Browsing by Author "Jarrett, Thomas"

Now showing 1 - 11 of 11
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    Open Access
    Deconstructing the WISE nearby galaxy population
    (2019) August, Tamlyn; Jarrett, Thomas; Cluver, Michelle
    This thesis aims to test the reliability of the Wide-Field Infrared Space Explorer (WISE) star formation indicators centred at 12 and 22 µm. To accomplish this, the total infrared (TIR) luminosity was used as a calibrator to understand the behaviour of the two indicators. Our sample consists of Spitzer galaxies from the SINGS and KINGFISH surveys, with a select few excluded. The TIR calibration also necessitates that most of our galaxies have FIR data from Herschel. The photometric analysis done on the raw WISE and Spitzer images of our galaxies is explained, including how the isophotes and background sky level were determined for each band/image. Once the reliability of the WISE indicators was established, new star formation rate relations were formulated. The new relations are comparable to previous relations in the literature, but improve on them in stellar mass-, metallicity-, and luminosity range. We also find that metallicity of the galaxies has minimal effect on our relations. In our comparison of emission from PAH molecules, the two bands in question, IRAC4 and W 3 are strongly correlated. This may suggest that the W3 band is dominated by emission from the 11.3 µm PAH molecule, or it might be an effect of the band overlap between IRAC4 and W3.
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    The integrated Sachs-Wolfe effect with Planck and the 2Mass Photometric Redshift Catalogue
    (2014) Steward, Louise; Jarrett, Thomas; Bilicki, M
    This thesis presents a measurement of the Integrated Sachs-Wolfe (ISW) effect through cross-correlation of the Cosmic Microwave Background (CMB) and the galaxy distribution tracing the large scale structure of the Universe. The CMB data used are from the 2013 release from the Planck satellite, and the large-scale structure data are from the 2MASS Photometric Redshift Catalogue (2MPZ). The galaxy data were divided into redshift shells, and HEALPix was used to create pixelised maps of the CMB temperature anisotropies and the galaxy overdensity. The linear galaxy bias, relating the galaxy density distribution to the underlying matter density distribution, was measured through least squares fitting of the theoretical prediction of the galaxy auto-correlation function in each redshift shell.The bias values were then used to rescale the theoretical predictions for the matter-CMB crosscorrelation functions in each shell. The observed cross-correlation function between the Planck and 2MPZ data in each shell was computed, and the uncertainties associated with each measurement were calculated using cross-correlation of simulated CMB and galaxy overdensity maps. To quantify the possible detection of the ISW effect, hypothesis testing was performed through computation of the covariance matrix and χ 2 statistic in each shell. Detection of the ISW effect was found to be preferred over no detection in every case, with a total likelihood ratio of 3.4:1. While this is not quite strong evidence of detection of the ISW effect, this ratio is more than twice better than previous measurements using photometric redshift shells. While the photometric redshifts in the 2MPZ are more accurate than those that have been used before, a strong detection is out of reach with this data, as the redshifts in the 2MPZ are too shallow for more decisive ISW detection.
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    Open Access
    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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    Multicolor analysis of galaxy clusters with radio halos and/or relics
    (2014) Legodi, L S; Oozeer, N; Jarrett, Thomas
    Galaxy clusters can be sites of considerable dynamic activity due to intra-cluster and inter-cluster interactions. These interactions include cluster-cluster mergers which are accompanied by large release of energy observed via thermal X-ray emission. The work presented here focuses on clusters that exhibit extended radio sources called radio relics and radio halos. The relics tend to be found in the peripheral regions of galaxy clusters while halos tend to be located at the central regions. These two types of sources share many of their characteristics but some of their key difference is radio polarization: relics tend to have polarised emission while halos do not. The observed polarisations and the non-thermal nature of the emission of these sources suggests the existence of large scale magnetic fields which permeate galaxy cluster volumes. Investigations into relic and halo sources may shed light into the formation of galaxy clusters and how they evolve. A way of analysing the evolution of clusters is to investigate the evolutionary state of their galaxy populations. This work aims to both characterise galaxy membership and probe galaxy evolution in these clusters through a spatial, kinematic and photometric analysis.
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    A new method of mapping cosmic flow fields : evaluating the sustainability of the infrared bands Tully-Fisher relation for ZoA work
    (2016) Affadi, Ikechukwu Patrick; Kraan-Korteweg, Renée C; Jarrett, Thomas
    This thesis aims at using the Infrared Survey Facility (IRSF) JHKₛ bands, Two Micron All Sky Survey (2MASS) JHKₛ bands and Wide Field Infrared Survey Explorer (WISE) W1-W4 bands isophotal magnitudes to understand the cosmic flow associated with Zone of Avoidance (ZoA) galaxies.
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    The NIR isophotal Tulley-Fisher relation and its cosmological applications in the Zone of Avoidance
    (2013) Said, Khaled; Kraan-Korteweg, Renée C; Jarrett, Thomas
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    Optimisation of galaxy identification methods on large interferometric surveys
    (2018) Gqaza, Themba; Kraan-Korteweg, Renee Christine; Jarrett, Thomas
    The astronomical size of spectral data cubes that will result from the SKA pathfinders planned large HI surveys such as LADUMA; Fornax HI survey; DINGO; WALLABY; etc. necessitate fully automated three-dimensional (3D) source finding and parametrization tools. A fraction of the percentage difference in the performance of these automated tools corresponds to a significant number of galaxies being detected or undetected. Failure or success to resolve satellites around big spirals will affect both the low and the high mass end of the HI mass function. As a result, the performance and efficiency of these automated tools are of great importance, especially in the epoch of big data. Here I present the comprehensive comparison of performance between the fully automated source identification and parametrization software: SOFIA, the visual galaxy identification method and the semi-automated galaxy identification method. Each galaxy identification method has been applied to the same ∼ 35 gigabytes 3D HI data cube. The data cube results from the blind HI imaging survey conducted using the Westerbork Synthesis Radio Telescope (WSRT). The survey mapped the overdensity corresponding to the Perseus-Pisces Supercluster filament crossing the Zone-of-Avoidance (ZoA), at (`, b) ≈ (160◦ , 0.5◦ ). A total of 211 galaxies detected using the semi-automated method by Ramatsoku et al. [2016]. In this work, I detected 194 galaxies (using the visual identification method) of which 89.7% (174) have cross-matches/counterparts on the galaxy catalogue produced through semi-automated identification method. A total of 130 detections were made using SOFIA of which 89 were also identified by the two other methods. I used the sample of 174 visual detections with semi-automated counterparts as a Testbed to calculate the reliability and completeness achieved by SOFIA. The achieved reliability is ∼ 0.68 whereas completeness is ∼ 0.51. Further parameter fine-tuning is necessary to have a better handle on all SOFIA parameters and achieve higher reliability and completeness values.
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    Probing the role of environment and HI content in galaxy evolution: a multi-wavelength study of isolated and paired galaxies
    (2021) Bok, Jamie; Jarrett, Thomas; Skelton, Rosalind; Cluver, Michelle, Blyth, Sarah
    This thesis records a detailed examination of the impact of the merger-pair galaxy environment on both the neutral hydrogen (Hi ) and mid-infrared (MIR) properties of galaxies in the nearby Universe. Making use of publicly available Hi profiles from the ALFALFA survey I construct the first statistically significant samples of close-pair galaxies (348 Hi -optical and 282 Hi - Hi pairs), and contrast their Hi content and star formation (SF) properties with a statistically significant Hisub-sample of isolated galaxies (544 galaxies) from the AMIGA project (Analysis of the interstellar Medium in Isolated GAlaxies; Verdes-Montenegro et al. (2005)). I present the first study of pairs using WISE data, and specifically examine their location on the MIR star-formation rate-stellar mass sequence (SFR-M★), or star-forming main sequence (SFMS), as a way to study how the close-pair environment influences the build up of galaxy stellar mass via SF. I also present the first MIR SFMS for isolated galaxies from the AMIGA catalogue to serve as the precedent for secular evolution. I derive an Hi scaling relation for isolated galaxies using WISE stellar masses, and thereby establish a baseline predictor of Hi content that can be used to assess the impact of environment on Hi content when compared with samples of galaxies in different environments. I use this updated relation to determine the Hi deficiency of both my paired and isolated galaxies, and invoke galaxy morphology (visual and MIR bulge-to-total ratios), the AMIGA isolation parameters 휂 (local number density) and Q (tidal influence), star formation efficiency (SFE), and Hi profile asymmetries to more closely inspect how these properties might be additionally driving the observed differences between the deficiency distributions of these two samples, as well as SFMS location. I also provide an analysis of the quantified Hi profile asymmetries of my pair and isolated galaxy samples, exploring not only the prevalence of asymmetry in Hi profiles, but also the possibility of using Hi profile asymmetries to trace merger activity. I find enhanced profile asymmetries in my pair sample, and propose that high profile asymmetries may be used to infer merger activity/identify close galaxy pairs at high redshifts, in lieu of the typically used 2D Hi maps we have for galaxies at low and intermediate redshifts, which are currently still limited. What my thesis shows is that although we have a plethora of data available (and coming), the key is to optimise how we use it, both in the questions we pose, and in understanding its limitations. We currently have large data-sets of Hi profiles in the local Universe, which, when incorporated into a cohesive multi- wavelength study, provide important clues as to how Hi forms, influences, and is processed in galaxies. These are the studies informing our theories for galaxy evolution, providing the incentive for superior telescopes (e.g. SKA), and ultimately guiding our decision-making in how to proceed in our ongoing endeavour to understand our Universe.
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    Seeing our world through science
    (2013) Sithaldeen, Riashna; Jarrett, Thomas
    We live in a time of exciting discoveries and developments in the field of science, allowing us to 'see' more than ever before, including 'seeing' into the past, the present and the future. For anyone who wants to learn more about seeing our world through science.
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    Star formation in the bars of nearby galaxies
    (2016) Ngcebetsha, Buntu; Jarrett, Thomas; Sheth, K
    Aims: In this study, we quantify the star formation activity in bars as a function of the host stellar mass and the morphological type. We use a sample of 70 barred spiral galaxies in the nearby universe taken from the Spitzer Survey of Stellar Structure in Galaxies (S⁴G). The data are combined with Hα data from various optical telescopes. (KPNO, JKT, OHP and CTIO) Methods: We overlay continuum-subtracted Hα emission on 3.6μm images from Spitzer (S⁴G) to quantify the distribution of star formation in each bar. We discern the star formation distribution into three classes: morph-(1): Minimal to no star formation in the bar but intense star formation activity at the bar ends and in the nuclear region; morph-(2): Star formation along the bar, in the nuclear region to the bar ends; morph-(3): Star formation only at the bar ends. To understand the location of the bar and its properties, we used ellipse fitting to identify the bar properties, including the bar size and the bar strength. We also use the host stellar mass of each galaxy measured from the 3.6μm integrated magnitudes from S⁴G to investigate how the three star formation distributions are related to the stellar mass. The host galaxies are divided into Early-Type (SB0-SBbc) and Late-Type Barred Spirals (SBc-SBm) - these morphological types correlate with a variety of galaxy properties that have historically been used to better understand galaxy formation and evolution. For the next stage of this work, we will compare the star formation activity in one galaxy from the sample, NGC 1097 with molecular gas distribution mapped using ALMA. This will be an investigation of the relationship between gas kinematics and star formation using numerical simulations of gas flow. [Please note: this thesis fulltext has been deferred until 9 December 2016]
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    WISEly deconstructing the Great Andromeda Galaxy
    (2014) Chauke, Priscilla; Jarrett, Thomas
    A global photometric investigation of the M 31 system is presented using the Wide-field Infrared Survey Explorer (WISE), a survey which mapped the entire sky at mid-infrared wavelengths 3.4 μm (W1), 4.6 μm (W2), 12 μm (W3) and 22 μm (W4), as well as previous studies across the electromagnetic spectrum. While numerous surveys of the galaxy exist, very few cover its extended disk and greater halo that incorporates its globular clusters and rich satellite system. WISE observed the entire region of M 31, and with multi-wavelength data and measurements having recently become available for M 31, viz., GALEX in the ultraviolet, Spitzer in the mid and far-infrared and Herschel in the far-infrared, WISE plays a complementary role towards the comprehension of the fundamental processes of formation and evolution in galaxies. A statistical classification scheme is developed to identify (and thereafter remove) the foreground Milky Way population from the WISE images, using WISE and 2MASS fluxes and colors, to obtain a clean measurement of the M 31 system. The scheme is tested using M 33, a smaller yet challenging galaxy, and is found to be an efficient method to trace the Galactic population (the results for this galaxy are, therefore, included). 104762 sources were identified as Milky Way, 70% of the total number of sources in a ~11 deg² elliptical area around the M31 region.