158 research outputs found
The colour-magnitude relations of ClJ1226.9+3332, a massive cluster of galaxies at z=0.89
(Abridged) The colour-magnitude relations of one of the most massive, high
redshift clusters of galaxies known have been studied. Photometry has been
measured in the V, R, I, z, F606W, F814W, J and K bands to a depth of K*+2.5
and spectroscopy confirms 27 K band selected cluster members. The V-K colours
are equivalent to a rest-frame colour of ~2700A-J, and provide a very sensitive
measure of star-formation activity. HST ACS imaging has been used to
morphologically classify the galaxies.
The cluster has a low early-type fraction compared to nearby clusters, with
only 33% of the cluster members having types E or S0. The early-type member
galaxies form a clear red-sequence in all colours. The scatter and slope of the
relations show no evolution compared to the equivalent Coma cluster relations,
suggesting the stellar populations are already very old. The normalisation of
the relations has been compared to models based on synthetic stellar
populations, and are most consistent with stellar populations forming at z>3.
Some late-type galaxies were found to lie on the red-sequence, suggesting that
they have very similar stellar populations to the early-types.
These results present a picture of a cluster in which the early-type galaxies
are all old, but in which there must be future morphological transformation of
galaxies to match the early-type fraction of nearby clusters. In order to
preserve the tight colour-magnitude relation of early-types seen in nearby
clusters, the late-type galaxies must transform their colours, through the
cessation of star-formation, before the morphological transformation occurs.
Such evolution is observed in the late-types lying on the colour-magnitude
relation.Comment: Accepted for publication in MNRAS. 14 pages, 5 figure
Evolution of the galaxy luminosity function in progenitors of fossil groups
Using the semi-analytic models based on the Millennium simulation, we trace
back the evolution of the luminosity function of galaxies residing in
progenitors of groups classified by the magnitude gap at redshift zero. We
determine the luminosity function of galaxies within , and for galaxy groups/clusters. The bright end of the galaxy
luminosity function of fossil groups shows a significant evolution with
redshift, with changes in by 1-2 mag between and
(for the central ), suggesting that the formation of the most
luminous galaxy in a fossil group has had a significant impact on the
galaxies e.g. it is formed as a result of multiple mergers of the
galaxies within the last Gyr. In contrast, the slope of the faint end,
, of the luminosity function shows no considerable redshift evolution
and the number of dwarf galaxies in the fossil groups exhibits no evolution,
unlike in non-fossil groups where it grows by towards low
redshifts. In agreement with previous studies, we also show that fossil groups
accumulate most of their halo mass earlier than non-fossil groups. Selecting
the fossils at a redshift of 1 and tracing them to a redshift 0, we show that
of the fossil groups () will lose their large magnitude gaps. However, about
of fossil clusters () will retain their large
gaps.Comment: Accepted for publication in A&A. 13 pages, 15 figure
The evolution of the radio luminosity function of group galaxies in COSMOS
To understand the role of the galaxy group environment on galaxy evolution,
we present a study of radio luminosity functions (RLFs) of group galaxies based
on the Karl G. Jansky Very Large Array-COSMOS 3 GHz Large Project. The
radio-selected sample of 7826 COSMOS galaxies with robust optical/near-infrared
counterparts, excellent photometric coverage, and the COSMOS X-ray galaxy
groups (M_200c > 10^13.3 M_0) enables us to construct the RLF of group galaxies
(GGs) and their contribution to the total RLF since z ~ 2.3. Using the Markov
chain Monte Carlo algorithm, we fit a redshift-dependent pure luminosity
evolution model and a linear and power-law model to the luminosity functions.
We compare it with past RLF studies from VLA-COSMOS on individual populations
of radio-selected star-forming galaxies (SFGs) and galaxies hosting active
galactic nuclei (AGN). These populations are classified based on the presence
or absence of a radio excess concerning the star-formation rates derived from
the infrared emission. We find that the fraction of radio group galaxies
evolves by a factor of ~ 3 from z ~ 2 to the present day. The increase in the
galaxy group contribution is due to the radio activity in groups being nearly
constant at z < 1, while it is declining in the field. We show that massive
galaxies inside galaxy groups remain radio active below redshift 1, contrary to
the ones in the field. This evolution in the GG RLF is driven mainly by
satellite galaxies in groups. Group galaxies associated with SFGs dominate the
GG RLF at z_med = 0.3, while at z_med = 0.8, the peak in the RLF, coinciding
with a known overdensity in COSMOS, is mainly driven by AGN. The study provides
an observational probe for the accuracy of the numerical predictions of the
radio emission in galaxies in a group environment.Comment: submitted to A&A; 15 pages, 6 figures, 8 table
On the Reliability of Photometric and Spectroscopic Tracers of Halo Relaxation
We characterize the relaxation state of galaxy systems by providing an assessment of the reliability of the photometric and spectroscopic probe via the semianalytic galaxy evolution model. We quantify the correlations between the dynamical age of simuglated galaxy groups and popular proxies of halo relaxation in observation, which are mainly either spectroscopic or photometric. We find the photometric indicators demonstrate a stronger correlation with the dynamical relaxation of galaxy groups compared to the spectroscopic probes. We take advantage of the Anderson Darling statistic (A(2)) and the velocity segregation (Delta V) as our spectroscopic indicators, and use the luminosity gap (Delta m(12)) and the luminosity decentering (D-offset) as photometric ones. First, we find that a combination of Delta m(12) and D-offset evaluated by a bivariant relation (B = 0.04 x Delta m(12) - 0.11 x Log(Doff-set) + 0.28) shows a good correlation with the dynamical age compared to all other indicators. Second, by using the observational X-ray surface brightness map, we show that the bivariant relation brings about some acceptable correlations with X-ray proxies. These correlations are as well as the correlations between A(2) and X-ray proxies, offering a reliable yet fast and economical method of quantifying the relaxation of galaxy systems. This study demonstrates that using photometric data to determine the relaxation status of a group will lead to some promising results that are comparable with the more expensive spectroscopic counterpart.Peer reviewe
Scaling relations in early-type galaxies belonging to groups
We present a photometric analysis of a large sample of early-type galaxies in
16 nearby groups, imaged with the Wide-Field Camera on the Isaac Newton
Telescope. Using a two-dimensional surface brightness decomposition routine, we
fit Sersic (r^{1/n}) and exponential models to their bulge and disk components
respectively. Dividing the galaxies into three subsamples according to the
X-ray luminosities of their parent groups, we compare their photometric
properties. Galaxies in X-ray luminous groups tend to be larger and more
luminous than those in groups with undetected or low X-ray luminosities, but no
significant differences in n are seen. Both normal and dwarf elliptical
galaxies in the central regions of groups are found to have cuspier profiles
than their counterparts in group outskirts.
Structural differences between dwarf and normal elliptical galaxies are
apparent in terms of an offset between their ``Photometric Planes'' in the
space of n, r_e and mu_0. Dwarf ellipticals are found to populate a surface,
with remarkably low scatter, in this space with significant curvature, somewhat
similar to the surfaces of constant entropy proposed by Marquez etal (2001).
Normal ellipticals are offset from this distribution in a direction of higher
specific entropy. This may indicate that the two populations are distinguished
by the action of galaxy merging on larger galaxies.Comment: Accepted for publication in MNRAS, 8 pages, 8 postscript figure
NGC1600 - Cluster or Field Elliptical?
A study of the galaxy distribution in the field of the elliptical galaxy
NGC1600 has been undertaken. Although this galaxy is often classified as a
member of a loose group, all the neighbouring galaxies are much fainter and
could be taken as satellites of NGC1600. The number density profile of galaxies
in the field of this galaxy shows a decline with radius, with evidence of a
background at approximately 1.3 Mpc. The density and number density profile are
consistent with that found for other isolated early-type galaxies. NGC1600
appears as an extended source in X-rays, and the center of the X-ray emission
seems not to coincide with the center of the galaxy. The velocity distribution
of neighbouring galaxies has been measured from optical spectroscopic
observations and shows that the mean radial velocity is approximately 85 km/s
less than that of NGC1600, indicating that the centre of mass could lie outside
the galaxy. The velocity dispersion of the `group' is estimated at 429 km/s.
The inferred mass of the system is therefore of the order of 10^14 solar
masses, a value that corresponds to a large group. NGC1600 therefore shares
some similarities, but is not identical to, the `fossil clusters' detected in
X-ray surveys. Implications of this result for studies of isolated early-type
galaxies are briefly discussed.Comment: 8 pages, 5 figures and 2 table, accepted for publication in the Ap
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