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A multi-frequency study of the star formation histories of galaxies in the integral field area survey CALIFA

AutorLópez Fernández, Rafael
DirectorGonzález Delgado, Rosa M. ; Cid Fernandes, Roberto
Palabras claveGalaxias
Galaxias: formación
Galaxias: evolución
Proyecto CALIFA
Fecha de publicación29-sep-2017
EditorUniversidad de Granada
CSIC - Instituto de Astrofísica de Andalucía (IAA)
ResumenThe Calar Alto Legacy Integral Field Area (CALIFA) survey is a large project of the Centro Astron omico Hispano Alem an at the Calar Alto observatory to obtain spatially resolved spectra for 600 local (0.005 < z < 0.03) galaxies by means of Integral Field Spectroscopy (IFS). CALIFA observations started in June 2010 with the Potsdam Multi Aperture Spectrograph (PMAS, Roth et al. 2005), mounted at the 3.5m telescope, employing the PPak wide-fi eld IFU Roth et al. 2005; Kelz et al. 2006. This project provides high quality spectra over a wide, hexagonal field of view of 7200 6400 (331 science fibres with a bre size of 2.77"), with a signal sampling of 1 arcsec per pixel and PSF of 2.5" (Garc a Benito et al. 2015). The IFS allows us to gather spectra of the sky over a two-dimensional field-of-view, obtaining a datacube with axes of x, y (the two spatial axes) and wavelength. IFS solves the main disadvantages of traditional long-slit and allows us to obtain high S/N spectra applying a suitable spatial binning. The IFS provides information to analyse the spatially resolved properties of galaxies, studying inner and outer parts, computing gradients, radial pro files and obtaining 2D images. Also we can analyse the integrated properties through a spectrum containing information from the whole galaxy.
The study in this thesis is based on the statistical analysis of the stellar population properties of galaxies, combining diff erent kinds of data. In particular we use Integral Field Spectroscopy (IFS) provided by CALIFA data and images in optical and UV range provided by SDSS and GALEX. The main goal is to obtain the cosmic evolution of the star formation and mass assembly history of galaxies using integrated and spatially resolved information of galaxies in the near Universe. The objects in this study are those for which CALIFA+GALEX+SDSS data are available. This is a sub-sample of 366 galaxies which is unbiased with respect to the CALIFA mother sample, including from E to late type spirals and with stellar masses from 10^9 to 10^11 solar masses . The CALIFA mother sample is not a purely volume-limited sample, but can be "volume-corrected". As the sub-sample is representative of the CALIFA mother sample, our results can be used to estimate the star formation rate density, the specfi c SFR, and the stellar mass density up to z > 2, and the contribution of central (<0.5 HLR) and outermost regions (1 < R < 2 HLR) in nearby galaxies to these fundamental observables in astrophysical cosmology. We have developed two di fferent methodologies to analyse the stellar population properties of galaxies, which use diff erent kinds of data.
The first one is a new version of the full spectral fi tting code starlight of Cid Fernandes et al. 2005, which in principle allows any combination of spectroscopy + photometry, although the actual application explored in this thesis focuses on the combined analysis of a 3700 - 7000 A optical spectrum and NUV ( 2274 A) plus FUV ( 1542 A) photometry from GALEX. Using this code, we fit the observed data with a combination of N SSP. The code returns the fractional contribution xj (j = 1,.....,N) of the SSP with age tj and metallicity Zj to the model. The second one is based on parametric SFHs, which allows any kind of parametrization. In this thesis nine di fferent models are used. The code returns the PDFs for the parameters that de fine the SFH by fitting a combination of UV+optical photometry and di fferent spectral features: H b, [MgFe]', and the 4000 A break index (D4000). Hb and D4000 are more sensitive to the stellar age while [MgFe]' is used as a metallicity tracer. D4000 and [MgFe]' are measured in the CALIFA spectrum and H b in the starlight synthetic spectrum. Previously to the application to real data, the codes have been tested through a set of simulations to gauge the performance of the codes under di fferent levels of signal-to-noise (S=N). The main specifi c conclusions are: 1) Purely optical spectral fits are poor predictors of the UV properties, having a tendency to overpredict the UV fluxes. Besides, the new optical+UV fi ts reduce the uncertainties in the derived stellar properties. Including UV photometry in the fi ts, we better constrain the contribution of stellar populations younger than 300 Myr. UV+optical fits tend to replace 30 Myr components by populations in the neighbourhood of 100 Myr. Despite their poor performance in predicting the UV fluxes the optical fits yield stellar population properties which agree with those obtained with UV+optical fits to within the expected uncertainties. The di fferences are exclusively found in low-mass, late-type galaxies, precisely the systems where, because of their signi ficant 300 Myr population, one would expect the addition of UV constraints to play a more relevant role. Also, the inclusion of UV constraints helps to mitigate degeneracies between age and metallicity. 2) With starlight no initial assumption on the SFR(t) is done, while with the parametric method we set a prior for the parameters that defi ne SFR(t). The main advantage with parametric models is that we can clearly explain the evolution of the star formation rate through an analytic function. Also the parameters of the model provide very useful information about star formation in galaxies. For example, in the delayed- model used in this thesis, t0 gives information about when the star formation begins and is the SFR e-folding time, which measures the time interval required to turn gas into stars. On the other hand, a troubled point is that for a statistical analysis of galaxies two di fferent paths are possible, which produce di fferent results: by averaging the mean parameters or by averaging the individual SFR(t), as we do with starlight. The second and main disadvantage is that if it is precisely the form of this function that we are trying to recover from the data we shall assume absolutely nothing about the SFR(t). 3) The comparison with cosmological observational results from literature indicates that a delayed-model (M1 in this thesis) provides the best results for describing the cosmic evolution of the star formation and mass assembly history of galaxies. The stellar population properties, stellar mass, extinction, mean stellar age and mean stellar metallicity, derived with this model are in agreement with those obtained with starlight to within the expected uncertainties.
4) A purely exponential profi le (M2) and the "Sandage" pro file (M3), the last one also being a delayed- model, provide similar results to those obtained with M1. However, they are not able to describe the evolution of the star formation rate density, the mass density and the speci fic star formation rate as well as M1 does. On the other hand, the rising models have been used to study high redshift galaxies, but our results indicate that they can be discarded as representative of the star formation history of nearby galaxies. The two component models used in this thesis provide better fits in terms of 2 as they introduce more parameters into the model, but they are worse than M1 for describing the evolution of the fundamental observables analysed in this thesis. The main specfii c results obtained with M1 are: 1) For Sa, Sb and Sbc we obtain t0 12 Gyr, which start to form earlier than E and S0, t0 = 10 Gyr. For Sc galaxies, t0 = 10 Gyr, similar to E galaxies, while Sd starts to form later on. We obtain that increases with the Hubble type, with more extended periods of star formation in late type spirals than early on. For E galaxies tau = 1.5 Gyr and for Sbc and later spirals tau = 4 Gyr. 2) The results agree with a "downsizing" scenario, in which more massive galaxies form at higher redshift. t80 decreases from 7.5 Gyr for massive galaxies to 5.5 Gyr for lower mass galaxies. The same scenario is obtained with starlight, although with a larger range of variation, decreasing t80 from 7.5 Gyr for massive galaxies to 4 Gyr for lower mass galaxies.
3) The analysis of the inner and outer regions suggest an inside-out formation scenario of galaxies, with t_in_0 > t_out_0 for all the Hubble types. Sbc and earlier type galaxies start to form at the same epoch 12 Gyr ago. Envelope of E and S0 start to form at later epoch than outer regions in Sa, Sb and Sbc. Sc and Sd galaxies start to form later than earlier types, but at t0 > 10Gyr. Their envelopes form later on. We find that t80 decreases with the morphology for the inner and outer regions, the values for the outer regions being lower than those for the inner ones for Sc and earlier types. For Sb and earlier types t80 = 7-9 Gyr for the inner regions and 6-7 Gyr for the outer ones. For Sbc, Sc and Sd galaxies we obtain t80 = 7, 5 and 4.5 Gyr for the inner regions, respectively, while for the outer ones t80 = 7, 5 and 4.5 Gyr. 4) The SFR density results indicate that the majority of the star formation at z = 0 takes place in late type spirals. We find that most of the star formation at z = 0 occurs outside galaxy centres, mainly in the disks of spirals, while for z > 1 it is dominated by the actual inner regions. Both structural components are competing in building the SFR density at z = 1. Our values are in agreement with those obtained by Fardal et al. 2007, although other cosmological results show a larger peak of SFR at z = 2. For z = 0, 2 and 5 we obtain log SFR density (M yr^-1 Mpc^-3) = -2.04, -1.34 and -1.4, respectively. 5) Mass density increases from z = 5 to z = 0. Our results are in agreement with those obtained from cosmological surveys, and in particular with Madau & Dickinson 2014 from z = 0 to z = 2. For z = 0, 2 and 5 we obtain log Mass density (M Mpc^-3) = 8.47, 7.97 and 7.35 , respectively. The analysis of Mass indicates that regions at 1 < R < 2 HLR have assembled their mass more slowly than inner regions and over a more extended period of time. We obtain that the central regions of galaxies have accreted most of their mass at z > 1. In terms of morphology, we obtain that E and S0 galaxies have assembled their mass in a shorter period of time than later types, Sc and Sd galaxies being those with a slower growth. 6) The sSFR declines as the universe evolves. This indicates that, on average, galaxies become progressively less e fficient at forming stars. Our estimation of sSFR agrees well with the cosmological surveys results, which find that sSFR decreases as (1 + z)^2 from z = 2 to z = 0 (Elbaz et al. 2011). However, for z > 2 our derived sSFR increases with a lower slope. For the di fferent Hubble types, we obtain similar curves for z > 1. For z < 1 the sSFR decreases with a di fferent slope, according to morphological type. For E and S0 the sSFR decreases reaching log sSFR (Gyr^-1) = -2.3 at z = 0, while for Sd galaxies we obtain log sSFR (Gyr^-1) = -0.5. The values at z = 0 correlate with morphology, decreasing from Sd to E galaxies. For the inner regions we obtain similar results to the whole galaxies, but for outer regions in E and S0 galaxies we obtain that sSFR increases at 0-2 < z < 1. This is in agreement with the two phase formation scenario for early type galaxies, where the central part formed most of its mass at high redshift and an outer envelope during a more extended period, in which galaxies grow in mass and signi ficantly in size through dry mergers.
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