Thick discs and truncations in spiral galaxies

Resum

Our understanding of the evolution of galaxies within their optical radius has made great advances in the last decades. These have been achieved by analysing in detail the internal secular processes, the star formation history, and the Interstellar medium. However, the study of galaxy outskirts, so-called low surface brightness science, is still in its infancy. In this thesis, we will use the deepest available data from ground-based and space telescopes to study the formation and evolution of galaxy structures, in particular, their thick discs and truncations, in unprecedented detail. Additionally, we analyse high-resolution data of complex central structures in three barred lenticular galaxies by using similar fitting techniques to that applied in the outskirts. The study of low surface brightness structures is a technical challenge from an observational point of view. Going beyond 30 mag arcsec^−2 (i.e., ~1500 times fainter than the darkest sky on Earth), is still very complex. The development of this field is held back not so much by the collection capability of the telescopes and their detectors which provide deep imaging and photon statistics, but rather by systematic errors. Firstly, it is crucial to perform meticulous data reduction and treatment of the sky and the images. Very careful sky subtraction is important in order not to confuse the light coming from the faintest extended structures with the background and, consequently, oversubtract the image. A key point in this process is to produce a reliable mask over the whole field of view, avoiding any undesirable light from foreground objects. Second, we need to accurately characterize the point spread function (PSF) of the images over an extension as large as possible to remove the scattered light produced by sources in the field. Third, there are many artifacts that may affect the quality of the images: fringing, flat fielding, ghosts, Galactic cirrus, etc. Once all undesirable light is discarded, it is possible to build a PSF-deconvolved model of the target galaxy. We use the astronomy software package imfit to perform reliable 2D fitting of the galaxies and their components. Thick discs can give invaluable information on the formation and evolution history of galaxies as most if not all, disc galaxies have a thin (classical) disc and a thick disc. We study the structure of thick discs in extraordinary depth by reaching a surface brightness limit of μrdeep ~28.5−29 mag arcsec^−2 with combined g,r,i bands images from the IAC Stripe 82 Legacy Project. Our methodology begins with a careful analysis of the background and masking processes. Then, the effects of the PSF are considered through galaxy modelling. We present the characterization of the thick discs in a fairly diverse sample of five edge-on galaxies. A study of the radial and vertical surface brightness profiles is presented, comparing our data with PSF- deconvolved models. The galaxy disc components are fitted considering that the thin and thick discs are two stellar fluids in hydrostatic equilibrium. We find that effects due to the PSF are significant when reaching lower surface brightness, especially in the vertical profiles, but can be accounted for by careful modelling. The galaxy outskirts are strongly affected by the faint wings of the PSF, mainly by PSF-redistributed light from the thin disc. This is a key issue when dealing with ultra-deep imaging. The thick disc component is required to reach satisfactory fit results in the more complex galaxies in our sample, although it is not required for all galaxies. If the PSF is ignored, the brightness of these structures may be overestimated by up to a factor of ~4. In general, our results are in good agreement with previous works, although we reach deeper surface brightness levels, so the PSF effects are more important. We obtain scale heights and mass ratios values of thin and thick discs (z_t, z_T, M_T/M_t, respectively), which provide excellent agreement with preceding studies. Disc truncations are the closest thing a galaxy will come to having an ‘edge’, but the nature of this phenomena is not yet fully understood. In this thesis, we explore the truncations in two nearby (D ~ 15 Mpc) Milky Way-like galaxies: NGC 4565 and NGC 5907. By covering a wide wavelength range from the NUV and optical, to 3.6 μm, we find that the radius of the truncation (26 ± 0.5 kpc) is independent of wavelength. Surprisingly, truncations are identified (at all wavelengths) at altitudes as high as 3 kpc above the mid-plane, which implies that the thin disc in those outer regions has a width of at least this value. We recognize the characteristic U-shape radial colour profile associated with a star formation threshold at the location of the truncation. Further supporting such an origin, the stellar mass density at the position of the truncation is ~ 1 − 2 M⊙ pc^−2, in good agreement with the critical gas density for transforming gas into stars. Beyond the truncation, the stellar mass density in the mid-plane of the disc drops to just 0.1 − 0.2% of the total stellar mass density of the galaxies. The detection of the truncation at high altitude in combination with the U-shape of the radial colour profiles allows us to establish, for the first time, an upper limit to the present-day growth rate of galactic discs. We find that, if the discs of galaxies are growing inside-out, their growth rate is less than 0.6 − 0.9 kpc Gyr^−1. Finally, we investigate three barred lenticular galaxies (NGC 2681, NGC 3945 and NGC 4371) which were previously reported to have complex central structures yet had not, until now, been subject to a detailed structural analysis using high-resolution data. We have therefore performed four- to six-component, (pseudo-)bulge/disc/bar/ring/point source, decompositions of the composite (Hubble Space Telescope plus ground-based) surface brightness profiles. We find that NGC 2681 hosts three bars, while NGC 3945 and NGC 4371 are double- and single- barred galaxies, respectively, in agreement with past isophotal analyses. We find that the bulges in these galaxies are compact, and have Sérsic indices of n ~ 2.2 − 3.6 and stellar masses of M∗ ~ 0.28 × 10^10 − 1.1 × 10^10M⊙. NGC 3945 and NGC 4371 have intermediate-scale ‘pseudo- bulges’ that are well described by a Sérsic model with low n ≤ 0.5 instead of an exponential (n = 1) profile as used in the past. We measure emission-line fluxes enclosed within nine different elliptical apertures, finding that NGC 2681 has a LINER-type emission inside R ~ 3 arcsec, but the emission line due to star formation is significant when aperture size is increased. In contrast, NGC 3945 and NGC 4371 have composite (AGN plus star forming) and LINER-type emissions inside and outside R ∼ 2 arcsec, respectively. Our findings suggest that the three galaxies have experienced a complex evolutionary path. The bulges appear to be consequences of an earlier violent merging event while subsequent disc formation via gas accretion and bar-driven perturbations may account for the eventual build-up of pseudo-bulges, bars, rings and point sources.