[Abridged] In high density environment, the gas content of galaxies isstripped, leading to a rapid quenching of their star formation activity. Thisdramatic environmental effect is generally not taken into account in the SFHsusually assumed to perform spectral energy distribution (SED) fitting of thesegalaxies, yielding to a poor fit of their stellar emission and, consequently, abiased estimate of the SFR. We aim at reproducing the SFH of galaxies thatunderwent a rapid star formation quenching using a truncated delayed SFH thatwe implemented in the SED fitting code CIGALE. We show that the ratio betweenthe instantaneous SFR and the SFR just before the quenching ($r_{SFR}$) is wellconstrained as long as rest frame UV data are available. This SED modelling isapplied to the Herschel Reference Survey (HRS) containing isolated galaxies andsources falling in the dense environment of the Virgo cluster. The latter areHI-deficient due to ram pressure stripping. We show that the truncated delayedSFH successfully reproduces their SED while typical SFH assumptions fail. Agood correlation is found between $r_{SFR}$ and HI-def, the parameterquantifying the gas deficiency of cluster galaxies, meaning that SED fittingresults can be used to provide a tentative estimate of the gas deficiency ofgalaxies for which HI observations are not available. The HRS galaxies areplaced on the SFR-$M_*$ diagram showing that the HI-deficient sources lie inthe quiescent region confirming previous studies. Using the $r_{SFR}$parameter, we derive the SFR of these sources before quenching and show thatthey were previously on the main sequence relation. We show that the $r_{SFR}$parameter is also well recovered for deeply obscured high redshift sources, aswell as in absence of IR data. SED fitting is thus a powerful tool to identifygalaxies that underwent a rapid star formation quenching.

We investigate the degree to which the inclusion of baryonic physics canovercome two long-standing problems of the standard cosmological model ongalaxy scales: (i) the problem of satellite planes around Local Group galaxies,and (ii) the "too big to fail" problem. By comparing dissipational anddissipationless simulations, we find no indication that the addition ofbaryonic physics results in more flattened satellite distributions aroundMilky-Way-like systems. Recent claims to the contrary are shown to derive inpart from a non-standard metric for the degree of flattening, which ignores thesatellites' radial positions. If the full 3D positions of the satellitegalaxies are considered, none of the simulations we analyse reproduce theobserved flattening nor the observed degree of kinematic coherence of the MilkyWay satellite system. Our results are consistent with the expectation thatbaryonic physics should have little or no influence on the structure ofsatellite systems on scales of hundreds of kiloparsecs. Claims that the "toobig to fail" problem can be resolved by the addition of baryonic physics arealso shown to be problematic.

In a pioneering effort, Preston et al. reported that the colors of bluehorizontal-branch (BHB) stars in the halo of the Galaxy shift with distance,from regions near the Galactic center to about 12 kpc away, and interpretedthis as a correlated variation in the ages of halo stars, from older toyounger, spanning a range of a few Gyrs. We have applied this approach to asample of some 4700 spectroscopically confirmed BHB stars selected from theSloan Digital Sky Survey to produce the first "chronographic map" of the haloof the Galaxy. We demonstrate that the mean de-reddened g$-$r color,<(g$-$r)o>, increases outward in the Galaxy from $-$0.22 to $-$0.08 (over acolor window spanning [$-$0.3:0.0]) from regions close to the Galactic centerto ~40 kpc, independent of the metallicity of the stars. Models of the expectedshift in the color of the field BHB stars based on modern stellar evolutionarycodes confirm that this color gradient can be associated with an age differenceof roughly 2-2.5 Gyrs, with the oldest stars concentrated in the central ~15kpc of the Galaxy. Within this central region, the age difference spans a meancolor range of about 0.05 mag (~0.8 Gyrs). Furthermore, we show thatchronographic maps can be used to identify individual substructures, such asthe Sagittarius Stream, and overdensities in the direction of Virgo andMonoceros, based on the observed contrast in their mean BHB colors with respectto the foreground/background field population.