Historia del contenido bariónico de las galaxias Enanas

Tesis (Doctor en Astronomía)--Univesidad Nacional de Córdoba. Facultad de Matemática, Astronomía y Física, 2015.Las galaxias enanas son los objetos extragalácticos más comunes en el Universo y cualquier teoría exitosa sobre la formación y evolución de galaxias debería ser capaz de predecir y explicar sus características principales. El modelo cosmológico aceptado actualmente, que asume un Universo de materia oscura fría dominado por una constante cosmológica (LCDM por sus siglas en ingles), ha sido capaz de reproducir muchos de los resultados observacionales de las estructuras en gran escala, pero tiene mayores dificultades intentado reproducir la formación de estructuras en escalas galácticas, debido probablemente a la complejidad de los procesos astrofísicos involucrados durante la formación y evolución de las galaxias. Para que este modelo siga siendo viable, debe, entre otras cosas, explicar el faltante de galaxias enanas comparado con simulaciones numéricas, el origen de su diversidad (tanto en contenido estelar como en contenido gaseoso), las diferencias sistemáticas entre sus poblaciones estelares, entre otras. En esta tesis exploramos distintos mecanismos que alteran la historia del contenido bariónico de las galaxias enanas formadas en un Universo LCDM. En particular, mostramos cómo las estructuras gaseosas en gran escala, tales como “panqueques” o “láminas”, afectan al contenido bariónico de las galaxias enanas. También estudiamos el impacto de la reionización cósmica en sus historias de formación estelar y el origen de las sistematicidades en las componentes estelares múltiples que ellas contienen

Determinación de parámetros fundamentales de cúmulos abiertos mediante espectroscopía integrada

Tesis (Lic. en Astronomía)--Universidad Nacional de Córdoba. Facultad de Matemática, Astronomía y Física, 2011.Los cúmulos abiertos constituyen excelentes laboratorios para delinear parámetros fundamentales de la Vía Láctea. Los más jóvenes son muy buenos trazadores de su estructura espiral en tanto que los viejos o de edades intermedias proveen información sobre la historia temprana del disco galáctico. En las últimas décadas, la determinación de los parámetros fundamentales de estos objetos, como ser el enrojecimiento, la edad y la metalicidad, se ha basado en diagramas color-magnitud y estudios fotométricos de estrellas individuales. Una técnica particularmente eficiente para determinar parámetros básicos de cúmulos abiertos de pequeño diámetros angulas es la espectroscopía. En este trabajo realizamos un análisis de la técnica de espectrosocopía integrada, lo que nos permite justificarla y utilizarla para analizar una muestra de cúmulos abiertos galácticos sin estudios previos, determinando sus enrojecimientos y edades. Asimismo, damos estimaciones de sus distancias. Presentamos también avances en cuanto a la sistematización y automatización de la técnica utilizando un código computacional desarrollado para tal fin.Alejandro Benítez Llamba

Simulations of the reionization of the clumpy intergalactic medium with a novel particle-based two-moment radiative transfer scheme

The progress of cosmic reionization depends on the presence of over-dense regions that act as photon sinks. Such sinks may slow down ionization fronts as compared to a uniform intergalactic medium (IGM) by increasing the clumping factor. We present simulations of reionization in a clumpy IGM resolving even the smallest sinks. The simulations use a novel, spatially adaptive and efficient radiative transfer implementation in the SWIFT SPH code, based on the two-moment method. We find that photon sinks can increase the clumping factor by a factor of ∼10 during the first ∼100 Myrs after the passage of an ionization front. After this time, the clumping factor decreases as the smaller sinks photoevaporate. Altogether, photon sinks increase the number of photons required to reionize the Universe by a factor of η ∼2, as compared to the homogeneous case. The value of η also depends on the emissivity of the ionizing sources

Dwarf Galaxies and the Cosmic Web

We use a cosmological simulation of the formation of the Local Group of Galaxies to identify a mechanism that enables the removal of baryons from low-mass halos without appealing to feedback or reionization. As the Local Group forms, matter bound to it develops a network of filaments and pancakes. This moving web of gas and dark matter drifts and sweeps a large volume, overtaking many halos in the process. The dark matter content of these halos is unaffected but their gas can be efficiently removed by ram-pressure. The loss of gas is especially pronounced in low-mass halos due to their lower binding energy and has a dramatic effect on the star formation history of affected systems. This "cosmic web stripping" may help to explain the scarcity of dwarf galaxies compared with the numerous low-mass halos expected in \Lambda CDM and the large diversity of star formation histories and morphologies characteristic of faint galaxies. Although our results are based on a single high-resolution simulation, it is likely that the hydrodynamical interaction of dwarf galaxies with the cosmic web is a crucial ingredient so far missing from galaxy formation models.Comment: Submitted to ApJL. 6 pages, 4 figures. A set of movies showing the interaction between dwarf galaxies and the Cosmic Web can be found at mirror 1 http://www.astro.uvic.ca/~mario/dwarf-web/ or at mirror 2 http://www.iate.oac.uncor.edu/~alejandro/dwarf-web/ . Comments are welcome

The detailed structure and the onset of galaxy formation in low-mass gaseous dark matter haloes

We present a model for the formation of the first galaxies before and after the reionization of hydrogen in the early universe. In this model, galaxy formation can only take place in dark matter haloes whose mass exceeds a redshift-dependent critical value, which, before reionization, is equal (in the simplest case) to the mass at which atomic hydrogen cooling becomes effective and, after reionization, is equal to the mass above which gas cannot remain in hydrostatic equilibrium. We define the Halo Occupation Fraction (HOF) as the fraction of haloes that host a luminous galaxy as a function of halo mass. The HOF is established by the interplay between the evolution of the critical mass and the assembly history of haloes and depends on three factors: the minimum halo mass for galaxy formation before reionization, the redshift of reionization, and the intensity of the (evolving) external photoheating rate. Our fiducial model predicts a cutoff in the galaxy mass function at a present-day halo mass, M200 ∼ 3 × 108M⊙; 100% occupation at M200 > 5 × 109M⊙; and a population of starless gaseous haloes of present-day mass in the range 106 ≲ M200/M⊙ ≲ 5 × 109, in which the gas is in thermal equilibrium with the ultraviolet background radiation and in hydrostatic equilibrium in the gravitational potential of the halo. The transition between HOF = 0 and HOF = 1 reflects the stochastic nature of halo mass growth. We explore how these characteristic masses vary with model assumptions and parameter values. The results of our model are in excellent agreement with cosmological hydrodynamic simulations of galaxy formation

The beginning and the end of star formation in faint field dwarf galaxies

We use the APOSTLE suite of cosmological simulations to examine the role of the cosmic ionizing background in regulating star formation (SF) in low-mass LCDM halos. In agreement with earlier work, we find that, after reionization, SF can only proceed in halos whose mass exceeds a redshift-dependent ``critical'' virial mass determined by the structure of LCDM halos and the thermal pressure of UV-heated gas. This critical mass increases from M_crit 10^8 Msun at z~10 to 10^9.7 Msun at z=0, roughly following the average mass growth history of halos in that mass range. This implies that most halos above or below critical at present have remained so since early times. The halos of most galaxies today were already above-critical (and thus forming stars) at high redshift, explaining the ubiquitous presence of ancient stellar populations in dwarfs, regardless of luminosity. Sub-critical halos may still host luminous galaxies if they were above-critical at some point in the past. SF ceases if a halo falls into the sub-critical regime; depending on each halo's accretion history this can occur over a wide range of times, explaining why SF in many dwarfs seems to continue well past the reionization epoch. The episodic nature of SF in some dwarfs, in this interpretation, would be linked to temporary halo excursions above and below the critical boundary. In the simulations, M_crit(z) cleanly separates star-forming from non-star-forming systems at all redshifts, indicating that the ionizing UV background, and not stellar feedback, is what regulates the end of SF in the faintest field dwarfs. Galaxies in sub-critical halos should make up a sizable population of faint field dwarfs, distinct from those in more massive halos because of their lack of ongoing star formation. Few such galaxies are known at present, the discovery of this population would provide strong support for our results.Comment: 12 pages, 12 figures. Submitted to MNRA

The vertical structure of gaseous galaxy discs in cold dark matter haloes

We study the vertical structure of polytropic centrifugally supported gaseous discs embedded in cold dark matter (CDM) haloes. At fixed radius, R, the shape of the vertical density profile depends weakly on whether the disc is self-gravitating (SG) or non-self-gravitating (NSG). The disc ‘characteristic’ thickness, zH, set by the midplane sound speed and circular velocity, zNSG = (cs/Vc)R, in the NSG case, and by the sound speed and surface density, zSG=c2s/GΣ zSG=cs2/GΣ , in SG discs, is smaller than zSG and zNSG. SG discs are typically Toomre unstable, NSG discs are stable. Exponential discs in CDM haloes with roughly flat circular velocity curves ‘flare’ outwards. Flares in mono abundance or coeval populations in galaxies like the Milky Way are thus not necessarily due to radial migration. For the polytropic equation of state of the Evolution and Assembly of GaLaxies and their Environments (EAGLE) simulations, discs that match observational constraints are NSG for Md < 3 × 109 M⊙ and SG at higher masses, if fully gaseous. We test these analytic results using a set of idealized smoothed particle hydrodynamic simulations and find excellent agreement. Our results clarify the role of the gravitational softening on the thickness of simulated discs, and on the onset of radial instabilities. EAGLE low-mass discs are NSG so the softening plays no role in their vertical structure. High-mass discs are expected to be SG and unstable, and may be artificially thickened and stabilized unless gravity is well resolved. Simulations with spatial resolution high enough to not compromise the vertical structure of a disc also resolve the onset of their instabilities, but the converse is not true