Browsing by Author "Vaisanen, Petri"

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    Bent tail radio sources as tracers of galaxy clusters at high redshift and SMBH mass estimates
    (2021) Mguda, Zolile Martin; Van Der, Heyden Kurt; Vaisanen, Petri
    Bent tail radio sources (BTRSs) are radio galaxies which have jets that show a characteristic C‐shape that is believed to be due to ram pressure caused by the motion of the galaxy through the ambient medium. They are generally found in galaxy clusters in the local Universe. They have already been used in observations as tracers of galaxy clusters at redshifts of up to z _ 1. They have, however, been shown to be numerous in galaxy groups as well. The ability to find high redshift galaxy clusters is important in cosmology because they are important cosmological probes. According to the _ CDM model, galaxy clusters form around redshift of z _ 2 and finding clusters of halo mass greater than 1014 M_ at redshift greater than z = 2:5 would disprove the current concordance model. Finding galaxy clusters at those redshifts is more feasible with the new generation of radio telescopes and the upcoming square kilometer array (SKA). In this work we look at some SMBH mass measurements, which are crucial in the determination of the correlations between the SMBH mass and some galaxy characteristics including jet length and luminosity. The high redshift SMBH mass measurement methods are calibrated using local Universe correlations. This makes SMBH mass measurement an important aspect in the study of high redshift radio galaxies and hence BTRSs. We use cosmological simulations from the MareNostrum Universe simulation to look at the efficacy of using BTRSs as tracers of clusters assuming the ram pressure is the cause of the jet bending. This is the first step in predicting the possible number of BTRSs that we may observe with the SKA. We find that SMBH masses can be measured up to redshift of z = 4:5 using the virial mass estimator method. The BTRSs are equally likely to be found in galaxy clusters and galaxy groups in the local Universe. This means that around 50% of the BTRSs that we are likely to find at high redshift will be in galaxy clusters. However, finding a pair of BTRSs in close proximity is a sign of a galaxy cluster environment. These results are still dependent on the resolution of degeneracies in our understanding of the duty cycles of AGN radio jets, projection effects of the radio jets, the environmental dependence of radio‐loudness in galaxies and other open questions.
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    Characterising star forming and luminous infrared galaxies with the Southern African Large Telescope (SALT)
    (2018) Ramphul, Rajin Anand; Vaisanen, Petri; van Der Heyden, Kurt
    Context: Stellar population modelling is a popular technique that has been extensively applied to main sequence galaxies. Yet starburst galaxies and Luminous InfraRed Galaxies (LIRGs) have, so far, not been studied as much using the method. LIRGs in the local universe are known to be highly interacting galaxies with strong star formation in obscured environments. Still, LIRGs also have diversity in terms of morphology and mode and location of star formation. Aim: This thesis investigates the stellar population properties of a group of 52 starbursts and luminous infrared galaxies (LIRGs) in the local universe that forms part of the SUperNovae and starBurst in the InfraReD (SUNBIRD) survey. The galaxies in a distance range of 3.5 < Dl < 280 Mpc and infrared luminosity of 10.30 < LIR < 11.91 L were observed with the Southern African Large Telescope in long-slit spectroscopy mode. Method: The stellar populations of the galaxies are derived by fitting Bruzual & Charlot (2003) templates to the reduced spectra using STARLIGHT software with a Monte Carlo method implemented to recover uncertainties on age, metallicity and extinction. The derived stellar population models are then subtracted from the observed spectra to produce emission spectra from which emission line fluxes are measured. Both integrated spectra and spatially resolved apertures are extracted to be analysed in this work. Results: The light-weighted and mass weighted age of the sample is found to be 160 Myr and 7.2 Gyr respectively. The star formation history of the sample shows a rise of activity in the past ∼ 50 Myr and with a jump of an order of magnitude in the past 3 Myr. Analysis of the stellar metallicity hints at inflow of pristine gas, which decreases the observed metallicity content as well as ignites SF-activity. Analysis of the oxygen abundances shows that while LIRGs and SF galaxies are under abundant, their under-abundance may have previously been over-estimated as compared to main sequence galaxies. The radial age profile of the sample is flat, similar to that of late-type Sd galaxies. Interaction is found to cause a drop in the age of apertures although the post-merging stages shows continued star forming activity in the nuclear region. The stellar metallicity gradient is found to be −0.029 ± 0.018 dex/kpc, comparable to Sb or Sbc galaxies favouring an inside-out formation scenario for the galaxies. As interaction stage increases, both age and metallicity gradients are seen to get flatter, eventually getting slightly positive. The more active interaction stages are HII driven, while isolated and post merging stages shows higher AGN activity. The current work offers an update on the abundances of IR dominated galaxies from the previous work done by Rupke et al. (2008). The formation scenario of our LIRGs in the local universe is shown to be in line with the scenario put forward by Hopkins et al. (2008). Future works with medium resolution spectra acquired during the course of this thesis should allow for detection of gas inflows and better constrain the different ionising mechanisms involved at different interaction stages.
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    Gas flows and feedback in star-forming galaxies
    (2018) Tafere, Melaku S.; Vaisanen, Petri; Van Der Heyden, Kurt
    In this MSc thesis, I investigated the kinematics of Hα emission from the hot ionized and NaD absorption from cool neutral gas in a sample of 40 nearby Luminous Infrared Galaxies (LIRGs: LIR ≥1011L) from Supernovae and starbursts in the infrared, SUNBIRD survey imaged with the Southern African Large Telescope (SALT). This project can be seen in two major areas. The first is analyzing the emission and absorption spectra, in this case from the hot ionized gas and cool gas respectively. Two routines were used for the profile fitting, to get the centroid wavelength of both the emission and absorption lines; Gaussian and Gaussian-Hermite functions were used. The fitting result provides an indication of the origin of an outflowing gas; nuclear or disk driven?. Secondly, I plotted and compared the velocities of hot ionized and cool neutral gas. Three different methods were used to estimate the systemic velocity Vsys for the accurate estimation of outflowing velocity. For instance, an outflowing NaD upto 380 kms−1 and inflowing gas up to -100 kms−1 relative to Hα is seen at the optical nucleus of the NGC6240[PA=45] and NGC1204[PA=253] galaxies respectively. I tested if there was a relation between star formation rate (SFR) and an outflowing gas and also whether the inflowing gas triggering new nuclear SF. I find the gas of two of our targets escaping the potential well of the host galaxy. There was also an expectation that the out flowing velocity would correlate with SFR, unfortunately I do not see any correlation, though it needs further investigation of all offset velocities of the gas with respect to stellar absorption lines and it also requires detail analysis of the of projection or orientation effect on the galaxy.