Context. The Hubble Ultra Deep field (HUDF) is the deepest region ever observed with the Hubble Space Telescope. With the main objective of unveiling the nature of galaxies up to z ∼ 7 − 8, the observing and reduction strategy have focused on the properties of small and unresolved objects, rather than the outskirts of the largest objects, which are usually over-subtracted. Aims. We aim to create a new set of WFC3 IR mosaics of the HUDF using novel techniques to preserve the properties of the low surface brightness regions. Methods. We created ABYSS: a pipeline that optimises the estimate and modelling of low-level systematic effects to obtain a robust background subtraction. We have improved four key points in the reduction: 1) creation of new absolute sky flat fields, 2) extended persistence models, 3) dedicated sky background subtraction and 4) robust co-adding. Results. The new mosaics successfully recover the low surface brightness structure removed on the previous HUDF published reductions. The amount of light recovered with a mean surface brightness dimmer than µ = 26 mag arcsec −2 is equivalent to a m = 19 mag source when compared to the XDF and a m = 20 mag compared to the HUDF12. Conclusions. We present a set of techniques to reduce ultra-deep images (µ > 32.5 mag arcsec −2 , 3σ in 10 × 10 arcsec boxes), that successfully allow to detect the low surface brightness structure of extended sources on ultra deep surveys. The developed procedures are applicable to HST, JWST, EUCLID and many other space and ground-based observatories. We made the final ABYSS WFC3 IR HUDF mosaics publicly available at http://www.iac.es/proyecto/abyss/.

Disc truncations are the closest feature to an edge that galaxies have, but the nature of this phenomenon is not yet understood. In this paper we explore the truncations in two nearby (D ∼15 Mpc) Milky Way-like galaxies: NGC 4565 and NGC 5907. We cover 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, we identify (at all wavelengths) the truncation 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 find the characteristic U shaped 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 in the mid-plane of the disc drops to just 0.1-0.2 % of the total stellar mass of the galaxies. The detection of the truncation at high altitude in combination with the U shape of the radial colour profile 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 these galaxies are growing inside-out, their growth rate is less than 0.6-1 kpc Gyr −1 .

We investigate three barred lenticular galaxies (NGC 2681, NGC 3945 and NGC 4371) which were previously reported to have complex central structures but without a detailed structural analysis of these galaxies' high-resolution data. We have therefore performed four-to six-component (pseudo-)bulge/disk/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 analysis. 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 10 M ⊙ . 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 done in the past. We measure emission line fluxes enclosed within 9 different elliptical apertures, finding that NGC 2681 has a LINER-type emission inside R ∼ 3 ′′ , 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 ′′ , 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 disk formation via gas accretion and bar-driven perturbations may account for the build-up of pseudo-bulges, bars, rings and point sources.