We find two chemically distinct populations separated relatively cleanly in the [Fe/H]-[Mg/Fe] plane, but also distinguished in other chemical planes, among metal-poor stars (primarily with metallicities [Fe/H] < -0.9) observed by the Apache Point Observatory Galactic Evolution Experiment (APOGEE) and analyzed for Data Release 13 (DR13) of the Sloan Digital Sky Survey. These two stellar populations show the most significant differences in their [X/Fe] ratios for the alpha-elements, C+N, Al, and Ni. In addition to these populations having differing chemistry, the low metallicity high-Mg population (which we denote "the HMg population") exhibits a significant net Galactic rotation, whereas the low-Mg population (or "the LMg population") has halo-like kinematics with little to no net rotation. Based on its properties, the origin of the LMg population is likely an accreted population of stars. The HMg population shows chemistry (and to an extent kinematics) similar to the thick disk, and is likely associated with in situ formation. The distinction between the LMg and HMg populations mimics the differences between the populations of low-and high-a halo stars found in previous studies, suggesting that these are samples of the same two populations.

Abstract The discs in galaxies are radially extended, rotationally supported, flattened systems. In the cosmological Lambda Cold Dark Matter model the formation of the discs is intimately connected with galaxy formation. Generally it is assumed that the discs have exponentially decreasing stellar surface brightness profiles, but completely satisfactory theoretical explanation for this has not been presented. Large number of studies in the past decade have challenged this view, and have found a change in the slope of the surface brightness profile in the outer regions of many galaxies discs: the surface brightness can decrease more, or less, steeply than in the inner regions. The transition between the two slopes is often called a disc break. Consequently, the discs are divided in three major categories: single exponential Type I, down-bending break Type II, and up-bending break Type III. Formation of these break features has been linked to the initial formation of the discs, internal evolution, and also with the interactions between galaxies. By studying the detailed properties of the disc break features, the evolutionary history of discs, and galaxies in general, can be better understood. The thesis work focuses on the structural analysis of the galaxies in the Spitzer Survey of Stellar Structure in Galaxies (S4G), which consists of 2352 galaxies observed in the 3.6 and 4.5 µm mid-infrared wavelengths with the Spitzer space telescope. Work has been carried out as a part of the data-analysis pipelines of the S4G survey, utilizing surface photometry. In addition, special emphasis has been put on the study of the disc and disc break properties in a wide range of galaxy morphological types and stellar masses. The thesis work attempts to at least partially understand how galaxy stellar mass and observed wavelength affect the properties of the discs and breaks, and how galaxy structural components are connected with the breaks. The data comprises mainly of the 3.6 µm infrared data, providing a view to the stellar mass distribution of galaxies. We find that the Type II breaks are the most common disc profile type, found in 45 ± 2% of the sample galaxies, consisting of 759 galaxies in the stellar mass range 8.5 ≲ log10(M*/M⊙) ≲ 11. Type I discs are found in 31 ± 2%, and the Type III breaks in 23 ± 2% of the sample. The fraction of the profile types also depends of the galaxy stellar mass: fractions of the Types II and III increase, while Type I fraction decreases, with increasing stellar mass. We attribute these changes with stellar mass to the increased frequency of bar resonance structures in higher mass galaxies, which are commonly associated with a Type II break, and to the increased fraction of Type III profiles in generally more massive early-type disc galaxies. In addition to the Type II breaks associated with bar resonance structures, we find that nearly half of these breaks relate to the visual spiral outer edge, confirming previous results of the Type II break connection with galaxy structure, and thus the internal evolution rather than initial formation of discs. Complementary data in optical wavelengths from the Sloan Digital Sky Survey shows a strong change in the properties of the discs inside the Type II breaks, indicating that the inner discs are evolving via star formation. In late-type spiral galaxies (T ≳ 4) with a Type II break, possible evidence of radial stellar migration is found in the outer disc: the slope of the surface brightness profile is shallower in the infrared compared to optical wavelengths, indicating that older stellar populations are more evenly spread throughout the disc. Formation of the Type I and III profiles remain poorly understood. However, indication that some of the Type III profiles are formed by environmentally driven processes is found, with a correlation between the properties of the local environment and the disc profile parameters. Furthermore, indication of star formation possibly causing the up-bends in spiral galaxies is found through a presence of young stellar population in the outer disc section.

>Magister Scientiae - MSc / We quantitatively examine the effects of merger and environment within a cosmological hydrodynamic simulation. We show that our simulation model broadly reproduces the observed scatter in H I at a given stellar mass as quantified by the HI mass function in bins of stellar mass, as well as the H I richness versus local galaxy density. The predicted H I fluctuations and environmental effects are roughly consistent with data, though some discrepancies are present at group scales. For satellite galaxies in & 1012Mhalos, the H I richness distribution is bimodal and drops towards the largest halo masses. The depletion rate of H I once a galaxy enters a more massive halo is more rapid at higher halo mass, in contrast to the specific star formation rate which shows much less variation in the attenuation rate versus halo mass. This suggests that, up to halo mass scales probed here (. 1014M), star formation is mainly attenuated by starvation, but H I is additionally removed by stripping once a hot gaseous halo is present. In low mass halos, the H I richness of satellites is independent of radius, while in high mass halos they become gas-poor towards the center, confirming the increasing strength of the stripping with halo mass. By tracking the progenitors of galaxies, we show that the gas fraction of satellite and central galaxiesdecreases from z =5 ! 0, tracking each other until z⇠1 after which the satellites’ H I content drops much more quickly, particularly for the highest halo masses. Mergers somewhat increase the H I richness and its scatter about the mean relation, but these variations are consistent with arising form inflow fluctuations, unlike in the case of star formation where mergers boost it above that expected from inflow fluctuations. In short, our simulations suggest that the H I content in galaxies is determined by their ability to accrete gas from their surroundings, with stripping effects playing a driving role once a hot gaseous halo is present.

I present Hubble Space Telescope photometry in optical (F475X, 475 nm) and near-infrared (F110W, 1.1 μm) bands of the globular cluster (GC) system of the inner haloes of a sample of 15 brightest cluster galaxies (BCGs). I also present a quantitative model of the relation between (F475X - F110W) colour and cluster metallicity, using simulated GCs. The sample comprises massive elliptical galaxies in a range of environments, from sparsely populated groups to dense clusters. Because the material available for large galaxies to accrete varies with environment and GC systems of such galaxies are built up through accretion, I expect the metallicity distribution of the GC systems in my sample to vary with galaxy environment. GC systems in massive elliptical galaxies tend to follow a bimodal colour distribution, with two subpopulations of blue (metal-poor) and red (metal-rich) clusters. The photometry is used to create a completeness-corrected metallicity histogram for each galaxy in my sample, and to fit a double Gaussian curve to each histogram in order to model the two subpopulations. Finally, the properties of the GC metallicity distribution are correlated against each BCG environment. I found that almost no GCS properties and host galaxy environmental properties are correlated, with the exception of weak but consistent correlations between number of GCs and nth-nearest neighbour surface density and between blue fraction and nth-nearest neighbour surface density. / Thesis / Master of Science (MSc)

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In this thesis I present an investigation into the presence, nature and origin of the thick disk component in late-type galaxies. I use ground-based wide-field observations to study two edge-on low-mass galaxies in the Local Universe: NGC 4244 and NGC 55. The large field-of-view of the ground-based data enables me to inspect the radial and vertical structure of each galaxy. The vertical profiles are studied up to larger distances fromthemid-plane than in any previous study and the presence of a second disk component beyond the thin disk with a larger scale height is revealed for both galaxies. The high-quality data allows me to carry out stellar population and metallicity studies for stars above and below the plane. Furthermore, direct comparisons with two simulated low-mass galaxies provided by the Preston group at the University of Central Lancashire are carried out. By putting the results for NGC 4244 and NGC 55 into context with the thick disk properties from these simulations and from the literature, the most likely thick disk formation scenarios can be pointed out. The bulgeless low-mass systemNGC 4244 lies at a distance of 4.4 Mpc and is studied using V- and I-band wide-field images taken with Suprime-Cam on the Subaru telescope, Hawaii. The extra-planar regions of NGC 4244 show the presence of a large population of Red Giant Branch (RGB) stars and some Asymptotic Giant Branch (AGB) stars. The best strategy to study the presence and structure of a thick disk component is to use the vertical diffuse light profiles in the crowded central regions and RGB star counts in the sparser and sky background dominated outskirts. The profiles show evidence for the presence of a sparsely populated second structural component beyond ∼ 2 kpc above and below the mid-plane. The profiles are fitted with a twodisk model, where each disk is approximated by an isothermal, self-gravitating sheet. A Bayesian model comparison confirms the need for a second disk component in the profile fit. Furthermore, the AGB profiles are inspected and are found to have a lower scale height than the RGB profiles. Metallicity studies of the RGB population in the thick disk component reveal that the metallicity is much lower than the solar metal-licity. Last, the scale length of the thin disk is quantified from the diffuse light radial profiles. At a distance of 1.9 Mpc the Magellanic type low-mass galaxy NGC 55 is studied using V- and I-band images from the VIsible MultiObject Spectrograph (VIMOS) in imaging mode on the Very Large Telescope (VLT, UT3) on Cerro Paranal, Chile. The very central regions of the galaxy are not covered by the VIMOS pointings so I use additional images from the Curtis-Schmidt telescope on Cerro Tololo. As for NGC 4244, I find that the RGB star count profiles extend to larger scale heights than the AGB profiles. The combined diffuse light + RGB profiles show evidence for a very prominent second disk component beyond ∼ 2 kpc above and below the plane. The metallicity studies of the RGB population show, that there is no trend in themetallicity with height above or below the plane. As for NGC 4244 the metallicity is significantly lower than the solar metallicity. Furthermore, the scale length of the thin and thick disk is derived from the radial profiles. The properties of thick disks in galaxies of all masses is studied by compiling the results fromobserved and simulated galaxies in the literature in addition to the results for NGC 4244, NGC 55 and the two simulated low-mass galaxies provided by the Preston group. By studying the similarities, differences and global trends with mass in the thick disk properties it is possible to confront the thick disk formation models. I find that none of the formation scenarios can be ruled out and even a hybrid scenario is possible.

Nós realizamos um estudo estatístico do comportamento de perfis de cor em bandas largas (UBV) para 257 galáxias espirais do tipo Sbc, ordinárias e barradas, utilizando dados obtidos através de fotometria fotoelétrica de abertura, disponíveis na literatura (Longo & de Vaucouleurs 1983,1985). Nós determinamos os gradientes de cor (B-V) e (U-B) para as galáxias da amostra total, bem como os índices de cor (B-V) e (U-B) de bojos e discos separadamente, utilizando métodos estatísticos robustos. Utilizamos uma técnica de decomposição bi-dimensional para modelar os perfis de brilho de bojos e discos em imagens dos arquivos do ``Digitised Sky Survey' (DSS), obtendo parâmetros estruturais característicos para 39 galáxias. A aquisição de imagens de 14 galáxias no Laboratório Nacional de Astrofísica permitiu-nos realizar um estudo fotométrico comparativo, e atestar a validade dos resultados obtidos neste estudo. Entre os principais resultados obtidos, destacam-se: (i) - 65% das galáxias possuem gradientes de cor negativos (mais vermelhos no centro), 25% possuem gradientes nulos, e 10% apresentam gradientes positivos; (ii) - galáxias que apresentam gradientes de cor nulos tendem a ser barradas; (iii) - os índices de cor ao longo das galáxias com gradientes nulos são similares aos índices de cor dos discos das galáxias com gradientes negativos; (iv) - confirmamos a correlação entre os índices de cor de bojos e discos, já obtida por outros autores; (v) - a ausência de correlação entre os gradientes de cor e de metalicidade sugere que o excesso de galáxias barradas com gradientes de cor nulos ou positivos reflete uma diferença no comportamento da idade média da população estelar ao longo de galáxias barradas e ordinárias; (vi) - galáxias com gradientes de cor nulos ou positivos têm uma leve tendência a apresentar bojos maiores e com maior concentração central de luz; e (vii) - confirmamos a correlação entre as escalas de comprimento de bojos e discos, já obtida por outros autores. Estes resultados são compatíveis e favoráveis ao cenário de evolução secular, no qual barras produzem fluxos radiais de massa para as regiões centrais de galáxias, não somente homogeneizando as populações estelares ao longo de galáxias, produzindo discos e bojos com índices de cor semelhantes, mas também contribuindo para a formação e/ou construção de bojos. / We have done a statistical study of the behaviour of the broadband color profiles (UBV) for 257 Sbc galaxies, barred and unbarred, collecting data obtained through photoeletric aperture photometry, available in the literature (Longo & de Vaucouleurs 1983,1985). We have determined (B-V) and (U-B) color gradients for the total sample of galaxies, as well as (B-V) and (U-B) color indices of bulges and disks separately, using robust statistical methods. Applying a bi-dimensional decomposition technique to model the brightness profiles of bulges and disks in images from the Digitised Sky Survey (DSS), we obtained characteristic structural parameters for 39 galaxies. The acquisition of images for 14 galaxies in the Laboratório Nacional de Astrofísica (Astrophysics National Laboratory) allowed us to do a comparative photometric study, and verify the validity of the results obtained in this work. Among the main results obtained, we point out: (i) - 65% of the galaxies have negative color gradients (reddish inward), 25% have zero gradients, and 10% show positive gradients; (ii) - galaxies that show zero color gradients tend to be barred; (iii) - the color indices along the galaxies with zero color gradients are similar to the color indices of the disks of the galaxies with negative color gradients; (iv) - we confirm the correlation between the color indices of bulges and disks, already found by other authors; (v) - the absence of correlation between color and metallicity gradients suggests that the excess of barred galaxies with zero or positive color gradients reflects a difference in the behaviour of the mean age of the stellar population along barred and unbarred galaxies; (vi) - galaxies with zero or positive color gradients show a slight tendency of having larger bulges, with a greater central concentration of light; and (vii) - we confirm the correlation between the scale lenghts of bulges and disks, already found by other authors. These results are compatible and favourable to the secular evolutionary scenario, in which stellar bars induce radial mass fluxes to the central regions of galaxies, not only turning homogeneous the stellar populations along the galaxies, producing disks and bulges with similar color indices, but also contributing to the formation and/or building of galactic bulges.

Nesta tese, investigamos a discrepância existente entre o número de galáxias satéli-tes da Via Láctea que é previsto e aquele que é observado, questão conhecida como ``problema da falta de satélites\'\' (PFS). Este problema pode ser reformulado em termos de um desacordo entre a função de luminosidades das galáxias satélites (FLS) que é estimada a partir de dados observacionais e a FLS predita por modelos numéricos de formação de galáxias. Nós revisamos tanto propriedades observacionais da população de satélites quanto a teoria associada à modelagem da formação de galáxias e estruturas. Para abordar o PFS, estudamos diferentes soluções possíveis. Nós desenvolvemos uma modificação simples ao potencial do inflaton usual e mostramos que esta leva a uma redução no número de halos de matéria escura de pequena massa. Nós usamos, então, um modelo semi-analítico de formação de galáxias para confirmar que supressões similares do espectro de potências de pequena escala produzem uma FLS com a forma correta. Em uma outra direção, nós discutimos outros mecanismos astrofísicos capazes de reduzir o número de galáxias pequenas, especificamente: os ventos gerados por explosões de supernovas e o aquecimento do meio intergalático durante a reionização do Universo. Finalmente, nós estudamos como um campo magnético primordial pode influenciar a formação de galáxias de pequena massa. Para isso, nós inicialmente mostramos que a pressão devida a um campo magnético leva a uma alteração significativa na massa de filtragem, levando a uma importante supressão na acresção de gás por galáxias de baixa massa. Introduzindo estas modifi-cações em um modelo numérico de formação de galáxias, mostramos que, para valo-res realistas de intensidade de campo, a pressão devido ao campo magnético leva a um bom acordo entre a FLS prevista e observada. / In this thesis we investigate the discrepancy between the predicted and observed number of satellite galaxies in the Milky Way, known as ``the missing satellites problem\'\' (MSP). This problem can be translated into the disagreement between the satellite luminosity function (SLF), which is estimated from the observational (particularly the SDSS) data and the SLF predicted by numerical models of galaxy formation. We review both the observational properties of the satellite population and the essentials of galaxy and structure formation modelling. To tackle the MSP, we study different possible solutions. We develop a small modification to the usual chaotic inflaton potential and show that it leads to a reduction in the number of small mass haloes. We use a semi-analytic model of galaxy formation to confirm that suppressions of the small scale power spectrum can produce a SLF with the correct shape. In a different direction, we discuss other astrophysical mechanisms that can reduce the number of small mass galaxies, namely: the outflows generated by supernovae explosions and the heating of the intergalactic medium during the reionization of the Universe. Finally, we study how a primordial magnetic field can influence the formation of small mass galaxies. We first find that small primordial magnetic field significantly change the filtering mass, leading to an important suppression in the gas accretion by small mass haloes. ( The filtering mass is the mass for which the baryon accretion is reduced to approximately 1/2 its normal value.) Introducing these modifications in the galaxy formation model, we show that for realistic field strengths, the pressure due to the magnetic field can result in a good match between the observed SLF and the model predictions.