Characterising star forming and luminous infrared galaxies with the Southern African Large Telescope (SALT)

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 &amp; 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

The Kiloparsec Scale Influence of the AGN in NGC 1068 with SALT RSS Fabry-P\'erot Spectroscopy

We present Fabry-P\'erot (FP) imaging and longslit spectroscopy of the nearby Seyfert II galaxy NGC 1068 using the Robert Stobie Spectrograph (RSS) on the Southern African Large Telescope (SALT) to observe the impact of the central Active Galactic Nucleus (AGN) on the ionized gas in the galaxy on kiloparsec scales. With SALT RSS FP we are able to observe the H$\alpha$+[N II] emission line complex over a $\sim$2.6 arcmin$^2$ field of view. Combined with the longslit observation, we demonstrate the efficacy of FP spectroscopy for studying nearby Type II Seyfert galaxies and investigate the kiloparsec-scale ionized gas in NGC 1068. We confirm the results of previous work from the TYPHOON/Progressive Integral Step Method (PrISM) survey that the kiloparsec-scale ionized features in NGC 1068 are driven by AGN photoionization. We analyze the spatial variation of the AGN intensity to put forward an explanation for the shape and structure of the kiloparsec-scale ionization features. Using a toy model, we suggest the ionization features may be understood as a light-echo from a burst of enhanced AGN activity $\sim$2000 years in the past.Comment: 18 pages, 9 figures. Accepted for publication in The Astronomical Journa

Star formation and AGN activity in a sample of local Luminous Infrared Galaxies through multi-wavelength characterization

Nuclear starbursts and AGN activity are the main heating processes in luminous infrared galaxies (LIRGs) and their relationship is fundamental to understand galaxy evolution. In this paper, we study the star-formation and AGN activity of a sample of 11 local LIRGs imaged with subarcsecond angular resolution at radio (8.4GHz) and near-infrared ($2.2\mu$m) wavelengths. This allows us to characterize the central kpc of these galaxies with a spatial resolution of $\simeq100$pc. In general, we find a good spatial correlation between the radio and the near-IR emission, although radio emission tends to be more concentrated in the nuclear regions. Additionally, we use an MCMC code to model their multi-wavelength spectral energy distribution (SED) using template libraries of starburst, AGN and spheroidal/cirrus models, determining the luminosity contribution of each component, and finding that all sources in our sample are starburst-dominated, except for NGC6926 with an AGN contribution of $\simeq64$\%. Our sources show high star formation rates ($40$ to $167M_\odot\mathrm{yr}^{-1}$), supernova rates (0.4 to $2.0\mathrm{SN}\mathrm{yr}^{-1}$), and similar starburst ages (13 to $29\mathrm{Myr}$), except for the young starburst (9Myr) in NGC6926. A comparison of our derived star-forming parameters with estimates obtained from different IR and radio tracers shows an overall consistency among the different star formation tracers. AGN tracers based on mid-IR, high-ionization line ratios also show an overall agreement with our SED model fit estimates for the AGN. Finally, we use our wide-band VLA observations to determine pixel-by-pixel radio spectral indices for all galaxies in our sample, finding a typical median value ($\alpha\simeq-0.8$) for synchrotron-powered LIRGs.Comment: Accepted for publication in MNRAS. 20 pages, 12 figure

Shutting down or powering up a (U)LIRG? Merger components in distinctly different evolutionary states in IRAS 19115-2124 (the Bird)

We present new SINFONI near-infrared (NIR) integral field unit (IFU) spectroscopy and Southern African Large Telescope (SALT) optical long-slit spectroscopy characterizing the history of a nearby merging luminous infrared galaxy, dubbed the Bird (IRAS19115-2124). TheNIR line-ratio maps of the IFU data cubes and stellar population fitting of the SALT spectra now allow dating of the star formation (SF) over the triple system uncovered from our previous adaptive optics data. The distinct components separate clearly in line-ratio diagnostic diagrams, both thermal and non-thermal excitation is present. An off-nuclear starburst dominates the current SF of the Bird with 60-70 per cent of the total, with a 4-7 Myr age. The most massive nucleus, in contrast, is quenched with a starburst age of >40 Myr and shows hints of budding active galactic nucleus (AGN) activity. The secondary massive nucleus is at an intermediate stage. The two major components have signs of an older stellar population, consistent with a starburst triggered 1 Gyr ago in a first encounter. The simplest explanation of the history is that of a triple merger, where the strongly star-forming component has joined later. We detect multiple gas flows. The Bird offers an opportunity to witness multiple stages of galaxy evolution in the same system; triggering as well as very recent quenching of SF, and, perhaps, an early appearance of AGN activity. It also serves as a cautionary note on interpretations of observations with lower spatial resolution and/or without infrared data. At high redshift the system would look like a clumpy starburst with crucial pieces of its puzzle hidden in danger of misinterpretations.Peer reviewe

The Optical and Infrared Photometric Evolution of the Recent Stellar Merger, V1309 Sco

Nova Sco 2008 was recently shown to have resulted from the merger of the two stars in the contact binary V1309 Sco. This is the first stellar merger ever observed between two convective stars. We present archival data, new infrared photometry, and Hubble Space Telescope WFC3 imaging of V1309 Sco. Spitzer observations show that it had a large infrared excess in the 3.6 μm to 8 μm range more than a year before the merger. Standard color diagnostics of the pre-merger infrared colors place V1309 Sco in the same region where evolved stars with chemically complex mass loss are located. Since the nova outburst subsided in optical bandpasses in 2008, the merger remnant's brightness in optical bandpasses, near-IR bandpasses, and the Spitzer 3.6 μm and 4.5 μm channels has varied by several magnitudes and in complex ways. A temporary, strong increase in the reddening during 2010 suggests the occurrence of a dust formation event. We point out several peculiarities in the relative fluxes and time behavior of the optical and near-IR magnitudes, which could be explained if some of the photometric bandpasses in the 1–5 μm range are strongly affected by emission lines