Ariel Zandivarez

Galaxy Groups in the SDSS-DR3

We present a new sample of galaxy groups identified in the Sloan Digital Sky Survey Data Release 3. Following previous works we use the well tested friend-of-friend algorithm developed by Huchra & Geller which take into account the number density variation due to the apparent magnitude limit of the galaxy catalog. To improve the identification we implement a procedure to avoid the artificial merging of small systems in high density regions and then apply an iterative method to recompute the group centers position. As a result we obtain a new catalog with 10864 galaxy groups with at least four members. The final group sample has a mean redshift of 0.1 and a median velocity dispersion of

On the luminosity function of galaxies in groups in the Sloan Digital Sky Survey

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Assessing the reliability of friends-of-friends groups on the future Javalambre Physics of the Accelerating Universe Astrophysical Survey

.We present a new sample of galaxy groups identified in the Sloan Digital Sky Survey Data Release 3. Following previous works, we use the well-tested friends-of-friends algorithm developed by Huchra and Geller, which takes into account the number density variation due to the apparent magnitude limit of the galaxy catalog. To improve the identification, we implement a procedure to avoid the artificial merging of small systems in high-density regions and then apply an iterative method to recompute the group centers position. As a result, we obtain a new catalog with 10,864 galaxy groups with at least four members. The final group sample has a mean redshift of 0.1 and a median velocity dispersion of 230 km s-1.

The power spectrum of flux-limited X-ray galaxy cluster surveys

Using galaxy groups identified in the fourth data release of the Sloan Digital Sky Survey (SDSS), we compute the luminosity function for several subsamples of galaxies in groups. In all cases, the luminosity functions are well described by Schechter functions, down to the faintest magnitudes we probe, M0.1r - 5 log(h) ~ -16. For the general luminosity function of galaxies in groups in the five SDSS bands, we observe that the characteristic magnitude is brighter at ~0.5 mag than those obtained for field galaxies by Blanton et al. Even when the observed faint-end slope is steeper in galaxy groups, it is statistically comparable with the field value. We analyze the dependence of the galaxy luminosity function with system masses, finding two clear trends: a continuous brightening of the characteristic magnitude and a steepening of the faint-end slope as mass increases. The results in 0.1g, 0.1r, 0.1i, and 0.1z bands show the same behavior. Using the u - r color to split the galaxy sample into red and blue galaxies, we show that the changes observed as a function of the system mass are mainly seen in the red, passively evolving galaxy population, while the luminosities of blue galaxies remain almost unchanged with mass. Finally, we observe that groups having an important luminosity difference between the two brightest galaxies of a system show a steeper faint-end slope than the other groups. Our results can be interpreted in terms of galaxy mergers as the main driving force behind galaxy evolution in groups.

Are compact groups hostile towards faint galaxies

Aims. We have performed a detailed analysis of the ability of the friends-of-friends algorithm to identify real galaxy systems in deep surveys such as the future Javalambre Physics of the Accelerating Universe Astrophysical Survey. Our approach was twofold: assessing the reliability of the algorithm in real and in redshift space. In the latter, our intention was also to determine the degree of accuracy that could be achieved when using spectroscopic or photometric-redshift determinations as a distance indicator. Methods. We built a light-cone mock catalogue using synthetic galaxies constructed from the Millennium Run Simulation I plus a semi-analytical model of galaxy formation. We explored different ways to define the proper linking length parameters of the algorithm to identify the best-suited galaxy groups in each case. Results. We found that when one identifies systems in redshift space using spectroscopic information, the linking lengths should take into account the variation of the luminosity function with redshift as well as the linear redshift dependence of the radial fiducial velocity in the line-of-sight direction. When we tested the purity and completeness of the group samples, we found that the best resulting group sample reaches values of ∼40% and ∼70% of systems with high levels of purity and completeness, when spectroscopic information was used. To identify systems using photometric redshifts, we adopted a probabilistic approach to link galaxies in the line-of-sight direction. Our result suggests that it is possible to identify a sample of groups with fewer than ∼40% false identifications at the same time as we recover around 60% of the true groups. Conclusions. This modified version of the algorithm can be applied to deep surveys provided that the linking lengths are selected appropriately for the science to be made with the data.

On the properties of compact groups identified in different photometric bands

We compute the redshift space power spectrum of two X-ray cluster samples: the X-ray Brightest Abell Cluster Sample (XBACS) and the Brightest Cluster Sample (BCS) using the method developed by Feldman, Kaiser & Peacock. The power spectrums derived for these samples are in agreement with determinations of other optical and X-ray cluster samples. For XBACS we find the largest power spectrum amplitude expected given the high richness of this sample (R ≥ 2). In the range 0.05hMpc 1 < k < 0.4hMpc 1 the power spectrum shows a power law behavior P(k) ∝ k n with an index n ≃ −1.2. In a similar range 0.04hMpc 1 < k < 0.3hMpc 1 BCS power spectrum has a smaller amplitude with index n ≃ −1.0. We do not find significant evidence for a peak at k ≃ 0.05hMpc 1 suggesting that claims such of feature detections in some cluster samples could relay on artificial inhomogeneities of the data. We compare our results with power spectrum predictions derived by Moscardini et al. within current cosmological models (LCDM and OCDM). For XBACS we find that both models underestimate the amplitude of the power spectrum but for BCS there is reasonably good agreement at k ∼ 0.03hMpc 1 for both models.

Fossil groups in the Millennium simulation - From the brightest to the faintest galaxies during the past 8 Gyr

The goal of this work is to understand whether the extreme environment of compact groups can affect the distribution and abundance of faint galaxies around them. We performed an analysis of the faint galaxy population in the vicinity of compact groups and normal groups. We built a light-cone mock galaxy catalogue constructed from the Millennium Run Simulation II plus a semi-analytical model of galaxy formation. We identified a sample of compact groups in the mock catalogue as well as a control sample of normal galaxy groups and computed the projected number density profiles of faint galaxies around the first- and the second-ranked galaxies. We also compared the profiles obtained from the semi-analytical galaxies in compact groups with those obtained from observational data. In addition, we investigated whether the ranking or the luminosity of a galaxy is the most important parameter in the determination of the centre around which the clustering of faint galaxies occurs. There is no particular influence of the extreme compact group environment on the number of faint galaxies in such groups compared to control groups. When selecting normal groups with separations between the 1st and 2nd ranked galaxies similar to what is observed in compact groups, the faint galaxy projected number density profiles in compact groups and normal groups are similar in shape and height. We observed a similar behaviour of the population of faint galaxies in observations and simulations in the regions closer to the 1st and 2nd ranked galaxies. Finally, we find that the projected density of faint galaxies is higher around luminous galaxies,regardless of the ranking in the compact group. The semi-analytical approach shows that compact groups and their surroundings do not represent a hostile enough environment to make faint galaxies to behave differently than in normal groups.

Luminosity function of galaxies in groups in the Sloan Digital Sky Survey Data Release 7: the dependence on mass, environment and galaxy type: Luminosity function of galaxies in groups

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:

The dynamical state of galaxy groups and their luminosity content

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Using the large scale quasar clustering to constrain flat quintessential universes

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