Browsing by Author "De Blok, W J G"

Now showing 1 - 7 of 7
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    A comparison of the HI and CO velocity dispersions of nearby galaxies
    (2012) Mogotsi, Keoikantse Moses; De Blok, W J G
    Velocity dispersions are used to determine the stability of galactic disks against gravitational collapse to form stars, in some star formation recipes, in studies of gas dynamics, and to determine how much turbulence there is in the interstellar medium. Atomic hydrogen (HI) dispersions have been used in the studies of star formation and large-scale turbulence, despite stars forming in molecular clouds and the inner regions of galaxies being dominated by molecular gas. Carbon monoxide (CO) has been used as a tracer for molecular gas. In this work HI and CO dispersions were determined for a sample of nearby galaxies and they were compared to determine what the relationship between HI and molecular gas dispersions is.
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    The dynamics of molecular gas in nearby galaxies
    (2013) Frank, Bradley Stanton; De Blok, W J G; Carignan, C
    Includes abstract. Includes bibliographical references.
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    The HI super profiles of the THINGS galaxies
    (2010) Ianjamasimanana, Roger; De Blok, W J G
    We use the HI velocity profiles of The HI Nearby Galaxy Survey (THINGS) galaxies to study the phase structure of the interstellar medium (ISM) and its relation to galaxy properties and morphology. To construct high signal-to-noise (S/N) profiles, we use a method analogous to the stacking method sometimes used in high redshift HI observations. We call these high S/N profiles 'super profiles'. By decomposing the super profiles into Gaussian components, we find broad and narrow components in all our analyzed galaxies.
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    Interstellar medium properties and star formation in nearby galaxies
    (2014) Ianjamasimanana, Roger; De Blok, W J G; Jarrett, Thomas
    We study the properties of the interstellar medium (ISM) of nearby galaxies from The HI Nearby Galaxy Survey (THINGS) by analyzing the shapes of their HI emission velocity profiles. We apply a stacking method to increase the signal-to-noise (S/N) of the profiles and obtain what we call a super profile. We quantify all the relevant systematic effects that could confuse the interpretation of the shapes of the super profiles. We identify a sample of 22 galaxies from THINGS that are mostly free from these effects and analyze their super profile shapes. We derive super profiles from the entire HI disks of galaxies, inside and outside the optical radius r25, as a function of radius, column density, and star formation rate surface density. The super profiles can be described as the sum of a narrow and a broad Gaussian components. We associate the narrow component with the cold HI phase of the ISM and the broad component with the warm HI phase of the ISM. We find that the shapes of the super profiles correlate with star formation indicators such as metallicity, HK and far-UV near-UV colors. We also find that the mass fraction of the narrow component tends to be higher inside the optical radius r25. In addition, the velocity dispersions of the narrow and broad components decline exponentially with radius for virtually all the analyzed galaxies. Moreover, the flux ratio or mass ratio of the narrow and broad components, which serves as an estimate of the fraction of HI gas in the cold phase, tends to decrease with radius. Our results also show that regions having higher narrow component fractions usually corresponds to regions of higher HI or total (HI+H2) gas column density. Furthermore, the velocity dispersions of the broad and narrow components increase with increasing column density and star formation rate surface density. We have also investigated the physical mechanisms that can explain the observed width of the super profiles. These are supernova explosions (SNe), thermal effects from ultra-violet (UV) photons and magnetorotational instability (MRI). We find that SNe can explain the observed width of the super profile within the star forming disk (r25) and our data implies a supernova efficiency between 0.01 and 0.1. In the outer disk, the observed width of the super profiles can be attributed to thermal effects from extragalactic background UV photons. Finally, in most cases, MRI is not sufficient to explain the width of the super profile.
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    Investigating the relationship between the HI and optical diameter for a sample of galaxies from the local volume HI survey (LVHIS)
    (2008) Pekeur, N W; Kraan-Korteweg, Renée C; De Blok, W J G; Koribalski, B S
    Includes abstract. Includes bibliographical references (leaves 82-88).
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    The star formation and dynamics of nearby galaxies
    (2016) Mogotsi, Keoikantse Moses; Carignan, Claude; Meurer, G R; De Blok, W J G
    A scaling relation between the surface density of star formation and gas in the disks of galaxies has become the basis of our understanding of extragalactic star formation on scales of hundreds of parsecs and larger. This is an empirical law but star formation is a complex process - the presence of gas at sufficiently high densities to collapse and form stars depends on a wide variety of physical processes. These processes can be thought of in terms of the stability of galaxy disks, which is a balance between the gravitational force and competing forces such as the outward force due to pressure. In this study I explore how star formation is related to galaxy dynamics in the central regions of galaxies. This is done by determining the dominant contributor to the inner dynamics of galaxies and developing star formation models based on self-regulating disks that maintain a constant sub-critical stability parameter. Stability parameters for a gas-only disk and a two- uid disk containing both gas and stars are considered. These models are tested in the central regions of a sample of galaxies with a wide range of Hi masses, sizes, morphologies and stellar masses. The analysis is performed using Hα integral field spectroscopy, R-band, narrowband Hα, and near-infrared photometry to determine the star formation rates and kinematics of the galaxies. In agreement with previous studies I find that the central stellar surface density is tightly correlated with the central velocity gradient, which traces the steepness of the inner gravitational well. The baryonic fractions found in the analysis suggest that baryons dominate the central density of most galaxies in the sample, but better constraints on these are needed to make more firm conclusions. There are correlations between the star formation surface density and velocity gradient, however the observed relations do not match predictions from the models. Tests suggest that the failure of the models is due to the implied stability parameters in the galaxy centers not being constant across the galaxy sample, and that the star formation laws used in the analysis may not hold over the full parameter space of the sample.
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    A Westerbork blind HI imaging survey of the Perseus-Pisces filament in the Zone of Avoidance
    (2017) Ramatsoku, Mpati Analicia; Kraan-Korteweg, Renée C; Verheijen, M A W; De Blok, W J G
    In this thesis we study a structure associated with the Perseus-Pisces Supercluster (PPS; cz ~ 6000 km s⁻¹), where it crosses the Zone of Avoidance (ZoA) at the Galactic longitude of ℓ ≈ 160ᵒ. This largely unexplored region contains the X-ray emitting 3C 129 cluster which hosts two strong radio sources with bent morphologies, thus indicative of a rich intra-cluster medium (ICM). Due to its low Galactic latitude of b = 0.27, where Galactic extinction is severe at optical wavelengths (AB = 1.8 − 8.0), the details of its galaxy population and dynamics had not been investigated in detail before. Additionally, the diverse and dynamic cosmic environments of the PPS with the embedded galaxy cluster make this region an ideal laboratory to study the effects on these environments on the galaxy properties and galaxy transformation processes therein. We used the Westerbork Synthesis Radio Telescope (WSRT) to blindly map this ZoA crossing of the PPS in the 21 cm HI-wavelength. This was conducted through 35 individual pointings observed for a total of 420 hours, covering a large area of about 9.6 sq.deg and a velocity range of approximately cz ~ 2000− 16000 km s⁻¹, thus mapping the immediate surrounding regions to enable investigations of the various cosmic environmental effects on the HI properties of galaxies. With the resulting spectral line data cubes of an angular resolution of 23" x 16" and a velocity resolution of 16.5 km s⁻¹ with a rms sensitivity of ~ 0.4 mJy/beam, we detect 211 galaxies within the entire surveyed volume. These new HI detections reveal a total of four distinct overdensities. Of these overdensities, the two major ones are located at the distance of the PPS at cz ~ 4000 − 8000 km s⁻¹ and behind it at cz ~ 8000 − 12000 km s−1. The galaxy overdensity associated with the PPS is used to demonstrate how this supercluster connects across the ZoA and confirms earlier indications of a filamentary connection between Perseus, Pisces and the A569 clusters through this region of the sky. Galaxies in the background of the PPS seem to be part of the CID15 structure that had been earlier predicted in the reconstructed density and velocity maps from the 2MASS Redshift Survey. We carry out a detailed census of the galaxy population in the 3C129 cluster by combining the HI-data of the gas-rich galaxies with the near-infrared (NIR) images of the gas-poor galaxies. The NIR galaxies are identified from the high resolution (0.2"/pix, seeing ~0.8") images of the UKIDSS Galactic Plane Survey. We obtain photometry in the J,H and K bands for about 9700 galaxies identified in the NIR images within the WSRT HI surveyed area. These measurements are used to derive the red-sequence of this cluster through the (J − K) vs K colour-magnitude diagram to identify the gas-poor cluster member candidates. Within the spatial extent of the cluster of radius ~ 1.7 Mpc, a total of 261 galaxies are identified as cluster members, with 23 detected in HI. An assessment of the morphologies of these galaxy members reveals a clear morphological segregation, with E and E/S0 galaxies dominating the inner regions of the 3C129 cluster and the late-type spirals found in the cluster outskirts. We also examine the richness of the cluster by comparing it to two well-known clusters at similar redshifts. One being a massive cluster in the Great Attractor region, namely the Norma cluster, also located in the ZoA but in the South, and the other being the Coma cluster. The comparison shows that the 3C129 cluster is quite rich as was suspected. The galaxy density in the core of the 3C129 cluster is similar to that in the Norma cluster and slightly less than in the Coma cluster. Furthermore, an assessment of the spatial distribution of galaxies in the core shows a slight asymmetry aligned with the irregular distribution of the X-ray emission, thus consistent with the results from the X-ray analysis which surmised that the 3C129 cluster seems to have undergone a merger and has not yet reached a dynamically relaxed state. In support of this we find a large substructure dominated by gas-rich galaxies North of the main cluster at a slightly higher recession velocity which could be falling into the cluster. All these findings seem to support a scenario where the 3C129 cluster is still growing through accretion of galaxies from the PPS filament. The wealth of HI data allows an analysis of the environmental effects on the galaxy properties within the WSRT surveyed volume. We first characterise the cosmic environments found within the two major overdensities as outlined by the HI-detected galaxies. This is conducted by perfoming tests to search for substructures. A total of four distinct substructures are found within the PPS ZoA overdensity (cz ~ 4000 − 8000 km s−1 ) and three in the background galaxy overdensity (cz ~ 8000 − 12000 km s−1). Our analysis of the HI properties of galaxies in these varying cosmic environments shows that highly disturbed HI-disks are found in larger and tightly bound groups, due to the higher incidence of tidal interactions between the galaxies. Furthermore, an indication of HI-gas deficiency is found in the core of the 3C129 cluster. We assess the cause of this deficiency through models of the ICM of the 3C129 cluster and find ram-pressure stripping to be the dominant gas removal process in this region. Additionally, highly HI-gas deficient galaxies are also found in the cluster outskirts. Most of these galaxies are located within galaxy groups where galaxy-galaxy interactions and mergers are prevalent, thus highlighting the importance of these processes in removing gas from galaxies in groups.