We study the luminosity function of satellite galaxies around isolated primaries using the Sloan Digital Sky Survey (SDSS) spectroscopic and photometric galaxy samples. We select isolated primaries from the spectroscopic sample and search for potential satellites in the much deeper photometric sample. For primaries of similar luminosity to the Milky Way and M31, we are able to stack as many as ∼20 000 galaxy systems to obtain robust statistical results. We derive the satellite luminosity function extending almost 8 mag fainter than the primary galaxy. We also determine how the satellite luminosity function varies with the luminosity, colour and concentration of the primary. We find that, in the mean, isolated primaries of comparable luminosity to the Milky Way and M31 contain about a factor of 2 fewer satellites brighter than MV=−14 than the average of the Milky Way and M31.

We compare global predictions from the EAGLE hydrodynamical simulation, and two semianalytic (SA) models of galaxy formation, L-GALAXIES and GALFORM. All three models include the key physical processes for the formation and evolution of galaxies and their parameters are calibrated against a small number of observables at z ≈ 0. The two SA models have been applied to merger trees constructed from the EAGLE dark matter only simulation. We find that at z 2, both the galaxy stellar mass functions for stellar masses M * < 10 10.5 M and the median specific star formation rates (sSFRs) in the three models agree to better than 0.4 dex. The evolution of the sSFR predicted by the three models closely follows the mass assembly history of dark matter haloes. In both EAGLE and L-GALAXIES there are more central passive galaxies with M * < 10 9.5 M than in GALFORM. This difference is related to galaxies that have entered and then left a larger halo and which are treated as satellites in GALFORM. In the range 0 < z < 1, the slope of the evolution of the star formation rate density in EAGLE is a factor of ≈ 1.5 steeper than for the two SA models. The median sizes for galaxies with M * > 10 9.5 M differ in some instances by an order of magnitude, while the stellar mass-size relation in EAGLE is a factor of ≈ 2 tighter than for the two SA models. Our results suggest the need for a revision of how SA models treat the effect of baryonic self-gravity on the underlying dark matter. The treatment of gas flows in the models needs to be revised based on detailed comparison with observations to understand in particular the evolution of the stellar mass-metallicity relation.

The accretion of satellites onto central galaxies along vast cosmic filaments is an apparent outcome of the anisotropic collapse of structure in our Universe. Numerical work (based on gravitational dynamics of N -body simulations) indicates that satellites are beamed towards hosts along preferred directions imprinted by the velocity shear field. Here we use the Sloan Digital Sky Survey to observationally test this claim. We construct 3D filaments and sheets and examine the relative position of satellite galaxies. A statistically significant alignment between satellite galaxy position and filament axis in observations is confirmed. We find a qualitatively compatible alignments by examining satellites and filaments similarly identified in the Millennium simulation, semi-analytical galaxy catalogue. We also examine the dependence of the alignment strength on galaxy properties such as colour, magnitude and (relative) satellite magnitude, finding that the alignment is strongest for the reddest and brightest central and satellite galaxies. Our results confirm the theoretical picture and the role of the cosmic web in satellite accretion. Furthermore our results suggest that filaments identified on larger scales can be reflected in the positions of satellite galaxies that are quite close to their hosts.

We investigate whether the satellite luminosity function (LF) of primary galaxies identified in the Sloan Digital Sky Survey (SDSS) depends on whether the host galaxy is in a filament or not. Isolated primary galaxies are identified in the SDSS spectroscopic sample while potential satellites (that are up to 4 magnitudes fainter than their hosts) are searched for in the much deeper photometric sample. Filaments are constructed from the galaxy distribution by the "Bisous" process. Isolated primary galaxies are divided into two subsamples: those in filaments and those not in filaments. We examine the stacked mean satellite LF of both the filament and non-filament sample and find that, on average, the satellite LFs of galaxies in filaments is significantly higher than those of galaxies not in filaments. The filamentary environment can increases the abundance of the brightest satellites (M sat. < M prim. +2.0), by a factor of ∼ 2 compared with non-filament isolated galaxies. This result is independent of primary galaxy magnitude although the satellite LF of galaxies in the faintest magnitude bin, is too noisy to determine if such a dependence exists. Since our filaments are extracted from a spectroscopic fluxlimited sample, we consider the possibility that the difference in satellite LF is due to a redshift, colour or environmental bias, finding these to be insufficient to explain our result. The dependence of the satellite LF on the cosmic web suggests that the filamentary environment may have a strong effect on the efficiency of galaxy formation.

We study the spatial distribution of satellite galaxies around isolated primaries using the Sloan Digital Sky Survey (SDSS) spectroscopic and photometric galaxy catalogues. We select isolated primaries from the spectroscopic sample and search for potential satellites in the much deeper photometric sample. For specific luminosity primaries we obtain robust statistical results by stacking as many as ∼50 000 galaxy systems. We find no evidence for any anistropy in the satellite galaxy distribution relative to the major axes of the primaries. We derive accurate projected number density profiles of satellites down to 4 mag fainter than their primaries. We find that the normalized satellite profiles generally have a universal form and can be well fitted by projected NFW profiles. The NFW concentration parameter increases with decreasing satellite luminosity while being independent of the luminosity of the primary except for very bright primaries. The profiles of the faintest satellites show deviations from the NFW form with an excess at small galactocentric projected distances. In addition, we quantify how the radial distribution of satellites depends on the colour of the satellites and on the colour and concentration of their primaries.

We investigate the luminosity functions (LFs) and projected number density profiles of galactic satellites around isolated primaries of different luminosity. We measure these quantities for model satellites placed into the Millennium and Millennium II dark matter simulations by the GALFORM semi-analytic galaxy formation model for different bins of primary galaxy magnitude and we investigate their dependence on satellite luminosity. We compare our model predictions to the data of Guo et al. from the Sloan Digital Sky Survey Data Release 8 (SDSS DR8). First, we use a mock light-cone catalogue to verify that the method we used to count satellites in the SDSS DR8 is unbiased. We find that the radial distributions of model satellites are similar to those around comparable primary galaxies in the SDSS DR8, with only slight differences at low luminosities and small projected radii. However, when splitting the satellites by colour, the model and SDSS satellite systems no longer resemble one another, with many red model satellites, in contrast to the dominant blue fraction at similar luminosity in the SDSS. The few model blue satellites are also significantly less centrally concentrated in the halo of their stacked primary than their SDSS counterparts. The implications of this result for the GALFORM model are discussed.

To shed light onto the circumnuclear environment of 22 GHz (λ ∼ 1.3 cm) H 2 O maser galaxies, we have analyzed some of their multi-wavelength properties, including the far infrared luminosity (FIR), the luminosity of the [O III]λ5007 emission line, the nuclear X-ray luminosity, and the equivalent width of the neutral iron Kα emission line (EW (K α )). Our statistical analysis includes a total of 85 sources, most of them harboring an active galactic nucleus (AGN). There are strong anti-correlations between EW (K α ) and two "optical thickness parameters", i.e. the ratios of the X-ray luminosity versus the presumably more isotropically radiated [O III] and far infrared (FIR) luminosities. Based on these anticorrelations, a set of quantitative criteria, EW (K α )>300 eV, L 2−10 keV <2 L [O III] and L FIR >600 L 2−10 keV can be established for Compton-thick nuclear regions. 18 H 2 O maser galaxies belong to this category. There are no obvious correlations between the EW (K α ), the [O III] luminosity and the isotropic H 2 O maser luminosity. When comparing samples of Seyfert 2s with and without detected H 2 O maser lines, there seem to exist differences in EW (K α ) and the fraction of Compton-thick nuclei. This should be studied further. For AGN masers alone, there is no obvious correlation between FIR and H 2 O maser luminosities. However, including masers associated with star forming regions, a linear correlation is revealed. Overall, the extragalactic FIR-H 2 O data agree with the corresponding relation for Galactic maser sources, extrapolated by several orders of magnitude to higher luminosities.

Although structures in the Universe form on a wide variety of scales, from small dwarf galaxies to large super clusters, the generation of angular momentum across these scales is poorly understood. Here we investigate the possibility that filaments of galaxies-cylindrical tendrils of matter hundreds of millions of light years across-are themselves spinning. By stacking thousands of filaments together and examining the velocity of galaxies perpendicular to the filament's axis (via their redshift and blueshift), we find that these objects too display vortical motion consistent with rotation, making them the largest objects known to have angular momentum. The strength of the rotation signal is directly dependent on the viewing angle and the dynamical state of the filament. Filament rotation is more clearly detected when viewed edge-on. In addition, the more massive the haloes that sit at either end of the filaments, the more rotation is detected. These results signify that angular momentum can be generated on unexpectedly large scales.