Star Formation Desert in GASP galaxies: IFU perspective

Galactic evolution is driven by the combined action of both internal and environmental processes. In particular, the presence or absence of star-formation significantly defines galaxies’ physical conditions and any suppression or enhancement of the SF itself must be regarded as the product of the interplay of the various physical mechanisms that concurrently mould the evolutionary history of galaxies. A peculiar case of heavily suppressed SF is represented by the ‘Star Formation Desert’ phenomenon, central regions devoid of HII-emission, which, to date, have been found only in a very small number of galaxies within the local Universe. SFDs have, moreover, never been analysed by means integral-field spectroscopy with high spatially resolved power outside the local Universe . In this thesis, a systematic analysis of all the SFD-galaxies within the GASP-survey has, thus, been performed by employing, for the first time, integral field spectroscopy obtained with MUSE. The analysis has focused on the spatially resolved emission-line maps, together with the distinct diagnostic tools, as BPT- and WHAN-diagrams, with the aim of characterising the galaxies properties and their underlying ionisation mechanisms. The results has evidence the presence, within the deserts, of emission from ‘Hot Evolved Low-Masses Stars’ (HOLMES), indicative of old stellar populations whose residual emission is still observable. SFHs within the galaxy central regions have, hence, been derived by means of SINOPSIS code with the purpose of investigating the differences between the galaxies with and without the SFDs, thus enabling their peculiarities to emerge. SFD-hosting galaxies have proven to be, systematically, defined by a common evolutionary pattern characterised by a really pronounced primordial burst of SF, plausibly explained by the formation of the today SFD-galaxies within environments richer in gas than the average. The, consequently, unusually rich gas reservoirs in galaxies in primordial epochs might have led to a substantial consumption of gas within the first burst of SF. According to the suggested scenarios, AGNs or, to a less extent, stellar bars might have, thereafter, affected the already gas-deficient central regions of these galaxies, eventually playing only a secondary role, if present, in exposing the underlying stellar populations, whose HOLMES emission has been found to be consistent with the primordial episode of SF. Bars, secular mechanisms within galaxies, could, in fact, not have formed and influenced the central regions within the adequately small timescales required in order to halter a further SF within the deserts. Considering this perspective, the often emphasised in literature role of bars as main responsible for the onset of SFDs has likely turns out to be flawed. In particular, the SFD phenomenon, still largely unexplained, has, hence, resulted to arise from a common condition of galaxies at the epoch of their formation, which may have later promoted the development of the deserts. Nevertheless, many issues linked to the intrinsic nature and origin of SFDs still need to be addressed. Moreover, the here proposed hypothesis demands verification on a larger sample, and future investigations will, thus, be necessary in order to confirm it robustly.