We present cosmological zoom-in hydro-dynamical simulations for the formation of disc galaxies, implementing dust evolution and dust promoted cooling of hot gas. We couple an improved version of our previous treatment of dust evolution, which adopts
the two-size approximation to estimate the grain size distribution, with the MUPPI star formation and feedback sub-resolution model. Our dust evolution model follows carbon and silicate dust separately. To distinguish differences induced by the chaotic behaviour of simulations from those genuinely due to different simulation set-up, we run each model six times, after introducing tiny perturbations in the initial conditions. With this method, we discuss the role of various dust-related physical processes and the effect of a few possible approximations adopted in the literature. Metal depletion and dust cooling affect the evolution of the system, causing substantial variations in its stellar, gas and dust content. We discuss possible effects on the Spectral Energy Distribution of the significant variations of the size distribution and chemical composition of grains, as predicted by our simulations during the evolution of the galaxy. We compare dust surface density, dust-to-gas ratio and small-to-big grain mass ratio as a function of galaxy radius and gas metallicity predicted by our fiducial run with recent observational estimates for three disc galaxies of different masses. The general agreement is good, in particular taking into account that we have not adjusted our model for this purpose.
Compact groups (CGs) of galaxies are defined as isolated and dense galaxy systems that appear to be a unique site of multiple galaxy interactions. Semi-analytical models of galaxy formation (SAMs) are a prime tool to understand CGs. We investigate ho
w the frequency and the three-dimensional nature of CGs depends on the SAM and its underlying cosmological parameters. Extracting 9 lightcones of galaxies from 5 different SAMs and selecting CGs as in observed samples, we find that the frequency and nature of CGs depends strongly on the cosmological parameters. Moving from the WMAP1 to the WMAP7 and Planck cosmologies (increasing density of the Universe and decreasing normalisation of the power spectrum), the space density of CGs is decreased by a factor 2.5, while the fraction of CGs that are physically dense falls from 50 to 35 percent. The lower $sigma_8$ leads to fewer dense groups, while the higher $Omega_{rm m}$ causes more chance alignments. However, with increased mass and spatial resolution, the fraction of CGs that are physically dense is pushed back up to 50 percent. The intrinsic differences in the SAM recipes also lead to differences in the frequency and nature of CGs, particularly those related to how SAMs treat orphan galaxies. We find no dependence of CG properties on the flux limit of the mock catalogues nor on the waveband in which galaxies are selected. One should thus be cautious when interpreting a particular SAM for the frequency and nature of CGs.
In this work we present an algorithm to identify compact groups (CGs) that closely follows Hicksons original aim and that improves the completeness of the samples of compact groups obtained from redshift surveys. Instead of identifying CGs in project
ion first and then checking a velocity concordance criterion, we identify them directly in redshift space using Hickson-like criteria. The methodology was tested on a mock lightcone of galaxies built from the outputs of a recent semi-analytic model of galaxy formation run on top of the Millennium Simulation I after scaling to represent the first-year Planck cosmology. The new algorithm identifies nearly twice as many CGs, no longer missing CGs that failed the isolation criterion because of velocity outliers lying in the isolation annulus. The new CG sample picks up lower surface brightness groups, which are both looser and with fainter brightest galaxies, missed by the classic method. A new catalogue of compact groups from the Sloan Digital Sky Survey is the natural corollary of this study. The publicly available sample comprises $462$ observational groups with four or more galaxy members, of which $406$ clearly fulfil all the compact group requirements: compactness, isolation, and velocity concordance of all of their members. The remaining $56$ groups need further redshift information of potentially contaminating sources. This constitutes the largest sample of groups that strictly satisfy all the Hicksons criteria in a survey with available spectroscopic information.
Historically, compact group catalogues vary not only in their identification algorithms and selection functions, but also in their photometric bands. Differences between compact group catalogues have been reported. However, it is difficult to assess
the impact of the photometric band in these differences given the variety of identification algorithms. We used the mock lightcone built by Henriques et al. (2012) to identify and compare compact groups in three different photometric bands: $K$, $r$, and $u$. We applied the same selection functions in the three bands, and found that compact groups in the u-band look the smallest in projection, the difference between the two brightest galaxies is the largest in the K-band, while compact groups in the r-band present the lowest compactness. We also investigated the differences between samples when galaxies are selected only in one particular band (pure compact groups) and those that exist regardless the band in which galaxies were observed (common compact groups). We found that the differences between the total samples are magnified, but also some others arise: pure-r compact groups are the largest in projection; pure-u compact groups have the brightest first ranked galaxies, and the most similar two first ranked galaxies; pure-K compact groups have the highest compactness and the most different two first ranked galaxies; and common compact groups show the largest percentage of physically dense groups. Therefore, without a careful selection and identification of the samples, the characteristic features of group properties in a particular photometric band could be overshadowed.
