking experiments. We focus on the effects of spatial and spectral aperture sizes and redshift uncertainties on co-added HI spectra, and by implication on the usefulness of results from HI line spectral stacking experiments. Using large spatial apertures to extract constituent galaxy spectra yields co-added spectra with high levels of contamination and with relatively low signal-to-noise ratios. These properties are also affected by the size of the spectral aperture as well as HI redshift uncertainties of galaxies. When redshift uncertainties are high, S/N decreases while the contamination level remains roughly constant. Using small spectral apertures in the presence of large HI redshift uncertainties can yield significant decreases in S/N without the expected decrease in amount of contaminant flux. Our simulations show that a co-added spectrum rarely yields an accurate measure of the total HI mass of a galaxy sample. Total mass is generally over/under-estimated for large/small spatial apertures, regardless of spectral aperture size. Our findings strongly suggest that any co-added HI galaxy spectrum needs to be fully modelled in the ways presented in this paper in order to apply accurate corrections for flux contamination and derive realistic uncertainties in total HI galaxy mass. Failing to do so will result in unreliable inferences of galaxy and cosmological parameters.

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arXiv2018

A Multi-Scale Study of Star Formation in Messier 33

For the Local Group Scd galaxy M 33 this paper presents a multi-scale study of the relationship between the monochromatic star formation rate (SFR) estimator based on 12 $μ$m emission and the total SFR estimator based on a combination of far-ultraviolet and 24 $μ$m emission. We show the 12 $μ$m emission to be a linear estimator of total SFR on spatial scales from 782 pc down to 49 pc, over almost four magnitudes in SFR. These results therefore extend to sub-kpc length scales the analogous results from other studies for global length scales. We use high-resolution HI and $^{12}\mathrm{CO}(J=2-1)$ image sets from the literature to compare the star formation to the neutral gas. For the full range of length scales we find well-defined power-law relationships between 12 $μ$m-derived SFR surface densities and neutral gas surface densities. For the H$_\mathrm{2}$ gas component almost all correlations are consistent with being linear. No evidence is found for a breakdown in the star formation law at small length scales in M 33 reported by other authors. We show that the average star formation efficiency in M 33 is roughly $10^{-9}$ yr$^{-1}$ and that it remains constant down to giant molecular cloud length scales. Toomre and shear-based models of the star formation threshold are shown to inaccurately account for the star formation activity in the inner disc of M 33. Finally, we clearly show that the HI saturation limit of $\approx 9$ M$_{\odot}$ pc$^{-2}$ reported in the literature for other galaxies is not an intrinsic property of M 33 - it is systematically introduced as an artefact of spatially smoothing the data.

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arXiv2017

An Overview of the MHONGOOSE Survey: Observing Nearby Galaxies with MeerKAT

MHONGOOSE is a deep survey of the neutral hydrogen distribution in a representative sample of 30 nearby disk and dwarf galaxies with HI masses from 10^6 to ~10^{11} M_sun, and luminosities from M_R ~ -12 to M_R ~ -22. The sample is selected to uniformly cover the available range in log(M_HI). Our extremely deep observations, down to HI column density limits of well below 10^{18} cm^{-2} - or a few hundred times fainter than the typical HI disks in galaxies - will directly detect the effects of cold accretion from the intergalactic medium and the links with the cosmic web. These observations will be the first ever to probe the very low-column density neutral gas in galaxies at these high resolutions. Combination with data at other wavelengths, most of it already available, will enable accurate modelling of the properties and evolution of the mass components in these galaxies and link these with the effects of environment, dark matter distribution, and other fundamental properties such as halo mass and angular momentum. MHONGOOSE can already start addressing some of the SKA-1 science goals and will provide a comprehensive inventory of the processes driving the transformation and evolution of galaxies in the nearby universe at high resolution and over 5 orders of magnitude in column density. It will be a Nearby Galaxies Legacy Survey that will be unsurpassed until the advent of the SKA, and can serve as a highly visible, lasting statement of MeerKAT's capabilities.

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arXiv2017

The relation between specific baryon angular momentum and mass for a sample of nearby low-mass galaxies with resolved HI kinematics

This paper investigates the relationship between specific baryon angular momentum $j_\mathrm{b}$ and baryon mass $M_\mathrm{b}$ for a sample of nearby late-type galaxies with resolved HI kinematics. This work roughly doubles the number of galaxies with $M_\mathrm{b} \lesssim 10^{10}$ $M_{\odot}$ used to study the $j_\mathrm{b} - M_\mathrm{b}$ relation. Most of the galaxies in the sample have their baryon mass dominated by their gas content, thereby offering $j_\mathrm{b}$ and Mb measures that are relatively unaffected by uncertainties arising from the stellar mass-to-light ratio. Measured HI surface density radial profiles together with optical and rotation curve data from the literature are used to derive a best-fit relation given by $j_\mathrm{b}=qM_\mathrm{b}^α$, with $α=0.62\pm 0.02$ and $\log_{10}q=-3.35\pm 0.25$. This result is consistent with the $j_\mathrm{b}\propto M_\mathrm{b}^{2/3}$ relation that is theoretically expected and also measured by Obreschkow & Glazebrook (2014) for their full sample of THINGS spiral galaxies, yet differs to their steeper relation found for subsets with fixed bulge fraction. The 30 arcsec spatial resolution of the HI imaging used in this study is significantly lower than that of the THINGS imaging used by Obreschkow & Glazebrook (2014), yet the results presented in this work are clearly shown to contain no significant systematic errors due to the low-resolution imaging.

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arXiv2017

Towards a Full Census of the Obscure(d) Vela Supercluster using MeerKAT

Recent spectroscopic observations of a few thousand partially obscured galaxies in the Vela constellation revealed a massive overdensity on supercluster scales straddling the Galactic Equator (l $\sim$ 272.5deg) at $cz \sim 18000$km/s. It remained unrecognised because it is located just beyond the boundaries and volumes of systematic whole-sky redshift and peculiar velocity surveys - and is obscured by the Milky Way. The structure lies close to the apex where residual bulkflows suggest considerable mass excess. The uncovered Vela Supercluster (VSCL) conforms of a confluence of merging walls, but its core remains uncharted. At the thickest foreground dust column densities (|b| < 6 deg) galaxies are not visible and optical spectroscopy is not effective. This precludes a reliable estimate of the mass of VSCL, hence its effect on the cosmic flow field and the peculiar velocity of the Local Group. Only systematic HI-surveys can bridge that gap. We have run simulations and will present early-science observing scenarios with MeerKAT 32 (M32) to complete the census of this dynamically and cosmologically relevant supercluster. M32 has been put forward because this pilot project will also serve as precursor project for HI MeerKAT Large Survey Projects, like Fornax and Laduma. Our calculations have shown that a survey area of the fully obscured part of the supercluster, where the two walls cross and the potential core of the supercluster resides, can be achieved on reasonable time-scales (200 hrs) with M32.

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arXiv2016

SHIELD: Comparing Gas and Star Formation in Low Mass Galaxies

We analyze the relationships between atomic, neutral hydrogen (HI) and star formation (SF) in the 12 low-mass SHIELD galaxies. We compare high spectral (~0.82 km/s/channel) and spatial resolution (physical resolutions of 170 pc - 700 pc) HI imaging from the VLA with Hαand far-ultraviolet imaging. We quantify the degree of co-spatiality between star forming regions and regions of high HI column densities. We calculate the global star formation efficiencies (SFE, $Σ_{\rm SFR}$ / $Σ_{\rm HI}$), and examine the relationships among the SFE and HI mass, HI column density, and star formation rate (SFR). The systems are consuming their cold neutral gas on timescales of order a few Gyr. While we derive an index for the Kennicutt-Schmidt relation of N ~ 0.68 $\pm$ 0.04 for the SHIELD sample as a whole, the values of N vary considerably from system to system. By supplementing SHIELD results with those from other surveys, we find that HI mass and UV-based SFR are strongly correlated over five orders of magnitude. Identification of patterns within the SHIELD sample allows us to bin the galaxies into three general categories: 1) mainly co-spatial HI and SF regions, found in systems with highest peak HI column densities and highest total HI masses, 2) moderately correlated HI and SF regions, found in systems with moderate HI column densities, and 3) obvious offsets between HI and SF peaks, found in systems with the lowest total HI masses. SF in these galaxies is dominated by stochasticity and random fluctuations in their ISM.

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arXiv2016

SHIELD: Neutral Gas Kinematics and Dynamics

We present kinematic analyses of the 12 galaxies in the "Survey of HI in Extremely Low-mass Dwarfs" (SHIELD). We use multi-configuration interferometric observations of the HI 21cm emission line from the Karl G. Jansky Very Large Array (VLA) to produce image cubes at a variety of spatial and spectral resolutions. Both two- and three-dimensional fitting techniques are employed in an attempt to derive inclination-corrected rotation curves for each galaxy. In most cases, the comparable magnitudes of velocity dispersion and projected rotation result in degeneracies that prohibit unambiguous circular velocity solutions. We thus make spatially resolved position-velocity cuts, corrected for inclination using the stellar components, to estimate the circular rotation velocities. We find circular velocities <30 km/s for the entire survey population. Baryonic masses are calculated using single-dish HI fluxes from Arecibo and stellar masses derived from HST and Spitzer imaging. Comparison is made with total dynamical masses estimated from the position-velocity analysis. The SHIELD galaxies are then placed on the baryonic Tully-Fisher relation. There exists an empirical threshold rotational velocity <15 km/s, below which current observations cannot differentiate coherent rotation from pressure support. The SHIELD galaxies are representative of an important population of galaxies whose properties cannot be described by current models of rotationally-dominated galaxy dynamics.

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arXiv2016

Synthetic data products for future HI galaxy surveys: a tool for characterising source confusion in spectral line stacking experiments

Much of our current understanding of neutral, atomic gas in galaxies comes from radio observations of the nearby Universe. Until the next generation of instruments allow us to push to much higher redshifts, we must rely mostly upon theoretical models of galaxy formation to provide us with key insights into the likely cosmic evolution of HI in the Universe, and its links to molecular clouds and star formation. In this work, we present a new set of methods to convert mock galaxy catalogues into synthetic data cubes containing model galaxies with realistic spatial and spectral HI distributions over large cosmological volumes. Such synthetic data products can be used to guide observing and data handling/analysis strategies for forthcoming HI galaxy surveys. As a demonstration of the potential use of our simulated products we use them to conduct several mock HI stacking experiments for both low and high-redshift galaxy samples. The stacked spectra can be accurately decomposed into contributions from target and non-target galaxies, revealing in all co-added spectra large fractions of contaminant mass due to source confusion. Our results are consistent with similar estimates extrapolated from z=0 observational data. The amount of confused mass in a stacked spectrum grows almost linearly with the size of the observational beam, suggesting potential over-estimates of Omega_HI by some recent HI stacking experiments. Our simulations will allow the study of subtle redshift-dependent effects in future stacking analyses.

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arXiv2015

HI observations of the nearest starburst galaxy NGC 253 with the SKA precursor KAT-7

We present HI observations of the Sculptor Group starburst spiral galaxy NGC 253, obtained with the Karoo Array Telescope (KAT-7). KAT-7 is a pathfinder for the SKA precursor MeerKAT, under construction. The short baselines and low system temperature of the telescope make it very sensitive to large scale, low surface brightness emission. The KAT-7 observations detected 33% more flux than previous VLA observations, mainly in the outer parts and in the halo for a total HI mass of $2.1 \pm 0.1$ $\times 10^{9}$ M$_{\odot}$. HI can be found at large distances perpendicular to the plane out to projected distances of ~9-10 kpc away from the nucleus and ~13-14 kpc at the edge of the disk. A novel technique, based on interactive profile fitting, was used to separate the main disk gas from the anomalous (halo) gas. The rotation curve (RC) derived for the HI disk confirms that it is declining in the outer parts, as seen in previous optical Fabry-Perot measurements. As for the anomalous component, its RC has a very shallow gradient in the inner parts and turns over at the same radius as the disk, kinematically lagging by ~100 km/sec. The kinematics of the observed extra planar gas is compatible with an outflow due to the central starburst and galactic fountains in the outer parts. However, the gas kinematics shows no evidence for inflow. Analysis of the near-IR WISE data, shows clearly that the star formation rate (SFR) is compatible with the starburst nature of NGC 253.

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arXiv2014

The initial mass function and star formation law in the outer disc of NGC 2915

Using Hubble Space Telescope (HST) ACS/WFC data we present the photometry and spatial distribution of resolved stellar populations in the outskirts of NGC 2915, a blue compact dwarf with an extended HI disc. These observations reveal an elliptical distribution of red giant branch stars, and a clumpy distribution of main-sequence stars that correlate with the HI gas distribution. We constrain the upper-end initial mass function (IMF) and determine the star formation law (SFL) in this field, using the observed main-sequence stars and an assumed constant star formation rate. Previously published Hα observations of the field, which show one faint HII region, are used to provide further constraints on the IMF. We find that the main-sequence luminosity function analysis alone results in a best-fitting IMF with a power-law slope α=-2.85 and upper-mass limit M$_\rm{u}$ = 60 M$_\odot$. However, if we assume that all Hα emission is confined to HII regions then the upper-mass limit is restricted to M$_\rm{u}$ $\le$20 M$_\odot$. For the luminosity function fit to be correct we have to discount the Hα observations implying significant diffuse ionized gas or escaping ionizing photons. Combining the HST photometry with HI imaging we find the SFL has a power law index $N=1.53 \pm 0.21$. Applying these results to the entire outer HI disc indicates that it contributes 11--28% of the total recent star formation in NGC 2915, depending on whether the IMF is constant within the disc or varies from the centre to the outer region.

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arXiv2014

Discovery of a Gas-Rich Companion to the Extremely Metal-Poor Galaxy DDO 68

We present HI spectral-line imaging of the extremely metal-poor galaxy DDO 68. This system has a nebular oxygen abundance of only 3% Z$_{\odot}$, making it one of the most metal-deficient galaxies known in the local volume. Surprisingly, DDO 68 is a relatively massive and luminous galaxy for its metal content, making it a significant outlier in the mass-metallicity and luminosity-metallicity relationships. The origin of such a low oxygen abundance in DDO 68 presents a challenge for models of the chemical evolution of galaxies. One possible solution to this problem is the infall of pristine neutral gas, potentially initiated during a gravitational interaction. Using archival HI spectral-line imaging obtained with the Karl G. Jansky Very Large Array, we have discovered a previously unknown companion of DDO 68. This low-mass (M$_{\rm HI}$ $=$ 2.8$\times$10$^{7}$ M$_{\odot}$), recently star-forming (SFR$_{\rm FUV}$ $=$ 1.4$\times$10$^{-3}$ M$_{\odot}$ yr$^{-1}$, SFR$_{\rm Hα}$ $<$ 7$\times$10$^{-5}$ M$_{\odot}$ yr$^{-1}$) companion has the same systemic velocity as DDO 68 (V$_{\rm sys}$ $=$ 506 km s$^{-1}$; D $=$ 12.74$\pm$0.27 Mpc) and is located at a projected distance of 42 kpc. New HI maps obtained with the 100m Robert C. Byrd Green Bank Telescope provide evidence that DDO 68 and this companion are gravitationally interacting at the present time. Low surface brightness HI gas forms a bridge between these objects.

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arXiv2013

HI synthesis observations of the blue compact dwarf NGC 1705

Australia Telescope Compact Array HI-line observations of the nearby dwarf galaxy NGC 1705 are presented. The data are used to trace the gravitational potential of the galaxy out to several stellar disc scale lengths. A rotation curve is derived for the system and used to generate mass models. Dark matter dominates the gravitational potential at nearly all galactocentric radii. NFW and pseudo-isothermal sphere halo parameterisations allowing for good reproductions of the observations. The models suggest NGC 1705 to have a dark matter halo that it much denser and more compact than previously thought.

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arXiv2011

Quantified HI Morphology II : Lopsidedness and Interaction in WHISP Column Density Maps

Lopsidedness of the gaseous disk of spiral galaxies is a common phenomenon in disk morphology, profile and kinematics. Simultaneously, the asymmetry of a galaxy's stellar disk, in combination with other morphological parameters, has seen extensive use as an indication of recent merger or interaction in galaxy samples. Quantified morphology of stellar spiral disks is one avenue to determine the merger rate over much of the age of the Universe. In this paper, we measure the quantitative morphology parameters for the HI column density maps from the Westerbork observations of neutral Hydrogen in Irregular and SPiral galaxies (WHISP). These are Concentration, Asymmetry, Smoothness, Gini, M20, and one addition of our own, the Gini parameter of the second order moment (GM). Our aim is to determine if lopsided or interacting disks can be identified with these parameters. Our sample of 141 HI maps have all previous classifications on their lopsidedness and interaction. We find that the Asymmetry, M20, and our new GM parameter correlate only weakly with the previous morphological lopsidedness quantification. These three parameters may be used to compute a probability that an HI disk is morphologically lopsided but not unequivocally to determine it. However, we do find that that the question whether or not an HI disk is interacting can be settled well using morphological parameters. Parameter cuts from the literature do not translate from ultraviolet to HI directly but new selection criteria using combinations of Asymmetry and M20 or Concentration and M20, work very well. We suggest that future all-sky HI surveys may use these parameters of the column density maps to determine the merger fraction and hence rate in the local Universe with a high degree of accuracy.

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arXiv2011

Quantified HI Morphology I: Multi-Wavelengths Analysis of the THINGS Galaxies

Galaxy evolution is driven to a large extent by interactions and mergers with other galaxies and the gas in galaxies is extremely sensitive to the interactions. One method to measure such interactions uses the quantified morphology of galaxy images. Well-established parameters are Concentration, Asymmetry, Smoothness, Gini, and M20 of a galaxy image. Thus far, the application of this technique has mostly been restricted to restframe ultra-violet and optical images. However, with the new radio observatories being commissioned (MeerKAT, ASKAP, EVLA, WSRT/APERTIF, and ultimately SKA), a new window on the neutral atomic hydrogen gas (HI) morphology of a large numbers of galaxies will open up. The quantified morphology of gas disks of spirals can be an alternative indicator of the level and frequency of interaction. The HI in galaxies is typically spatially more extended and more sensitive to low-mass or weak interactions. In this paper, we explore six morphological parameters calculated over the extent of the stellar (optical) disk and the extent of the gas disk for a range of wavelengths spanning UV, Optical, Near- and Far-Infrared and 21 cm (HI) of 28 galaxies from The HI Nearby Galaxy Survey (THINGS). Though the THINGS sample is small and contains only a single ongoing interaction, it spans both non-interacting and post-interacting galaxies with a wealth of multi-wavelength data. We find that the choice of area for the computation of the morphological parameters is less of an issue than the wavelength at which they are measured. The signal of interaction is as good in the HI as in any of the other wavelengths in which morphology has been used to trace the interaction rate to date, mostly star-formation dominated ones (near- and far-ultraviolet). The Asymmetry and M20 parameters are the ones which show the most promise as tracers of interaction in 21 cm line observations.

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arXiv2011

Three-dimensional modeling of the HI kinematics of NGC 2915

The nearby blue compact dwarf, NGC 2915, has its stellar disc embedded in a large, extended (~ 22 B-band scale-lengths) HI disc. New high-resolution HI synthesis observations of NGC 2915 have been obtained with the Australia Telescope Compact Array. These observations provide evidence of extremely complex HI kinematics within the immediate vicinity of the galaxy's star-forming core. We identify and quantify double-peaked HI line profiles near the centre of the galaxy and show that the HI energetics can be accounted for by the mechanical energy output of the central high-mass stellar population within time-scales of 10^6-10^7 yr. Full three-dimensional models of the HI data cube are generated and compared to the observations to test various physical scenarios associated with the high-mass star-forming core of NGC 2915. Purely circular HI kinematics are ruled out together with the possibility of a high-velocity-dispersion inter-stellar medium at inner radii. Radial velocities of ~ 30 km/s are required to describe the central-most HI kinematics of the system. Our results lend themselves to the simple physical scenario in which the young stellar core of the galaxy expels the gas outwards from the centre of the disc, thereby creating a central HI under-density. These kinematics should be thought of as being linked to a central HI outflow rather than a large-scale galactic blow-out or wind.

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arXiv2010

A search for non-circular flows in the extended HI disc of NGC 2915

NGC 2915 is a nearby blue compact dwarf with a differentially rotating HI disc extending out to ~ 5 R-band R_{25} radii. This disc serves as an ideal tracer of the system's gravitational potential in regions of the galaxy that are dominated by dark matter. We use new HI synthesis observations of NGC 2915, obtained with the Australia Telescope Compact Array, to search for non-circular flows within the outer HI disc. Two independent methods are used, and the results of each interpreted in the context of relevant axisymmetric and non-axisymmetric perturbations of the potential. We find evidence for: (1) elliptical streaming associated with the spiral structure of the HI disc and the central bar-like feature in the mass distribution, (2) a spherical dark matter halo, and (3) an axisymmetric radial outflow of ~ 5-17 km/s (~ 6-20 percent of the circular speed). A possible bar-like perturbation of the potential hinders attempts to unambiguously detect kinematic signatures of radial flows in the HI velocity field. The radial outflows are inconsistent with the plausible disc formation scenario in which gas from the surrounding inter-galactic medium is deposited on the outer HI disc and then transported towards the centre of the galaxy. They are, however, consistent with the possibility of some material being re-distributed towards the outer disc in order to conserve angular momentum as material flows inwards along a bar.

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arXiv2010

The dark matter content of the blue compact dwarf NGC 2915

NGC 2915 is a nearby blue compact dwarf with the HI properties of a late-type spiral. Its large, rotating HI disk (extending out to R ~ 22 B-band scale lengths) and apparent lack of stars in the outer HI disk make it a useful candidate for dark matter studies. New HI synthesis observations of NGC 2915 have been obtained using the Australian Telescope Compact Array. These data are combined with high-quality 3.6 $μ$m imaging from the Spitzer Infrared Nearby Galaxies Survey. The central regions of the HI disk are shown to consist of two distinct HI concentrations with significantly non-Gaussian line profiles. We fit a tilted ring model to the HI velocity field to derive a rotation curve. This is used as input for mass models that determine the contributions from the stellar and gas disks as well as the dark matter halo. The galaxy is dark-matter-dominated at nearly all radii. At the last measured point of the rotation curve, the total mass to blue light ratio is ~ 140 times solar, making NGC 2915 one of the darkest galaxies known. We show that the stellar disk cannot account for the steeply-rising portion of the observed rotation curve. The best-fitting dark matter halo is a pseudo-isothermal sphere with a core density $ρ_0\sim 0.17 \pm 0.03$ \msun pc$^{-3}$ and a core radius $r_c\sim 0.9 \pm 0.1$ kpc.

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