RT info:eu-repo/semantics/doctoralThesis
T1 The star formation history in the local group
A1 Fusco, Federica
K1 Formación de galaxias
K1 Astrofísica
AB In this thesis, I focused on the study of the dwarf galaxies in the LocalGroup (LG). Since they can be resolved into stars, this group of galaxiesoffers a perfect laboratory in addressing the problems still open in galaxyformation and evolution. In addition, the LG is composed of a unique mixture of all the morphological types of dwarfs, namely dwarf irregulars (dIrr),spheroidals (dSph), and the recently discovered ultra faints (UFD). The starformation histories (SFH) of these systems are fundamental to obtain deeperinsights into their stellar populations, and especially on the presence of radialgradients.The sample of galaxies analysed in the present study is composed of adIrr, two dSphs, and a UFD. These are NGC 6822, Draco, Ursa Minor (UMi),and Hercules, respectively.In all the cases the photometry was obtained using DAOPHOT/ ALLSTAR/ ALLFRAME (Stetson 1987, 1994), which resulted in the derivation ofvery accurate colour-magnitude diagrams (CMD). The study of NGC 6822is based on HST data, whereas in all the other cases Subaru data havebeen used. In addition, in the case of NGC 6822 and Draco the accuratestar formation history was derived using the IAC method composed of IACstar/ IAC-pop/ MinnIAC (Aparicio & Gallart 2004; Aparicio & Hidalgo 2009;Hidalgo et al. 2011).The data sample of NGC 6822 is divided into six fields, which cover thewhole bar of this dwarf galaxy. The SFH solutions we derived show an enhanced star formation rate (SFR) in Fields 1 and 3 during the past 500 Myr,whereas the SFRs in the other fields are almost extinguished at very recent epochs. In addition, I studied the radial gradients of the populations by considering the total mass converted into stars in two time intervals (between 0and 0.5 Gyr ago and between 0.5 and 13.5 Gyr ago). The scale lengths of theyoung and intermediate-to-old populations result perfectly compatible, withthe exception of the young populations in Fields 1 and 3. The recent SFR inthese two fields is greater than in the other ones. This might be an indicationthat in these two fields we are sampling incipient spiral arms. In addition,the age-metallicity relations were derived. As expected, the metallicity increases with time for all of the fields. No radial gradient in the metallicity isobserved.The data available for the dSph Draco are composed of five fields, whichcover as far out as 3 core radii. The SFH of Draco is characterized by aunique burst which took place ∼ 12.9 Gyr ago, with a width of ∼ 0.5 Gyr.This is compatible with a quenching of the SF activity due to the effect ofreionization, even though a signature in the spectroscopic measurement ofthe metallicity rises doubts about this interpretation. In addition, I studiedthe presence of radial gradients in Draco. I calculated the scale lengthsof different evolutionary features of this galaxy. All the scale lengths arecompatible, hence there is evidence that Draco is composed of a uniquestellar population.For UMi the SFH was not obtained, because of a significant differencebetween the predictions of the evolutionary models and the observed CMD.A real peculiarity in this galaxy is excluded if archival HST photometry isconsidered, since it agrees with the models. Hence I requested two additionalsets of data, obtained with the IAC80 and NOT telescopes. The first datasetwas used in order to verify the calibration equations obtained. The secondone was used to investigate if a non linearity between the NOT and Subaruphotometries was present. None of the two data sets were useful in solvingthe discrepancy, hence I performed a study of the populations in this galaxybased on star counts. In particular, I compared the width of the observedsub-giant branch with that predicted by stellar evolutionary models. Thiscomparison sheds light on the duration of the SF burst in UMi, which results to be between 0.15 and 2 Gyr. Moreover, I investigated the radial gradientsin this dSph. As in the previous cases I derived the scale lengths for differentevolutionary features in this galaxy, which result compatible. Hence, UMiappears to be dominated by only one stellar population.The photometry of Hercules resulted to be too shallow to derive the accurate SFH, therefore, I compared it to a set of GGC and isochrones belongingto the BaSTI library (Pietrinferni et al. 2004). This comparison disclosesthat this galaxy hosts a population of age ≥ 12 Gyr and of very low metalcontent.
YR 2016
FD 2016
LK http://riull.ull.es/xmlui/handle/915/25985
UL http://riull.ull.es/xmlui/handle/915/25985
LA en
DS Repositorio institucional de la Universidad de La Laguna
RD 18-abr-2024