Integral Field Spectroscopy of (U) LIRGs and Post-Starburst QSOs: the role of mergers in galaxy evolution

Abstract

In this thesis we have characterized and compared the star formation histories, average stellar population properties and ionized gas properties in two small samples of galaxies in different stages across the merger sequence, three LIRGs (the two pre-mergers IC 1623 and NGC 6090 and the merger NGC 2623) and nine PSQSOs, by analysing high quality Integral Field Spectroscopy (IFS) data in the rest-frame optical range 3700 - 7000 °A, and high resolution HST imaging. Additionally, for NGC 2623 we have narrow band imaging in Hα and [NII]λ6583 from OSIRIS@GTC Tunable Filters, that allow us to study the outer parts of the tidal tails. The results from the LIRGs and PSQSOs have been compared with control Sbc and Sc galaxies from CALIFA survey. The methodology applied has been the same for all, a full spectral fitting analysis was performed using the Starlight code with a combination of single stellar population (SSP) models from the literature. With our data we find a evolutionary sequence related to the merger progression that is in agreement with recent simulations. In the initial stages (pre-mergers) the induced star formation (SF) is extended, and enhanced, on average, by a factor ∼ 4 with respect to the control spirals. When we resolve it spatially we find that for IC 1623 W the enhancement is the same in the central region and in the disk (by a factor 7), while for NGC 6090 NE the enhancement is higher in the central region (by a factor 9), still significant at one half light radius (by a factor 5) and less significant in the ”disk” (only enhanced by a factor 1.5). Attending to the importance of stellar populations ∼ 300 Myr, we find that the merger-induced star formation started earlier in IC 1623 W than in NGC 6090 NE. In more advanced mergers, as NGC 2623, we find that most of the young SF is concentrated in the central region, enhanced by a factor 9 with respect to control spirals. However, there exists also lower level of star formation in the outer parts, enhanced by . 3 in comparison to spirals. From the global average across the whole galaxy we find than is a factor ∼ 3 higher than in spirals. In addition, in NGC 2623 we detect fossil emission of an extended merger-induced burst occurred ∼ 1.5 Gyr ago, probably when it was at the pre-merger stage. The mass formed during this first burst is enhanced by a factor 2 both in the center and in the outer parts with respect to the mass formed in the same period in isolated spirals. From the average of the nine PSQSOs we find that, both in terms of light and mass, the SFHs are comparable to NGC 2623 ones. Attending to the average ages we found that they are ∼ 400 Myr older than NGC 2623. It seems that PSQSOs are slightly more evolved, however, given the uncertainties related to the model base choice, and the heterogeneity of the sample, we can not confirm exactly to what degree. We note, however, that both present a significant contribution to mass of stellar populations younger than 1.5 Gyr, which is not present in the pre-merger LIRGs. However, we think that the current starburst seen in the pre-mergers is in fact forming this intermediate age mass that will look like NGC 2623 in . 1.5 Gyr. The stellar mass in SSPs younger than 1.5 Gyr is ∼ 6.6 × 109 M⊙ (Chabrier IMF) in NGC 2623 and ∼ 1010 M⊙ in the PSQSOs, hence, we find that if the pre-merger LIRGs keep forming stars at the current rate ∼ 16 M⊙ yr−1 during ∼ 400 - 600 Myr, then they can account for the mass in NGC 2623 and PSQSOs. This time is approximately consistent with the 500 Myr upper limit expected for the duration of merger triggered starbursts from numerical simulations of mergers. With respect to the evolution from NGC 2623 to PSQSO, we note that in few 100 Myr molecular outflows like the ones in some U/LIRGs would be able to remove dust from the core leaving the AGN uncovered. This is in consistent with the average time delay of ∼ 400 Myr found by us between NGC 2623 and the PSQSOs. Finally, given the stellar masses measured in the LIRGs and in the PSQSOs, most of them . 1011 M⊙, we find that they will form ellipticals of intermediate mass (∼ 1011 M⊙), or the core of future giant ellipticals, in agreement with the major-merger evolutionary scenario.