Found 219 talks archived in Galaxies

The flow of gas from the cosmic web into galaxies provides the necessary fuel for star formation and galaxy assembly. I will review our current knowledge about gas accretion into galaxies and its consequences for galaxy formation at high and low redshifts. Special attention will be given to the detectability of cold streams as Lyman-alpha blobs or Lyman-Limit systems, as well as the current challenges to the cold-flow picture.

It is now clear that supermassive black holes (M>1e6 Msun) live in the center of most (all) galaxies, including our own Milky Way. Furthermore, the energy released during the growth of this black hole is a critical ingredient in understanding galaxy formation and evolution. In this talk, I will show what we know about how, when and where these supermassive black holes are acquiring their masses. In particular, I will focus on the effects of obscuration, as it is now clear that the majority of this black hole growth is hidden from our view by large amounts of gas and dust. I will present statistical evidence suggesting that while most nuclear activity is triggered by internal secular processes, the most violent episodes are linked to major galaxy mergers. Finally, I will show how future data obtained combining observations with the ALMA radio telescope and the NuSTAR X-ray observatory will allow us to understand the physical details of the connection between black hole growth and galaxy evolution.

Crucial issues in cosmology and astrophysics are to understand the
process of galaxy formation and evolution and the nature of what
appears to be the dominant form of matter in the Universe, i.e. dark
matter. Dwarf galaxies provide important information on both of these
issues. In this talk, I will focus on the dwarf galaxies found in the
Local Group, as it is the galaxy population that can be studied in
the greatest detail than any other from the properties of their
resolved stellar populations. I will show how wide-area surveys have
led to a leap forward in our observational understanding of these
galaxies and discuss future prospects.

AzTEC is a sensitive bolometer camera that, coupled with 10-15m-class sub-mm telescopes, has mapped more than 3 sq. deg of the extragalactic sky to depths between 0.7 and 1.1 mJy at 1.1mm, prior to its current installation and operation on the 32m Large Millimeter Telescope (LMT). These extragalactic surveys targeted towards blank-fields and biased high-z environments alike have allowed us to identify regions of the sky where submillimeter galaxies (SMGs), powerful obscured starbursts at high-redshifts (z>1), cluster, and possibly mark the potential wells of accelerated galaxy-formation that will eventually form massive clusters. In this talk I will describe the evidence we have found for these overdense regions of massive galaxies, their structure and possible interpretation, as well as the follow-up observations we are carrying out with the LMT nowadays.

The direct accretion of pristine gas streams is predicted to be the main mode of galaxy disk growth in the early universe (cold-flows). We (think we) have discovered this physical process at work in the local Universe. The finding is one of the outcomes of our in-depth study of local extremely metal poor (XMP) galaxies. I will explain the main observational properties of XMPs, in particular, their tendency to have cometary or tadpole morphology, with a bright peripheral clump (the head) on a faint tail. Tadpole galaxies are rare in the nearby universe but turn out to be very common at high redshift, where they are usually interpreted as disk galaxies in early stages of assembling. We have found the heads to be giant HII regions displaced with respect to the rotation center, with the galaxy metallicity being smallest at the head and larger elsewhere. The resulting chemical abundance gradient is opposite to the one observed in local spirals, and suggests a recent gas accretion episode onto the head. Thus, local XMP galaxies seem to be primitive disks, with their star formation sustained by accretion of external metal poor gas. I will argue how the same mechanism may be driving the star formation in many other local galaxies. Ongoing observational projects to confirm these findings and conjectures will be briefly mentioned.

 Chemical abundances derived using emission-line spectra in ionized gaseous nebulae are between the most useful properties that can be derived to understand the evolution of galaxies from the local Universe up to very high redshifts. Since nitrogen is one of the most abundant metals in the gas-phase of galaxies and its emission-lines can be measured many times instead of those emitted by oxygen, it is important to be aware of the implications of the variations in the nitrogen-to-oxygen ratio for the derivation of total metallicity and what are the advantages of using this abundance ratio to derive other evolutionary properties in different emission-line objects. We will also see the utility of some observational techniques, such integral field spectroscopy, to disentangle between different processes implied in the excess of observed nitrogen as derived from integrated observations.

The general picture of galaxy formation and evolution includes bars as the main drivers of the internal secular processes affecting the lifetime of disc galaxies. Bars are present in a very high fraction of all the spiral galaxies found at different redshifts, and the processes inducing their formation or the effects they may have on their host galaxies are still under discussion. Particularly interesting is the case of double-barred galaxies: at least 20% of all spirals have turned out to host not only one but two bars embedded in them. These two bars appear randomly oriented and independently rotating. The formation of such a double-barred system has been the goal of several numerical simulations and the results obtained so far can be roughly divided in two big groups: gas-rich and gas-free formation scenarios. In the same way a single bar does, double-bar systems might also promote gas inflow and contribute to the internal secular evolution. Moreover, they have also been proposed as a very efficient mechanism for the feeding of the active galactic nuclei.

All the previous theoretical hypothesis on the formation and evolution of double-barred galaxies have not been tested due to the lack of observational works focused on these systems. With this motivation, during my PhD I observed a sample of double-barred galaxies in order to fully analyse their kinematics and stellar populations. Among the most interesting results, it is important to highlight the discovery of the sigma-hollows, which are the only known kinematical signature of the presence of inner bars, or the fact that inner bars are younger and more metal-rich than their surrounding regions. In this talk I will present the whole work and discuss the results in the framework of the different formation scenarios and the role that these inner bars may be playing in the evolution of their host galaxies.

Although there is increasing speculation that the evolution of galaxy bulges may be regulated by AGN-induced outflows associated with the growth of the central supermassive black holes, the importance of AGN-induced outflows relative to those driven by starbursts has yet to be established observationally. In this context we have recently presented a study focusing on AGN-induced outflows in a sample of local Seyftert-ULIRGs. Perhaps, our most interesting result is related to the energy that the AGN returns to the galaxy in the form of feedback. We find that the typical mass outflows rates and kinetic powers of the emission line outflows are, in general, less energetically significant than the neutral and molecular outflows in ULIRGs and moreover, than those required today in the majority of the current hydrodynamic simulations that include AGN feedback. However, the uncertainties in the existing measurements are large, and more accurate estimates of the radii, densities and reddening of the outflows are required to put these results on a firmer footing. In this context, we are using HST /ACS+STIS and VLT-Xhsooter observations to accurately estimate sizes, electron densities and reddening to eventually provide the most accurate estimates of the kinetic powers associated with the ionized gas. In this talk I will describe in detail the results of this study focussing on testing the current simulations of hierarchical galaxy evolution.

How do the first galaxies form and evolve? Optical and near-infrared deep surveys are now finding galaxies at very high redshifts. However, they are typically small, not massive and present some but not very high star formation. But now the Herschel Multi-tiered Extragalactic Survey (HerMES), the largest project that has being carried out with the Herschel Space Observatory, in collaboration with other groups, has discovered a massive, maximum-starburst galaxy at a redshift of 6.34. The presence of galaxies like HFLS3 in the early Universe challenges current theories of galaxy fomation and evolution. I will describe the method we have developed to find these galaxies, the follow-up observations with different facilities and the main physical properties of this extreme object.

Taking advantage of the ultra-deep near-infrared imaging obtained with the Hubble Space Telescope on the Hubble Ultra Deep Field, we detect and explore for the first time the properties of the stellar haloes of two Milky Way-like galaxies at z~1. We find that the structural properties of those haloes (size and shape) are similar to the ones found in the local universe. However, these high-z stellar haloes are approximately three magnitudes brighter and exhibit bluer colours ((g-r)<0.3 mag) than their local counterparts. The stellar populations of z~1 stellar haloes are compatible with having ages <1 Gyr. This implies that the stars in those haloes were formed basically at 1<z<2. This result matches very well the theoretical predictions that locate most of the formation of the stellar haloes at those early epochs. A pure passive evolutionary scenario, where the stellar populations of our high-z haloes simply fade to match the stellar halo properties found in the local universe, is consistent with our data.