Found 185 talks archived in Galaxies
Cold gas streaming along the dark-matter filaments of the cosmic web is predicted to be the major provider of resources for disc buildup and star formation in massive galaxies in the early universe. We use hydrodynamical simulations to study to what extent these cold streams are traceable in the extended circum-galactic environment of galaxies via Ly alpha emission, Ly alpha absorption and selected low ionisation metal absorption lines. We predict the strength of the absorption signal produced by the streams and find that it is consistent with observations in high redshift galaxies. The characteristics of the Ly alpha emission of our simulated galaxies are similar in luminosity, morphology and extent to the observed Ly alpha blobs, with distinct kinematic features. We analyse the characteristics of the cold streams in simulations and present scaling relations for the amount of infall, its velocity, distribution and its clumpiness and compare our findings with observations.
The exponential radial profiles of spiral galaxies have been observed for decades with various explanations offered, but none of these explanations hold up now that the profiles have been traced to 10 scale lengths in some cases. Profiles with breaks also have their outer parts remarkably close to exponential. This is true in both spirals and dwarfs, with a particular problem for non-barred dwarfs where there are no strong torques or shears in the stellar disks for radial scattering. We have shown that scattering from disk clumps can make exponentials, and indeed dwarfs have fairly massive clumps and irregularities in their HI distributions. Spiral waves also have clumps in the form of persistent wave enhancements at corotation and at points of wave interference. This talk reviews the observations of exponential profiles and profile breaks in spiral and dwarf galaxies, and considers various theories to explain them.
Galaxies at redshifts z~2-5 are dominated by clumpy morphologies instead of the more familiar elliptical and spiral types seen in the local Universe. The clumpy galaxies are gas-rich and highly turbulent, with star-forming regions 100 times more massive than in today’s galaxies. Clumpy galaxies should evolve into spirals and barred spirals when the disks become less turbulent. We find transitional types midway between clumpy and spiral, primarily around z~1-2. We observed local analogs of these transitional spirals in the Kiso Survey of Ultraviolet Galaxies, whose clumps are comparable in mass and surface density to the clumps at high redshift. We also find local analogs of high redshift tadpole galaxies, whose measured low metallicities in star-forming regions suggest ongoing external gas accretion.
Many stars are observed to belong to multiple systems. Interactions between binary stars may change the evolutionary track of a star, creating atypical stars like Blue Stragglers and explaining the existence of extreme horizontal branch (EHB) stars. Using evolutionary population synthesis models including binary star evolutionary tracks from Hurley et al. and including the two He white dwarfs merger channel, suggested by Han et al., for the formation of EHB stars we compute a series of isochrones which include these atypical stars. We derive the integrated spectral energy distributions and the colors corresponding to these populations. The predictions of this model are in good agreement with traditional population synthesis models, except when the spectrum of the stellar population is dominated by binary stars or their products, e.g., EHB stars in the ultraviolet (UV) of early-type galaxies (ETGs) (Hernández-Pérez and Bruzual 2013). Using this binary population synthesis model we reproduce successfully the observed colour-colour diagram of a sample of 3417 ETGs observed both in the optical (SDSS -DR8) and the UV (GALEX-GR6) (Hernández-Pérez and Bruzual 2014). I will show how important is to consider binary interactions in evolutionary synthesis models.
I will talk about how resolved stellar populations in the nearby Local Group dwarf galaxies have been used to study the detailed chemical, kinematic and star formation history of these systems and the link to the properties of the Milky Way. I will mainly discuss the results from the DART spectroscopic surveys of nearby dwarf spheroidal galaxies, determining detailed abundances, looking for CEMP stars and also combining spectroscopy with colour-magnitude diagram analysis to measure the time scale for star formation and chemical evolution.
Any successful model of galaxy formation needs to explain the low rate of star formation in the small progenitors of today’s galaxies. This inefficiency is necessary for reproducing the low stellar-to-virial mass fractions. A possible driver of this low efficiency is the radiation pressure exerted by ionizing photons from massive stars. The effect of radiation pressure in cosmological, zoom-in galaxy formation simulations is modelled as a non-thermal pressure that acts only in dense and optically thick star-forming regions. The main effect of radiation pressure is to regulate and limit the high values of gas density and the amount of gas available for star formation. By using these simulations, I will address the early formation of compact spheroids by violent disc instabilities (VDI). Due to the inefficiency of star formation, this process is gas rich, so the dissipation naturally leads to compact spheroids. These VDI-driven spheroids, much like merger-driven spheroids, have steep surface density profiles, consistent with a classical, de-Vaucouleurs profile at all times.
In order to understand galaxy formation it is crucial to obtain sensitive observations of the emission of dust and molecular gas both of which constrain the on-going star formation or AGN activity and the future potential of the galaxy to grow. Constraining the growth of ensemble of galaxies in the distant universe and not simply the most active ones, is one of the primary goals of current and planned (sub)mm facilities such as ALMA or SPICA. I will discuss two major questions in galaxy formation and assembly: 1) are dusty galaxies vigorously forming stars embedded within large scale structures at z>1.5; and 2) do dusty starbursts exist at the highest redshift. To shed light on these obscure topics, I will present our on-going observations of dust and molecular gas with a number of different (sub)mm facilities such as Herschel, APEX, IRAM or ALMA of one important star forming galaxy population in the distant universe: submillimeter selected galaxies (SMGs). My presentation will be complemented by our recently initiated census of the molecular gas reservoirs of nearby galaxies with optical IFU coverage. The local analogs serve as a reference sample for current and future studies of high-z galaxy populations.
What can the shape and size of a galaxy tell us about how it has evolved across cosmic time? Which evolutionary mechanisms are important, or relevant, and which not? How do galaxies form in the early Universe? As we enter a new era of big-data astronomy, our capacity to further pursue answers to these questions is increasingly limited not by Human ingenuity but by our use of 20th century data analysis techniques. In this talk, I will summarise my work with the Galaxy And Mass Assembly (GAMA) Survey in measuring the multi-wavelength light profile and stellar mass properties of ~200,000 galaxies in the local Universe. I will show how the stellar mass function may be broken down by morphology and structural component, and the implications this has for our understanding on which evolutionary mechanisms are important in shaping the galaxies around us over the course of the last 1 billion years.
Dwarf galaxies are the most common type of galaxy in the Universe andinclude the most dark-matter-dominated objects known. They offerintriguing insights into evolutionary processes at low halo masses and low metallicities. Moreover, as survivors of a once much more numerous population of building blocks of larger galaxies, they are key to understanding very early star formation processes. The Local Group and particularly the Milky Way's dwarf galaxy entourage offer us the unique possibility to compare in detail dwarf and Galactic populations. This is an important step towards quantifying the magnitude and time scales of dwarf contributions to the build-up of the Milky Way and allows us to test predictions of cosmological theories and hierarchical structure formation.
Modern imaging surveys provide a fundamental tool in order to study the morphological
properties of galaxy populations in the nearby and the distant Universe. In order to
process a complete set of survey images, we designed GALAPAGOS-C. GALAPAGOS-C
unifies the detection of sources (via source extractor), postage stamp cutting, object
mask preparation, sky background estimation and complex two-dimensional light profile
Sérsic modeling (via GALFIT) in one automatic program. GALAPAGOS-C is designed
around the concept of MPI-parallelization, allowing the processing of large data sets
in a quick and efficient manner. Further, GALAPAGOS-C is capable of fitting multiple-
Sérsic profiles to each galaxy, each representing distinct galaxy components (e.g. bulge,
disc, bar), in addition to the option to fit asymmetric distortions with a Fourier mode
expansion to the axis-symmetric single-Sérsic isophotes. The modeling reliability of our
core single-Sérsic fitting capability and the optional Fourier mode expansion are tested
thoroughly using image simulations.
GALAPAGOS-C is applied to a sample of 2063 galaxies in the A901/902 galaxy cluster
(z ∼ 0.165) from the Space Telescope A901/902 Galaxy Evolution Survey (STAGES) and
an additional sample of 2876 field galaxies from the Galaxy Evolution From Morphology
And SEDs Survey (GEMS). We measure the distribution of Sérsic indices as a function of
local object density in the A901/902 cluster sample to provide one of the first measures
of the Sérsic index–density relation. In addition, we measure the distribution of lopsided
galaxies in the A901/902 cluster sample and quantify the intensity of lopsidedness in
the galaxies in the field since z ∼ 0.9 in order to study the evolution of lopsidedness as
a function of redshift. In each application, we study the correlations of the measured
parameters with other intrinsic and structural variables, e.g. the stellar mass, the color
or the presence or absence of a disk. Our results provide further clues on the evolution
of galaxy structure with cosmic time and the dependence on environment.