Recent Talks

Solar surface convection displays highly localized sinks where cold plasma returns to the solar interior. On its way to being engulfed by a downdraft the plasma can also advect and intensify magnetic fields up to kG field strengths. Such theoretical predictions strengthen the idea that localized downdrafts are places where the concentration of magnetic fields is favored. The observational discovery of convectively driven sinks is rather recent, however, and its role in the formation and evolution of quiet-Sun magnetic features is still poorly characterized. In our work, we provide both quantitative and qualitative bases for the association between sinks and magnetic fields using high spatial resolution spectropolarimetric data acquired with the Imaging Magnetograph eXperiment on board Sunrise. We find 3.1 x 10-3 sinks Mm-2 minute-1 located at mesogranular vertices. These sinks are associated to (1) horizontal velocity flows converging to a central point and (2) long-lived downdrafts.  The spatial distribution of magnetic fields in the quiet Sun is also examined. The strongest magnetic fields are preferentially located at sinks. We find that 40% of the pixels with longitudinal component of the magnetic field higher than 500 G are located in the close neighborhood of sinks.  The study of individual examples reveal that sinks can play an important role in the evolution of quiet-Sun magnetic features.

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.

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.

In mid nineteens, it was discovered that the Sun had a dipolar global magnetic field, whose temporal evolution followed the Solar Cycle. Polar regions, as well as sunspots that appear in the activity belts, changed their polarity every 11 years: sunspots during each activity minima, and the poles in activity maxima. This fact, made people think that the poles reversal was related to the arrival of opposite polarity magnetic flux dragged from active regions by a meridional flow. Such new flux reduced the dominant polarity at the poles by cancellation, and built the opposite one until next minimum of activity. In our study, we have used the high quality full disc magnetograms, recorded by the HMI instrument onboard the SDO satellite since the beginning of the mission, in april 2010. We perform a deep study of the evolution with time of the line of sight component of the magnetic field at the solar poles. In our data, we see many aspects of the solar cycle as the decay of the dominant polarity of both poles while we approach to the activity maximum. But the main result is the detection of a monthly oscillatory pattern of the pole's magnetic field. Such oscillation, related to solar rotation is a clear evidence of a non-axisymmetric component of the magnetic field. One of the possible explanations is that the global field is tilted with respect to the rotation axis. This rather usual finding in other stars, here represents a breach of modern solar dynamo theories for the generation and maintenance of the Sun's magnetic field.

Planck satellite provides for the first time the possibility to detect galaxy clusters using their Sunyaev-Zeldovich (SZ) effect signature covering the full sky (Planck Col XXIX, 2013). Planck SZ catalogs I and II include more than 1900 sources, of which 700 remain unknown. The study of the purity of these samples and the characterization of SZ sources is essential to perform cosmology with cluster counts. With this aim in mind, the IAC-Planck group is performing the optical validation and characterization of these samples through two long-term observing programs at Canary Island observatories, the ITP 13B15A and the large-term 15B-17A. In this talk we will present intermediate results of this validation program. Using photometric and spectroscopic information (mainly multi-object techniques) we estimate redshifts and dynamical masses in order to minimize the errors in the Msz-Mdyn scaling relation and the SZ clusters mass function which allow a better determination of cosmological parameters (mainly Omega_m, sigma_8 and neutrino mass) from Planck SZ survey.

I will summarize the two well proved techniques for high spatial resolution: Lucky Imaging and Adaptive Optics and the work of our group in this field. I will also introduce the state-of-the-art new instrument Adaptive Optics Lucky Imager (AOLI). On AOLI, both techniques merge providing a very versatile answer on the visible range. Some first science on the T-Tauri system LkHa 262/263 in the MBM 12 cloud will be reported together with a review of the next steps to be developed.

Radiative transfer is underlying physical phenomenon in many astrophysical problems and among the most difficult phenomena to deal with. The main difficulty arises from the non-local and, in general, non-linear coupling of the radiation field and the state of the gas. A variety of numerical methods have been developed so far to solve the NLTE radiative transfer in spectral lines (NLTE line formation problem). An overview of selected methods will be given in the talk. Special emphasize is put on two extremely fast convergent methods: Iteration Factors Method (IFM) and Forth-and-Back Implicit Lambda Iteration (FBILI) that use the so-called iteration factors in an explicit and implicit way, respectively. Although these methods are developed to solve the general non-linear multi-level problem, their basic ideas and properties are demonstrated on the well-known test-problem of the two-level atom line formation under the assumption of complete redistribution.

I will make an overview of the classical pulsating variable stars (i.e. SX Phoenicis, RR Lyrae, Anomalous Cepheids, Classical Cepheids) in our Local Group and beyond. I will focus on the evolution and pulsation theory that drives our interpretation of their observed properties in terms of stellar evolutionary tracers and powerful distance indicators. The analysis of low mass (and thus ancient) variable stars can provide sound constraints in our understanding of how galaxies form and evolve. Particular attention will be devoted to the idea of whether the current dwarf spheroidal satellites (dSphs) of the MW are surviving representatives of the Halo protogalactic fragments (Fiorentino et al. 2015). Furthermore, I will discuss the use of classical Cepheids to calibrate SNIa host galaxies with the aim of accurately measure the local Hubble constant, Ho.
I will discuss in detail the possible exploitation of the public variable star catalogues coming from the ESA Gaia mission and the next quantum leap that will be possible in the near future thanks to full sky project with extremely deep limiting magnitude such as the Large Synoptic Survey Telescope. I will conclude describing the importance of resolved stellar population studies in preparing the era for extremely large telescopes.

 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.

In the first part of the talk, I discuss the chromospheric activity of quiet Sun. The so-called basal flux, the minimum chromospheric emission of main sequence stars, is discussed in terms of magnetic and non-magnetic heatings. The second half of the talk is about variability of the solar cycle. The extended minimum of the last cycle caused speculations about a possible long-term decline in the solar activity. Based on our observations in past 15 years, I argue that the Sun does not cease to generate sunspots in the next cycle. Finally I outline new tools to evaluate chromospheric activities.