Context. The detection of extragalactic sources beyond the Milky Way is limited by the location of the Solar system. In the zone of avoidance (ZOA), optical observations are hindered by stellar crowding and Galactic absorption. To fully understand the background galaxy population in this region, observations at longer wavelengths are needed. Aims. The objective of this study is to determine the characteristics of galaxies that are situated behind the Milky Way bulge through the use of near-infrared (NIR) data obtained from the VISTA Variables in V & iacute;a L & aacute;ctea (VVV) survey. Methods. The galaxy candidates were visually confirmed, and a morphological classification of all sources was performed based on their appearance, by differentiating between elliptical, spiral, undefined, and doubtful galaxies. Additionally, the surrounding area was classified based on the number of sources that are detected in the same target field. Isolated galaxies, galaxies with close companions, and galaxies in denser environments were catalogued. Results. Following a visual inspection, 14 480 galaxy candidates were classified morphologically: 3702 as elliptical, 1704 as spiral, 6425 as undefined, and 2649 as doubtful. The environmental classification showed that 73.9% were isolated, 12.4% had close companions, and 13.7% were in denser environments. Our catalogue shows slower proper motions than star catalogues. The majority of galaxies that are classified as elliptical and spiral manifest SExtractor parameters that are indicative of this category of sources. Additionally, elliptical galaxies in denser environments appear to be redder than spiral galaxies in isolated environments. Conclusions. A total of 14 480 galaxy candidates were subjected to a morphological classification in the region of the Milky Way bulge. This result verifies the efficacy of the VVV/VVVX survey in identifying and characterising a diverse range of galaxies and extragalactic structures that are obscured by the Milky Way. The survey thereby contributes to a more profound understanding of the Universe in the ZOA.

We present a statistical analysis of different astrophysical properties of a sample of galaxy pairs in cosmic voids. The sample consists of 72 galaxy pairs with projected separations and relative radial velocities rp < 100 h(-1) kpc, Delta V < 500 km s(-1) in the redshift range z < 0.1. We compared the different results for this pair sample to those derived for matched samples configured in absolute magnitude, stellar mass, and concentration residing in void, wall, and global averaged environments. We find that the pair galaxies in voids tend to have bluer optical colors than the corresponding galaxies in the wall and field environments, regardless of their stellar mass and concentration, which indicates a more recent formation of the bulk of stars. We also obtained larger mid-IR color indices for the void paired galaxies with respect to the corresponding matched samples in the wall and field environments. However, we found significantly larger differences for galaxies with a high mass and concentration. We also note that a mid-IR color-color diagram of the results showed void pair members consistent with the locus of star forming galaxies, in contrast to the other environments that exhibit a bimodal behavior comprising both passive and star forming objects. The Dn(4000) parameter also shows a significant younger stellar population in paired galaxies in voids. This is also reflected in the higher star formation rate values, which show a greater efficiency for void paired galaxies. We note that the star formation efficiency is greater for void paired galaxies with a high stellar mass and concentration. We also find that the efficiency of star formation associated with galaxy interactions is significantly greater in pairs residing in cosmic voids. This increased star formation activity could be associated with the expected richer gas environment and a gentler dynamical behavior as compared to the more eccentric orbits and stronger interactions and mergers more likely to occur in the wall and field environments.

Aims. We explore galaxies with ringed structures in the TNG50 simulation to study their frequency and the influence of physical processes on their properties and morphologies. Methods. The catalog of ringed galaxies was compiled through visual classification of synthetic images from the TNG50 simulation. Galaxies were selected based on specific criteria: a redshift range of 0.0110(9) M-circle dot, stellar half-mass radius r(50)>1 kpc, and a specific star formation rate (sSFR), log(sSFR/yr(-1))>-13. Our classification allowed for differentiation between inner rings, outer rings, combinations of rings - inner + outer (i+o) -, and partial rings (pseudo-rings), including barred and non-barred ringed galaxies. We constructed a suitable control sample of non-ringed galaxies with similar redshift, stellar mass, and environmental density distributions to those of the ringed ones. The comparison between the galaxies in both samples enabled the analysis of their properties and differences. Finally, we analyzed the surface density of stellar mass (Sigma(*)(r)) to better understand the distribution and structure of the rings in relation to the properties of their host galaxies. Results. We identified 807 ringed galaxies within the TNG50 simulation. Approximately 59% of these galaxies possess an inner ring, 22% a partial ring, 12% an outer ring, and 7% have i+o rings. Furthermore, our statistical analysis reveals that a notable 64% (507 galaxies) exhibit bars. In terms of physical properties, we find that ringed galaxies exhibit a lower efficiency for star formation activity, reduced gas fractions, redder colors, and higher metallicities with respect to non-ringed disk objects. Moreover, ringed galaxies display greater variability in metallicity for a given stellar mass compared to their non-ringed counterparts, indicating distinct evolutionary processes. From the analysis of radial profiles, we find that galaxies with outer rings exhibit a r(50) similar to or slightly larger than their control group, while those with inner or partial rings tend to have smaller sizes. A deeper exploration of radial density profiles revealed a pronounced central mass deficit preceding the ring structures, with inner and outer rings located approximately at r(50) and 1.5 r(50), respectively. Galaxies with both i+o rings have inner rings that are more compact and massive. Additionally, galaxies with partial rings exhibit deeper mass profiles than their controls, particularly in central areas where pseudo-rings extend beyond r(50). These findings not only improve our understanding of galactic evolution in ringed galaxies but also underline the complex interplay between mass distribution and galactic morphology.

The catalog of ringed galaxies was compiled through visual classification of synthetic images from the TNG50 simulation. Galaxies were selected based on specific criteria: a redshift range of 0.01 < z < 0.1, stellar mass M_⋆ >10^9 M_⊙, stellar half-mass radius r_50 > 1 kpc, and specific star formation rate (sSFR), log(sSFR/yr^-1) > -13. Our classification allowed for differentiation between inner rings, outer rings, combinations of rings, and partial rings (pseudo-rings), including barred and non-barred ringed galaxies. We constructed a control sample of non-ringed galaxies with similar redshift, stellar mass, and environmental density distributions. We identified 807 ringed galaxies. Approximately 59 outer ring, and 7 (507 galaxies) exhibit bars. Ringed galaxies exhibit lower efficiency for star formation, reduced gas fractions, redder colors, and higher metallicities compared to non-ringed disk objects. They also show greater variability in metallicity for a given stellar mass. From the analysis of radial profiles, galaxies with outer rings exhibit a r_50 similar to or slightly larger than their control group, while those with inner or partial rings tend to have smaller sizes. A deeper exploration of radial density profiles revealed a pronounced central mass deficit preceding the ring structures, with inner and outer rings located at r_50 and 1.5 , r_50, respectively. Galaxies with both i+o rings have inner rings that are more compact and massive. Additionally, galaxies with partial rings exhibit deeper mass profiles than their controls, particularly in central areas. These findings improve our understanding of galactic evolution and the complex interplay between mass distribution and morphology.

Context. The location of the Solar System complicates the detection extragalactic sources beyond the Milky Way plane. The optical observations are hampered in the so-called zone of avoidance (ZOA), where stellar crowding and Galactic absorption are severe. Observations at longer wavelengths are needed in order to discover new background galaxies and complete the census in the ZOA. Aims. The goal of this work is to identify galaxies behind the Milky Way bulge using near-infrared (NIR) data from the VISTA Variables in Via Lactea (VVV) survey. Methods. To this end, we made use of different VISTA Science Archive (VSA) tools in order to extract relevant information from more than 32 billion catalogued sources in the VVV bulge region. We find that initial photometric restriction on sources from the VSA vvvSource table combined with restrictions on star-galaxy separation parameters obtained from Source Extractor is a successful strategy for achieving acceptable levels of contamination (60%) and high completeness (75%) in the construction of a galaxy target sample. To remove contaminating Galactic sources from the initial target sample, our methodology also incorporates a visual inspection of false-colour RGB images, a crucial quality control carried out following a specifically defined procedure. Results. Under this methodology, we find 14 480 galaxy candidates in the VVV bulge region, making this sample the largest catalogue to date in the ZOA. Moreover, these new sources provide a fresh picture of the Universe hidden behind the curtain of stars, dust, and gas in the unexplored Milky Way bulge region. Conclusions. The results from this work further demonstrate the potential of the VVV/VVVX survey to find and study a large number of galaxies and extragalactic structures obscured by the Milky Way, expanding our knowledge of the Universe in this challenging and impressive region of the sky.

Aims. To unveil the influence of galaxy-galaxy interactions on the material transport driven by galactic bars toward the central regions of active galactic nucleus (AGN) galaxies, and to assess the efficiency of the combined mechanisms of interactions and bars in fueling massive black holes, we meticulously examine barred active galaxies in paired systems. Methods. Our study focuses on barred AGN galaxies in pairs with projected separations of r(p) < 100 kpc h(-1) and relative radial velocities of Delta V < 500 km s(-1) within z < 0.1, identified by the Sloan Digital Sky Survey (SDSS). To quantify the impact of interactions on material transport by galactic bars, we also constructed a suitable control sample of barred active galaxies without paired companions, matched in redshift, absolute r-band magnitude, stellar mass, color, and stellar age distributions. Additionally, we calculated the structural characteristics of galactic bars through two-dimensional image modeling, considering that bars exhibit a wide range of shapes and sizes, which may influence their ability to channel material. Results. From this study, we clearly found that nuclear activity (derived from the Lum[OIII]) increases as the projected separations between galaxy pair members decrease. Notably, barred AGN galaxies in close pairs (r(p) (sic) 25 kpc h(-1)) exhibit significantly higher nuclear activity compared to galaxies in the control sample. Additionally, barred galaxies with a close pair companion show enhanced nuclear activity across all ranges of luminosity, stellar mass, and color. We also found that barred AGN galaxies with longer bar structures exhibit more efficient nuclear activity compared to those with shorter bars. This trend is especially pronounced in barred AGN galaxies within close pair systems, which show a significant excess of high Lum[OIII] values. Furthermore, we examined the central nuclear activity in barred AGNs undergoing major and minor interactions. Our findings show a clear escalation in nuclear activity as the pair projected separations decrease, particularly pronounced in major systems. Additionally, nuclear activity distributions in barred AGN samples within major and minor pairs exhibit similar trends. However, a significant deviation occurs among barred AGN galaxies in close pair systems within major interactions, showing a substantial excess of high Lum[OIII] values. This result is also reflected in the analysis of the accretion strength onto central black holes. These findings indicate that external perturbations from a nearby galaxy companion can influence gas flows induced by galactic bars, leading to increased nuclear activity in barred AGN galaxies within pair systems. Thus, the coexistence of both - bars and interactions - significantly amplifies central nuclear activity, thereby influencing the accretion processes onto massive black holes.

Context. The location of the Solar System constrains the detection of extragalactic sources beyond the Milky Way(MW) plane. The optical observations are hampered in the so–called Zone of Avoidance (ZOA) where stellar crowding and Galactic absorption are severe. Observations at longer wavelengths are needed to discover new background galaxies and complete the census in the ZOA. Aims. The goal of this work is to identify galaxies behind the MW Bulge using near-infrared (NIR) data from the VISTA Variables in Vía Láctea (VVV) survey. Methods. To this end we made use of different VISTA Science Archive (VSA) tools in order to extract relevant information from more than 32 billion catalogued sources in the VVV Bulge region. We find that initial photometric restriction on sources from VSA table combined with restrictions on star–galaxy separation parameters obtained from Source Extractor is a successful strategy to achieve acceptable levels of contamination (60%) and a high completeness (75%) in the construction of a galaxy target sample. To decontaminate the initial target sample from Galactic sources our methodology also incorporates the visual inspection of false colour RGB images, a crucial quality control which was carried out following a specifically defined procedure. Results. Under this methodology we find 14480 galaxy candidates in the VVV Bulge region making this sample the largest catalogue to date in the ZOA. Moreover these new sources provide a fresh picture of the Universe hidden behind the curtain of stars, dust and gas in the unexplored MW Bulge region. Conclusions. The results from this work further demonstrate the potential of the VVV/VVVX survey to find and study a large number of galaxies and extragalactic structures obscured by the MW, enlightening our knowledge of the Universe in this challenging and impressive region of the sky.

Aims. We explore galaxies with ringed structures inhabiting poor and rich groups with the aim of assessing the effects of local density environments on ringed galaxy properties. Methods. We identified galaxies with inner, outer, nuclear, inner+outer (inner and outer rings combined), and partial rings that reside in groups by cross-correlating a sample of ringed galaxies with a group catalog obtained from Sloan Digital Sky Survey (SDSS). The resulting sample was divided based on group richness. To quantify the effects of rings and the role of local density environment on galaxy properties, we constructed a suitable control sample for each catalog of ringed galaxies, consisting of non-ringed galaxies with similar values for the z, magnitude, morphology, group masses, and environmental density distributions as those of ringed ones. We explored the occurrence of ringed galaxies in poor and rich groups and analyzed several galaxy properties, such as SFR, stellar populations, and colors. Results. We obtained a sample of 637 ringed galaxies residing in groups. With 76% of these galaxies inhabiting poor groups and 24% rich groups. In addition, ringed galaxies in groups display a reduction in their star formation activity and aged stellar populations, compared to non-ringed ones in the control samples. However, the SFR is higher for nuclear rings in poor groups than for other types. This disparity may stem from the environmental influence on the internal processes of galaxies, either enhancing or diminishing star formation. Ringed galaxies also show an excess of red colors and tend to populate the green valley and the red sequence of color-magnitude and color-color diagrams, with a surplus of galaxies in the red sequence, while non-ringed galaxies are found in the green valley and the blue region. These trends are more significant in galaxies with ringed structures residing in rich groups.

Context. The zone of avoidance (ZoA) does not allow for clear optical observations of extragalactic sources behind the Milky Way due to the meaningful extinction of the optical emission of these objects. Observations in near-infrared (NIR) wavelengths represent a potential source of astronomical discoveries that support the detection of new galaxies and potentially complete the picture of the large-scale structures in this as-yet poorly explored area of the sky. Aims. Our aim is to decipher the nature of the overdensity located behind the Milky Way in tile b204 of the VISTA Variables in Vía Láctea (VVV) survey. Methods. We studied an area of six arcmin around a galaxy concentration located at l = 354.82° and b = −9.81°. We selected five galaxies, taking into account the source distribution on the sky to optimise the requested time for the observations, and we obtained the spectra with Flamingos 2 long-slit spectrograph at Gemini South 8.1-meter telescope. To identify and characterise the absorption features, we fit the galaxies underlying spectrum using the STARLIGHT code together with the IRTF stellar library. In addition, the spectroscopic findings are reinforced using complementary photometric techniques such as red-sequence and photometric redshift estimation. Results. The mean spectroscopic redshift estimated from the NIR spectra is z = 0.225 ± 0.014. This value presents a good agreement with that obtained from photometric analysis, photoz = 0.21 ± 0.08, and the probability distribution function of the galaxies in the studied region. Also, the red-sequence slope is consistent with the one expected for NIR observations of galaxy clusters. Conclusions. The redshifts obtained from both, photometric and spectroscopic techniques are in good agreement, allowing for the confirmation of the nature of this structure at z = 0.225 ± 0.014, thereby unveiling a new galaxy cluster, VVVGCl-B J181435-381432, behind the Milky Way bulge.

Context. The extragalactic vision we have through the Milky Way is very unclear. There is significant extinction of the optical emission from objects located in the region called the zone of avoidance (ZOA). NIR wavelengths are less affected by extinction, and therefore the infrared surveys in this zone are a potential source of astronomical discoveries. Nevertheless, these observations need to be compared with cosmological simulations in order to carry out high−accuracy studies. Aims. Our aim is to identify extragalactic sources in the ZOA, using infrared images of the VVV survey. We consider mock galaxy catalogues in order to interpret observational results. Methods. We studied a region of 1.636 square degrees corresponding to the VVV tile b204. Using SExtractor, we analysed photometric data generating a catalogue of extended sources in this area. In order to confirm these sources as galaxy candidates we visually inspected RGB images looking for typical galaxy features. Using 2MASX and GCMW catalogued sources we tested completeness and contamination of our catalogue and define suitable colour cuts to select galaxies. We also compared the observational results with those obtained from two semi-analytical models on Dark Matter simulations. One galaxy catalogue was constructed with the SAG semi-analytic model of galaxy formation, and the other one was constructed with the L-Galaxies semi-analytic model. Results. By adopting CLASS_STAR < 0.5, r1/2 > 0.7 arcsec and specific colour cuts (J − Ks > 0.97, J − H > 0 and H − Ks > 0) we generated an automatic catalogue of extended sources. After visual inspection we identified 624 sources with 10 < Ks < 17 as galaxy candidates. The contamination of the automatic catalogue is 28% when considering visually confirmed galaxies as reliable objects. The estimated completeness is 87% up to magnitude Ks = 13.5. We analysed the spatial distribution of galaxy candidates, finding a high concentration of galaxies in a small region of 15 arcmin radius. This region has three times higher density than similar areas in the tile. We compared the number of galaxies in this small area with the mean density values obtained from a suitable sample of galaxies from semi-analytic models finding that our results are consistent with an overdensity region. Conclusions. Using VVV near-infrared data and mock catalogues we detect new extragalactic sources that have not been identified by other catalogues. We demonstrate the potentiality of the VVV survey in finding and studying a large number of galaxy candidates and extragalactic structures obscured by the Milky Way.

We use SDSS-DR14 to construct a sample of galaxy systems consisting of a central object and two satellites. We adopt projected distance and radial velocity difference criteria and impose an isolation criterion to avoid membership in larger structures. We also classify the interaction between the members of each system through a visual inspection of galaxy images, finding ∼ 80% of the systems lack evidence of interactions whilst the remaining ∼ 20% involve some kind of interaction, as inferred from their observed distorted morphology. We have considered separately, samples of satellites and central galaxies, and each of these samples were tested against suitable control sets to analyse the results. We find that central galaxies showing signs of interactions present evidence of enhanced star formation activity and younger stellar populations. As a counterpart, satellite samples show these galaxies presenting older stellar populations with a lower star formation rate than the control sample. The observed trends correlate with the stellar mass content of the galaxies and with the projected distance between the members involved in the interaction. The most massive systems are less affected since they show no star formation excess, possibly due to their more evolved stage and less gas available to form new stars. Our results suggest that it is arguable a transfer of material during interactions, with satellites acting as donors to the central galaxy. As a consequence of the interactions, satellite stellar population ages rapidly and new bursts of star formation may frequently occur in the central galaxy.

We perform a comparative study of AGNs in pairs, triplets and groups. To this end we use the Duplancic et al. catalogue of small galaxy systems and consider BPT and WHAN diagnostic diagrams to select optical AGNs. Also we identify mid-IR AGNs by using WISE data. We performed a comparison between the different AGN classification methodologies and study the AGN fraction in pairs, triplets, and groups with four to six members. We also analyse the main properties of Optical and mid-IR AGN hosts and the influence of environment on the active nuclei phenomena in these small galaxy systems. Our results show that, regardless the specifically adopted classification scheme, the fraction of AGN in pairs and triplets is always higher than the corresponding fraction in groups. Moreover, the fraction of powerful AGNs in pair and triplets is about twice the fraction of regular AGNs. We also find a remarkable difference between Optical and mid-IR AGNs in groups, where host galaxies of WISE AGNs are less massive and concentrated, with young stellar populations and blue colours. Also all WISE AGNs in groups have a very close companions and reside in an intermediate global density environment. Galaxy triplets show a larger AGN fraction for galaxies with a close nearest neighbours, while pairs present a nearly constant AGN fraction regardless the distance to the nearest companion. Our studies highlight the important role of interactions, besides the global environment dependence, in the activation of the AGN phenomenon in small galaxy systems.

Aims. We present a statistical analysis of different characteristics of ringed spiral galaxies with the aim of assessing the effects of rings on disk galaxy properties. Methods. We built a catalog of ringed galaxies from the Sloan Digital Sky Survey Data Release 14 (SDSS-DR14). Via visual inspection of SDSS images, we classified the face-on spiral galaxies brighter than g < 16.0 mag into galaxies with: an inner ring, an outer ring, a nuclear ring, both an inner and an outer ring, and a pseudo-ring. In addition to rings, we recorded morphological types and the existence of bars, lenses, and galaxy pair companions with or without interaction. With the goal of providing an appropriate quantification of the influence of rings on galaxy properties, we also constructed a suitable control sample of non-ringed galaxies with similar redshift, magnitude, morphology, and local density environment distributions to those of ringed ones. Results. We found 1868 ringed galaxies, accounting for 22% of the full sample of spiral galaxies. In addition, within galaxies with ringed structures, 46% have an inner ring, 10% an outer ring, 20% both an inner and an outer ring, 6% a nuclear ring, and 18% a partial ring. Moreover, 64% of the ringed galaxies present bars. We also found that ringed galaxies have both a lower efficiency of star formation activity and older stellar populations (as derived with the D n (4000) spectral index) with respect to non-ringed disk objects from the control sample. Moreover, there is a significant excess of ringed galaxies with red colors. These effects are more important for ringed galaxies that have inner rings and bars with respect to their counterparts that have some other types of rings and are non-barred. The color-magnitude and color-color diagrams show that ringed galaxies are mostly concentrated in the red region, while non-ringed spiral objects are more extended to the blue zone. Galaxies with ringed structures present an excess of high metallicity values compared to non-ringed ones, which show a 12 + Log(O/H) distribution toward lower values. These findings seem to indicate that rings are peculiar structures that produce an accelerating galactic evolution, strongly altering the physical properties of their host galaxies.

The main goal of this work is to investigate the influence of environment at different scales on the properties of galaxies in systems with a low number of members. To this end, we used a catalogue of small galaxy systems comprising compact and locally isolated pairs, triplets, and groups with four and up to six galaxies. We consider fixed aperture estimators and found that at scales lower than 5 Mpc pairs are associated to lower density environments than triplets and groups. Moreover, a nearest neighbour approach highlights that triplets prefer denser environments than pairs and slightly less dense environments than groups. When considering the position within the cosmic web we found that pairs and triplets in our sample are associated to void environments while galaxy groups are more likely to reside in void walls. In agreement with these results, the system-galaxy cross-correlation function shows that pairs inhabit environments of lesser density compared to triplets and groups, and on small scales (< 3 Mpc) triplets appear to behave as an intermediate system. Related to the properties of neighbour galaxies of small systems we found that the neighbours of groups present a lower fractions of star-forming, young stellar population, and blue colour galaxies with respect to neighbours of triplet and pair systems. These results suggest that differences in the properties of galaxies in pairs, triplets, and groups are not only related to the existence of an extra galaxy member but also to the large-scale environment inhabited by the systems.

Here we analyze a sample of close galaxy pairs (relative projected separation < 25 h−1 kpc and relative radial velocities < 350 km s−1) using a weak-lensing analysis based on the Canada-France-Hawaii Telescope Stripe 82 Survey (CS82). We determine halo masses for the total sample of pairs as well as for interacting, red, and higher-luminosity pair subsamples with ∼3σ confidence. The derived lensing signal for the total sample can be fitted either by a Singular Isothermal Sphere (SIS) with σV = 223 ± 24 km s−1 or a Navarro–Frenk–White (NFW) profile with R200 = 0.30 ± 0.03 h−1 Mpc. The pair total masses and total r band luminosities imply an average mass-to-light ratio of ∼200 h M⊙/L⊙. On the other hand, red pairs which include a larger fraction of elliptical galaxies, show a larger mass-to-light ratio of ∼345 h M⊙/L⊙. Derived lensing masses were compared to a proxy of the dynamical mass, obtaining a good correlation. However, there is a large discrepancy between lensing masses and the dynamical mass estimates, which could be accounted for by astrophysical processes such as dynamical friction, by the inclusion of unbound pairs, and by significant deviations of the density distribution from SIS and NFW profiles in the inner regions. We also compared lensing masses with group mass estimates, finding very good agreement with the sample of groups with two members. Red and blue pairs show large differences between group and lensing masses, which is likely due to the single mass-to-light ratio adopted to compute the group masses.

Aims. With the aim of performing an analysis of the orientations of galaxy pair systems with respect to the underlying large-scale structure, we study the alignment between the axis connecting the pair galaxies and the host cosmic filament where the pair resides. In addition, we analyze the dependence of the amplitude of the alignment on the morphology of pair members as well as filament properties. Methods. We build a galaxy pair catalog requiring rp < 100 h−1 kpc and ΔV < 500 km s−1 within redshift z < 0.1 from the Sloan Digital Sky Survey (SDSS). We divided the galaxy pair catalog taking into account the morphological classification by defining three pair categories composed by elliptical–elliptical (E–E), elliptical–spiral (E–S) and spiral–spiral (S–S) galaxies. We use a previously defined catalog of filaments obtained from SDSS and we select pairs located closer than 1 h−1 Mpc to the filament spine, which are considered as members of filaments. For these pairs, we calculate the relative angle between the axis connecting each galaxy, and the direction defined by the spine of the parent filament. Results. We find a statistically significant alignment signal between the pair axes and the spine of the host filaments consistent with a relative excess of ∼15% aligned pairs. We obtain that pairs composed by elliptical galaxies exhibit a stronger alignment, showing a higher alignment signal for pairs closer than 200 h−1 kpc to the filament spine. In addition, we find that the aligned pairs are associated with luminous host filaments populated with a high fraction of elliptical galaxies. The findings of this work show that large-scale structures play a fundamental role in driving galactic anisotropic accretion as induced by galaxy pairs exhibiting a preferred alignment along the filament direction.

We aim at defining homogeneous selection criteria of small galaxy systems in order to build catalogues suitable to compare main properties of pairs, triplets, and groups with four or more members. To this end we use spectroscopic and photometric Sloan Digital Sky Survey (SDSS) data to identify systems with a low number of members. We study global properties of these systems and the properties of their member galaxies finding that galaxies in groups are systematically redder and with lower star formation activity indicators than galaxies in pairs which have a higher fraction of star-forming galaxies. Triplet galaxies present intermediate trends between pairs and groups. We also find an enhancement of star formation activity for galaxies in small systems with companions closer than 100 kpc, irrespective of the number of members. We have tested these analyses on SUSS mock catalogues derived from the Millennium simulation, finding as conservative thresholds 76 per cent completeness and a contamination of 23 per cent in small galaxy systems, when considering an extreme case of incompleteness due to fiber collisions. Nevertheless, we also found that the results obtained are not likely affected by projection effects. Our studies suggest that an extra galaxy in a system modify the properties of the member galaxies. In pairs, galaxy-galaxy interactions increase gas density and trigger starbursts. However, repeated interactions in triplets and groups can generate gas stripping, turbulence, and shocks quenching the star formation in these systems.

Aims. With the aim of performing a suitable comparison of the internal process of galactic bars with respect to the external effect of interactions on driving gas toward the inner most region of the galaxies, we explored and compared the efficiency of both mechanisms on central nuclear activity in optically selected active galactic nuclei (AGNs) in spiral galaxies. Methods. We selected homogeneous samples of barred AGNs and active objects residing in pair systems, derived from the Sloan Digital Sky Survey (SDSS). In order to carry out a reliable comparison of both samples (AGNs in barred hosts in isolation and in galaxy pairs), we selected spiral AGN galaxies with similar distributions of redshift, magnitude, stellar mass, color and stellar age population from both catalogs. With the goal of providing an appropriate quantification of the influence of strong bars and interactions on nuclear activity, we also constructed a suitable control sample of unbarred spiral AGNs without a companion and with similar host properties to the other two samples. Results. We found that barred optically selected AGNs show an excess of nuclear activity (as derived from the Lum[OIII]) and accretion rate onto a central black hole (ℛ) with respect to AGNs in pairs. In addition, both samples show an excess of high values of Lum[OIII] and ℛ with respect to unbarred AGNs in the control sample. We also found that the fractions of AGNs with powerful nuclear activity and high accretion rates increase toward more massive hosts with bluer colors and younger stellar populations. Moreover, AGNs with bars exhibit a higher fraction of galaxies with powerful Lum[OIII] and efficient ℛ with respect to AGN galaxies inhabiting pair systems, in bins of different galaxy properties. Regarding AGNs belonging to pair systems, we found that the central nuclear activity is remarkably dependent on the galaxy pair companion features. The Lum[OIII] for AGNs in pairs is clearly enhanced when the galaxy companion exhibits a bright and more massive host with high metallicity, blue color, efficient star formation activity and young stellar population. The results of this work reveal an important capacity of both mechanisms, bars and interactions, to transport material towards the galaxy central regions. In this context, it should also be noted that the internal process of the bar is more efficient at improving the central nuclear activity in AGN objects than that corresponding to the external mechanism of the galaxy–galaxy interactions.