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  • 1
    Publication Date: 2015-08-29
    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.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
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  • 2
    Publication Date: 2015-04-02
    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.
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    Electronic ISSN: 1365-2966
    Topics: Physics
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  • 3
    Publication Date: 2016-01-02
    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.
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  • 4
    Publication Date: 2015-08-27
    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.
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  • 5
    Publication Date: 2015-09-27
    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.
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  • 6
    Publication Date: 2016-01-16
    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.
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  • 7
    Publication Date: 2014-03-21
    Description: We report the results of Gemini/GMOS long-slit spectroscopic observations along the major and minor axes of the central galaxies in two fossil groups, SDSS J073422.21+265133.9 and SDSS J075828.11+374711.8 (the NGC 2484 group). Spatially resolved kinematics and stellar population parameters (ages, metallicities and α-element abundance ratios) derived using ~20 Lick indices are presented. Despite remarkable similarities in their morphologies, photometric properties (luminosity and colour) and kinematics, the two galaxies exhibit significantly different stellar population parameters. SDSS J073422.21+265133.9 exhibits a strong metallicity gradient ( [ Z /H]/ R  ~ –0.4) all the way into the centre of the galaxy. It also exhibits an age profile that suggest a relatively recent, centrally concentrated burst of star formation superimposed on an older, more spatially extended population. NGC 2484, a well-known X-ray AGN, exhibits a flat core-like structure in its metallicity gradient, but no detectable age gradient. The α-element abundance ratio ([ E /Fe]) profiles of the two galaxies are also significantly different. SDSS J073422.21+265133.9 exhibits a slightly positive gradient ( [ E /H]/ R  ~ 0.1), perhaps again suggesting a more recent central burst of star formation, while NGC 2484 shows a negative gradient ( [ E /H]/ R  ~ –0.1), indicating that star formation may have happened ‘inside out’. Our analysis of these two galaxies of similar mass, morphology and kinematics therefore suggests two different mechanisms to have been in action during their formation. Consequently, we conclude that the central galaxies of fossil groups cannot be considered a homogeneous group with regard to their formation processes or star formation histories.
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  • 8
    Publication Date: 2014-05-04
    Description: We present an analysis of the colour–magnitude relation for a sample of 56 Abell X-ray underluminous clusters. Our aim is to unveil properties that might elucidate the evolutionary stages of the galaxy populations that compose such systems. To do so, we have compared the parameters of their colour–magnitude relations with the ones found for another sample of 50 normal Abell X-ray emitting clusters, both samples selected in an objective way. The g and r magnitudes from Data Release 7 of the Sloan Digital Sky Survey were used for constructing the colour–magnitude relations. We have found that both samples show the same trend: the red-sequence slopes change with redshift, but the slopes for X-ray underluminous clusters are always flatter than those for normal clusters, with a difference of about 69 per cent along the surveyed redshift range of 0.05 ≤ z  〈 0.20. Also, the intrinsic scatter of the colour–magnitude relation was found to grow with redshift for both samples; however, for the X-ray underluminous clusters, this is systematically larger by about 28 per cent. By applying the Cramér test to the result of this comparison between X-ray normal and underluminous cluster samples, we obtain probabilities of 92 and 99 per cent that the red-sequence slope and intrinsic scatter distributions, respectively, differ, in the sense that X-ray underluminous clusters red sequences show flatter slopes and higher scatters in their relations. No significant differences in the distributions of red-sequence median colours are found between the two cluster samples. This points to X-ray underluminous clusters being younger systems than normal clusters, possibly in the process of accreting groups of galaxies, individual galaxies and gas.
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  • 9
    Publication Date: 2014-01-29
    Description: We present new Gemini/GMOS spectroscopic and archival imaging data of the interacting galaxy NGC 92, which is part of a compact group and displays an extended tidal tail. We have studied the physical properties of 20 star-forming complexes in this system. We found that the star-forming regions located in the tidal tail of NGC 92 have ages younger than ~8 Myr, which suggests that these objects were formed in situ. The spectroscopic data reveal that these regions have slightly sub-solar metallicities, suggesting that they were formed from pre-enriched material. Using the oxygen abundances derived for each system, we found that the extended tidal tail of NGC 92 has a flat metallicity distribution. Although this scenario is consistent with N -body simulations of interacting systems, where there is gas mixing triggered by the interaction, archival Hα Fabry–Perot data cubes of NGC 92 have not shown a velocity gradient along the tail of this galaxy, which under certain assumptions could be interpreted as a lack of gas flow in the tail. Our results suggest that a fraction of the enriched gas that was originally located in the centre of the galaxy was expelled into the tidal tail when the interacting process that formed the tail happened. However, we cannot exclude the scenario in which the star formation in the tail has increased its original oxygen abundance.
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  • 10
    Publication Date: 2014-06-20
    Description: We present new Fabry-Perot observations for a sample of 42 galaxies located in 12 compact groups of galaxies: HCG 1, HCG 14, HCG 25, HCG 44, HCG 53, HCG 57, HCG 61, HCG 69, HCG 93, VV 304, LGG 455 and Arp 314. From the 42 observed galaxies, a total of 26 objects are spiral galaxies, which range from Sa to Im morphological types. The remaining 16 objects are E, S0 and S0a galaxies. Using these observations, we have derived velocity maps, monochromatic and velocity dispersion maps for 24 galaxies, where 18 are spiral, three are S0a, two are S0 and one is an Im galaxy. From the 24 velocity fields obtained, we could derive rotation curves for 15 galaxies; only two of them exhibit rotation curves without any clear signature of interactions. Based on kinematic information, we have evaluated the evolutionary stage of the different groups of the current sample. We identify groups that range from having no Hα emission to displaying an extremely complex kinematics, where their members display strongly perturbed velocity fields and rotation curves. In the case of galaxies with no Hα emission, we suggest that past galaxy interactions removed their gaseous components, thereby quenching their star formation. However, we cannot discard that the lack of Hα emission is linked with the detection limit for some of our observations.
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