ALBERT

Description: We present a local sample ( z 〈 0.15) of 280 star-forming compact groups (SFCGs) of galaxies identified in the ultraviolet Galaxy Evolution EXplorer ( GALEX ) All-sky Imaging Survey (AIS). So far, just one prototypical example of SFCG, the Blue Infalling Group, has been studied in detail in the Local Universe. The sample of SFCGs is mainly the result of applying a Friends-of-Friends group finder in the space of celestial coordinates with a maximum linking-length of 1.5 arcmin and choosing groups with a minimum number of four members of bright UV-emitting 17〈FUV〈20.5 sources (mostly galaxies) from the GALEX /AIS catalogue. The result from the search are 280 galaxy groups composed by 226, 39, 11 and 4 groups of four, five, six and seven bright ultraviolet (UV) members, respectively. Only 59 of these 280 newly identified SFCGs have a previous catalogued group counterpart. Group redshifts are available for at least one member in 75 per cent of the SFCGs, and over 40 per cent of the SFCGs have redshifts measured for two or more galaxies. 26 of the SFCGs appear to be located in the infalling regions of clusters with known redshift. The SFCG sample presents a combination of properties different from the group samples studied up to now, such as low-velocity dispersions ( $\sigma _{\rm {}\text{l-o-s}}$ ~ 120 km s –1 ), small crossing-times ( H 0 t c ~ 0.05) and high star formation content (95 per cent of star-forming galaxies). This points to the SFCGs being in an evolutionary stage distinct from those groups selected in the optical and near-infrared ranges. Once redshifts are obtained to discard interlopers, SFCGs will constitute a unique sample of SFCGs.

Description: We report the results of a multiwaveband analysis of the masses and luminosities of ~600 galaxy groups and clusters identified in the maxBCG catalogue. These data are intended to form the basis of future work on the formation of the ‘ m 12 gap’ in galaxy groups and clusters. We use SDSS spectroscopy and g -, r - and i -band photometry to estimate galaxy group/cluster virial radii, masses and total luminosities. In order to establish the robustness of our results, we compare them with literature studies that utilize a variety of mass determinations techniques (dynamical, X-ray and weak lensing) and total luminosities estimated in the B , r , i and K wavebands. We also compare our results to predictions derived from the Millennium Simulation. We find that, once selection effects are properly accounted for, excellent agreement exists between our results and the literature with the exception of a single observational study. We also find that the Millennium Simulation does an excellent job of predicting the effects of our selection criteria. Our results show that, over the mass range ~10 13 –10 15  M , variations in the slope of the mass–luminosity scaling relation with mass detected in this and many other literature studies is in part the result of selection effects. We show that this can have serious ramifications on attempts to determine how the mass-to-light ratio of galaxy groups and cluster varies with mass.

Description: Globular clusters (GCs) can be considered discrete, long-lived, dynamical tracers that retain crucial information about the assembly history of their parent galaxy. In this paper, we present a new catalogue of GC velocities and colours for the lenticular galaxy NGC 1023, we study their kinematics and spatial distribution, in comparison with the underlying stellar kinematics and surface brightness profile, and we test a new method for studying GC properties. Specifically, we decompose the galaxy light into its spheroid (assumed to represent the bulge+halo components) and disc components and use it to assign to each GC a probability of belonging to one of the two components. Then we model the galaxy kinematics, assuming a disc and spheroidal component, using planetary nebulae and integrated stellar light. We use this kinematic model and the probability previously obtained from the photometry to recalculate for each GC its likelihood of being associated with the disc, the spheroid, or neither. We find that the reddest GCs are likely to be associated with the disc, as found for faint fuzzies in this same galaxy, suggesting that the disc of this S0 galaxy originated at z ~= 2. The majority of blue GCs are found likely to be associated with the spheroidal (hot) component. The method also allows us to identify objects that are unlikely to be in equilibrium with the system. In NGC 1023 some of the rejected GCs form a substructure in phase space that is connected with NGC 1023 companion galaxy.

Description: We present a study of the kinematics and the physical properties of the central region of the Hickson Compact Group 31 (HCG 31), focusing on the HCG 31A+C system, using integral field spectroscopy data taken with the Gemini South Telescope. The main players in the merging event (galaxies A and C) are two dwarf galaxies, which have had one close encounter, given the observed tidal tails, and may now be in their second approach, and are possibly about to merge. We present new velocity fields and Hα emission, stellar continuum, velocity dispersion, electron density, Hα equivalent-width and age maps. Considering the high spatial resolution of the integral field unit data, we were able to measure various components and estimate their physical parameters, spatially resolving the different structures in this region. Our main findings are the following: (1) We report for the first time the presence of a super stellar cluster next to the burst associated with the HCG 31C central blob, related to the high values of velocity dispersion observed in this region as well as to the highest value of stellar continuum emission. This may suggest that this system is cleaning its environment through strong stellar winds that may then trigger a strong star formation event in its neighbourhood. (2) Among other physical parameters, we estimate L (Hα) ~ 14  x  10 41  erg s –1 and the star formation rate, SFR ~11 M  yr –1 for the central merging region of HCG 31A+C. These values indicate a high star formation density, suggesting that the system is part of a merging object, supporting previous scenarios proposed for this system.

Description: We have obtained Gemini/Gemini Multi-Object Spectrograph spectra of 28 regions located across the interacting group NGC 6845, spanning from the inner regions of the four major galaxies (NGC 6845A, B, C, D) to the tidal tails of NGC 6845A. All regions in the tails are star-forming objects with ages younger than 10 Myr. We derived the gas-phase metallicity gradients across NGC 6845A and its two tails and we find that these are shallower than those for isolated galaxies. NGC 6845A has a gas-phase oxygen central metallicity of 12 + log(O/H) ~ 8.5 and a flat gas-phase metallicity gradient (β = 0.002 ± 0.004 dex kpc –1 ) out to ~4 R 25 (to the end of the longest tidal tail). Considering the mass–metallicity relation, the central region of NGC 6845A displays a lower oxygen abundance than the expected for its mass. Taking into account this fact and considering the flat oxygen distribution measured along the eastern tidal tail, we suggest that an interaction event has produced a dilution in the central metallicity of this galaxy and the observed flattening in its metal distribution. We found that the star formation process along the eastern tidal structure has not been efficient enough to increase the oxygen abundances in this place, suggesting that this structure was formed from enriched material.

Description: The impending Javalambre Physics of the accelerating Universe Astrophysical Survey (J-PAS) will be the first wide-field survey of 8500 deg 2 to reach the ‘stage IV’ category. Because of the redshift resolution afforded by 54 narrow-band filters, J-PAS is particularly suitable for cluster detection in the range z 〈1. The photometric redshift dispersion is estimated to be only ~0.003 with few outliers 4 per cent for galaxies brighter than i  ~ 23 AB, because of the sensitivity of narrow band imaging to absorption and emission lines. Here, we evaluate the cluster selection function for J-PAS using N -body+semi-analytical realistic mock catalogues. We optimally detect clusters from this simulation with the Bayesian Cluster Finder, and we assess the completeness and purity of cluster detection against the mock data. The minimum halo mass threshold we find for detections of galaxy clusters and groups with both 〉80 per cent completeness and purity is M h  ~ 5  x  10 13 M up to z  ~ 0.7. We also model the optical observable, $M^{\ast }_{\rm CL}$ –halo mass relation, finding a non-evolution with redshift and main scatter of $\sigma _{M^{\ast }_{\rm CL} | M_{\rm h}}\sim 0.14 \,{\rm dex}$ down to a factor 2 lower in mass than other planned broad-band stage IV surveys, at least. For the M h  ~ 1  x  10 14 M Planck mass limit, J-PAS will arrive up to z  ~ 0.85 with a $\sigma _{M^{\ast }_{\rm CL} | M_{\rm h}}\sim 0.12 \, {\rm dex}$ . Therefore, J-PAS will provide the largest sample of clusters and groups up to z  ~ 0.8 with a mass calibration accuracy comparable to X-ray data.

Description: We observed the late-type peculiar galaxy NGC 4424 during the Virgo Environmental Survey Tracing Galaxy Evolution (VESTIGE), a blind narrow-band Hα+[NII] imaging survey of the Virgo cluster carried out with MegaCam at the Canada-French-Hawaii Telescope (CFHT). The presence of a ∼110 kpc (in projected distance) HI tail in the southern direction indicates that this galaxy is undergoing a ram pressure stripping event. The deep narrow-band image revealed a low surface brightness (Σ(Hα) ≃ 4 × 10−18 erg s−1 cm−2 arcsec−2) ionised gas tail ∼10 kpc in length extending from the centre of the galaxy to the north-west, thus in the direction opposite to the HI tail. Chandra and XMM X-rays data do not show a compact source in the nucleus or an extended tail of hot gas, while IFU spectroscopy (MUSE) indicates that the gas is photo-ionised in the inner regions and shock-ionised in the outer parts. Medium-resolution (MUSE) and high-resolution (Fabry-Perot) IFU spectroscopy confirms that the ionised gas is kinematically decoupled from the stellar component and indicates the presence of two kinematically distinct structures in the stellar disc. The analysis of the SED of the galaxy indicates that the activity of star formation was totally quenched in the outer disc ∼250–280 Myr ago, while only reduced by ∼80% in the central regions. All this observational evidence suggests that NGC 4424 is the remnant of an unequal-mass merger that occurred ≲500 Myr ago when the galaxy was already a member of the Virgo cluster, and is now undergoing a ram pressure stripping event that has removed the gas and quenched the activity of star formation in the outer disc. The tail of ionised gas probably results from the outflow produced by a central starburst fed by the collapse of gas induced by the merging episode. This outflow is sufficiently powerful to overcome the ram pressure induced by the intracluster medium on the disc of the galaxy crossing the cluster. This analysis thus suggests that feedback can participate in the quenching process of galaxies in high-density regions.

Description: We present R c -band surface photometry for 170 of the 203 galaxies in GHASP, the Gassendi H-alpha survey of spirals, a sample of late-type galaxies for which high-resolution Fabry–Perot Hα maps have previously been obtained. Our data set is constructed using new R c -band observations taken at the Observatoire de Haute-Provence, supplemented with Sloan Digital Sky Survey archival data, obtained with the purpose of deriving homogeneous photometric profiles and parameters. Our results include R c -band surface brightness profiles for 170 galaxies and ugriz profiles for 108 of these objects. We catalogue several parameters of general interest for further reference, such as total magnitude, effective radius and isophotal parameters (magnitude, position angle, ellipticity and inclination). We also perform a structural decomposition of the surface brightness profiles using a multi-component method to separate discs from bulges and bars, and to observe the main scaling relations involving luminosities, sizes and maximum velocities. We determine the R c -band Tully–Fisher relation using maximum velocities derived solely from Hα rotation curves for a sample of 80 galaxies, resulting in a slope of –8.1 ± 0.5, zero-point of –3.0 ± 1.0 and an estimated intrinsic scatter of 0.28 ± 0.07. We note that, unlike the Tully–Fisher relation in the near-infrared derived for the same sample, no change in the slope of the relation is seen at the low-mass end (for galaxies with V max  〈 125 km s –1 ). We suggest that this different behaviour of the Tully–Fisher relation (with the optical relation being described by a single power law while the near-infrared has two), may be caused by differences in the stellar mass-to-light ratio for galaxies with V max  〈 125 km s –1 .

Description: We have obtained Gemini/Gemini Multi-Object Spectrograph spectra of 28 regions located across the interacting group NGC 6845, spanning from the inner regions of the four major galaxies (NGC 6845A, B, C, D) to the tidal tails of NGC 6845A. All regions in the tails are star-forming objects with ages younger than 10 Myr. We derived the gas-phase metallicity gradients across NGC 6845A and its two tails and we find that these are shallower than those for isolated galaxies. NGC 6845A has a gas-phase oxygen central metallicity of 12 + log(O/H) ~ 8.5 and a flat gas-phase metallicity gradient (β = 0.002 ± 0.004 dex kpc –1 ) out to ~4 R 25 (to the end of the longest tidal tail). Considering the mass–metallicity relation, the central region of NGC 6845A displays a lower oxygen abundance than the expected for its mass. Taking into account this fact and considering the flat oxygen distribution measured along the eastern tidal tail, we suggest that an interaction event has produced a dilution in the central metallicity of this galaxy and the observed flattening in its metal distribution. We found that the star formation process along the eastern tidal structure has not been efficient enough to increase the oxygen abundances in this place, suggesting that this structure was formed from enriched material.

Description: We study the role of systematic effects in observational studies of the cusp–core problem under the minimum disc approximation using a suite of high-resolution (25-pc softening length) hydrodynamical simulations of dwarf galaxies. We mimic realistic kinematic observations and fit the mock rotation curves with two analytic models commonly used to differentiate cores from cusps in the dark matter distribution. We find that the cored pseudo-isothermal sphere (ISO) model is strongly favoured by the reduced $\chi ^2_\nu$ of the fits in spite of the fact that our simulations contain cuspy Navarro–Frenk–White profiles (NFW). We show that even idealized measurements of the gas circular motions can lead to the incorrect answer if velocity underestimates induced by pressure support, with a typical size of order ~5 km s –1 in the central kiloparsec, are neglected. Increasing the spatial resolution of the mock observations leads to more misleading results because the inner region, where the effect of pressure support is most significant, is better sampled. Fits to observations with a spatial resolution of 100 pc (2 arcsec at 10 Mpc) favour the ISO model in 78–90 per cent of the cases, while at 800-pc resolution, 41–77 per cent of the galaxies indicate the fictitious presence of a dark matter core. The coefficients of our best-fitting models agree well with those reported in observational studies; therefore, we conclude that NFW haloes cannot be ruled out reliably from this type of analysis.