Found 217 talks archived in Galaxies

Thursday August 24, 2017
Dr. Sébastien Comerón
University of Oulu, Finland


The reports of thick discs' death are greatly exaggerated: Thick discs are NOT artefacts caused by diffuse scattered light

Recent studies have made the community aware of the importance of accounting for scattered light when examining low surface brightness galaxy features such as thick discs. In past studies on thick discs of edge-on galaxies the point spread function (PSF) effects were not taken into account or were modelled with a Gaussian kernel.

We have re-examined results on photometric decompositions of discs in the Spitzer Survey of Stellar Structure of Galaxies (S4G) using a revised PSF model that accounts for extended wings out to more than 2.5 arcminutes. We study 141 edge-on galaxies, so we more than double the samples examined in past studies. This is the largest sample of extragalactic thick discs studied so far.

The main difference between our current fits and those presented in the past is that now the scattered light from the thin disc dominates the surface brightness at levels below 26 mag arcsec-2. This change, however, does not affect drastically any of our previously presented results: 

- Thick discs are nearly ubiquitous. They are not an artefact caused by scattered light as has been suggested elsewhere. 
- Thick discs have masses comparable to those of thin discs in low-mass galaxies (with circular velocities v_c<120 km s-2) whereas they are typically less massive than the thin discs in high-mass galaxies.
- Thick discs and central mass concentrations seem to have formed at the same epoch from a common material reservoir.
- Approximately sixty per cent of the up-bending breaks in face-on galaxies are caused by the superposition of a thin and a thick disc where the scale-length of the latter is the largest.

Friday July 14, 2017
Prof. David Nidever


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).

Tuesday March 14, 2017
Dr. Bärbel S. Koribalski
Australia Telescope National Facility, CSIRO, Australia


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.

Thursday October 20, 2016
Prof. Giuseppe Bono
Univ. of Rome Tor Vergata


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.

Tuesday October 4, 2016
Dr. Remco Van-Der-Burg
CEA (France)


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.

Thursday June 23, 2016
Dr. Sébastien Comerón
University of Oulu


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.

Thursday June 2, 2016
Dr. Mariya Lyubenova
Kapteyn Astronomical Institute


Tests of the concordance cold dark matter model on the scale of galaxies are so far inconclusive due to our poor understanding of the interplay between baryons and dark matter (DM). Two critical limitations in previous efforts to disentangle the baryonic and DM distributions have been the lack of (i) two-dimensional, spatially complete and radially extended kinematics to infer the total mass distribution, and (ii) coverage in wavelength to robustly constrain the baryonic mass distribution and isolate the DM contribution. Both are now provided by existing integral-field spectroscopic data from the CALIFA survey of a statistically well-defined sample of ~600 nearby galaxies of all Hubble types. We apply dynamical and stellar population modelling in a homogeneous way to the same data. In this way we for the first time constrain both the normalisation (ratio of dwarf to giant stars) and shape (single versus broken power-law slope) of the stellar initial mass function (IMF). We then robustly characterise the mass distribution of galaxies, from dwarf-star dominance at the high-mass end to dark matter excess in low-mass spirals. In this way, CALIFA yields physical insights into the baryonic and DM interplay for a statistically well-defined sample of nearby galaxies, providing in turn crucial constraints on galaxy formation and evolution models.

Tuesday April 5, 2016
Dr. Chia-Yu Hu
MPA, Garching


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.

Wednesday March 30, 2016
Dr. Ignacio Trujillo
Instituto de Astrofísica de Canarias


Current state-of-the-art imaging surveys deliver images with limiting surface brightness of 26.5 mag/arcsec^2. This depth is around 100 times fainter than the brightness of the sky in professional observatories. This view of the Universe is the basis of most of our visual understanding of the closest (galactic and extragalactic) objects. However, going deeper is absolutely mandatory if we want to understand a plethora of astrophysical phenomena that manifest themselves at lower surface brightness limits. To understand from the smallest scales of our local Galactic cirrus to the huge extensions of the intra-cluster light of massive galaxy clusters I will present in this talk two important steps forward conducted here at the IAC.  The first one is The IAC Stripe82 Legacy Project: a public survey for the astronomical community which includes 275 square degrees in 5 optical bands reaching 28.5 mag/arcsec^2 depth. The second one is the deepest ever imaging of the nearby Universe: 8h of GTC time on the near UGC00180 galaxy reaching a limiting surface brightness of 31.5 mag/arcsec^2 (100 times deeper than traditional surveys). A large amount of unexpected discoveries emerge in these unprecedented set of images.

Thursday February 18, 2016
Dr. Evanthia Hatziminaoglou


 Nuclear activity and intense star formation are two phenomena known to co-exist in a variety of galaxies, spanning several orders of magnitude in luminosity. I will present a compilation of results derived from studies of type 1 and type 2 AGN, using Spitzer and Herschel data that aim at quantifying the effects of the two phenomena in the mid- and far-infrared. I will address the incidence of
star formation in AGN using various mid- and far-infrared indicators, describe diagnostics, and discuss the effects of AGN on the properties of their hosts.