Found 219 talks archived in Galaxies

Galactic Archeology is today a vibrant field of research. The adoption and launch of the Gaia astrometric satellite by ESA has resulted in many spectroscopic Galactic surveys that aim to complement the Gaia data with information (for the fainter Gaia stars) about stellar elemental abundances, radial velocities, and stellar parameters. This results in multi-dimensional data sets which will allow us to put the Milky Way stellar populations into a much broader galactic context, eg by comparing with models and galaxies at large look-back times. In this talk I will review a selection of recent exciting developments in Galactic Archeaology found via on-going surveys as well as look to the future and see what surveys like 4MOST and WEAVE will bring.  The proposed surveys will be put into a wider context of past, on-going and future spectroscopic surveys and how this can all be combined to understand the Milky Way as a galaxy.

I will present new results regarding the first ~2 Gyrs of cosmic time using very wide-field Lyman-alpha (Lya) narrow-band surveys, including a large, matched Lya-Halpha survey to investigate how Lya and Lyman-continuum (LyC) photons escape from typical star-forming galaxies at high-redshift. We find that large Lya halos are ubiquitous in star-forming galaxies, and that the typical escape fraction of Lya and LyC photons is typically below a few percent. However, the escape fractions of Lya selected sources are significantly higher. We also find a much higher space density of very luminous Lyman-alpha emitters all the way from z~2 to z~7 than previously assumed, which we confirm spectroscopically with Keck, VLT and WHT. Many of our sources show high-ionisation lines in the rest-frame UV, and some have clear Lya blue wings. Our results also show that the steep drop in the Lya luminosity function into the epoch of re-ionisation happens only for the faint Lya emitters, while the bright ones likely ionise their own local bubbles very early on, and thus are visible at the earliest cosmic times. I will finish with new exciting ALMA detections of individual [CII] clumps at z~7 despite no dust continuum at the epoch of re-ionisation.

How galaxies form and evolve remains one of the cornerstone questions in our understanding of the universe on grand scales.  While much progress has been made by studying galaxy populations out to high redshifts, there is also much to be learned from near-field cosmology ? that is, investigating nearby galaxies in detail using observations of resolved stars.  I will highlight some recent results from several projects that are providing new insights into the structure and formation history of the Milky Way and the Magellanic Clouds.  First, I will discuss how I am mapping the stellar halos of the Milky Way and the Magellanic Clouds and thereby uncovering clues to their hierarchical buildup.  Second, I will summarize results from the APOGEE survey that, in combination with high-resolution simulations, are revealing the chemical structure, evolution and dynamical history of the Milky Way disk.  I will end with a brief summary of my recent work with the NOAO Data Lab to create an all-sky catalog of NOAO archive images (the NOAO Source Catalog or NSC).

I will present results from the "Local Volume HI Survey'' (LVHIS), including a multi-wavelength atlas of 82 nearby galaxies. The LVHIS project targets all nearby, gas-rich galaxies with vLG < 550 km/s or D < 10 Mpc that are detected in the "HI Parkes All-Sky Survey" (HIPASS). A declination limit of DEC < -30 degrees was chosen for observations with the Australia Telescope Compact Array (ATCA). The majority of LVHIS galaxies are dwarf galaxies, but we also mapped the disks and outskirts of several very large galaxies (eg M83).
I will also introduce the Australian Square Kilometre Array Pathfinder (ASKAP) which consists of 36 x 12-m dishes, each equipped with Phased-Array Feeds, operating from 0.7 to 1.8 GHz. With a field-of-view of 30 square degr ASKAP is a fast 21-cm survey machine. Early Science with 12 antennas has started and I will present first results on our target field: nearby groups and clusters.

We present new results concerning the radial gradients (Fe, alpha, s- and r-process elements) across the Galactic thin disk. We use young (t<300 Myr, classical Cepheids) stellar tracers for  which we collected high resolution spectra with UVES at VLT together with similar estimates avaialble in the literature. The investigated elements display well defined negative gradients when moving from the innermost to the outermost regions. Moreover, we also found that the radial gradients of the neutron capture elements are positive as a function of age (pulsation period). Thus suggesting an age dependence similar to alpha-elements. On the other hand, the slopes of [elements/Fe] vs Galactocentric distance are more positive than for alpha-elements. We discuss plausible working hypotheses to take account of the difference, and perform a detailed comparison with similar abundances for dwarf and giant stars available in the literature. We also discuss the abundance ratio between s- and r-process elements (La/Eu) and between heavy and light s-process elements (La/Y)  and outline their impact on the chemical enrichment history of the Galactic thin disk. Finally, we present new results concerning the iron gradient of the Galactic halo using old (t>10 Gyr, RR Lyrae) stellar tracers for which we collected high-resolution spectra  with UVES at VLT together with metallicity estimates based on low-resolution spectra available in the literature. We discuss the difference with the metallicity gradient and spread in metallicity of the M31 halo and the impact on their early formation and evolution.

Galaxy clusters are the most massive gravitationally collapsed structures in the universe, and they have important cosmological and astrophysical applications. Measurements of the radial distribution of galaxies in clusters show how galaxies trace the underlying dark matter distribution, and provide constraints on the physics related to their evolution in these environments. I will present measurements on the radial distribution of galaxies in two cluster samples, which span about 8 Gyrs of lookback time. By matching local galaxy clusters to their progenitors at high redshift, we study how clusters assemble their stellar mass content. Interestingly, this suggests that the central part of the stellar mass distribution of local galaxy clusters is already in place at redshift, and any further growth seems to happen in an inside-out fashion. I will put these findings into context by comparing them to the results from dark matter simulations. I will also focus on the abundance and spatial distribution of ultra-diffuse galaxies (UDGs, which have the luminosities of dwarfs but sizes of giant galaxies) in clusters. These mysterious galaxies have been found to be surprisingly abundant in local clusters, but their origin remains puzzling. I will discuss what we can learn about the properties of UDGs by studying their abundance as a function of halo mass, and their radial distribution in these haloes.

The disc of galaxies is made of the superposition of a thin and a thick disc. Thick discs are seen in edge-on galaxies as excesses of light a few thin disc scale-heights above the mid-plane. Star formation occurs in the thin discs whereas thick discs are made of old stars. The formation mechanisms of thick discs are under debate. Thick discs might have formed either at high redshift on a short time-scale or might have been built slowly over the cosmic time. They may have an internal or an external origin. To solve the issue of the thick disc origin we studied the kinematics and the stellar populations of the nearby edge-on galaxies ESO 533-4 and ESO 243-49. We present the first Integral Field Unit (IFU) spectroscopy works with enough depth and quality to study the thick discs. This was done with VIMOS@VLT and MUSE@VLT.

Our results point that thick discs formed in a relatively short event at high redshift and that the thin disc has formed afterwards within it. We also find that the thick disc stars have an internal origin as opposed to have their stars accreted during encounters. The work regarding ESO 533-4 has recently been published in Comer?n et al. 2015, A&A, 584, 34.

We study the connection of star formation to atomic (HI) and molecular hydrogen (H2) in isolated, low metallicity dwarf galaxies with high-resolution SPH simulations. The model includes self-gravity, non-equilibrium cooling, shielding from an interstellar radiation field, the chemistry of H2 formation, H2-independent star formation, supernova feedback and metal enrichment. We find that the H2 mass fraction is sensitive to the adopted dust-to-gas ratio and the strength of the interstellar radiation field, while the star formation rate is not. Star formation is regulated by stellar feedback, keeping the gas out of thermal equilibrium for densities n < 1 cm-3. Because of the long chemical timescales, the H2 mass remains out of chemical equilibrium throughout the simulation. Star formation is well-correlated with cold gas, but this dense and cold gas - the reservoir for star formation - is dominated by HI, not H2. In addition, a significant fraction of H2 resides in a diffuse, warm phase, which is not star-forming. The cold gas fraction is regulated by feedback at small radii and by the assumed radiation field at large radii. The decreasing cold gas fractions result in a rapid increase in depletion time (up to 100 Gyr) for total gas surface densities, in agreement with observations of dwarf galaxies in the Kennicutt-Schmidt plane.