We have performed a detailed analysis of the ability of the friends-of-friends algorithm in identifying real galaxy systems in deep surveys such as the future Javalambre Physics of the Accelerating ...Universe Astrophysical Survey. Our approach is two-fold, i.e., assessing the reliability of the algorithm in both real and redshift space. In the latter, our intention is also to determine the degree of accuracy that could be achieved when using spectroscopic or photometric redshift determinations as a distance indicator. We have 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 have explored different ways to define the proper linking length parameters of the algorithm in order to perform an identification of galaxy groups as suitable as possible in each case. We find that, when identifying 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 testing purity and completeness of the group samples, we find that the best resulting group sample reaches values of 40% and 70% of systems with high levels of purity and completeness, respectively, when using spectroscopic information. When identifying 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 less than 40% false identification at the same time as we recover around 60% of the true groups. 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 done with the data.
This work analyses the properties of groups of galaxies in the surroundings
of clusters. On the basis of a very large public Virgo Consortium Simulation,
we identified systems of galaxies in a wide ...range of masses. Systems with
masses greater than $ M_{cut}= 4 \times 10^{14} M_{\odot} h^{-1} $ are
considered "host", whereas smaller systems are taken as groups. Our results
show that groups properties are affected by the proximity of massive hosts.
Physical properties such as velocity dispersion, internal energy ($E$) and
virial radius, show an increment, whereas the mean density decreases as the
host-group distance is smaller. By analysing groups with different properties,
we find that the low mass and the weakly bounded ($E > 0$) subsamples, are
strongly affected by the presence of the host; on the other hand, massive
groups and groups with $E < 0$ do not show dependence on the host-group
distance. Using a sample of groups identified in the final version of the 2dF
Galaxy Redshift Survey, we find a very similar velocity dispersion behaviour in
the observational data compared to results in the simulation. We also study the
dependence of the groups velocity dispersion on the host masses in both,
observations and simulation; finding that the larger is the host mass the
higher is the effect on its vicinity.
This work analyses the properties of groups of galaxies in the surroundings of clusters. On the basis of a very large public Virgo Consortium Simulation, we identified systems of galaxies in a wide ...range of masses. Systems with masses greater than \( M_{cut}= 4 \times 10^{14} M_{\odot} h^{-1} \) are considered "host", whereas smaller systems are taken as groups. Our results show that groups properties are affected by the proximity of massive hosts. Physical properties such as velocity dispersion, internal energy (\(E\)) and virial radius, show an increment, whereas the mean density decreases as the host-group distance is smaller. By analysing groups with different properties, we find that the low mass and the weakly bounded (\(E > 0\)) subsamples, are strongly affected by the presence of the host; on the other hand, massive groups and groups with \(E < 0\) do not show dependence on the host-group distance. Using a sample of groups identified in the final version of the 2dF Galaxy Redshift Survey, we find a very similar velocity dispersion behaviour in the observational data compared to results in the simulation. We also study the dependence of the groups velocity dispersion on the host masses in both, observations and simulation; finding that the larger is the host mass the higher is the effect on its vicinity.
A comprehensive study on compactness has been carried out on the 2dF Galaxy
Group Catalogue constructed by Merch\'an & Zandivarez. The compactness indexes
defined in this work take into account ...different geometrical constraints in
order to explore a wide range of possibilities. Our results show that there is
no clear distinction between groups with high and low level of compactness when
considering particular properties as the radial velocity dispersion, the
relative fraction of galaxies per spectral type and luminosity functions of
their galaxy members.
Studying the trend of the fraction of galaxies per spectral type as a
function of the dimensionless crossing time some signs of dynamical evolution
are observed. From the comparison with previous works on compactness we realize
that special care should be taken into account for some compactness criteria
definitions in order to avoid possible biases in the identification.
Mon.Not.Roy.Astron.Soc. 337 (2002) 1441 Several statistics are applied to groups and galaxies in groups in the Two
degree Field Galaxy Redshift Survey. Firstly we estimate the luminosity
functions ...for different subsets of galaxies in groups. The results are well
fitted by a Schechter function with parameters $M^{\ast}-5\log(h)=-19.90 \pm
0.03$ and $\alpha=-1.13\pm0.02$ for all galaxies in groups, which is quite
consistent with the results by Norberg et al. for field galaxies. When
considering the four different spectral types defined by Madgwick et al. we
find that the characteristic magnitude is typically brighter than in the field.
We also observe a steeper value, $\alpha=-0.76\pm 0.03$, of the faint end slope
for low star-forming galaxies when compared with the corresponding field value.
This steepening is more conspicuous, $\alpha=-1.10\pm 0.06$, for those galaxies
in more massive groups (${\mathcal M} \gsim 10^{14} h^{-1} M_{\odot}$) than the
obtained in the lower mass subset, $\alpha=-0.71\pm 0.04$ (${\mathcal
M}<10^{14} h^{-1} M_{\odot}$). Secondly, we compute group total luminosities
using Moore, Frenk & White prescriptions. We define a flux-limited group sample
using a new statistical tool developed by Rauzy. The resulting group sample is
used to determine the group luminosity function finding a good agreement with
previous determinations and semianalytical models. Finally, the group mass
function for the flux-limited sample is derived. An excellent agreement is
obtained when comparing our determination with analytical predictions over two
orders of magnitude in mass.
Mon.Not.Roy.Astron.Soc. 333 (2002) L31 We estimate the fraction of star forming galaxies in a catalogue of groups,
constructed from the 2dF galaxy redshift survey by Merch\'an & Zandivarez. We
use ...the $\eta$ spectral type parameter of galaxies and subdivide the sample of
galaxies in groups into four types depending on the values of the $\eta$
parameter following Madgwick et al. We obtain a strong correlation between the
relative fraction of galaxies with high star formation and the parent group
virial mass. We find that even in the environment of groups with low virial
mass $M \sim 10^{13} M_{\odot}$ the star formation of their member galaxies is
significantly suppressed. The relation between the fraction of early-type
galaxies and the group virial mass obeys a simple power law spanning over three
orders of magnitude in virial mass. Our results show quantitatively the way
that the presence of galaxies with high star formation rates is inhibited in
massive galaxy systems.
Mon.Not.Roy.Astron.Soc. 335 (2002) 825 The behaviour of the relative fraction of galaxies with different spectral
types in groups is analysed as a function of projected local galaxy density and
the ...group-centric distance. The group sample was taken from the 2dF Group
Galaxy Calatogue constructed by Merch\'an & Zandivarez. Our group sample was
constrained to have a homogeneous virial mass distribution with redshift.
Galaxies belonging to this group sample were selected in order to minimize
possible biases such as preferential selection of high luminosity objects. We
find a clear distinction between high virial mass groups ($M_V\gsim 10^{13.5}
M_{\odot}$) and the less massive ones. While the massive groups show a
significant dependence of the relative fraction of low star formation galaxies
on local galaxy density and group-centric radius,groups with lower masses show
no significant trends. We also cross-correlate our group subsample with the
previously identified clusters finding that this sample shows a very similar
behaviour as observed in the high virial mass group subsample.
A comprehensive study on compactness has been carried out on the 2dF Galaxy Group Catalogue constructed by Merchán & Zandivarez. The compactness indexes defined in this work take into account ...different geometrical constraints in order to explore a wide range of possibilities. Our results show that there is no clear distinction between groups with high and low level of compactness when considering particular properties as the radial velocity dispersion, the relative fraction of galaxies per spectral type and luminosity functions of their galaxy members. Studying the trend of the fraction of galaxies per spectral type as a function of the dimensionless crossing time some signs of dynamical evolution are observed. From the comparison with previous works on compactness we realize that special care should be taken into account for some compactness criteria definitions in order to avoid possible biases in the identification.
Several statistics are applied to groups and galaxies in groups in the Two degree Field Galaxy Redshift Survey. Firstly we estimate the luminosity functions for different subsets of galaxies in ...groups. The results are well fitted by a Schechter function with parameters \(M^{\ast}-5\log(h)=-19.90 \pm 0.03\) and \(\alpha=-1.13\pm0.02\) for all galaxies in groups, which is quite consistent with the results by Norberg et al. for field galaxies. When considering the four different spectral types defined by Madgwick et al. we find that the characteristic magnitude is typically brighter than in the field. We also observe a steeper value, \(\alpha=-0.76\pm 0.03\), of the faint end slope for low star-forming galaxies when compared with the corresponding field value. This steepening is more conspicuous, \(\alpha=-1.10\pm 0.06\), for those galaxies in more massive groups (\({\mathcal M} \gsim 10^{14} h^{-1} M_{\odot}\)) than the obtained in the lower mass subset, \(\alpha=-0.71\pm 0.04\) (\({\mathcal M}<10^{14} h^{-1} M_{\odot}\)). Secondly, we compute group total luminosities using Moore, Frenk & White prescriptions. We define a flux-limited group sample using a new statistical tool developed by Rauzy. The resulting group sample is used to determine the group luminosity function finding a good agreement with previous determinations and semianalytical models. Finally, the group mass function for the flux-limited sample is derived. An excellent agreement is obtained when comparing our determination with analytical predictions over two orders of magnitude in mass.
We estimate the fraction of star forming galaxies in a catalogue of groups, constructed from the 2dF galaxy redshift survey by Merchán & Zandivarez. We use the \(\eta\) spectral type parameter of ...galaxies and subdivide the sample of galaxies in groups into four types depending on the values of the \(\eta\) parameter following Madgwick et al. We obtain a strong correlation between the relative fraction of galaxies with high star formation and the parent group virial mass. We find that even in the environment of groups with low virial mass \(M \sim 10^{13} M_{\odot}\) the star formation of their member galaxies is significantly suppressed. The relation between the fraction of early-type galaxies and the group virial mass obeys a simple power law spanning over three orders of magnitude in virial mass. Our results show quantitatively the way that the presence of galaxies with high star formation rates is inhibited in massive galaxy systems.
