The KLEVER Survey: spatially resolved metallicity maps and gradients in a sample of 1.2 < z < 2.5 lensed galaxies

We present near-infrared observations of 42 gravitationally lensed galaxies obtained in the framework of the KMOS Lensed Emission Lines and VElocity Review (KLEVER) Survey, a programme aimed at investigating the spatially resolved properties of the ionized gas in 1.2 3σ) ‘inverted’ gradients are also found, showing an anticorrelation between metallicity and star formation rate density on local scales, possibly suggesting recent episodes of pristine gas accretion or strong radial flows in place. Nevertheless, the individual metallicity maps are characterized by a variety of different morphologies, with flat radial gradients sometimes hiding non-axisymmetric variations on kpc scales, which are washed out by azimuthal averages, especially in interacting systems or in those undergoing local episodes of recent star formation

The chemical evolution of galaxies explored through multi-object integral field spectroscopy

Galaxies are expected to grow and evolve via a series of physical processes relating to gas flows into and out of the galaxy. Inflows of gas from the surrounding cosmic web provide fuel for star formation, which subsequently causes an enrichment of the interstellar medium (ISM) with the metals produced within stars, whilst supernovae-driven outflows drive gas out of the galaxy, re-distributing metals in the process. In this way, measurements of chemical abundances within galaxies can provide insight into the different physical processes that drive galaxy evolution. The interplay between these different processes has been well studied in the local Universe by large spectroscopic surveys that have established a number of scaling relations between stellar mass, star formation rate and gas-phase metallicity. However, the existence of such relations at earlier times in the Universe is less well studied. The aim of this thesis is to investigate the evolution of chemical abundances within galaxies across cosmic time, making use of integral field spectroscopic data obtained through the KLEVER survey. In the first part of this thesis, I compare the galaxy-integrated properties of galaxies at z=2 to those found in local galaxies, with a particular focus of the abundance of nitrogen relative to oxygen (N/O). I find that high redshift galaxies have similar N/O values to local galaxies at a fixed metallicity, but much lower N/O values than local galaxies at a fixed stellar mass. I then demonstrate that an anti-correlation exists locally between N/O and star formation rate, such that at a fixed stellar mass galaxies with higher star formation rates have lower N/O values. In light of this, I parameterise a three-dimensional relationship between stellar mass, star formation rate and N/O abundance, before demonstrating that this relationship accurately predicts the N/O ratios of galaxies at z=2 as well as those observed locally. As such, I name this relationship the fundamental nitrogen relation (FNR), in analogy to the fundamental metallicity relation (FMR). Furthermore, I show that the measured FNR is well described by a simple combination of the FMR and a non-evolving relationship between N/O and metallicity. These results suggest that the physical processes that govern the FMR must be sensitive not only to the metallicity, but also the N/O abundance. In the second part of this thesis I extend my analysis to the spatially resolved scale, studying the spatial distribution of N/O in galaxies at z=2. I present some of the first measurements of N/O gradients at z=2, finding they are generally flatter than those found locally. This is contrary to inside-out growth models, which predict steeper gradients at earlier times, however this difference may be reconciled by invoking star-formation driven feedback mechanisms that effectively mix metals within the ISM. I present observations of inverted N/O gradients, which I suggest may be a consequence of the inverted metallicity gradients also observed at high redshift. I also present evidence for negative Balmer decrement gradients within z=2 galaxies, consistent with high levels of star formation in the galaxy centre that may be associated with early bulge formation. I note that the slope of the N/O gradients is dependent on the choice of diagnostic used to determine the N/O, suggesting this may be driven by differences in the ionisation properties of sulphur relative to oxygen. Finally, in the third part of this thesis I present preliminary work analysing the scatter in the relationship between N/O and O/H for local galaxies. I present observations of a population of galaxies with low metallicities that have enhanced N/O abundances. I show that the galaxies with the highest N/O values also have higher stellar masses and star formation rates. I then investigate the possibility that these galaxies have undergone recent gas accretion, driving changes in their metallicities and N/O values whilst boosting their star formation. I compare to a simple gas mixing model, finding that the deviations of galaxies from their expected metallicities and N/O values can be well modelled by the accretion of metal rich gas with a metallicity equal to 55% of that of the galaxy. However, the models also predict that the gas fraction within the galaxy is expected to increase by between 0.64-1 dex during the accretion event, much larger than the changes in gas fraction inferred from the observed deviations from the star forming main sequence for local galaxies. I demonstrate that the expected changes in gas fraction are better matched by accretion of lower metallicity gas, however such models are unable to reproduce the observed decrease in N/O from the expected values. I conclude that improved models are needed that include prescriptions for star formation, chemical enrichment and gas outflows in order to better constrain the impact of dilution events on the N/O values and metallicities within galaxies

Being KLEVER at cosmic noon: ionised gas outflows are inconspicuous in low-mass star-forming galaxies but prominent in massive AGN hosts

We investigate the presence of ionised gas outflows in a sample of 141main-sequence star-forming galaxies at 1.2<2.6 from the KLEVER (KMOS LensedEmission Lines and VElocity Review) survey. Our sample covers an exceptionallywide range of stellar masses, 8.1<\log(M_\star/M_{\odot})<11.3, pushingoutflow studies into the dwarf regime thanks to gravitationally lensed objects.We stack optical rest-frame emission lines (H$\beta$, [OIII], H$\alpha$ and[NII]) in different mass bins and seek for tracers of gas outflows by using anovel, physically motivated method that improves over the widely used,simplistic double Gaussian fitting. We compare the observed emission lines withthe expectations from a rotating disc (disc+bulge for the most massivegalaxies) model, whereby significant deviations are interpreted as a signatureof outflows. We find clear evidence for outflows in the most massive,\log(M_\star/M_{\odot}) >10.8, AGN-dominated galaxies, suggesting that AGNsmay be the primary drivers of these gas flows. Surprisingly, at$\log(M_\star/M_{\odot})\leq 9.6$, the observed line profiles are fullyconsistent with a rotating disc model, indicating that ionised gas outflows indwarf galaxies might play a negligible role even during the peak of cosmicstar-formation activity. Finally, we find that the observed mass loading factorscales with stellar mass as expected from the TNG50 cosmological simulation,but the ionised gas mass accounts for only 2$\%$ of the predicted value. Thissuggests that either the bulk of the outflowing mass is in other gaseous phasesor the current feedback models implemented in cosmological simulations need tobe revised

The KLEVER survey: nitrogen abundances at z ∼ 2 and probing the existence of a fundamental nitrogen relation

We present a comparison of the nitrogen-to-oxygen ratio (N/O) in 37 high-redshift galaxies at z ∼ 2 taken from the KMOS Lensed Emission Lines and VElocity Review (KLEVER) Survey with a comparison sample of local galaxies, taken from the Sloan Digital Sky Survey (SDSS). The KLEVER sample shows only a mild enrichment in N/O of +0.1 dex when compared to local galaxies at a given gas-phase metallicity (O/H), but shows a depletion in N/O of −0.35 dex when compared at a fixed stellar mass (M*). We find a strong anticorrelation in local galaxies between N/O and SFR in the M*–N/O plane, similar to the anticorrelation between O/H and SFR found in the mass–metallicity relation (MZR). We use this anticorrelation to construct a fundamental nitrogen relation (FNR), analogous to the fundamental metallicity relation (FMR). We find that KLEVER galaxies are consistent with both the FMR and the FNR. This suggests that the depletion of N/O in high-z galaxies when considered at a fixed M* is driven by the redshift evolution of the mass–metallicity relation in combination with a near redshift-invariant N/O–O/H relation. Furthermore, the existence of an fundamental nitrogen relation suggests that the mechanisms governing the fundamental metallicity relation must be probed by not only O/H, but also N/O, suggesting pure-pristine gas inflows are not the primary driver of the FMR, and other properties such as variations in galaxy age and star formation efficiency must be important

The KLEVER survey: Nitrogen abundances at z ∼ 2 and probing the existence of a fundamental nitrogen relation

We present a comparison of the nitrogen-to-oxygen ratio (N/O) in 37high-redshift galaxies at $z\sim$2 taken from the KMOS Lensed Emission Linesand VElocity Review (KLEVER) Survey with a comparison sample of local galaxies,taken from the Sloan Digital Sky Survey (SDSS). The KLEVER sample shows only amild enrichment in N/O of $+$0.1 dex when compared to local galaxies at a givengas-phase metallicity (O/H), but shows a depletion in N/O of $-$0.36 dex whencompared at a fixed stellar mass (M$_*$). We find a strong anti-correlation inlocal galaxies between N/O and SFR in the M$_*$-N/O plane, similar to theanti-correlation between O/H and SFR found in the mass-metallicity relation(MZR). We use this anti-correlation to construct a fundamental nitrogenrelation (FNR), analogous to the fundamental metallicity relation (FMR). Wefind that KLEVER galaxies are consistent with both the FMR and the FNR. Thissuggests that the depletion of N/O in high-$z$ galaxies when considered at afixed M$_*$ is driven by the redshift-evolution of the mass-metallicityrelation in combination with a near redshift-invariant N/O-O/H relation.Furthermore, the existence of an fundamental nitrogen relation suggests thatthe mechanisms governing the fundamental metallicity relation must be probed bynot only O/H, but also N/O, suggesting pure-pristine gas inflows are not theprimary driver of the FMR, and other properties such as variations in galaxyage and star formation efficiency must be important