A comprehensive study of star formation processes in the circumnuclear environment of galaxies

Resumen

The inner zones of some galaxies showstar formation rates higher than the average observed in galactic discs, frequently arranged in ring structures with a diameter of about 1 kpc, named circumnuclear starforming regions (CNSFR). They are commonly referred to as hotspots and they appear in early spiral galaxies, many of them showing bars, with or without an active nucleus. They show large gas surface densities and are bright at ultraviolet wavelengths due to they containing hot and massive O and/or B stars. Due to the extreme conditions in terms of density and metallicity of their specific locations, they are excellent places to study how star formation proceeds in high metallicity and high-density environments. The main goal of this thesis is understanding the star formation processes in the young stellar clusters powering CNSFRs through observations of the ionised gas and the stellar emission in the optical range. In order to do so I have analysed two prototype galaxies,NGC7742 andNGC7469, using observations obtained with the Very Large Telescope (ESO/VLT) and Gran Telescopio CANARIAS (GTC) with 8.2 m and 10.4 m in diameter respectively. I have used integral field spectroscopy (IFS) from the Multi Unit Spectroscopic Explorer (MUSE) and Multi-Espectrógrafo en GTC de Alta Resolución para Astronomía (MEGARA) instruments. The first of the studied galaxies, NGC 7742, has been selected for having more than a hundred CNSFRs in a ring about 400 pc wide, and having a wealth of data obtained with different instrumentation at different wavelengths. The second galaxy, NGC 7469, has been selected due to the many studies performed on this galaxy, although lacking a study of the physical properties of its CNSFRs. One of the reasons for this is the high luminosity of the active nucleus producing saturation effects in the strongest emission lines, mainly H a. The first challenging issue which has been found in thiswork is the chemical abundance derivation in CNSFRs since the nebular auroral lines used to calculate the metallicity of HII regions by direct methods are intrinsically weak and the empirical calibrations commonly used for these regions are not reliable for higher abundances. Thus, I have selected high quality spectroscopic data from the literature divided in two samples, HII regions and HII galaxies, using them to establish a robust sulphur abundance calibration and analysing their dependencies with other properties of the ionised gas in these objects. I have applied this method to calculate chemical abundances in 88 CNSFRs observed in NGC 7742 together with other 158 HII regions outside its galaxy ring; and 28 CNSFRs observed in the two rings of NGC 7469, which probably constitute the largest CNSFR sample deeply studied. I have used the measured gas emission lines to derive H luminosities and rates of hydrogen emission of ionising photons; electron densities; ionisation parameters; predicted angular radius; filling factors, and masses of ionised hydrogen. Regarding the ionising cluster properties, I have measured the integrated r and i fluxes, the r-i colours, and the Balmer emission line equivalent widths. The relation between the last two properties, in conjunction with stellar population synthesis models, has been used to estimate the ages of the ionising clusters and the ionising and photometric cluster masses. The effective temperature of stars has also been estimated from the ratio of the numbers of ionising hydrogen and helium photons, using models of stellar atmospheres. Finally, I have derived the dynamical masses for each observed cluster, assuming a virialised system and using their measured sizes and stellar velocity dispersions obtained from the CaT absorption lines. The kinematical components of the gas and their origin and their comparison with those obtained for the stars are also presented for the first studied galaxy, NGC 7742. This work has required the development of two new analysis methodologies. The first one is the development of an alternative method to calculate the interestellar extinction of the ionised gas in the cases in which the strongest emission lines in the high surface brightness regions of some nebulae are saturated, not allowing the use of the common procedure of extinction derivation that makes use of observed HI Balmer recombination line ratios. A method to derive the logarithmic extinction coefficient at optical wavelengths using the emission lines of HeI has been developed and applied to observations obtained withMUSEfrom 30 Doradus, a nearby spatially resolved HII region in the Large Magellanic Cloud. The second method concerns the calculation of the velocity dispersion of stars using high spectral resolution data. We have revised and adapted the cross correlation technique for the spectral resolution of MEGARA, analysing the theoretical concepts and the assumptions underlying the procedure and estimating the best input parameters to be used and the velocity dispersion errors. The CNSFRs in NGC 7742, show chemical abundances between 0.25 and 2.4 times the solar value, consistent with the effective temperature calculated, between 35000Kand 40000K.Abundances in this high metallicity regime have not been derived by direct method up to now. Their measured sizes are compatible with those predicted by photoionisation models, thus these regions are radiation bounded and the photon escape fraction is negligible. Two different stellar populations seem to be present in these clusters, a young non-ionising stellar population and an ionising one with ages around 300 Ma and 4.7 Ma respectively and the ionised gas in the regions show a complex kinematics. Their dynamical masses are between 2.5x10^6 and 10.0x10^7 Md. A comparison of the gas and star velocity dispersion, seldom found in the literature, has been made revealing a complex internal structure for the gas. A scenario has been put forward suggesting that the observed clusters would have formed simultaneously in a first star formation episode whose high mass stars would have returned part of their gas to the interstellar medium during their evolution. A fraction of this gas could have left the cluster, while some of it would be unable to do so thus remaining trapped in the cluster and subject to the same gravitational potential as the cluster stars. The fraction of the gas that cooled down would form a new stellar population, responsible for the ionisation of the gas currently observed. This secondary young stellar population accounts for between 0.15 and 7.07 % of the total dynamical mass of the clusters. All the obtained results fits the minor merger scenario proposed by previous works. The CNSFRs in NGC 7469 show multiple kinematical components compatible with an outflow coming from the active galaxy nucleus and located at the North-East area of the galaxy, previously reported by other authors. On average the regions located in the outer ring, although being more luminous and massive than those of NGC 7742, seem to have similar characteristics to those in the ring of NGC 7742. In this case, their measured sizes can only be explained by the existence of an additional component, not present in the models, produced by the WR stars present in all the observed CNSFRs that seem to show a single ionising population with mean ages of 5.7 Ma for the outer ring and younger ages for the inner one. The dynamical masses of the inner ring regions are large, with a mean value of 1.07x10^9, which suggests that they may be survivors in the hostile environment powered by nuclear activity. For the outer ring this value is 6.58x10^8 Md. Some of the main original contributions of this thesis are: the development of a methodology to derive ionised gas chemical abundances in the high metallicity regime, up to 5 times the solar value; the detailed simultaneous spectroscopic study of a large number (88) of CNSFRs in a given galaxy; the development of a methodology to derive ionised gas extinctions using HeI emission lines to be used in the cases in which the H emission line is saturated; the detailed spectroscopic study of CNSFRs located within the central few handred pc of a luminous AGN galaxy; the accurate derivation of the dynamical masses of the star clusters powering CNSFR.