Found 219 talks archived in Galaxies
Abstract
Long gamma-ray bursts are supposed to be connected to the death of very massive stars. Due to their brightness, we can detect them to much larger distances than supernovae. Using them as powerful lightsources, they allow us to study star-forming high redshift galaxies and their interstellar medium in great detail with medium and high resolution spectroscopy. Despite the large redshift ranged spanned by GRBs, there is surprisingly little evolution in the properties of their host galaxies which might indicate that GRBs can only occur under certain conditions. This can be investigated from a few bursts at very low redshifts where we can resolve their host galaxies e.g. with integral field spectroscopy. The immediate surroundings might allow us some conclusions on the progenitors of GRBs.
Abstract
Observations have shown that massive galaxies at high redshift have much smaller sizes than galaxies of similar mass today. The mean stellar density of such objects was almost two orders of magnitude higher than the ones we measured in the most massive nearby galaxies, reaching, in some cases, densities similar to those observed in globular clusters. What is the nature of these objects? And, how these objects have been transformed into the present population of massive galaxies? We will summarize the recent findings our group has done on this topic. In particular, we will focus on our search for finding relics of these compact galaxies in the nearby universe, and the effort we have done for measuring the evolution of the velocity dispersion of these galaxies in the last 10 Gyr. The implications of this research within the galaxy formation scenario will be discussed.Abstract
In this work we have tried to verify what types of bulges are the descendants and the precursors of the bulges with blue colors observed at intermediate z. These may be the result of intense star formation in the central regions of the disks, related to the phenomenon of pseudo-bulges in the local universe or, alternatively, they may result from rejuvenation processes of old and red classical bulges, formed at high z, perhaps through secular evolution produced by internal or external agents. We can identify and distinguish between these processes analyzing the central surface brightness of the galaxies, μ0. For the general bulge population in the local universe, color is strongly correlated with surface density, in the sense that redder bulges are denser. Classical bulges and pseudo-bulges occupy different regions in a color-μ0 diagram. We have studied the redshift evolution of the relation between the colors and the central surface brightness for samples of spiral galaxy bulges selected from HST/ACS GOODS-N survey, and have analysed the ability of color-μ0 diagram to segregate different types of bulges at z ~ 1. The results show that, up to z ~ 0.8, galaxy nuclear and global colors are strongly correlated with the central surface brightness and, therefore, with the central surface density, in the sense that denser bulges are redder. This suggests that these formed the bulk of their stars at earlier epochs than less dense bulges. For z > 0.8, we find an important fraction of galaxies with high central surface brightness and nuclear colors much bluer than the rest of the galaxy, which probably corresponds to episodes of strong nuclear star formation that may result in the growth of the bulges inside the disks. From simple evolution models we can infer that these nuclei with star formation evolve towards the formation/growth of moderate central surface brightness, intermediate color z ~ 0 pseudobulges rather than classical bulges. These models also argue against rejuvenation processes for z ~ 1 dense and old bulges.
Abstract
Large-scale outflows from galaxies are a crucially important yet poorly understood aspect of galaxy evolution. They redistribute gas and metals into the IGM, regulate star formation, affect the galaxy luminosity function and mass-metallicity relation, etc. Unfortunately, their detailed context in galaxy evolution is difficult to understand: locally, they are identified and studied in heterogeneous manners, while we have only recently begun to study them on cosmological scales and then only in known bright, starbursting galaxies. I will discuss increasing evidence that the so-called ultra-strong MgII intervening quasar absorbers select galactic superwinds over a large range of redshift in a manner independent of luminosity. As superwinds cover a small fraction of the sky at any epoch, only with recent huge quasar absorption lines surveys has it been possible to identify significant numbers of outflows in this manner. I will present new results from several of our studies -- including the measurement of the average SFR of their hosts using [O II] emission from SDSS composite spectra, WIYN, Gemini and WHT imaging of the superwind environments, Gemini/GMOS spectroscopy of superwind host galaxies, and VLT/UVES echellegrams of the absorption lines -- with the aim of understanding the nature of the outflows, their host galaxies, environments, and their evolution over cosmic time.Abstract
I will propose a new theory to explain the formation and properties of rings and spirals in barred galaxies, focusing on a comparison of theoretical results to observations and giving some predictions for further comparisons. This theory can account for both spirals and rings, the latter both inner and outer. The model outer rings have the observed R1, R_1', R_2, R_2' and R_1R_2 morphologies, including the dimples near the direction of the bar major axis. It explains why the vast majority of spirals in barred galaxies are two armed and trailing and I will discuss what it takes for higher multiplicity arms to form. The shapes of observed and theoretical spirals agree and the theory predicts that stronger non-axisymmetric forcings at and somewhat beyond corotation will drive more open spirals. I will compare the ratio of ring diameters in theory and in observations and predict that more elliptical rings will correspond to stronger forcings. This theory also provides the right building blocks for the rectangular-like bar outline and for ansae.
Abstract
The colour distribution of globular cluster (GC) systems in the majority of galaxies is bi/multimodal in optical colours. It is widely accepted that multiple populations differing in metallicity exist implying different mechanisms/epochs of star formation, with small age differences still being allowed due to the large current uncertainties. Recently Yoon, Yi and Lee (2006) challenged this interpretation stating that the metallicity bimodality is an artifact of the horizontal branch (HB) morphologies that can transform a unimodal metallicity distribution in a bimodal (optical) colour distribution. The combination of optical and near-infrared (NIR) colours can in principal break the age/metallicity degeneracy inherent in optical colours alone, allowing age estimates for a large sample of GCs possible at the same time. It has been shown that the colours that best represent the true metallicity distributions are the combination of optical and NIR (eg. Puzia et al. 2002, Cantiello & Blakeslee 2007). Therefore studying GCs in the NIR is crucial to reveal their true metallicity distributions. We are currently building a homogeneous optical/NIR data set of GC systems in a large sample of elliptical and lenticular galaxies. I will present the sample, an attempt to estimate overall ages and metallicities for the GC systems and the optical/NIR colour distributions.Abstract
The Sloan Digital Sky Survey is currently the largest spectroscopic survey of extragalactic objects and one of the most ambitious observational programs ever undertaken in astronomy, measuring about 1 million redshifts and thus providing a three dimensional mapping of the local universe up to a depth of several hundreds of Mpc. The main characteristic of galaxy distribution in this survey, and in the Two degree Field Galaxy redshift Survey completed few years ago, is that large scale structures have been found to extend to scales of the order of hundreds of mega parsecs. However the standard determination of a characteristic length scale, statistically describing galaxy correlations, is of only few mega parsecs: the standard explanation of this apparent mismatch is that large scale structures have small amplitude relative to the average density. We show that, in the newest galaxy samples, large scale structures are quite typical and correspond to large fluctuation in the galaxy density field, making the standard interpretation untenable. We show that the standard statistical analysis is affected by systematics which are due to inconsistent assumptions. We point out that standard theoretical models of structure formation are unable to explain the existence of the large fluctuations in the galaxy density field detected in these samples. This conclusion is reached in two ways: by considering the scale, determined by a linear perturbation analysis of a self-gravitating fluid, below which large fluctuations are expected in standard models and through the determination of statistical properties of mock galaxy catalogs generated from cosmological N-body simulations. Finally we discuss the implications of this results in relation to recent attempts to describe inhomogeneous models in general relativity and to the recent discoveries of large scale coherent bulk flows.Abstract
From galaxy formation theory we expect galaxies to be embedded in massive dark matter haloes. For spiral and dwarf galaxies this has indeed been observationally confirmed, by modeling the kinematics from the large cold gas discs that often surround these galaxies. These gas discs are however rare in elliptical galaxies, so that we have to resort to other tracers when we want to probe their dark matter haloes, which are not always easily accessible. As a result, dark haloes for only a handful of early-type galaxies have been mapped. In this talk I will give an overview of the methods that can be used to find dark matter in early-type galaxies. I will then focus on two projects that I worked on with the integral-field spectrograph SAURON, using two different methods to constrain the dark halo. The first is based on the combination of two-dimensional ionised gas and cold gas kinematics. The second method uses SAURON as a 'photon collector', to obtain spectra at large radii in galaxies. From these spectra we can not only obtain the velocity profile and construct mass models to constrain the dark halo, but also infer the properties of the stellar halo population. I will show the results from these two projects and discuss some future prospects.Abstract
The centers of massive galaxies are special in many ways, not least because all of them are believed to host supermassive black holes. Since the discovery of a number of relations linking the mass of this central black hole to the large scale properties of the dynamically hot component of its host galaxy (bulge) it has become clear that the growth of the central black hole is intimately connected to the evolution of its host galaxy. However, for bulge-less galaxies, the situation is much less clear. Interestingly, these galaxy often host star clusters in their nuclei, and unlike black holes, these nuclear star clusters provide a visible record of the accretion of stars and gas into the nucleus. I will present my ongoing projects on nuclear star clusters that aim to understand their formation process and might give a hint on how black holes get to the centers of galaxies.Abstract
Peculiar velocities of galaxies, derived using distance estimators, are plagued with systematic effects and are unreliable beyond 100 Mpc/h. In Kashlinsky & Atrio-Barandela (2000) we proposed to measure peculiar velocities of clusters of galaxies using the temperature anisotropies on the Cosmic Microwave Background generated by the hot X-ray emitting. Using this technique we have recently found a bulk flow velocity of amplitude 600-1000 km/s in the same direction as the CMB dipole and encompassing a sphere of 300 Mpc/h radius. We shall discuss the cosmological implications of this measurement.Upcoming talks
- Properties and origin of thick disks in external galaxiesDr. Francesca PinnaThursday January 16, 2025 - 10:30 GMT (Aula)
- Seminar by Luigi TibaldoLuigi TibaldoTuesday January 21, 2025 - 12:30 GMT (Aula)