Found 183 talks archived in Galaxies

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.

Thursday December 3, 2015
Dr. Francesco La Barbera


Understanding the stellar initial mass function (IMF) is a key aspect to obtain a complete picture of galaxy formation and evolution. In the past years, we have carried out a systematic census of the IMF in the unresolved stellar populations of (massive) early-type galaxies (ETGs), using optical and NIR spectroscopy from different surveys (e.g.  SDSS, CALIFA) and dedicated observing programmes (OSIRIS@GTC, XSHOOTER@VLT).  I will present results on a non-universal IMF in ETGs - pointing to an excess of low-mass stars in high-, relative to low-, mass galaxies - and current constraints on the physical driver behind the IMF variations.

Thursday October 15, 2015
Dr. Jorge Sanchez Almeida


Models of galaxy formation predict that gas accretion from the cosmic web is a primary driver of star formation over cosmic history. Except in very dense environments where galaxy mergers are also important, model galaxies feed from cold streams of gas from the web that penetrate their dark matter haloes. Although these predictions are unambiguous, the observational support has been indirect so far. I will report spectroscopic evidence for this process in extremely metal-poor galaxies (XMPs) of the local Universe, taking the form of localized starbursts associated with gas having low metallicity. Because gas mixes azimuthally in a rotation timescale (a few hundred Myr),  the observed metallicity inhomogeneities are only possible if the metal-poor gas producing stars fell onto the disk recently. I will analyze several possibilities for the origin of the metal-poor gas, favoring the metal-poor gas infall predicted by numerical models. In addition, I will show model galaxies in cosmological numerical simulations with starbursts of low metallicity like to the star-forming regions in XMPs.

Thursday October 8, 2015
Prof. Joop Schaye
Univ. of Leiden


The realism of hydrodynamical simulations of the formation and evolution of galaxies has improved considerably in recent years. I will try to give some insight into the reasons behind this success, focusing in particular on the importance of subgrid models and the associated limitations. I will also present recent results from the cosmological EAGLE simulations as well as from higher-resolution simulations of individual galaxies.

Thursday October 1, 2015
Prof. Evan Skillman
Univ. of Minnesota


The epoch of reionization has been predicted to leave an imprint on the star formation histories of dwarf galaxies. I will briefly review theoretical ideas and observational constraints on reionization. I will then discuss the predicted effects on the evolution of dwarf galaxies.
This will be followed by a discussion of tests using star formation histories of nearby dwarf galaxies.

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