Wednesday Colloquium


"Blueshifts in the Lemaitre-Tolman and quasi-spherical Szekeres models"

Andrzej Krasiński (NCAC, Warsaw)

In Lemaitre-Tolman models that have nonconstant Big Bang (BB) function, light rays emitted radially from the BB reach every observer with infinite blueshift (i.e. z = -1). These blueshifted rays have several properties in common with the observed gamma-ray bursts. The quasi-spherical Szekeres (QSS) models have in general no symmetry, so there are no radial directions in them. However, in axially symmetric QSS models with nonconstant BB numerical calculations show that very strong blueshifts can appear on those rays that intersect every space of constant time on the symmetry axis. In general QSS models with nonconstant BB, numerical calculations show that two preferred directions exist, on which blueshifts are equally strong. Astrophysical applications of these results will be briefly discussed.


"Absorption line variability in BALQSOs and NGC 4151"

Conor Wildy (University of Leicester)

Absorption lines are commonly observed in AGN spectra, ranging in FWHM from a few hundred km/s (narrow) to >10000 km/s (broad absorption lines in BALQSOs). Many are observed at wavelengths blueward of the object rest-frame, indicating an origin in powerful outflowing winds, possibly contributing to galaxy feedback effects. Follow-up spectral observations indicate that AGN absorption lines can change strength over time, however there is no consensus regarding the dominant mechanism responsible for this variability. Two main possibilities exist: (1) varying emission-source covering fraction due to absorbing clouds moving across the line-of-sight, and (2) variable ionizing continuum flux changing the ionization state of the absorbing gas. Our study of 50 SDSS-selected BALQSOs showed no evidence of correlation between quasar luminosity and absorption line variability for Si IV λ1400 and C IV λ1549 BALs. This tentatively suggests ionization changes are not predominantly responsible for BAL variability in the quasar population, as less luminous quasars show greater continuum variability. One object in this sample showed extreme BAL variability, defying the predictions of a random-walk model which accurately predicted BAL behaviour in a large BALQSO study. Changes in BAL strength in this object are co-ordinated, despite large veloctiy separations. This, together with Cloudy photoionization simulations and the behaviour of the continuum, strongly suggests that changes in the ionization state of the absorber induced by continuum variability is responsible for the BAL variability in this quasar. The Seyfert 1 galaxy NGC 4151, notable for its continuum variability, also exhibits blueshifted absorption features. Examination of a variable, narrow metastable helium absorber suggests ionization changes are driving its variability. Taken together, these results indicate ionization changes definitely contribute to variability of very different types of AGN absorption line, without ruling out covering fraction changes as a contributor to AGN absorption line variability in general.


"Intrinsic morphology and global kinematics of Andromeda satellite galaxies"

Jean-Baptiste Salomon (CAMK, Warsaw)

The ΛCDM model represents nowadays the best understanding of the formation and the evolution of large scale structures in our Universe. Nevertheless, this paradigm is not predictive and successful yet at smaller scales. In this context, satellites in the Local Group (LG), the simpler and closer galactic systems, are one of our best chance to test this model and to improve our comprehension of galaxy formation at smaller scales. In this talk, I will present a method to derive analytically the intrinsic (3D) morphology of 25 Andromeda (M31) satellites. Results suggest that the LG is in fact more disturbed than what was previously thought. After this individual approach, I will further expose a recent result on the global kinematics of the M31 system. This new estimation suggests for the first time a high transverse velocity for this system with respect to the Milky Way. These values could lead to redefine the entire dynamic of the LG and its surroundings.


"The MESSIER orbiter: lifting the veil on the ultra-low surface brightness Universe"

David Valls-Gabaud (Observatoire de Paris, CNRS)

The S-class MESSIER satellite has been designed to explore the extremely low surface brightness universe at UV and optical wavelengths. The two driving science cases target the mildly- and highly non-linear regimes of structure formation to test two key predictions of the LCDM scenario: (1) the detection of the putative large number of galaxy satellites, and (2) the identification of the filaments of the cosmic web. The satellite will drift scan the entire sky in 6 bands covering the 200-1000 nm wavelength range to reach the unprecedented surface brightness levels of 34 mag/arcsec^2 in the optical and 37 mag/arcsec^2 in the UV. Many important secondary science cases will result as free by-products and will be discussed in some detail, such as the luminosity function of galaxies, the contribution and role of intracluster light, the cosmological background radiation at UV and optical wavelengths, the molecular hydrogen content of galaxies at z=0.25, time domain studies of supernovae, GRBs and tidal disruption events, the chemical enrichment of the interstellar medium through mass loss of red giant stars and the accurate measure of the BAO scale at z=0.7 with over 30 million galaxies detected in Lyman-alpha at this redshift. It will provide the astronomical community the first space-based reference UV-optical photometric catalogue of the entire sky, and synergies with GAIA, EUCLID and WFIRST will also be discussed. Technical issues will likewise be addressed for possible improvements on the current design.